By studying how net joint moments and individual muscle forces contribute to the control of human movement, we intend to link the capabilities of a physically challenged person to his or her ability to perform activities of daily living. Measuring these contributions is critical for defining adaptive movement control strategies, understanding the process by which movement control strategies for activities like walking, standing and rising from a seated position are selected and implemented, and determining the contribution that assistive technologies play in enhancing function. We have extended existing movement analysis methodologies that estimate the influence of joint moments or muscle forces on the motion of individual joints and body segments, as well as their contribution to overall task performance.? ? Earlier efforts to determine the contribution of net muscular joint moments and individual muscle forces to human movement control have been limited by the assumption that the relative contribution of muscular effort to the control of movement is independent of the position of the body segments. A generalized coordinate formulation of the equations of motion shows that this assumption is incorrect and can lead to errors in interpreting human movement analysis data. Our past research revealed that the generation of forward velocity during normal gait arises primarily from ankle joint plantar flexor push-off, rather than through a controlled fall. We found that the ankle joint plantar flexor muscle group produces forward acceleration of the upper body during periods in which the same muscle group acts eccentrically. We examined the relative contribution of the lower extremity joint moments to the maintenance of upright posture (support) and found that, during the single limb support phase, ankle moments generate the greatest contribution to support. We also examined the relative sensitivity of the joint accelerations to the joint moments during the foot-flat phase of gait and found that the ankle, knee and hip joint accelerations are almost twice as sensitive to moments generated at the knee than to the moments at any other lower extremity joint. During the heel-off phase of gait, the acceleration sensitivities at both the ankle and hip joints increase in response to moments at their own joint. It is clear from these data that there is significant redundancy in controlling the motion of the stance limb. Application of the model to patients with severe muscle weakness has shown how they can exploit this redundancy to develop a variety of alternative movement control strategies during gait and sit-to-stand movements to compensate for weak or lost muscle function. We have further extended the model used in these analyses to estimate the sources of mechanical energy exchange between segments, and have used it to determine the muscle groups responsible for the inter-segment transfer of mechanical energy during gait. We found that the energy transfer depends on the sign of the joint moment, rather than on the sign of the joint power, and that pairs of net joint moments function in combination to balance energy flow through the leg and trunk segments during normal gait.
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