Upper extremity multi-degree of freedom torque generating abilities in able-bodied individuals

The ability of able-bodied individuals to generate maximum static torques at the shoulder and elbow in flexion and extension while maintaining fixed levels of shoulder abduction/adduction torque was evaluated in six subjects. A six-axis load cell was used to measure the torques and forces generated at the elbow and shoulder. Visual feedback of the torque was provided during the experiment. Subjects were verbally encouraged to perform maximum voluntary torques in 24 directions on the shoulder/elbow flexion/extension plane while maintaining 20 and 50% of maximum shoulder abduction or adduction torques or zero level of abduction. These torques were significantly smaller than the torque generated in the plane when subjects were not required to control shoulder abduction/adduction. The maximum torques generated while subjects maintained zero level of abduction were also significantly smaller than in the 20 and 50% maximum abduction/adduction tasks. Muscles at the shoulder and elbow cross more than one joint and effect more than one degree of freedom, thus imposing constraints on tasks that demand simultaneous control of multiple degrees of freedom. However, these constraints do not seem to affect the ability to generate maximum torques at increasing levels once control of a third degree of freedom has been introduced.