Estimates of individual muscle power production in normal adult walking.

BACKGROUND: The purpose of this study was to determine the contribution of individual hip muscles to the net hip power in normal adult self-selected speed walking. A further goal was to examine each muscle's role in propulsion or support of the body during that task.

METHODS: An EMG-to-force processing (EFP) model was developed which scaled muscle-tendon unit (MTU) force output to gait EMG. Active muscle power was defined as the product of MTU forces (derived from EFP) and that muscle's contraction velocity. Passive hip power was estimated from passive moments associates with hip position (angle of flexion (extension)) and the hip's angular velocity. Net hip EFP power was determined by summing individual active hip muscle power plus the net passive hip power at each percent gait cycle interval. Net hip power was also calculated for these study participants via inverse dynamics (kinetics plus kinematics, KIN). The inverse dynamics technique - well accepted in the biomechanics literature - was used as a "gold standard" for validation of this EFP model. Closeness of fit of the power curves of the two methods was used to validate the model.

RESULTS: The correlation between the EFP and KIN methods was sufficiently close, suggesting validation of the model's ability to provide reasonable estimates of power produced by individual hip muscles. Key findings were that (1) most muscles undergo a stretch-shorten cycle of muscle contraction, (2) greatest power was produced by the hip abductors, and (3) the hip adductors contribute to either hip adduction or hip extension (but not both).

CONCLUSIONS: The EMG-to-force processing approach provides reasonable estimates of individual hip muscle forces in self-selected speed walking in neurologically-intact adults.