Theoretical analyses have shown that rotations of a rigid body about the principal axis corresponding to the intermediate principal moment of inertia are unstable. This poses a potential problem for gymnasts who perform double somersaults without twist in

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 torque and muscle activation patterns would be modified to generate steady-state pedaling at altered body orientations. Eleven healthy subjects pedaled a modified ergometer at different body orientations (from horizontal 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, triceps surae, rectus femoris, biceps femoris). Measures of muscle activation (joint torque and EMG activity) showed strong dependence on body orientation, indicating that muscle activity is not fixed and is modified in response to altered body orientation. Simulations confirmed that, while joint torque changes were not necessary to pedal at different body orientations, observed changes were necessary to maintain consistent crank angular velocity profiles. Dependence of muscle activity on body orientation may be due to neural integration of sensory information with an internal model that includes characteristics of the endpoint, to produce consistent pedaling trajectories. Thus, both sensory consequences and mechanical aspects of gravitational forces are important determinants of locomotor tasks such as pedaling.