Joint Moments
| Muscles produce moments of force across joints during the walking cycle. (Other soft tissues, like ligaments, can also produce forces and moments when they are elongated.)
These figures, derived from data published by Winter (1987, Table 4.24b, p.64) show typical moments at the hip, knee, and ankle during the gait cycle. The support moment is the sum of the extensor moments.
Calculation of joint moments is complex, but is validated by the moments' excellent correspondence with normal muscle activity during gait.
SAS program that produced these graphs
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Joint moments are calculated using sophisticated laboratory equipment and computer programs. To understand their derivation, and their importance to clinical gait analysis, consider an observed joint movement represents the effect of the total moment that acts at the joint at that instant during the gait cycle.
The total moment at a joint is calculated as the product of two measurable quantities:
- the joint segments' moments of inertia, which involves knowing thee segments' masses and lengths
- the joint's angular acceleration
The instantaneous total moment is the sum of individual moments produced by:
- ground reaction forces
- joint reaction forces
- muscle and soft tissue forces that produce the joint moment
ground reaction moment
+ joint reaction moment
+ (internal) joint moment
= total moment
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We can calculate the ground reaction and joint reaction moments if we also have information on ground reaction forces (from forceplate data) and segmental velocities.
We can then calculate the joint moment as a residual:
total moment
- ground reaction moment
- joint reaction moment
= joint moment
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We use calculated joint moments to calculate and understand joint power
Last updated 4-19-01 ©Dave Thompson PT