This document provides a review of joint kinetics methods, interpretation, and use in treatment decision-making for children with cerebral palsy and myelomeningocele. It begins with an introduction to how joint kinetics has become an important part of clinical gait analysis and decision making. It then describes the methods used to compute joint kinetics from kinematic and force platform data. The rest of the document discusses guidelines for interpreting joint kinetics data and provides examples of its use in decision making for orthosis selection and understanding the effects of surgery, though its role in surgical decision making is still limited.

1. ELSEVJER Gait Jr Posture 4 (1996)62-78
Review article
Joint kinetics: methods, interpretation and treatment decision-making
in children with cerebral palsy and myelomeningocele
Sylvia Ounpuu *, Roy B. Davis, Peter A. DeLuca
Connecticut Childrens Medical Center, 181 East Cedar St., Newington, CT 06111, USA
Accepted20 July 1995
Abstract
Computerized gait analysis has become an integral part of the treatment decision-making process in many clinical settings. The
integration of kinetic data, more specifically joint moments and powers, is a relatively new addition to other types of data including
joint kinematics, temporal and stride parameters and electromyography. Joint kinetic data is an important contribution to the
understanding of the cauSeof certain gait abnormalities which arenot provided by the other measures.Its utility isnot only limited
to the surgical decision-making processin personswith cerebral palsy and myelomeningocele but also in the orthosis decision-
making process.At the time of this writing, its useasasurgical decision-making tool islimited to a fewtypesof treatment. However,
systematic study of the effects of treatment on the joint kinetics and the relationship of deviations at one joint with adjacent joints
will improve our understanding of these data and how they can become an integral part of the treatment decision-making process.
A reviewof the methods, pointers on interpretation and specificdata examples will provide the reader with a detailed introduction
to joint kinetics.
Keywordr: Kinetics; Methods; Clinical decision-making; Cerebral palsy; Myelomeningocele
1. Introduction
In many clinical settings, computerized gait analysis
has become an integral part of the clinical decision-
making process for the treatment of gait abnormalities
[l-6]. The majority of clinical decisions derived from
computerized gait analysis have been directed by
kinematic and electromyography (EMG) data in com-
bination with clinical examination measures. The
precise assessment of these types of information has
been invaluable in contributing to the clinicians’
understanding of the mechanisms in normal gait as well
as in pathological gait of persons with complex
neuromuscular disorders such as cerebral palsy (CP)
and myelomeningocele (ML).
More recently, joint kinetics, specifically joint mom-
ents and joint powers, have been available as an ad-
ditional tool in the assessment of normal [7-lo] and
pathological gait [ 11- 171. Identifying specific joint
l Corresponding
author.
0966-6362/96/$15.00
0 1996
Elsevier
Science
B.V.All rightsreserved
SSDI 0966-6362(95)01044-A
kinematic and related joint kinetic patterns and their re-
lationship to associated clinical measures such as joint
range of motion is an important component to the
understanding of the mechanisms of gait. Joint kinetics
provides an opportunity to better appreciate the role of
trunk positioning and the relationship between joints
and limbs during gait. For example, the evaluation of
the relationship of power generation on the involved
versus non-involved side of persons with hemiplegia
suggests that the non-involved limb shows greater than
normal power generation to compensate for the weaker
non-involved limb [18,191. Understanding this general
mechanism of gait in persons with hemiplegia helps the
clinician recognize pathology specific concerns and may
eventually guide treatment protocols.
The two primary avenues of treatment of gait abnor-
malities in patients with CP and ML are surgical inter-
vention and orthotic management. The understanding
of the effects of orthoses in terms of joint kinetics is a
more straightforward task in comparison to surgical
decision-making. That is, the changes in kinetic patterns

2. S. dunpuu et al. /Gait and Posture 4 (1996) 62-78 63
are a direct function of the orthosis which is the only
parameter changed. Multiple trials of barefoot walking
can be immediately followed by multiple trials of brace
walking. Several studies have used joint kinetics in this
way to evaluate the effects and function of orthoses such
as the posterior leaf spring ankle-foot orthosis (PLS) in
patients with CP [20], the function of an ankle-foot or-
thosis on sagittal plane ankle and knee joint kinetics [21]
and to confirm the need for orthoses such as the knee-
ankle foot orthosis (KAFO) in patients with ML [22].
Joint kinetics may also be used effectively to evaluate
the function of an orthosis in a routine clinical gait anal-
ysis evaluation 111,221. The information obtained in a
routine test may be used for prescribing a new orthosis
if the present orthosis cannot be modified.
While the routine collection of joint kinetics is more
common in clinical gait laboratories, our understanding
of its clinical application in treatment decision-making
for orthopaedic surgery is still in its infancy. As with all
treatment decision-making, it is important to identify
correctly and treat the primary problem and not the sec-
ondary adaptation. Study of the effects of surgery on
joint kinetics can help the clinician identify between the
two. The task of determining the effects of surgery on
joint kinetics, however, is much more complex than
understanding the effects of an orthosis on joint kinet-
ics. In the year between surgery and the follow up gait
analysis, the patient may have grown in height and/or
weight, had some other intervention, like physical thera-
py, that might contribute to a change in the patient’s
joint kinetic patterns. Also, when multiple levels of sur-
gery are done simultaneously and frequently bilaterally,
change is difficult to attribute to a specific procedure.
Therefore, attributing a change in joint kinetics to a spe-
cific procedure or combination of procedures is diflicult.
There is a similar problem in the evaluation of outcome
in terms of any measure used to document gait including
joint kinematics. The work in this area has only just
begun [12,18,19]. Continued systematic study of the ef-
fects of surgery on joint kinetics will be critical to our
understanding of joint kinetics by providing us the data
to identify patterns of movement as well as the post op-
erative studies to evaluate the effects of surgery.
The purpose of this paper is to present the methods
of computation and examine the present role of joint ki-
netics in the treatment decision-making process for per-
sons with CP and ML. We will begin with the methods
of estimation, followed by guidelines for interpretation
and finally several case examples. Joint kinetics which
are one component of gait analysis are generally not in-
terpreted in isolation of joint kinematics, dynamic EMG
and related clinical examination information such as
joint range of motion and muscular strength. These
other components of computerized gait analysis will be
referred to as needed throughout the text.
2. Joint kinetics computation
Currently available technology does not allow the
direct measurement of joint kinetics, that is, joint forces,
moments, and powers, as part of routine clinical gait
analysis. Consequently, these joint kinetic quantities
must be estimated through the combination of
kinematic data associated with body segment locations
and spatial orientations with force platform data. In the
clinical setting, these empirical data are provided usually
by a motion measurement system capable of measuring
the trajectories of markers placed on the surface of the
lower extremities in conjunction with one or more force
platforms to monitor ground reaction forces. These
measured data provide the input to the biomechanical
model or computational process (Fig. 1) that quantifies
both the position and orientation of the lower ex-
tremities (i.e. kinematics) and the joint kinetics.
More specifically, the calculation of joint forces,
moments and powers over the gait cycle require the
following data sets:
l The locations of the hip, knee and ankle joints,
l The locations of the center of mass (CM) of the
thigh, shank and foot,
l The linear acceleration of the CM of the thigh, shank
and foot,
l The angular velocity and acceleration of the thigh,
shank and foot,
l The ground reaction forces and vertical torque, and
l The location of the point of application of the
ground reactions, referred to as the center of pres-
sure.
Fig. 1. A schematic representation of the joint kinetics computational
plWO2SS.

3. 64 S. bunpuu et al. /Gait and Posture 4 (19%) 62-78
These data are then incorporated into equations of
motion along with estimates of the mass and the mass
moment of inertia of each lower extremity segment. In
addition, anatomically fixed frames of reference are re-
quired for the thigh, shank and foot.
The computation of joint kinetics is a relatively
straightforward application of Newtonian mechanics.
Current computation strategies remain comparable to
the approaches described in several volumes by Braune
and Fisher [23,24] between 1889 and 1904 and Bresler
and Frankel [25] in 1950. Because of current computer
power, the most significant change is the speed by which
data can be collected and processed. For example, com-
pare the minutes or even seconds required currently for
each walking trial to the 500 person-hours per trial ex-
perienced by Bresler and Frankel in 1950.
It is important to appreciate each of the input quan-
tities listed above before describing the analytical ap-
proach in detail. For this discussion on joint kinetics, it
is assumed that the three-dimensional locations of the
hip, knee and ankle joint centers have been obtained
from some type of motion measurement system equip-
ped with force platforms for the measurement of ground
reaction forces, vertical torque and the locations of the
center of pressure. It is important to note that the
moments of force are computed about some reference
point. Consequently, a hip joint moment must be com-
puted about a hip joint center and not a marker placed
on the surface of the limb, e.g., ‘over’ the greater
trochanter. Moreover, it is assumed for this discussion
that anatomically fixed frames of reference have been
computed for the thigh, shank and foot throughout the
gait cycle [26]. These provide reference axes to describe
both the rotation of a distal segment relative to a prox-
imal segment, e.g., flexion (or extension) of the knee,
and the moment of force applied by the proximal seg-
ment on the distal segment, e.g., a knee flexor (or exten-
sor) moment. The importance of the selection of
reference coordinate system will be described below.
In addition to these measured quantities, other par-
ameters associated with the weights and the inertial pro-
perties of each segment are typically estimated from
statistically-based anthropometric relationships derived
from human data or from models of the human body as
a collection of simple geometric shapes, e.g., the seg-
ments as conic sections or stacks of elliptical slices. The
results of cadaver studies by Dempster [27], Clauser et
al. [28], Chandler et al. [29], and Liu and Wickstrom
[30] are representative of the former approach where the
locations of the CM of the thigh, shank and foot might
be expressed as a function of segment length, e.g., the
CM of the thigh is approximately 43% from the hip
center and approximately 57% from the knee center
[27]. Similarly, the mass of each segment is computed as
a percentage of total body mass. The mass moment of
inertia of each lower extremity segment is commonly
calculated from a statistically-based relationship for the
radius of gyration of the segment expressed as a function
of segment length and the segmental mass, e.g.,
Zi = m&Ii2 (1)
where
Zi = mass moment of inertia of the segment, i.e.
thigh, shank or foot, about a particular axis,
mi = mass of the segment, and
pi = radius of gyration of the segment about a partic-
ular axis.
An alternative method for the assessment of the inertial
properties of limb segments was first proposed by Wein-
bath [31] in 1938. In this approach, the surface
geometry of the limb segment is mapped and then divid-
ed analytically into regularly-shaped, e.g., elliptical,
slices of constant density. In this way, the inertial pro-
perties of the entire segment are determined by combin-
ing the contributions of the individual discrete elements.
More recently, Jenson [32] has applied this geometrical
method to document further the anthropometry of
children by digitizing the photographs of the front and
side views of the subjects. Alternatively, entire segments
may be approximated assimple solids of some geometric
shape, e.g., ellipsoidal, cylindrical, of constant density,
such as the 15 segment model of Hanavan [33]. Until
recently, the time required to quantify the surface
geometry of the segment from photographic images or
numerous anatomical measurements, e.g., over 200 in
the detailed model of Hatze [34], reduced the efficacy of
this approach in the clinical setting. The incorporation
of video technology and edge detection algorithms may
improve the clinical feasibility of the technique.
The review article by Cappozzo and Bet-me [35] pro-
‘vides an excellent summary of the potential error in an-
thropometric estimates. These authors conclude that
when extreme accuracy is desired, Hatze’s model ap-
pears to offer the best current approach. They recom-
mend that if a statistically-based strategy is used, then
the test subject should be consistent with the body com-
position and sex of the study population employed to
develop the regression relationships. For example, the
work of Dempster [27] is based on eight male subjects
ranging in age from 52 to 83 years. It should also be
noted that these techniques are intended to estimate the
principal moments of inertia about the principal axes
passing through the CM of the segment. Consequently,
the anatomically aligned coordinate systems fixed to
each segment should either be constructed to approx-
imate the principle axes of the segment or appropriate

4. S. &npuu et al. /Gait and Posture 4 (I 9%) 62- 78 65
rotational transformations used to relate the two sets of
body fixed axes.
Numerical differentiation methods must be employed
to compute the second derivatives of the displacement of
the CM of the thigh, shank and foot, i.e., the linear ac-
celeration of each segmental CM, and the first and sec-
ond derivatives of the angular displacement of each
segment, i.e., the angular velocity and acceleration,
respectively. A wide array of methods are available for
this requirement including high order finite difference
schemes, polynomial regression, spline functions, and
Fourier series approximations. Woltring [36] concluded
that quintic splines can be employed satisfactorily, but
that the Fourier series approach is computationally
faster and more numerically stable. One must appreciate
that a Fourier series strategy yields harmonic amplitude
and phase results that represent an average over a par-
ticular interval, e.g., results associated with a gait cycle
combines the different frequency contents of stance and
swing phases. Winter [37] describes an approach, com-
monly used in gait analysis, that combines digital filter-
ing (to smooth the motion data) with finite differences
(for derivative computation). For additional informa-
tion, the reader is referred to a number of review articles
that have been produced on the smoothing and differ-
entiation of noisy data, e.g., Hatze [38], Woltring [36],
and Wood [39].
Given the availability of the empirical data, the
segmental inertial estimates and the numerically gener-
ated derivatives, one may compute the net moment of
force and power at each lower extremity joint. As in-
dicated above the computational method is based upon
Newtonian mechanics and illustrated in the classical
contributions of Braune and Fischer [23,24], Elftman
[40], Bresler and Frankel [25], and Paul [41]. To
demonstrate the process, consider a free-body-diagram
of the foot in the stance phase of the gait cycle that in-
cludes the external loads due to gravity and ground in-
teraction and exposes the reactions produced by the
body at the ankle, i.e., net joint force and moment (Fig.
2) where
A = the 3-D location of the ankle center relative to
the fixed laboratory coordinate system ex-
pressed as A,, A, and A,,
C = the 3-D location of the CM of the foot relative
to the fixed laboratory coordinate system ex-
pressed as C,, CYand C,,
P= the 3-D location of the point of application of
the ground reactions on the foot, i.e., the
center of pressure, relative to the fixed labora-
tory coordinate system expressed as P, and Py
with P, generally set to zero,
FA = the net inter-segmental reactive force between
the shank and foot,
MA = the net ankle moment reaction vector,
MA
Y
Fig. 2. A free-body diagram of the foot segment, the first of the three
free-body diagrams required in the computation of joint kinetics.
W’= weight of the foot (the product of the mass of
the foot, mf, and the gravitational constant,
g)v
F* = the ground force reaction vector, and
T= the ground torque (vertical) reaction vector.
Newton’s second law provides the basis for the equation
of translational motion as
F=mpc (2)
where
mf=
ac=
F=
the mass of the foot,
the linear acceleration (a 3-D vector) of the
CM of the foot, and
the sum of the external forces acting on the
foot.
Expanding Eq. 1,
and solving for the net inter-segmental reactive force be-
tween the shank and foot yields,
FA= mAac-g)-Fg (4)
or
FAX-~cx - Fgx
FAN
-PC, - F.
FAZ-&cz - d - F.
(5)
It should be noted that care has been taken to define FA
asa net inter-segmental reactive force between the shank
and foot that is different from the joint contact (or bone-
on-bone) force produced between articulating surfaces
of the ankle joint. The net inter-segmental reactive force
reflects the effects of external loads such as segment
weight and acceleration as well asground reactions. The
compressive joint bone-on-bone force represents not
only these external loads but also muscle (and other soft

5. 66 S. hmpuu et al. /Gait and Posture 4 (19%) 62-78
tissue) forces that cross the joint. Consequently, joint
contact loads are generally greater than the correspon-
ding net inter-segmental reactive force magnitudes dur-
ing the stance phase of gait, e.g., Paul [41] reports peak
hip (bone-on-bone) force magnitudes of 3.9 times body
weight compared to an inter-segmental reactive force
magnitude of approximately 1.1 times body weight [25].
The computation of the joint moments is facilitated
through the use of the equation of rotational motion,
i.e.,
M=li (6)
where M is the sum of the external moments acting on
the segment and fiis the rate of change of the angular
momentum of the segment. Eq. 6 is valid when the refer-
ence point for calculating these quantities is taken at the
center of mass of the segment or a fixed point. Joint
moments may also be computed about an arbitrary
point, such as the joint center as in the work by Bresler
and Frankel [25] and Paul [41], through the use of the
following relationship
M,=&-+p x m,ae
where
(7)
y= the net joint moment reaction vector,
H& = the rate of change of the angular momentum
about the CM of the segment,
P= the vector from the joint center to the CM of
the segment,
m, = mass of the segment, and
ao = linear acceleration of the CM of the segment.
The expansion of either Eq. 6 or 7 produces a significant
number of scalar terms that relate the mass moment of
inertia, angular acceleration, and angular velocity of the
segment. Technically, nine values are required to de-
scribe the distribution of the mass of the segment rela-
tive to a chosen axis, i.e., three moments and six
products of inertia. In order to reduce this complexity,
investigators have often made assumptions. Bresler and
Frankel [25] assumed that the inertial properties of the
segments (relative to the fixed laboratory coordinate
system) did not change as the subject walked, i.e., the
segments remain approximately vertical (thigh and
shank) or pointed forward (foot). Paul [41] accounted
for the change in spatial orientation of the segments as
seen from the front or the side of the subject and assum-
ed that transverse rotation of the segments were negligi-
ble. Given the measurement and computational re-
sources available at the time, both were reasonable ap-
proaches.
The use of Eq. 6 is considerably simplified if the re-
quired reference coordinate system is chosen SOthat it
coincides with the principal axes of the segment and are
located at the CM of the segment. The resulting
simplification produces a set of three equations known
as Euler’s equation’s of motion. The anatomically fixed
segment coordinate systems described above required
for joint angle computation can be specified to satisfy
this requirement. Returning to the foot kinetics illustra-
tion, the foot coordinate system, x’ y’ z’, must be pro-
duced by the measurement process so asto be equivalent
to the principal axesof the foot. Under these conditions,
Eq. 6 reduces to
M z,a,~
x, = + (Z,* - Zy) WY’w,,
Mf = ZpYy’ + (I,* - I,,) w,, W.&f
M z’ = zza,* + (Zy’ - I,#) W,) WY’
where
(8)
M,,, MyI, M,, = the x’ y’ z’ components of sum
of the external moments about
the CM of the segment,
a,,, o!f, c-t,1= the x’ y’ z’ components of the
absolute angular acceleration of
the segment,
I,,, zy, Z,l = the principal mass moments of
inertia of the segment, and
w,,, WY’,w,* = the x’ y’ z’ components of the
absolute angular velocity of the
segment.
The left side of Eq. 8 includes the known external
moments due to the ground reaction force (rp,c- x Fr),
the ground reaction torque (Z’), the inter-segmental reac-
tive force (r&c x FA) as well as the unknown net ankle
moment, MA. On the right side of Eq. 8, the necessary
kinematic values, w and (Y, are provided through
numerical differentiation of the acquired data and the
inertial properties are estimated from anthropometric
relationships, both of which were described above. Note
that these vector quantities are first transformed into the
foot coordinate system before they may be used in Eq.
8. The computation result, MAP, is commonly reported
relative to the foot coordinate system, or alternatively,
it may be transformed into another reference coordinate
system for reporting, e.g., the fixed laboratory axes. The
net knee moment is computed using the reactions ob-
tained from this first step, i.e., FA I and MA,, are used as
the external load on the distal shank and the process re-
peated, treating the shank as the free-body. The proce-
dure is then repeated in like manner for the thigh to
calculate the instantaneous moment at the hip.
The mechanical power associated with joint rotation
is computed from the combination of the joint moment
and the joint angular velocity (the rotational velocity of
one segment relative to another), i.e.,
P= M, . whh, (9)

6. S. dunpuu et al. /Gait and Posture 4 (19%) 62-78 61
This quantity represents the rate at which work is
done by the joint moment in producing or controlling
joint rotational displacement. Joint power can
sometimes be related to a particular type of muscular
contraction. That is, positive joint power is sometimes
referred to as ‘power generation’ and related to the
dominance of a concentrically contacting muscle group.
Conversely, negative joint power may be referred to as
‘power absorption’ and related to the dominance of a ec-
centrically contracting muscle group. Care must be exer-
cised in this assumed relationship because joint
moments are produced by both active muscular contrac-
tion and passive soft tissue forces. Consequently, joint
power may be ‘absorbed’ by the elongation of passive
muscles, e.g., passive iliopsoas elongation during mid-
to-late stance.
Through the consideration of only the rotational joint
power (e.g., Eq. 9), one implicitly assumes that the
translational joint power (associated with the transla-
tion of one segment relative to another) is negligible. In
a recent article, Buczek [42] explored this assumption
through the application of a six degree-of-freedom gait
model that allowed three degrees of rotation and three
degrees of translation at each joint. This allows Eq. 9 to
be expanded to
(10)
where F, is the net joint force (i.e., reaction force be-
tween adjoining segments) and V’, is the translational
velocity of one segment relative to the other. In Buczek’s
examination of ankle kinetics, he found translational
joint power values were approximately an order of
magnitude smaller than the more dominant rotational
joint power contributions during normal gait. While
these translational components were found to be
statistically significant, their clinical relevance remains
unclear. It is important to note that the approach
employed by Buczek and the three degree of freedom
model described above (Eq. 9).both base the joint power
computation on the relative joint velocity, i.e., the veloc-
ity of one articulating surface of the joint relative to the
other articulating surface. These joint power parameters
are fundamentally different from those produced with
computational methods that combine a joint moment
with the absolute angular velocity of a segment and/or
the inter-segmental reaction force with the absolute
translational velocity of a joint center in the examina-
tion of the inter-segmental transfer of energy [43].
Assumptions that are incorporated commonly into
the joint kinetics model include
l Body segments are ‘rigid’ and do not deform when
loaded,
l Soft tissue movement relative to underlying bony
structures is small, and
l Joint center locations remain fixed relative to the
respective segment.
An appreciation of the implications of these assump-
tions is particularly important in the assessment of clini-
cal results. For example, an obese patient’s excessive
soft tissue movement can reduce the quality of estimates
of the joint center locations as well as distort kinematic
quantities such as angular velocities and accelerations.
Bony deformities of the foot and ankle can produce ar-
tifacts in the ankle results, e.g., show ankle power
changes due to reduced integrity of the midfoot struc-
ture.
In addition to an understanding of the modeling
assumptions, the clinician should possess a basic
understanding of the kinetics computational approach.
The general method described above is based on three-
dimensional data and algorithms. Other computational
approaches have been used in the clinical setting to pro-
duce estimates of joint moments. Less computationally
intensive, two dimensional approaches have been
employed to calculate joint moments during gait by
assuming that body segment displacement occurs within
a vertical plane that is parallel to the direction of for-
ward progression, referred to often as the ‘sagittal plane’
of the subject. It has been demonstrated that both nor-
mal and impaired ambulators produce segment and
joint displacements that are three-dimensional [8&l].
Consequently, one might surmise that just as joint
flexion-extension axes do not remain perpendicular with
this plane of progression, that flexor-extensor joint
moments also possess time-variant three-dimensional
spatial attitudes. This raises an important, albeit subtle,
point of discussion. A joint moment vector may be
referenced to a variety of coordinate systems. Three pos-
sible alternatives include a body-fixed (anatomically
aligned) coordinate system either proximal or distal to
the particular joint of interest, or an inertially-fixed (lab-
oratory based) coordinate system (Fig. 3). One might
argue that an anatomically based reference is mean-
ingful because it allows a more direct appreciation of the
relationship between the joint motion, the joint moment
and the activity of the muscles associated with the joint.
Conversely, one might propose that joint moments
referenced to a fixed laboratory coordinate system
allows the clinician to better understand how joint
moments produce the changes in linear and angular
momentum that are needed for propulsion in a particu-
lar direction. This choice of coordinate reference merits
further discussion and quantitative evaluation.
A strategy for joint moment estimation that is often
cited in the literature is the projection of the ground
reaction force vector upward toward the particular joint
center. The joint moment is then estimated by simply
combining the magnitude of the ground reaction force
vector with the distance between the vector and the joint

7. 68 S. dunpuu et al. /Gait and Posture 4 (19%) 62-78
- Net Hip Moment
- HIP Moment due to Ground Reactions
._.--.-. Hip Moment due to Segment Weight
.__-. Hip Moment due to Segment Inertia
Hip
Moment
(N-m/kg)
0
I I I 1
25 50 75 100
% Gait Cycle
Fig. 3. A plot of hip moments over one gait cycle for a normal am-
bulator illustrating the relative importance of the ground reactions
and the weight and momentum of the segments, i.e., each curve
represents the required net joint moment associated with a particular
type of external load.
center. This method does not include the joint moment
associated with the weight of the leg segments or the
joint moment required to either produce or control the
angular momentum of the leg. Consequently, while this
approach is straightforward and appealing, it over-
simplifies the gait mechanics, can result in misinter-
pretation, especially at the hip and at higher walking
speeds, and does not allow the computation of the joint
moments during swing [45]. As the example in Fig. 4 il-
lustrates, the hip moment associated with the ground
reaction force, segment weight, and segment inertia are
all significant in magnitude. In this normal ambulator,
however, the hip moment required to counteract the seg-
ment weight offsets the hip moment generated in re-
sponse to segment inertia throughout much of the stance
phase of the gait cycle. It is important to note that dur-
ing intervals of transition at the beginning and end of
the stance phase, the hip moments required to produce
or control segment inertia are relatively high. The rela-
tionships between these several mechanical entities war-
rant additional study and thought.
In closing, clinical users of joint kinetic information
should develop a fundamental understanding of the
computational approach used to generate the results as
well as the validity of the modeling assumptions in the
context of an individual data set, e.g., acceptable soft tis-
sue displacement relative to bone. Furthermore, both
those involved in the collection of data as well as those
responsible for the interpretation of the results need to
remain alerted to the potential for data collection ar-
tifact. For example, the peculiar deviation in the hip mo-
ment shown in Fig. 5 was not caused by some muscle
E!i+
elabx
Fig. 4. An illustration of the several reference systems that are avail-
able to quantify the ankle moments, including one distal to the joint
(&,,,,), one proximal to the joint (eshank ), and an inertially fixed frame
L
of reference (elab).
pathology, but by inadvertent force platform contact by
the subject’s swing limb.
3. Interpretation of joint kintics
Joint kinetics, like joint kinematics, do not necessarily
provide direct answers to clinical questions but give the
*r
Hip
I-
Moment
(N-wk&
0 J
Hip
Power
Watts/kg)
-L
0 25 50 75 100
% Gait Cycle
Fig. 5. Plots illustrating a data collection artifact due to swing limb
contact with stance limb force platform (the interval of contact is
represented by the gray band).

8. S. hpuu et al. /Gait and Posture 4 (19%) 62-78 69
clinician more information with which to make appro-
priate treatment decisions. As with any type of treat-
ment, the ultimate decision depends on the philosophies
of the clinician making the decisions. In general, when
using joint kinetics in the evaluation of and treatment
decision-making for pathological gait, emphasis should
be made on the pattern and timing of the specific curve
in comparison to normal with less emphasis on the
amplitudes of the individual peaks. Joint kinetic results,
which cannot be observed directly, are difficult to pre-
dict and challenging to visualize. However, a systematic
approach to the interpretation of joint kinetics along
with minimal background information will increase
their utility as an integral part of the treatment decision-
making process.
There are several basic concepts and guidelines which
are important in the interpretation of kinetic data. An
understanding of these, along with a knowledge of the
potential data collection errors, is also a prerequisite for
effective data interpretation. These guidelines are
covered in the following paragraphs. Joint kinetics are
a component of gait analysis and should be interpreted
with all other information collected including, joint ki-
nematics, temporal and stride variables, dynamic EMG
and pertinent clinical examination information such as
Hip
Flexion 45 c
Joint
Rotation
(degrees)
Extension
joint range of motion, estimates of tibia1 and femoral
torsion and muscle strength. With out all these sources
of information it is much more difficult to reach appro-
priate treatment decisions.
3.1. Joint angle definitions
First one needs an understanding of the joint angle
definitions which are based on marker set alignment and
the underlying mathematical models used to estimate
joint centers and axes of rotation. Unfortunately, the
marker locations and the mathematical models used in
gait analysis vary from laboratory to laboratory and as
a result, there are differences in the joint kinetic pat-
terns. This is primarily a problem at the hip joint where
many mathematical models and marker placement com-
binations are used for the determination of hip kinemat-
ics and kinetics. The joint angle definitions used in this
chapter have been previously published [8].
3.2. Normal data base
As in the interpretation of other types of gait analysis
data, pathological joint kinetic data are usually com-
pared to normal data (Fig. 6) to determine abnormal
Knee
Generation
~2 ;fijijm.g ‘I-”
(Watts/kg)
Absorption
75 100 0 25 50 75 100
% Gait Cycle % Gait Cycle
Ankle
25 50 75 100
% Gait Cycle
Fig. 6. The mean ( f 1 S.D.) sagittal plane hip, knee and ankle joint kinematic (top row), moment (middle row) and power (bottom row) for 26
normal children. All data are normalized to 100% of the gait cycle with stance phase separated from swing phase by the vertical line (toe-oflI.

9. IO S. &npuuet al. /Gait and Posture 4 (19%) 62-78
patterns. Because of differences in angle definitions,
plotting formats and often subtle methodological differ-
ences, it is necessary that normal reference data be col-
lected in the same laboratory and using the same
methods as during the collection of the patient data.
Generally, the joint kinetic ‘patterns’ are of the most in-
terest when comparing normal and pathological data as
the amplitudes of the patterns are velocity dependent
(seebelow). The normal data reference given in the cases
at the end of this chapter was collected in the same labo-
ratory and has been previously published [8].
3.3. Knowledgeof plotting conventions
Along with a knowledge of the methods used, the
clinician must also be aware of the conventions used for
plotting (Fig. 6). Unfortunately, like the angle detini-
tions, there is no consistency between laboratories on
the plotting format specifically in relation to whether the
internal [7,8] or external [13,46] moment is plotted. In
this chapter, the internal moment is plotted because it
represents the body’s response to the external load and
corresponds to the dynamic EMG data. Joint kinetic
data are divided by either body weight [7,8], body
weight and height [13,471, or body weight and leg length
[47]. Division by body weight reduces the inter subject
variability and facilitates comparison from individual to
individual. It has been reported that division by body
weight and height slightly decreases the inter subject
variability in comparison to body weight alone, how-
ever, this depends on the joint and the plane [47]. Al-
though these procedures do not at&t the overall
pattern of the joint kinetic plots, they do affect the
amplitude (units) and therefore must be noted before in-
terpretation. This is specifically important for the com-
parison of data from different laboratories.
3.4. Stride to stride consistency
As with all variables collected in computerized gait
analysis, the stride to stride consistency must be assessed
to determine if an individual trial is a reasonable
representation of how the patient walks. Therefore, the
collection of multiple trials per side is recommended. If
the patient appears to be ‘variable’ on observation of
gait, more trials may be necessary. If a patient is not
consistent, all trials should be plotted in an overlay and
not presented asmean curve. There are three reasons for
this: the mean curve does not represent any individual
trial, averaging is, curve smoothing and thus high fre-
quency content may be lost in the averaging process,
and finally, mean results may make the correlation be-
tweenjoints and segments difficult. Treatment protocols
for persons that do not show consistency may differ
significantly from those persons that are consistent,
specifically-when surgical intervention is being con-
sidered. Generally, joint kinetic data is very consistent
stride to stride in persons with CP [48] when gait pat-
terns have reached maturity.
3.5. Walking velocity
Within a normal individual, changing walking veloci-
ty results in significant changes in the peak moment and
power amplitudes with minor changes in the ‘pattern’ of
the joint kinetic curve [49]. More specifically, increasing
walking velocities are associated with increasing mo-
ment and power amplitudes and decreasing velocities
are associated with decreasing moment and power
amplitudes. In pathological gait, there are not only
changes in peak moment and power amplitudes, but the
kinetic patterns may also change. For example, a mildly
involved child with CP may show increasing amplitudes
as well as a change in the pattern of the ankle kinetics
from about normal to a double bump ankle pattern (see
description below) with increasing velocity. Therefore,
when evaluating amplitude and modulation differences
over multiple trials of data, the clinician should, check
the velocity data to make sure it is similar. This is par-
ticularly important if the right and left side data are
taken from different trials. Similarly, in pre versus post
treatment comparisons and barefoot versus orthosis
comparisons velocity may contribute to differences in
the peak joint kinetic amplitudes and patterns.
3.6. The role of the trunk position in the netjoint kinetic
In pathological gait, the trunk may be used to com-
pensate for lower extremity weakness especially about
the hip. For example, lateral lean of the trunk can be
used to reduce and even eliminate the normal hip abduc-
tor moment during stance when hip abductor strength is
limited. A forward trunk lean, will increase the hip ex-
tensor moment in terms of amplitude aswell as delay the
cross over time from hip extensor to hip flexor which
normally occurs at about 25% of the gait cycle. A for-
ward trunk lean may also be used to increase the knee
flexor moment in a patient who is quadriceps deficient
and wants to minimize the risk of knee flexion. Similar-
ly, a forward trunk lean will reduce a knee extensor mo-
ment when the knee is in severe flexion during stance.
Therefore, it important to know the position of the
trunk when interpreting unusual joint moments as it
may be an important compensation that results in ab-
normal moments.
3.7. Role of EMG in joint kinetic data interpretation
The primary problem in the interpretation of EMG
data on its own is there is no relationship of the
amplitude of the EMG signal with the force being pro-
duced unless the signal is normalized to some known
level of force 1501.The net joint moment is important in
that it provides information about which muscle group

10. S. &mpuu et al. /Gait and Posture 4 (19%) 62-78 11
is dominant. The net moment of force is a summation of
all agonist and antagonist muscle forces which also in-
clude the contribution of passive structures such ascon-
nective tissue. In patients with CP, this can be an
important contribution asit is very frequent that activity
is noted in the agonists and antagonists, simultaneously,
with no indication of which is the dominant muscle
group without joint moments. Treating an inappropri-
ate moment at one side of the joint may result in the op
posite deformity if the antagonist is ‘left’ to become the
dominant moment after surgery. It is therefore, impor-
tant to refer to the EMG results to determine the ‘con-
tent’ of the dominant moment. However, the joint
kinetics provide information as to whether the muscle
activity is agonist or antagonist which may not be possi-
ble to determine with the EMG alone.
Dynamic EMG results also are important when deter-
mining the cause of moments, that is, if the musculature
is not active, the moment may be produced by a joint
capsule or ligamentous structure. This would be of clini-
cal significance if the joint was at risk for damage due
to the absence of muscular support. For this to be deter-
mined, however, all muscles that are potentially involv-
ed need to be monitored. This is generally not practical
in routine clinical testing involving children.
There is also some level of confusion about the timing
of events when relating the EMG data to the joint mo-
ment. A good example of this is at the ankle in terminal
stance (about 4040% of the gait cycle). In normal gait
there is a plantar flexor moment continuing after the
raw EMG signal has terminated. This can be explained
by the electromechanical phase lag between the EMG
and the tension developed in a muscle which ranges
from 40- 100 ms depending on the muscle characteristics
(type one or type two) 1511.To represent the appearance
of this phase lag, the linear envelope form of EMG with
a critically damped low pass filter is used, which essen-
tially phase shifts the signal to bring it closely into phase
with the joint moment patterns.
3.8. Joint kinetic patterns
In general, specitic joint kinetic patterns are associ-
ated with specific abnormalities as defined by kinematic
patterns. The study of joint kinetic patterns and the
associated kinematic and EMG patterns and related
clinical information will help us in understanding the
mechanisms of pathological gait. Joint kinematic and
kinetic patterns may eventually be used for guidance in
treatment decision-making [12,521, that is, a certain pat-
tern could suggest a certain surgical procedure as
discussed below. These patterns may also be used for
error detection, that is, inconsistencies in these ‘ex-
pected’ relationships can alert the clinician of a possible
error. For example, a person walking on their toes dur-
ing the entire stance phase will have an associated net in-
ternal ankle plantar flexor moment pattern during the
entire stance phase. An ankle moment plot showing a
dorsitlexor moment for a person walking on the toes is
incorrect and would suggest an error.
The shape of the joint kinetic pattern may also pro-
vide further information for the clinician and suggest
specific treatment. An example of this use of joint kinet-
ics is given in the next section for the ‘double bump’
ankle pattern. The ‘double bump’ refers to the shape of
the moment and the power curve and not the amplitude.
This type of ankle kinetic pattern is now been routinely
used at our hospital as criteria for intramuscular heel
cord lengthening [121.The identification of specific joint
kinetic patterns may some day help direct surgical treat-
ment for all joints in the lower extremity but only in the
context of the personal philosophy of the physician.
When interpreting joint kinetic data we find it useful
to follow a systematic approach which is facilitated by
the plotting format used in Fig. 7. In this format, the
joint kinematic plot is followed by the internal joint mo-
ment and joint power plots which are all normalized to
body weight and to the gait cycle. This format is used for
all data presented in this paper.
ANKLE = ANKLE
ANKLE
POWER MOMENT x
ANGULAR
VELOCITY
n
dorsiflexion
KINEMATICS
plamuflexion
MOMENT
pltlIltdeXOr
dorsiflexor
POWER
generation
absorption
Fig. 7. Example of the standard format used for the presentation of
normal (mean f 1SD.) ankle joint kinetic data. The joint kinematic
is followed by the joint moment and the joint power. All kinetics are
normalized to body weight and represent the body’s response to the
external load, that is, they are internal moments. The mid stance phase
portion of the gait cycle is highlighted to facilitate the examination of
the three plots at this specific phase in the gait cycle.

11. 72 S. &npuu et al. /Gait and Posture 4 (19%) 62-78
Step 1 Select a specific phase in the gait cycle
Step 2 Note the joint motion on the kinematic (top)
plot
A variety of pathological conditions have been selected
to illustrate the broad spectrum to which joint kinetics
may be applied.
Step 3 Determine the moment which indicates domi-
nant muscle group on the moment (middle) plot 4.1. Orthosis decision-making for persons with myelomen-
Step 4 Confirm the power which indicates whether ingocele
there is a concentric or eccentric contraction by
examining the power (bottom) plot
In Fig. 7, the mid stance portion of the gait cycle for the
ankle joint has been selected for analysis. During mid
stance, the ankle is dorsiflexing as indicated on the
kinematic plot. The moment plot reveals that there is a
net ankle plantar flexor moment which indicates that the
plantar flexors are dominant. This can be confirmed on
dynamic EMG. The corresponding phase on the power
plot indicates a power absorption and that the ankle
plantar flexors are contracting eccentrically controlling
the forward movement of the tibia over the plantar
grade foot.
4. Applications
The use of joint kinetics in the treatment decision-
making process is relatively new. Through experience
and routine evaluation of the effects of treatment on the
joint kinetic patterns we will continue to improve our
understanding and treatment of pathological gait. The
following examples illustrate some of present uses of
joint kinetics in the treatment decision-making process.
Persons with myelomeningocele have very complex
gait patterns that involve abnormal motion in all three
planes [53]. Gait abnormalities in these persons are
usually treated with a combination of orthoses and sur-
gery. Joint kinetics can be a useful tool not only in im-
proving our understanding of the mechanisms of
pathological gait but in making decisions about appro-
priate orthoses in this patient population. In the follow-
ing example, the patient has an ; ‘apparent’ knee valgus
thrust which occurs at the initial-part of stance phase as
the stance limb accepts body weight (Fig. 8). A knee
valgus thrust is defined as a rapid abduction (opening of
the medial joint space) with associated stresson the soft
tissue of the knee joint during weight acceptance. Gait
analysis is recommended to determine if a knee-ankle-
foot orthosis (KAFO) is needed to protect the medial
aspect of the knee. In the case of a real knee valgus
thrust, the expected net knee moment would be an ad-
ductor moment which would resist further valgus posi-
tioning of the knee. In this case, the KAFO was
prescribed to prevent knee valgus seen on visual obser-
vation and thus protect or prevent further damage to the
medial capsule of the knee. Examination of the coronal
Fig. 8. photo of the ‘apparent’ knee valgus thrust during the initial part of stance when the knee is flexing, the hip is internally rotated and the
pelvis is rotating internally.

12. S. &npuu et al. /Gait and Posture 4 (I 9%) 62- 78 73
plane moments (Fig. 9) reveals a net knee abductor mo-
ment which indicates that there is no stress on the medi-
al compartment of the knee. This does not appear
possible on visual examination of the patients’ gait. Fur-
ther examination of the joint kinematics of the lower ex-
tremity, pelvis and trunk reveal a combination of
movements that result in the visual impression of a
valgus thrust and at the same time prevent a valgus
thrust. The combination of progressive knee flexion, in-
ternal pelvic rotation and a flail foot with associated ex-
cessiveexternal foot progression gives the appearance of
a valgus thrust. Large rotations in the transverse plane
at the pelvis, knee and ankle/foot allow this combination
of movements to occur. The complex kinematics of the
trunk also play a role in the net knee moment.
Therefore, in this case, the KAFO is not indicated for
medial protection of the knee joint. Appropriate fitting
of a solid ankle-foot orthosis would provide a reduction
in the progressive crouch and correction of the external
foot progression and eliminate the ‘visual’ valgus thrust.
It is important to note, however, that joint kinetic data
was needed to determine the knee coronal plane mo-
ment which can not be determined using visual observa-
tion of gait alone.
4.2. Evaluation of ankle-foot-orthosis function
Joint kinetics can provide excellent information about
40/
varus 30
20 : 1 I
Valgus
::LAL-l
Abductor
Ir----rT
Joint I
Moment t
(N-mllig)
oJ1- a.-
Adductor
0 25 50 75 100
% Gait Cvcle
Fig. 9. The coronal plane kinematic and moment for the knee durmg
a representative stride of a patient with myelomeningocele (solid line).
The normal (mean f 1 SD.) motion is indicated by the gray band.
The kinematic plot indicates that the knee is in varus and the moment
plot that there is a net abductor moment.
the function of a specific ankle-foot-orthosis (AFO) by
providing additional information that the joint kinemat-
ics alone cannot provide. In the following example, the
rear entry hinged floor reaction AFO was designed to
allow ankle plantar flexion through the hinged ankle
joint and prevent ankle dorsiflexion and associated
crouch through a dorsiflexion stop provided by a solid
anterior shank piece. A representative ankle stride for
both the barefoot and brace walk for this child with CP
is shown in Fig. 10. The kinematic data shows that the
orthosis reduces the excessive ankle dorsiflexion in mid
stance and the range of plantar flexion in terminal
stance as compared to barefoot walking. The joint mo-
ment data indicates a normal dorsiflexor moment during
loading response when barefoot and a plantar flexor
Flexion
Joint
Rotation
(degrees)
Extension
-10 -
-30 I ! I
Extensor
::11 r-r
Joint
Moment
(N-m/Q)
Flexor
Generation
Jomt
Power
(Watts/kg)
Absorption
25 50 15 1uo
8 Gait Cycle
Fig. 10. The sagittal plane joint kinematic, moment and power for a
selected trial of barefoot (thin line) and rear-entry hinged floor reac-
tion AFO (thick line) walking for a child with cerebral palsy. The
mean normal motion is indicated by the gray band. The data indicates
that hinged component of the AFO (which allows for free plantar flex-
ion) is not used for active plantar flexion in terminal stance.

13. 74 S. dunpuu et al. /Gait and Posture 4 (19%) 62-78
moment during loading response when walking with the In the following example, joint kinetics are used to
AFO. This is a result of the AFO which also reduces the improve our understanding of the posterior leaf spring
ankle dorsiflexion at initial contact and results in a toe orthosis (PLS). The proposed function of this orthosis,
contact due to simultaneous knee flexion. A rapid de- as suggested by its name, is to control the forward mo-
velopment of a plantar flexor moment indicates that tion of the tibia over the plantar grade foot (second
there is an early heel rise or premature weight bearing rocker) and then return some of this stored energy to
on the distal aspect of the foot. Similar ankle moments augment plantar flexion in terminal stance (third
in terminal stance indicate that weight bearing on the rocker). This capability of the PLS can be examined
distal aspect of the foot is similar in both conditions. with the ankle joint power plot which should
The joint power results show that with the AFO, power demonstrate an increase in power generation if the PLS
generation at the ankle is reduced significantly as com- augments ankle function in terminal stance. A com-
pared to barefoot walking and indicates that the hinged parison of the barefoot and PLS walk of a patient with
component of the AFO is not functional in this patient. CP (Fig. 1I), shows a small reduction in the peak power
This data would suggest that the ‘expensive’ addition of generation in terminal stance with the PLS. Although
the hinged component at the ankle was not necessary. the PLS improves ankle function by more appropriately
Joint
Rotation
(degrees)
-10
Extension
2.0 m
Extensor
Joint
Moment
(N-d@
Flexor
3
Generation
2
Joint
Power
1
(Wattsikg) o
Absorption _,
I I II I
25 50 75 100
% Gait Cycle
Fig. I 1. The sagittal plane ankle joint kinematic, moment and power Fig. 12. The sagittal plane ankle jomt kinematic, moment and power
for a selected trial of barefoot (thin line) and posterior leaf spring for a selected trial when walking barefoot just prior to surgery (thin
(thick line) walking for a child with cerebral palsy. The mean normal line) and one year after surgery (thick line) for a child with cerebral
motion is indicated by the gray band. The posterior leaf spring im- palsy. The mean normal motion is indicated by the gray band. Post-
proves modulation of the ankle kinematics and kinetics but reduces operatively, there was an improvement in the ankle kinematic and ki-
the peak power generation at toe-off in comparison to barefoot netic modulation with an increase in the ankle power generation in ter-
walking. minal stance post operatively.
Flexion
Joint
Rotation
(degrees)
Extension
Extensor
Joint
Moment
(N-m&9
Flexor
Generation
Joint
Power
(Watts/kg)
2.0r----
1.5
1.0 3:<j
0.5
0.0
b,.
-0.5
1
-1.01
-3 I I I
-0 25 50 75 11
% Gait Cycle

14. S. &npuu et al. /Gait and Posture 4 (19%) 62-78 15
positioning the ankle for initial contact, the PLS does
not function as the name suggests.
4.3. The evaluation of the baker type gastrocnemius
lengthening
The direct application of joint kinetics in the surgical
decision-making process in persons with CP is limited at
the time of this writing. As mentioned previously, more
systematic study is needed before the ultimate potential
of this tool is known. One noted exception is the role of
joint kinetics in the treatment decision-making process
for the spastic gastrocnemius. A spastic gastrocnemius is
one of the more common problems in children with CP
and results in a toe initial contact with minimal or no
heel contact with the ground during stance. This gait
pattern is primarily a result of gastrocnemius tightness
and/or spasticity although limited knee extension (or ex-
cessiveknee flexion in stance) can also contribute to lim-
ited heel contact. The ankle joint kinetics for this type
of toe walking are ‘double bump’ in shape as illustrated
in Fig. 12. The presence of this double bump ankle pat-
tern suggests that a Baker type gastrocnemius lengthen-
ing is the appropriate treatment [121.This is despite the
fact that the ankle comes into a normal degree of dor-
siflexion in early stance. A systematic study of 26 sides
that underwent Baker type gastrocnemius lengthening
shows that this surgery did not reduce the power
generating capabilities of the ankle but actually increas-
ed power generation in most casesand resulted in nor-
mal ankle kinetic modulation, that is, elimination of the
double bump pattern.
4.4. The effects of surgical treatment on the joint kinetics
of adjacent joints
As described in the example above, ankle joint kinet-
ics were integral in the decision-making for a Baker type
heel cord lengthening. The situation, however, becomes
more complicated astreatment decisions are being made
for the more proximal joints of the knee and hip. Move-
ment at a proximal joint can be a function of actual
pathology at the specific joint and/or pathology at a
more distal joint which results in compensatory move-
ments proximally. Therefore, it is important to differen-
tiate primary from secondary problems which can be
Hip Knee Ankle
Rotation
Extension -15.
2.0
Extensor
Joint
1.0
Moment
(N-m&) 0.0
Flexor
-1.0
-Generation
Joint
Power
(Watts/kg)
Absorption
Fig. 13. Comparison of the pre (thin line) versus post-operative (thick line) sagittal plane joint kinematics and kinetics for the right hip, knee and
ankle for a child with cerebral palsy spastic hemiplegia. The surgical treatment to the ankle joint alone resulted in changes to not only the ankle
kinematics and kinetics but also to the knee and hip.

15. 76 S. hpuu et al. /Gait and Posture 4 (19%) 62-78
facilitated by studying the effects of surgery on the joint
kinematics and kinetics at adjacent joints. In the follow-
ing example (Fig. 13), the pre-operative ankle joint kine-
matics and kinetics were consistent for significant
equinus, minimal plantar flexor moment through out
the stance phase and negligible power generation in ter-
minal stance. Clinical evaluation showed a severe heel
cord contracture of -20 degrees with the knee extended
and no ability to isolate the ankle musculature. The
child was also crouched with an associated excessive
knee extensor moment and prolonged hip extensor mo-
ment to prevent collapse. Even though pathology was
noted in terms of the EMG, kinematics and kinetics at
the more proximal joints, surgery was performed on the
ankle musculature alone (Baker type gastrocnemius
lengthening) because of variability in the joint kinematic
and kinetic patterns and no evidence of contracture on
clinical examination at the knee and hip. This led to the
conclusion that the hip and knee deformities were a sec-
ondary problem caused by the severe equinus. There-
fore, the delayed hip moment crossover was not a result
of a hip flexor problem that required surgery but due to
the hip flexion required to ambulate with severe
equinus.
Post-operatively, the data suggests that the hip and
knee motion were a secondary deviation as a result of
the equinus. There were significant kinematic and kinet-
ic changes at the ankle as well as at the more proximal
joints of the knee and hip. The ankle joint kinematics
showed improvement with elimination of the drop foot
in swing and equinus in stance, with normal moment
and power modulation. Changes at the knee included a
reduction in the crouch and associated knee extensor
moment. Changes at the hip included a major shift in
the cross-over (point of change from hip extensor to
flexor moment or hip power generation to absorption in
stance) of the hip extensor moment and power genera-
tion during the initial part of stance. Post-operatively,
cross-over waspremature instead of delayed asseen pre-
operatively. What this means clinically or long term is
unclear. This example, however, indicates that surgical
decisions based on joint kinetic patterns alone may lead
to inappropriate treatment decisions. The importance of
integrating all collected data and clinical examination
measures for treatment decision-making cannot be
underestimated.
5. Conclusions
It is our feeling, having worked with kinetics in the
clinical realm for the past live years, that the utility of
this information is in a formative state. It is very likely
that applying kinetics to help define gait pathology and
treatment in large numbers of patients will eventually
lead to more specific and sophisticated treatment
regimens based on joint kinetics. Each clinical group
develops their own preferred procedures for handling a
specific gait pathology. For example, a spastic hamstr-
ing causing crouched gait is dealt with by distal hamstr-
ing fractional lengthening at this institution [2,6] and
many other centers [54]. Other available and utilized
techniques include proximal release [55], or distal
hamstring transfers of some or all of the hamstrings
[56]. This diversity of treatment along with the more fre-
quent collection of joint kinetics will increase our
knowledge base more rapidly. At present, we continue
to use it in the patient with CP to help define those re-
quiring treatment of a spastic gastrocnemius-soleus [121,
and in certain brace modifications [11,20,53]. We intend
to apply kinetics to the study of the rectus femoris trans-
fer for the spastic knee [6,57] as well as the spastic hip
flexors and hope other clinical laboratories are also ap-
plying them to other problems and treatments. System-
atic study of the joint kinetic patterns and related gait
variables and the effects of treatment, specifically surgi-
cal, is needed before joint kinetics will be used as a
routine tool for surgical decision-making.
In addition to this synopsis of clinical utility, this
paper summarized the framework for the computational
process for joint kinetic quantities. The ‘building blocks’
for the process, include estimates for segmental mass,
mass moment of inertia, and center of mass location;
three dimensional subject motion data that leads to the
predictions of the instantaneous locations of the lower
extremity joint center locations as well as values for the
linear and angular accelerations of the body segments;
and measures of the magnitude and point of application
of the ground reactions. In the overview, the underlying
assumptions associated with the gait models were iden-
tified, including a discussion of soft tissue movement re-
lative to bone.
Work remains to be done in the modeling area. Algo-
rithms are needed to improve data reliability in the con-
text of soft tissue movement and joint center approx-
imation. Discussion is needed asto the merits of each of
the several reference systems described above with an
understanding that there may be no one ‘right’ ap-
proach, but different approaches, each with strength
and weakness.
The clinical scenario described by Dr. James Gage
[58] describing two children that look the same, have the
same surgery but the affect of surgery is different is
quoted as one of the primary reasons why computerized
gait analysis is needed. It is possible that not until the
joint kinetics are evaluated can the real differences in the
mechanisms behind pathological gait be revealed and
understood. Hopefully, the accurate computation and
interpretation of joint kinetics in combination with the
other components of computerized gait analysis will
eventually lead to significant improvements in treatment
decision making in complex gait abnormalities such as
those of persons with CP and ML.

16. S. &npuu et al. / Gait and Posture 4 (I 9%) 62- 78 77
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