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Cruciate ligament loading during common knee rehabilitation
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Article in Proceedings of the Institution of Mechanical
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6. loading during commonly used therapeutic exercises. In
general, weight-bearing exercise produces smaller loads on the
anterior cruciate ligament and posterior cruciate liga-
ment compared with non-weight-bearing exercise. The anterior
cruciate ligament is loaded less at higher knee angles
(i.e. 50–100�). Squatting and lunging with a more forward
trunk tilt and moving the resistance pad proximally on the leg
during the seated knee extension unloads the anterior cruciate
ligament. The posterior cruciate ligament is less loaded
at lower knee angles (i.e. 0–50�), and may be progressed from
level ground walking to a one-leg squat, lunges, wall squat,
leg press, and the two-leg squat (from smallest to greatest).
Exercise type and technique variation affect cruciate ligament
loading, such that the clinician may prescribe therapeutic
exercises to progress ligament loading safely, while ensuring
optimal recovery of the musculoskeletal system.
Keywords
Anterior cruciate ligament, anterior shear force, exercise
therapy, reconstruction, strain
Date received: 3 May 2011; accepted: 23 March 2012
Introduction
Cruciate ligament injuries are common. After sustaining
an injury to the cruciate ligaments, or after anterior cruci-
ate ligament (ACL) or posterior cruciate ligament (PCL)
reconstruction, it is important to properly rehabilitate the
tibiofemoral joint to ensure optimal recovery of the heal-
ing tissues, keep the joint healthy, and to prevent lower
extremity muscle atrophy. Understanding cruciate liga-
ment loading during commonly prescribed rehabilitation
exercises helps the clinician maximize treatment efficacy
and minimize the likelihood of injury.
7. The scientific literature on cruciate ligament loading
has not been recently reviewed and summarized to give
the clinician a current understanding of how ligament
loading is quantified, or to give an updated understand-
ing of loading across a wider range of exercises than pre-
viously described.
1–13
Rehabilitation exercises commonly
used include both weight-bearing exercises (WBE), also
referred to as closed kinetic chain exercises, and non-
weight-bearing exercises (NWBE), also referred to as
open kinetic chain exercises. The specific exercises
included in this review include squatting, lunging, step-
ping (e.g. stepping up and down stairs), leg press, seated
knee extension and knee flexion, stair climbing, station-
ary bicycling, drop landing, and walking (samples of
squatting and lunging exercises are shown in Figures 1
and 2). These exercises were chosen because using these
exercises as part of a cruciate ligament rehabilitation
1
Department of Physical Therapy, California State University
Sacramento, USA
2
Department of Radiology and Biomedical Imaging, University
of
California San Francisco, USA
3
Champion Sports Medicine, USA
4
8. Paulos Sports Injury and Joint Preservation Clinic, USA
5American Sports Medicine Institute, USA
6
Andrews Institute, USA
This paper was submitted as part of the Lower Limb
Musculoskeletal
Modelling Special Issue.
Corresponding author:
Rafael F Escamilla, Department of Physical Therapy, California
State
University Sacramento, 6000 J Street, Sacramento, CA 95819-
6020, USA.
Email: [email protected]
program after cruciate ligament injury or reconstruction
has been shown to significantly improve short- and long-
term knee function and enhance a successful return to
sport or activity.
14–16
Cruciate ligament loading will be investigated during
these exercises with varying resistance, speeds of
movement, and techniques. Understanding how the
cruciate ligaments are loaded during WBE and NWBE
rehabilitation can help clinicians better prescribe train-
9. ing and rehabilitation regimens in a safe manner, to
enhance recovery and the rehabilitation process.
Techniques commonly used to measure
cruciate ligament biomechanics
Both in-vivo
1–7
and experimental
8–13
biomechanical
models have been developed to evaluate ACL strain or
tensile force during WBE and NWBE, and both these
approaches have advantages and limitations. The obvi-
ous advantage of in vivo studies is that they calculate
ACL strain directly by using strain sensors within the
ACL. The subjects in these in vivo studies were patients
that had strain sensors implanted within the anterome-
dial bundle of their healthy ACL during arthroscopic
surgery to repair damaged knee structures (partial
meniscectomy; capsule, and patellofemoral joint debri-
dement). Immediately after surgery, these patients were
asked to perform a variety of NWBE and WBE, includ-
ing lunging, squatting, leg press, step-up and step-down,
stair climbing, bicycling, and seated knee extension and
knee flexion. The strain within the anteromedial bundle
of the ACL was measured and referenced to an instru-
mented Lachman Test with 150 N of resistance.
There are several limitations to measuring ACL strain
in vivo, such as, the procedure is invasive, time consum-
ing, costly, performed in a patient population under sur-
gical conditions, and that the types of activities are
limited. Moreover, the exercise technique employed
10. while these patients performed selected WBE was gener-
ally not controlled. For example, there are many ways
to perform a squat that could affect muscle forces and
cruciate ligament loading, such as, using narrow stance
or a wide stance, turning the feet in or out, having a near
vertical trunk position or tilting the trunk forward 30�–
45� relative to vertical, and during the squat descent
moving the knees forward beyond the toes or keeping
the knees from moving forward beyond the toes.
Another limitation to in vivo studies is that both athletes
and non-athletes are employed in performing WBE, gen-
erally only body weight or light external resistance is
employed during the exercises, and usually only selected
knee flexion angles are chosen for ACL strain data col-
lection. Therefore, the ability to generalize the results of
ACL strain in vivo from studies during WBE to the
active athletic population, which comprise the majority
of ACL injuries and who often trains with moderate to
heavy external resistance over a large knee range of
motion, is limited and should be interpreted cautiously.
Experimental biomechanical knee models, which
also have advantages and limitations, have been previ-
ously developed and described.
8,9,11–13,17–21
The advan-
tage of using experimental models is that the estimated
loads are better generalized to the active athletic popu-
lation because variables are often better controlled. For
Figure 2. One leg squat.
Figure 1. Forward lunge.
11. Escamilla et al. 671
example, moderate to heavy resistance could be used
during exercise using the experimental model, such is
more consistent to how athletes train, but only body
weight or light resistance could be used with the in vivo
model, because the subjects are all patients that are just
coming out of knee surgery. The obvious limitation of
experimental biomechanical knee models is that they
do not measure ACL loading directly, but only esti-
mate its value. However, if the same experimental
model is used for all the exercises, it still provides a
good relative comparison (assuming that the models
are physiologically realistic). Another limitation in
using experimental biomechanical knee models is that
these models were primarily limited to sagittal plane
motion because squatting, lunging, and similar exer-
cises are performed primarily in the sagittal plane with
only minimal transverse plane rotary motions and fron-
tal plane valgus/varus motions. However, performing
these types of exercises with excessive transverse plane
rotary motions and frontal plane valgus/varus motions
could affect cruciate ligament loading, and this should
be the focus of future studies. Further, most experimen-
tal models are constructed with the assumption that the
cruciate ligaments are the only restraints to tibiofe-
moral shear forces, and do not account for other soft
or hard tissues (meniscus, tibial slope, etc.) that likely
play a role.
Both in vivo and experimental models have draw-
backs, but there is evidence to suggest their validity
because several in vivo experiments found similar results
to studies using experimental models examining ACL
12. loading. Studies using in vivo modeling
1–7
reported peak
ACL loading for squatting and lunging of approxi-
mately 2.8% to 4% (about 100–150 N) at knee flexion
angles between 0� and 30�, corresponding to the peak
ACL forces calculated from experimental models
8,9,11–
13,17–21
for the same exercises. This example demon-
strates that the magnitude of predicted forces from
experimental models are in general agreement with in
vivo direct measurement, thus providing some validity
to the measurements and suggesting that the two mea-
sures may be very cautiously compared between and
within techniques. There is another drawback related
to the two models, in that the ultimate tensile force is
not readily determined using live subjects, representing
a potential disagreement between at maximal force lev-
els between in vivo and experimental modeling studies.
Finally, because in vivo studies only include subjects
that have otherwise undergone surgery, the majority of
studies use experimental models and, therefore, the
majority of the work presented within this article is
based upon experimental models.
Commonly used graft mechanical
properties
Both autograft, which is tissue harvested from the
patient undergoing surgery, and allograft, which is
13. tissue harvested from a cadaver, are commonly used to
reconstruct the cruciate ligaments in the United
States.
22,23
In healthy adults, the ultimate strength of
the native ACL is approximately 2000 N,
24
and the
reconstructed ACL has similar ultimate strengths com-
pared with the healthy ACL, although these values can
change considerably depending on graft type, donor’s
age, and donor characteristics (e.g, autograft versus
allograft, patellar tendon versus hamstrings graft,
etc.).
25
However, the healing graft and graft site may
be injured with considerably less force compared with
the ultimate strength of the graft, especially when it
involves soft tissue to bone fixation, such as the ham-
strings graft. The graft must mature, and as the
maturation process continues, the ultimate tensile prop-
erties of the graft increase in strength. Unfortunately it
is not known how much force to the graft site is too
much and how soon force can be applied to the healing
tissues after reconstruction. However, concerns regard-
ing the loading properties of the cruciate ligaments
highlight the importance of biomechanical testing of
the cruciate ligaments.
ACL loading during selected rehabilitation
14. exercises
Both WBE and NWBE have been employed and shown
to be effective in enhancing ACL rehabilitation and
return to sport.
26
However, it is believed by some clini-
cians that, compared with NWBE, individuals that per-
form predominately WBE in their rehabilitation tend
to have less knee pain, more stable knees, are generally
more satisfied with the end result, and return to their
sport sooner than expected. Tables 1–3 present ACL
strain, ACL tensile force, and anterior shear force (the
force on the tibia in the anterior direction that loads
the ACL) data from selected articles in scientific litera-
ture, and each of these will now be summarized.
ACL strain
The ACL strains reported in Table 1 are from several
in vivo studies
1–6
performed during a variety of NWBE
and WBE. Key points from Table 1 are as follows.
1. It should be emphasized that peak ACL strain
occurs at knee angles of less than 30�. Therefore, if
the rehabilitation goal is to minimize ACL loading,
such as during the early phases after ACL recon-
struction surgery, training both NWBE and WBE
at higher knee angles (i.e. 50�–100�) is recom-
mended, compared with training these exercises at
15. lower knee angles (i.e. 0�–50�). In addition, it
should be emphasized that ACL loading from both
NWBE and WBE at knee angles less than 60� are
of relatively small strain magnitudes (typically less
than 3.7% from Table 1), which is similar to the
ACL strain observed with a 150 N Lachman test,
672 Proc IMechE Part H: J Engineering in Medicine 226(9)
which produced 3.7% strain at a 30� knee flexion
angle.
2. Peak ACL strain is typically greatest at around
10�–15� knee flexion, and gradually decreases
between 15�–50� knee angle, and between approxi-
mately 50�–90� knee angles there is minimal or no
ACL strain. For example, ACL strains during the
isometric seated leg extension using a 30 Nm tor-
que as resistance were 4.4% at 15� knee angle, 2%
at 30� knee angle, and no ACL strain at 60� and
90� knee angles.6 Moreover, when tested at 30�,
50�, and 70� knee angles, squatting, lunging and
step-up and step-down exercises had the greatest
ACL strain at 30� knee angle.6 ACL strain at full
knee extension (0�) has not been reported during
exercise, but is assumed to be minimal owing to the
knee being in a very stable closed pack position.
3. Peak ACL strain was not significantly different
between squatting with or without 136 N of exter-
nal resistance, or between stair climbing at slower
versus faster rates.
1,4,6,30
16. It can be concluded from
these WBE data that increasing resistance during
the squat, or increasing the rate of stepping during
stair climbing, may not increase ACL strain. This
may have occurred because adding resistance or
stepping faster may affect muscle recruitment pat-
terns, such as recruiting the hamstrings to a greater
extent (perhaps owing to changes in technique,
such as a greater forward trunk tilt). Muscle force
from the hamstrings helps unload the ACL owing
to their posterior directed force on the leg. This
finding, during the WBE (such as squatting and
lunging), is different compared with the NWBE
seated knee extension, in which ACL strain
increased from 2.8% without external resistance to
3.8% with adding only 45 N (10 lbs) of external
resistance
5
One possible explanation for this is that
technique variations typically do not occur during
the seated knee extension exercise (and the ham-
strings are not recruited to unload the ACL), but
techniques variations do occur during WBE, such
as the squat and lunge.
4. Peak ACL strain was generally greater in the
NWBE seated knee extension compared with most
WBE.
5
For example, performing a leg press type
17. exercise with 40% bodyweight resistance, stair
climbing, and forward lunging, all produced less
ACL strain compared with performing a seated
knee extension with no external resistance.
5
Interestingly, performing a NWBE seated knee
extension with no external resistance (quadriceps
activation only), produced the same amount of
ACL strain compared with performing a WBE one
leg sit-to-stand or stair climbing, with the WBE
being much more challenging in recruiting impor-
tant hip and thigh musculature (e.g. quadriceps,
hamstrings, and gluteals) that help stabilize the
knee and protect the ACL.
6
Therefore, WBEs
minimize ACL strain to a greater extent compared
with the NWBE seated knee extension, and WBEs
Table 1. Peak ACL strain and knee angle for commonly
performed rehabilitation exercises.
Author Rehabilitation exercise Peak ACL
strain (%)
Knee flexion
angle (�)
Beynnon et al. (1997)1 Squatting with or without 136 N (30 lb)
resistance 3.6-4.0 10
Beynnon et al. (1995)
18. 4
Dynamic seated leg extension using with a 45 N (10 lb) force as
resistance 3.8 10
Dynamic seated leg extension without external resistance 2.8 10
Isometric seated leg extension using a 30 Nm torque as
resistance 4.4 15
Isometric seated leg extension using a 30 Nm torque as
resistance 2.0 30
Isometric seated leg extension using a 30 Nm torque as
resistance 0 60
Isometric seated leg extension using a 30 Nm torque as
resistance 0 90
Beynnon et al. (1992)
3
Isometric seated leg extension using a 27 Nm torque as
resistance 3.2 30
Isometric seated leg extension using a 27 Nm torque as
resistance 0 90
150 N (33 lbs) Lachman test 3.7 30
Anterior drawer test 150 N (33 lbs) 1.8 90
Heijne et al. (2004)6 One-legged sit to stand (without external
resistance) - tested at 30�, 50�,
and 70� knee angle
2.8 30
Step-up (without external resistance) – tested at 30�, 50�, and
70� knee angle 2.5 30
Step-down (without external resistance) – tested at 30�, 50�,
and 70� knee
angle
2.5–2.6 30
19. Leg press using 40% bodyweight resistance 2.1 20
Forward lunge (without external resistance) – tested at 30�,
50�, and 70�
knee angle
1.8–2.0 30
Stationary bicycling 1.7 30
Fleming et al. (1999)
5
Stair climbing (112 steps per minute) (without external
resistance) 2.8 20
Stair climbing (80 steps per minute) (without external
resistance) 2.7 11
Fleming et al. (1998)
7
Stationary bicycling (175 W, 60 r/min) 0 Near full
extension
150 N (33 lbs) Lachman test 3.0 30
ACL: anterior cruciate ligament.
Escamilla et al. 673
Table 2. Peak ACL tensile forces and knee angles for commonly
performed rehabilitation exercises.
Author Rehabilitation exercise ACL
20. Peak
force (N)
Knee flexion
angle (�)
Escamilla et al. (1998)9 Barbell squat using 12 repetition
maximum resistance** 0
Leg press using 12 repetition maximum resistance** 0
Dynamic seated knee extension using 12 repetition maximum
resistance** 158 15
Escamilla et al. (2001)27 Barbell squat with narrow stance
using 12 repetition maximum resistance** 0
Barbell squat with wide stance using 12 repetition maximum
resistance** 0
Leg press with narrow stance with high foot placement using 12
repetition
maximum resistance**
0
Leg press with wide stance with high foot placement using 12
repetition
maximum resistance**
0
Leg press with narrow stance with low foot placement using 12
repetition
maximum resistance**
0
Leg press with wide stance with low foot placement using 12
repetition
21. maximum resistance**
0
Escamilla et al. (2009)8 Wall squat with heels position far from
wall using 12 repetition maximum
dumbbell resistance**
0
Wall squat with heels positioned close to wall using 12
repetition maximum
dumbbell resistance**
0
One-leg squat using 12 repetition maximum dumbbell
resistance** 59 30
Escamilla et al. (2010)11 Forward lunge while taking a long
step forward using 12 repetition maximum
dumbbell resistance**
0
Forward lunge while taking a short step forward using 12
repetition maximum
dumbbell resistance**
0
Escamilla et al. (2010)
11
Forward lunge while taking a normal length step forward using
12 repetition
maximum dumbbell resistance**
22. 0
Side lunge while taking a normal length step sideways using 12
repetition
maximum dumbbell resistance**
0
Lunging forward and sideways while taking a normal length
step using 12
repetition maximum dumbbell resistance**
0
Lunging forward and sideways while keeping both feet
stationary using 12
repetition maximum dumbbell resistance**
0
Toutoungi et al. (2000)
18
Isokinetic seated knee extension at 60�/s 349 35–40
Isokinetic seated knee extension at 120�/s 325 35–40
Isokinetic seated knee extension at 180�/s 254 35–40
Isokinetic seated knee flexion at 60�/s 0
Isokinetic seated knee flexion at 120�/s 0
Isokinetic seated knee flexion at 180�/s 0
Isometric seated knee extension 396 35–40
Isometric seated knee flexion 0
Squat with heel-off-ground without external resistance 95 50
Squat with heel-on-ground without external resistance 28 50
Squat one-legged without external resistance 142 50
23. Shelburne et al. (2005)19 Level ground walking 303 15–20
Shelburne et al. (2002)
20
Dynamic squat-to-stand 20 25
Pflum et al. (2004)
13
Two-feet drop landing stepping off of 60 cm height platform
253 33–48
Shin et al. (2007)21 Single leg landing from running to a stop
1294 25–30�
**Used the heaviest resistance possible that allowed the
performance of 12 consecutive repetitions with proper form and
technique.
ACL: anterior cruciate ligament.
Table 3. Peak anterior shear force (N) (ACL loading) and knee
angle (�) for commonly performed rehabilitation exercises.
Author Rehabilitation exercise Anterior shear
force (N)
Knee flexion
angle (�)
Wilk et al. (1996)28 Barbell squat using 12 repetition maximum
resistance** 0
Leg press using 12 repetition maximum resistance** 0
Dynamic seated knee extension using 12 repetition
maximum resistance**
248 14
24. Nagura et al. (2006)
29
Full squat using no external resistance 66 10.9
Rising from kneeling 111 40.9
Level ground walking 355 16.8
Stair climbing 146 50.8
Pflum et al. (2004)
13
Drop landing 220 33–48
**Used the heaviest resistance possible that allowed the
performance of 12 consecutive repetitions with proper form and
technique.
674 Proc IMechE Part H: J Engineering in Medicine 226(9)
are more functional multi-joint, multi-muscle exer-
cises that are effective in developing important hip
and thigh musculature, such as the gluteals, ham-
strings, quadriceps, and adductors and abductors.
Peak ACL tensile force
ACL tensile force levels that are injurious to the recon-
structed ACL are unknown, although it likely depends,
in part, on the number of weeks post reconstruction.
ACL tensile forces are generally lower during WBE
compared with NWBE,
6,9,28
and are typically absent in
25. both WBE and NWBE between 50�–100� knee
angles.
6,12,28
Therefore, employing higher knee angles
of between 50�–100� during WBE and NWBE mini-
mizes the risk of injury to the healing graft site.
Table 2 presents ACL tensile force during a variety
of NWBE and WBE. Key points of emphasis from
Table 2 are as follows.
First, like the ACL strain data from Table 1, peak
ACL tensile force is of relatively low magnitude (typi-
cally under 150 N for WBE, between approximately
150–350 N for the NWBE seated knee extension) com-
pared with loading of the posterior cruciate ligament,
9
and occurred at lower knee angles, typically between
15�–35�. The highest ACL tensile forces between
NWBE and WBE occurred during maximal-effort iso-
kinetic seated knee extension exercises, in which ACL
tensile force was approximately 40% greater at a
slower 60�/s speed, compared with faster 180�/s speed.
Rapid deceleration activities, such as one-leg landing
from a jump, or running and cutting movements, have
been shown to generate very high ACL loading and are
often implicated in ACL injuries.
31
For example, dur-
26. ing a running plyometric-type exercise involving a
single-leg landing and rapidly coming to a stop, high
deceleration forces are produced that result in approxi-
mately 1300 N of ACL tensile force.
31
This high ACL
loading demonstrates that high explosive deceleration-
type plyometric exercises should not be performed until
the later stages of ACL rehabilitation, after the ACL
graft has healed, revascularized, and strengthened ade-
quately. In contrast, a two-leg drop jump from a 60 cm
platform only resulted in approximately 250 N of ACL
tensile force,
31
which is similar to the ACL loading that
occurred during the NWBE seated knee extension.
Therefore, lower-intensity plyometric exercises, such as
the two-leg drop jump, should precede higher intensity
plyometric exercises, such as the single-leg drop jump.
The rate of deceleration should also be considered
when performing plyometric exercises, as a higher rate
of deceleration will result in greater ACL loading.
Second, squatting typically resulted in minimal or no
ACL tensile force, and one-leg squatting producing
slightly greater ACL loading compared with two-leg
squatting. The minimal or absence of ACL loading dur-
ing the squat is, in part, owing to the increased ham-
strings activity and force generated during squatting.
For example, peak hamstring activity during the barbell
27. squat was approximately 50% of a maximum voluntary
isometric contraction, which helps unload the ACL.
9
Moreover, peak hamstring force reported during the
one-leg squat has been reported to be approximately
200 N.
8
The increased hamstring activity and force
from one- and two-leg squatting was, in part, owing to
a forward trunk tilt of approximately 30�–40� at maxi-
mum knee flexion.
8,9
Progressively increasing the for-
ward trunk tilt during the squat tends to increase
hamstring activity and decrease quadricep activity,
both which result in ACL unloading at knee angles of
less than 60�.32 Also, squatting with the heels off the
ground, which typically results in increased forward
knee movement beyond the toes, resulted in over three
times the ACL loading compared with squatting with
the heels on the ground. Like the squat, the absence of
ACL loading during the forward and side lunge is, in
part, owing to the relatively high hamstring force, peak-
ing at approximately 150 N at knee angles of less than
30�.11,12
Wall squat exercises may be a better choice com-
pared with the one-leg squat early after ACL recon-
struction because of greater ACL forces generated
during the one-leg squat compared with the wall squat.
28. However, because peak ACL force during the one leg
squat was only approximately 60 N, it is not likely that
the one leg squat will produce forces that would be
injurious to the healing ACL graft, and mild strain to
the graft may enhance the healing process.
33
During
both the wall squat and one-leg squat, as well as other
WBE such as the leg press and lunge, employing larger
knee angles (i.e. 50�–100�) before progressing to smaller
knee angles (i.e. 0�–50�) may be desirable during the
early stages of ACL rehabilitation because ACL forces
primarily occur at smaller knee angles of less than 50�.
The knees moving forward beyond the toes during
squatting and lunging may also increase ACL loading,
especially if excessive (approximately 8–10 cm or
more).
8,11,12
ACL loading was significantly greater in
the one-leg squat, in which the knees moved forward
beyond the toes 10 6 2 cm, and in the lunge using a
short step, in which the knees moved forward beyond
the toes 9 6 2 cm, compared with a lunge with a long
step and a wall squat exercise in which the knees did
not move forward beyond the toes.
8,11,12
Moreover,
squatting with a more erect trunk position at the lowest
position of the squat compared with squatting with a
29. 30�–40� forward trunk tilt position tends to cause more
forward movement of the knees beyond the toes, as
well as greater quadricep activation (which increases
ACL tensile force at lower knee angles) and less ham-
string activation (which results in less unloading of the
ACL).
8,9
Furthermore, as the knee goes forward
beyond the toes, the tibia plateau slopes anteriorly,
resulting in an increase in ACL loading.
29,34
Forward trunk tilt may also affect ACL loading dur-
ing the squat and forward lunge exercises. Squatting
and lunging with increased forward trunk tilt, com-
pared with a more erect trunk position, has been shown
Escamilla et al. 675
to increase hamstrings, which may decrease ACL load-
ing.
9,28,32,35
For example, Ohkoshi et al.
32
reported no
ACL loading at all knee angles tested (15�, 30�, 60�,
and 90�) while maintaining a squat position with trunk
tilted forward from 0�–90�, with 30� or more forward
30. trunk tilt optimal for recruiting relatively high ham-
strings activity and minimizing ACL loading.
Technique variations during the NWBE seated knee
extension also can affect ACL tensile force. For exam-
ple, given a constant external knee torque applied to the
leg, ACL force decreases when the resistance pad is
moved up the leg more proximal to the knee compared
with being more distal to the knee closer to the ankle
(Figure 3).
17
From Figure 3, when a constant external
knee torque is applied to the leg at 30� knee angle, the
ACL tensile force is approximately twice as great when
the resistance pad is positioned near the ankle (approxi-
mately 400 N) compared with when it is placed near the
middle of the leg (approximately 200 N). Also, Figure 3
demonstrates that ACL loading decreases progressively
from 15� knee angle (approximately 500 N when the
resistance pad is near the ankle and approximately
325 N when the resistance pad is placed near the middle
of the leg) to 60� knee angle (approximately 100 N when
the resistance pad is near the ankle and approximately
0 N when the resistance pad is positioned near the mid-
dle of the leg), with no ACL loading at knee angles
greater than 60�. Nisell et al.34 reported a similar find-
ing of less ACL loading with a more proximally posi-
tion resistance pad on the leg during isokinetic seated
knee extension at 30�/s and 180�/s. It can be concluded
from these data that, when the goal is to minimize ACL
loading while using the seated knee extension exercise,
this exercise should be performed at higher knee angles
(50�–100�) and with the resistance pad position more
31. proximal on the leg compared with a more distal posi-
tion. Moreover, it should be emphasized that if the
ACL is torn, there is no ligament to restrain anterior
tibial translation on the femur. Therefore, performing
exercises that would normally load the ACL may cause
anterior tibial translation, which may result in altered
and possibly injurious tibiofemoral joint loading.
36,37
Wilk and Andrews
38
have reported that ACL-deficient
knees during isokinetic exercises tibial translation can
be reduced by utilizing a proximal pad and performing
higher angular velocities (e.g. 180 �/s and 300 �/s) com-
pared with slower speeds (e.g. 60 �/s).
Peak anterior shear force
Peak anterior shear force during WBE and NWBE are
shown in Table 3. From Table 3, it is interesting that
level ground walking resulted in greater anterior shear
force (ACL loading) compared with both NWBE and
WBE. Compared with WBE, anterior shear forces were
greater in the NWBE seated knee extension. Peak ante-
rior shear forces occurred at knee angles of 50� or less.
Moreover, from Table 2 peak ACL tensile force during
level walking was approximately 300 N and occurred
near opposite foot toe-off (approximately 15�–20� knee
flexion). Therefore, peak ACL loading during level
walking is similar to peak ACL loading during NWBE
seated isokinetic and isometric knee extension exercises,
and these ACL loading magnitudes are several times
greater than the ACL tensile forces reported for the
32. WBE.
Figure 3. Changes in ACL loading during the seated knee
extension exercise with proximal or distal resistance applied on
the leg.
The location of the restraining force is given relative to the
distance from the knee joint. Given a constant external knee
torque
applied to the leg, moving the restraining force closer to the
knee joint axis decreases ACL force. Adapted from Pandy and
Shelburne17 with kind permission of Elsevier.
ACL: anterior cruciate ligament.
676 Proc IMechE Part H: J Engineering in Medicine 226(9)
PCL loading during selected rehabilitation
exercises
Table 4 presents PCL tensile force data from selected
articles in the scientific literature during a variety of
NWBE and WBE commonly used in cruciate ligament
rehabilitation. Key points of emphasis from Table 4
are as follows.
First, in contrast to the peak, ACL loading at lower
knee angles between approximately 0�–50� shown in
Tables 1 and 2, peak PCL loading occurs at higher
knee angles between approximately 50�–100� (typically
around 80�–90� knee angles). Thus, to control PCL
loading, the rehabilitation process is the complete
opposite of that of the ACL program. Therefore, if the
rehabilitation goal is to minimize PCL loading, such as
during the early phases after PCL reconstruction sur-
33. gery, training both NWBE and WBE at lower knee
angles (e.g. 0�–50�) would be recommended compared
with training these exercises as higher knee angles (e.g.
50�–100�).
Second, peak PCL tensile force was generally greater
in the NWBE seated knee flexion compared with WBE.
For example, performing the NWBE seated isometric
knee flexion at 90� knee angle produced greater PCL
Table 4. Peak PCL tensile force and knee angle for commonly
performed rehabilitation exercises.
Author Rehabilitation exercise Peak PCL
force (N)
Knee flexion
angle (�)
Escamilla et al. (2009)8 Wall squat with heels position far from
wall using 12 repetition
maximum dumbbell resistance**
757 80
Wall squat with heels positioned close to wall using 12
repetition
maximum dumbbell resistance**
786 90
One-leg squat using 12 repetition maximum dumbbell
resistance**
414 90
34. Escamilla et al. (2010)
11
Forward lunge while taking a long step forward using 12
repetition maximum dumbbell resistance**
765 70
Forward lunge while taking a short step forward using 12
repetition maximum dumbbell resistance**
612 90
Escamilla et al. (2010)11 Forward lunge while taking a normal
length step forward using
12 repetition maximum dumbbell resistance**
765 70
Side lunge while taking a normal length step sideways using 12
repetition maximum dumbbell resistance**
641 50
Lunging forward and sideways while taking a normal length
step
using 12 repetition maximum dumbbell resistance**
733 60
Lunging forward and sideways while keeping both feet
stationary
using 12 repetition maximum dumbbell resistance**
652 80
35. Escamilla et al. (1998)9 Barbell squat using 12 repetition
maximum resistance** 1868 63
Leg press using 12 repetition maximum resistance** 1866 95
Dynamic seated knee extension using 12 repetition maximum
resistance**
959 79
Escamilla et al. (2001)
39
Barbell squat with narrow stance using 12 repetition maximum
resistance**
2066 77
Barbell squat with wide stance using 12 repetition maximum
resistance**
2212 76
Leg press with narrow stance with high foot placement using 12
repetition maximum resistance**
1703 94
Leg press with wide stance with high foot placement using 12
repetition maximum resistance**
1726 88
Leg press with narrow stance with low foot placement using 12
repetition maximum resistance**
1690 95
36. Leg press with wide stance with low foot placement using 12
repetition maximum resistance**
1726 95
Toutoungi et al. (2000)18 Isokinetic seated knee extension at 60
�/s 74 90
Isokinetic seated knee extension at 120 �/s 59 90
Isokinetic seated knee extension at 180 �/s 55 90
Isokinetic seated knee flexion at 60 �/s 2701 90
Isokinetic seated knee flexion at 120 �/s 2394 90
Isokinetic seated knee flexion at 180 �/s 1952 90
Isometric seated knee extension 0
Isometric seated knee flexion 3330 90
Squat with heel-off-ground without external resistance 2222 90–
100
Squat with heel-on-ground without external resistance 2704 90–
100
Shelburne et al. (2005)19 Level ground walking Approximately
160 15–20
**Used the heaviest resistance possible that allowed the
performance of 12 consecutive repetitions with proper form and
technique.
PCL: posterior cruciate ligament.
Escamilla et al. 677
tensile force (3330 N) compared with all WBE. The
NWBE seated isokinetic knee flexion at 60 �/s, 120 �/s,
and 180 �/s, and the two-leg squat, produced the next
highest PCL tensile force, ranging between approxi-
37. mately 1900�2700 N. PCL tensile forces were approxi-
mately 1700�1900 N during the leg press, approximately
750�800 N during the wall squat, approximately
650�750 N during the forward and side lunges, approxi-
mately 400 N during the one-leg squat, and approxi-
mately 160 N during level ground walking.
Comparing technique variations within exercises,
performing the forward lunge while taking a long step
forward, produces a significantly greater PCL tensile
force compared with performing the forward lunge
while taking a short step forward (the short step lunge
causes the knees to move forward over the toes approx-
imately 8 cm). Moreover, performing the forward lunge
produced a significantly greater PCL tensile force com-
pared with performing the side lunge, and performing
the forward and side lunge while taking a normal
length step (and then pushing back to the upright start-
ing position) produced a significantly greater PCL ten-
sile force compared with performing the forward and
side lunge while keeping both feet stationary (and sim-
ply lunging up and down). Therefore, there are differ-
ent progressions both within an exercise and between
exercises that can be employed during PCL rehabilita-
tion, and as a general rule NWBE and WBE should
first begin with no external resistance and progress to
increasing amounts of external resistance. Given the
PCL loading shown in Table 4, one example of exercise
progression for lower to higher PCL loading may
include performing NWBE and WBE initially between
0�–50� knee flexion and progressing to 50�–100� knee
flexion. Exercises that may be appropriate early in
rehabilitation might include seated knee extensions
between 0�–50� knee flexion, level ground walking,
one-leg squats with no resistance, and forward and side
lunges with no resistance. Resistance can slowly be
38. added to these exercises, and the leg press, squat, and
seated knee flexion exercises can be added later, ini-
tially without resistance and progressing to resistance.
It is not well understood what PCL force magnitudes
become injurious to the healthy or reconstructed PCL.
In healthy adults, the ultimate strength of the PCL is
approximately 4000 N,
40
although these values depend
on age and anatomical factors. Therefore, the PCL
loads generated during both NWBE and WBE appear
to be well within a safe limit for the healthy PCL. The
reconstructed PCL has similar ultimate strengths com-
pared with the healthy PCL. However, the healing graft
site may be injured with considerably less force com-
pared with the ultimate strength of the graft, although
it is not well understood how much force to the healing
graft site is too much and how soon force can be
applied after reconstruction. Therefore, the peak PCL
forces that occur during NWBE and WBE may be pro-
blematic early after PCL reconstruction when the graft
site is still healing, especially between 50�–100� of knee
angles. It may be prudent to employ smaller knee angles
(e.g. 0�–50�) before progressing to larger knee angles
(e.g. 50�–100�) during NWBE and WBE, because PCL
forces increase as knee angle increases.
Summary
This review allows the clinician to select specific thera-
peutic exercises, broken down by weight bearing status,
knee range of motion, and technique variations, to
39. progress cruciate ligament loading over the course of
rehabilitation safely while ensuring optimal recovery of
the musculoskeletal system. In general, WBE produces
smaller loads on the ACL and PCL compared with
NWBE. For the ACL, performing seated knee exten-
sions with resistance produces significantly greater
ACL loading compared with most WBE. Further, peak
ACL loading during level walking is similar to peak
ACL loading during NWBE seated isokinetic and iso-
metric knee extension exercises, and these ACL loading
magnitudes are several times greater than the ACL ten-
sile forces reported for the WBE. For the PCL, the
highest loading occurred in the two-leg squat, followed
by the leg press, wall squat, forward and side lunges,
one-leg squat, and level ground walking. WBE has the
benefit of being much more functional and challenging
in terms of hip and thigh muscle recruitment compared
with NWBE. Therefore, early after injury or recon-
struction of the cruciate ligament the clinician should
prescribe WBE rather than NWBE, and progress to
NWBE as tolerated and to facilitate isolated muscle
functional groups – such as the quadriceps.
Peak ACL loading occurs at knee angles of between
10�–15�, and progressively decreases between 15�–60�
knee angles. Beyond 50�–60� knee angles there is mini-
mal or no ACL loading. In contrast, PCL loading
occurs at higher knee angles (i.e. 50�–100�), with peak
PCL loading typically occurring around 80�–90� knee
angles. These arc ranges of motion should serve as
guidelines to limit the therapeutic exercise technique
early on in the rehabilitation protocol.
Exercise technique variation should also be considered
when prescribing a rehabilitation protocol. For ACL
rehabilitation, anterior knee movement of 8 cm or more
40. beyond the toes may also increase ACL loading during
squatting, lunging, leg press, and other WBE. Moreover,
squatting with the heels off the ground, which typically
results in increased anterior knee movement beyond the
toes, resulted in over three times the ACL loading com-
pared with squatting with the heels on the ground.
Squatting and lunging with a more forward trunk tilt
tends to unload the ACL to a greater extent compared
with squatting and lunging with a more erect trunk posi-
tion. Moving the resistance pad up the leg proximally
towards the knee when performing the seated knee
extension, rather than being positioned closer to the
ankle, decreases ACL loading. Rapid deceleration activi-
ties, such as one-leg landing from a jump, or running
678 Proc IMechE Part H: J Engineering in Medicine 226(9)
and cutting movements, have been shown to generate
very high ACL loading and are often implicated in ACL
injuries, so cautious progression is required. These higher
intensity plyometric type exercises should be performed
only during the later stages of ACL rehabilitation. When
prescribing therapeutic exercise for the PCL, the forward
lunge with a long step forward produced greater PCL
loading compared with forward lunge with a short step
forward. The forward lunge produced greater PCL load-
ing compared with the side lunge, and lunging by taking
a forward or sideways step and pushing back to the
starting position produced greater PCL loading com-
pared with lunging up and down with both feet station-
ary. The exercise guidelines provided in this review may
be used by the clinician to progress a cruciate ligament
injured individual to maximize the potential benefits and
minimize the chance for injury.
41. Funding
This research received no specific grant from any fund-
ing agency in the public, commercial, or not-for-profit
sectors.
This article was submitted as part of the Lower Limb
Musculoskeletal Modelling Special Issue.
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