The Sit-to-Stand Manoeuvre
Recorded at 25 pictures per second (total duration 4.26 seconds), manually digitised and low-pass filtered at 3 Hz. Ground reaction forces halved (assuming symmetry). Anthropometry from New York study.
Ground Reaction Forces
Ground Reaction Vector (butterfly diagram)
Kinematics + Ground Reaction Vector
Joint Angles, Moments & Powers
Segment Energies & Velocities + Ground Reaction Forces
Segment Powers & Power Flows
Angle-angle-angle (AAA) diagram
Moment-moment-moment (MMM) diagram
How is standing initiated?
When is trunk energy at its maximum?
When is trunk velocity at its maximum? Same time as energy is max? If not, why not?
Which muscles are mainly responsible for standing?
Which muscle is more active standing up, and which sitting down?
At what point in the movement is knee power generation and absorption at a maximum?
When is ankle activity at a maximum?
What is the difference between concentric & eccentric contraction?
To which direction (forwards/backwards?) is the ground reaction vector directed?
How valid are the inverse dynamics?
Some hints from Motion Toolbox
...and what we said on the CGA list!
The effect of starting position
Following the suggestions made on the CGA list, two further analyses were made on a subject rising to standing from two different starting positions. This starting position can be defined in several ways:
Position of buttocks on the chair
Position of feet on the floor with respect to chair
Ankle angle (degrees of initial dorsiflexion or plantarflexion)
In these experiments the position of the feet was kept constant, and the subject was asked to sit either forwards or backwards in the chair (equivalent to a dorsiflexed or plantarflexed ankle, respectively).
Here are the movies:
Standing from a forward position in the chair
Standing from a backward position in the chair
... and here are the analyses:
Kinetics from front of the chair
Energetics from front of the chair
Kinetics from back of the chair
Energetics from back of the chair
In summary, here is the work done by each joint:
From forward position in chair .... backward position in chair
Duration ........ 2.32 s 2.76 s
Ankle Work ...... 1 J 1 J
Knee Work ...... 43 J 16 J
Hip Work ....... 56 J 88 J
Total Work .....100 J 104 J
Thus, it seems that when the subject sits back in the chair, more work is done by the hip extensors (glutei), but the less by the knee extensors (quadriceps).