MUSCLE-ACTIVITY PATTERNS ALTERED DURING PEDALING AT DIFFERENT BODY ORIENTATIONS

Gravity is a contributing force that is believed to influence strongly the control of limb movements since it affects sensory input and also contributes to task mechanics. By altering the relative contribution of gravitational force to the overall forces used to control pedaling at different body orientations, we tested the hypothesis that joint to rque and muscle activation patterns would be modified to generate stea dy-state pedaling at altered body orientations. Eleven healthy subject s pedaled a modified ergometer at different body orientations (From ho rizontal to vertical), maintaining the same workload (80 J), cadence ( 60 rpm), and hip and knee kinematics. Pedal reaction forces and crank and pedal kinematics were measured and used to calculate joint torques and angles. EMG was recorded from four muscles (tibialis anterior, tr iceps surae, rectus femoris, biceps femoris). Measures of muscle activ ation (joint torque and EMG activity) showed strong dependence on body orientation, indicating that muscle activity is not fixed and is modi fied in response to altered body orientation. Simulations confirmed th at, while joint torque changes were not necessary to pedal at differen t body orientations, observed changes were necessary to maintain consi stent crank angular velocity profiles. Dependence of muscle activity o n body orientation may be due to neural integration of sensory informa tion with an internal model that includes characteristics of the endpo int, to produce consistent pedaling trajectories. Thus, both sensory c onsequences and mechanical aspects of gravitational forces are importa nt determinants of locomotor tasks such as pedaling. Published by Else vier Science Ltd.