Simulation analysis of muscle activity changes with altered body orientations during pedaling

Testing hypotheses related to the effect of gravitational orientation on ne ural control mechanisms is difficult for most locomotor tasks, like walking , because body orientation with respect to gravity affects both sensorimoto r control and task mechanics. To examine the mechanical effect of body orie ntation independently from changes in workload and posture, Brown et al. (J . Biomech. 29 p. 1349, 1996) studied pedaling at altered body orientations. They found that subjects pedaling at different orientations changed needle ssly their muscle excitations, putatively to preserve body-upright pedaling kinematics. We tested the feasibility of this hypothesis using simulations based on a three biomechanical-function pair organization for control of l ower limb muscles (limb extension/flexion pair, extension/flexion transitio n pair, and foot plantarflexion/dorsiflexion pair), where each pair consist s of alternating agonistic/antagonistic muscles. Adjustment of only three p arameters, one to scale the muscle excitations of each pair; was sufficient to preserve pedaling kinematics to altered body orientation. Because these adjustments produced changes in muscle excitation and net joint moments si milar. to those observed in pedaling subjects, the hypothesis is supported. Moreover, the effectiveness of a decoupled gain adjustment procedure where each parameter was adjusted by error in only one aspect of the pedaling tr ajectory during each iteration (i.e., cadence adjusted the Ext/Flex paramet er; peak-to-peak variation in crank velocity over the cycle adjusted the tr ansition parameter; average ankle angle over the cycle adjusted the foot pa rameter) further supports the distinct function of each muscle pair. Publis hed by Elsevier Science Ltd.