1) The sit-to-stand movement involves raising the body from a sitting to standing position and requires coordination of limbs to transfer weight while maintaining balance. 2) Individuals with knee osteoarthritis display different movement strategies during sit-to-stand, such as greater muscle co-contraction, earlier hamstring activation, and reduced hip and knee range of motion. 3) Altered movement strategies in osteoarthritis are thought to compensate for pain and weakness, helping to accomplish the task while protecting the affected knee joint.

1. Sit to Stand Mechanism and
Osteoarthritis

2. The Sit-to-Stand (STS) transition is a voluntary daily activity that
consists of rising from a sitting position to a standing position, an
activity that is typically performed by a person several times a day.

3. To undertake the activity successfully requires the
coordination of the body limbs in order to transfer the body
weight between the sitting and standing positions, maintaining
the balance, in order to avoid a fall

4. The STS movement has four principal phases: sitting, initiation,
ascending, and standing
Each phase is composed of events, which change the orientation
and position (pose) of the body components (segments and joints).

5. The STS transition begins when the person is sitting on a firm
surface such as a chair or the edge of a bed, it’s called first base of
support.

6. Quadriceps are required to generate enough concentric moment to
extend the knee against the combined effects of gravity and body
weight
Quadriceps have to resist the antagonistic action of the
hamstrings.

7. Transfer of angular momentum
When upper body swings forward then stops, the angular
momentum is transferred from the upper body to thighs, lower
legs, etc.
The angular momentum is the product of the moment of inertia
and the angular velocity around an axis.
The units of angular momentum are kg∙m2/s.
Angular momentum can be determined by multiplying moment of
inertia by angular velocity

8. Inertia is the resistance to change in motion, specifically to
change in a body’s velocity
Momentum is a resistance to change in velocity of a moving
body
If a person is unable to generate sufficient momentum to
stand up, it will back into the chair
When a person is unable to control the direction of the
momentum they generated they have to use the stepping
strategy to regain balance

9. Inertia for angular motion depends on how the mass was
distributed relative to the axis of rotation
Moment of inertia depends not only how much mass
someone has, but also on how that mass is distributed in
relation to the axis of rotation

10. The moment of inertia, otherwise known as the angular
mass or rotational inertia, of a rigid body is a tensor that
determines the torque needed for a desired angular
acceleration about a rotational axis; similar to how mass
determines the force needed for a desired acceleration

11. The support base function is to help to balance and stabilize the
body throughout the STS transition. The most challenging
moment of the whole transition is when the body leaves the chair,
because that is when the body changes its support base: from the
chair to the feet. This transition produces an abrupt movement
mainly caused by the forces and torques produced by segments
and joints of the body in order to ascend and stand

12. In order to rise from a chair trunk flexion with associated
hip flexion occurs.
Excessive hip flexion is resisted by contraction in the
hamstrings, which simultaneously induces knee flexion this
‘unwanted’ knee flexion then has to be overcome by
additional quadriceps activity

13. Initial phase used to generate
upper-body momentum.
Centre of mass predominantly
translates horizontally forwards

14. Transitional phase- momentum
from upper body is transferred
to the whole body as the centre
of mass changes from horizontal
to vertical translation

15. Extension phase- vertical ascent
of body takes place

16. Standing up requires an initial impulse in the horizontal
direction, changing to the vertical direction when the thighs
are lifted off the supporting surface.
The horizontal distance moved by the centre of body mass
(CBM) and the timing of lower limb extensor force in
relation to the position of the CBM appear to be critical to
ensure both a change in direction and the preservation of
equilibrium

17. The center of mass of the human body depends on the
gender and the position of the limbs. In a standing posture,
it is typically about 10 cm lower than the navel, near the
top of the hip bones.
In the anatomical position, the Centre of gravity lies
approximately anterior to the second sacral vertebra.

18. There are 3 significant challenges to sit-to-stand:
(a) Bringing the center of mass forward
(a) Vertically raising the center of mass from the sitting to
standing position
(a) Transition from a relatively large and stable base of
support in sitting to a considerably smaller base of
support when standing.

19. Transfer of angular momentum
When upper body swings forward then stops, the angular
momentum is transferred from the upper body to thighs,
lower legs, etc.
Angular momentum can be determined by multiplying
moment of inertia by angular velocity

20. Reaction force
For every force there is an equal and opposite reaction
(Newton's 1 st law).
In this picture the lady needs to pushing downwards and
backwards to ensure a propulsive (upwards and forward)
reaction force is produced.

21. Rotary Stability
Rotary stability is affected by height of Centre of Mass,
Base of support and position of line of gravity within Base
of support
Base of support is the area inside a line joining the parts of
the system in contact with the ground (in this picture, feet
of chair and person).
In this picture, the lady needs to move her LOG over her
feet, so she is stable upon standing.

22. Friction
FF = g X FN
Frictional properties of shoes and feet of chair with ground
such that the lady and/or chair doesn't slip when applying
force (horizontal component) when rising

23. There are 3 significant challenges to sit-to-stand:
(a) Bringing the center of mass forward
(a) Vertically raising the center of mass from the sitting to
standing position
(a) Transition from a relatively large and stable base of
support in sitting to a considerably smaller base of
support when standing.
If the chair height is lower the starting position of the
centre of gravity is lower making lift off from the seat more
demanding.

24. Ankle joint motion during sit-to stand task
The ankle motion is initially in slight dorsiflexion, although
this will vary slightly with different initial foot positions.
The ankle then moves smoothly into an increasing
dorsiflexed position.
As the person leaves the chair the ankle moves back towards
the ankle neutral position.
The angular velocity shows an initial dorsiflexion velocity
(positive) followed by a plantarflexion velocity (negative)

25. Knee-joint motion during sit-to-stand task
The knee joint is initially flexed at 90degree, then after a
short delay smoothly extends to near full extension when the
person is upright.
The angular velocity shows a smooth increase and decrease
in the extension velocity (negative) demonstrating a
controlled movement into extension.

26. Hip-joint motion during sit-to-stand
The hip joint is initially flexed at 90degree at the onset there
is an immediate movement into further flexion as the trunk
is moved forward over the feet.
Then at approximately the same time as the onset of knee
extension the hip starts to extend until the upright position
is attained.
The velocity shows an initial flexion angular velocity
(positive) as the trunk is inclined forwards, followed by an
extension velocity (negative) until the upright position is
attained.

27. Sit to stand movement with moderate knee osteoarthritis
Patients with moderate OA rise from the chair
using greater muscle co-contraction of the knee
muscles and earlier and greater activation of the
hamstrings which results in reduced hip and knee
range of motion.
This may be a way to overcome the pain and
potential muscle atrophy of knee extensor muscles
without compromising overall task duration.

28. Sit to stand movement with moderate knee osteoarthritis
Patients with osteoarthritis displayed significantly
lower vastus lateralis coupled with a higher biceps
femoris electromyographic activity and higher
agonist–antagonist co-contraction and co-activation
than asymptomatic patients.

29. Sit to stand movement with moderate knee osteoarthritis
Research studies have shown that individuals with
knee OA display different movement strategies
when performing a Sit to stand task than
asymptomatic individuals.

30. Sit to stand movement with moderate knee osteoarthritis
OA patients perform the STS task at a slower speed
a lower knee range of motion, extension and flexion
torque, a higher hip extension torque and a more
posterior position of the center of pressure than
asymptomatic controls.

31. Sit to stand movement with moderate knee osteoarthritis
There is evidence that OA affects the activity of major trunk
extensors muscles.
Research reported that patients with knee OA showed a
higher maximal trunk flexion and a higher lateral trunk
lean on the contralateral side when compared with the
control group, making the trunk a major contributor to Sit
to stand task.

32. Sit to stand movement with moderate knee osteoarthritis
Pathological or pain-related constraints set by knee OA on
the musculoskeletal system, may lead to altered
intersegmental coordination solutions to accomplish the
same overall task goal.
The redundancy of the degrees of freedom available in the
human motor system allows for the development of new
movement affordances ensuring effective interaction with
the physical environment for the successful accomplishment
of the tasks of daily living.

33. Sit to stand movement with moderate knee osteoarthritis
Altered neuromuscular patterns are mostly translated into
different muscle co-contraction patterns while individuals
perform a given movement.
Co-contraction is defined as the simultaneous activity of
agonist and antagonist muscles surrounding a joint.
Higher co-contraction may be linked with higher joint
stiffness and altered control of force transfer from the hip to
the knee via the work of the bi-articular components of the
hamstrings and quadriceps

34. Sit to stand movement with moderate knee osteoarthritis
Research studies have found that individuals with knee OA
display a high vastus lateralis (VL) to biceps femoris (BF)
activation ratio (lateral muscle co-contraction)
In a systematic review, co-contraction was considered as “a
prevalent muscle adaptation” when the knee is fully loaded,
in order to protect the medial knee joint from excessive
loading

35. Sit to stand movement with moderate knee osteoarthritis
Research over Women with knee OA performed the sit to
stand movement by using the biceps femoris muscles
predominantly as hip extensors so that the upper body is
transferred to full extension.
This maybe a way to overcome the pain and the potential
muscle atrophy of knee extensor muscles without
compromising overall task duration.