Contributions of joint mechanics and neural control to the generation of torque during movement

Completing motor tasks that require contact is dependent on an ability to regulate the relationship between limb motions and interaction forces with the environment. This can be achieved by exploiting the mechanical properties of a limb or through active regulation of joint torques through changes in muscle activation. Leveraging the mechanical properties of a joint might simplify neural control when they are matched to the functional requirements of a task. The purpose of this study was to determine if humans change their control strategy, relying on limb mechanics rather than regulated muscle activation, when feasible. This was accomplished by measuring ankle impedance and muscle activation strategies in three tasks requiring joint torques to: oppose movement, assist movement, or remain constant during movement. We found that subjects produced more torque due to impedance and less torque due to muscle activation in the torque-oppose task, the only task that could feasibly be completed through impedance modulation. These results demonstrate that people do leverage the mechanical properties of a joint to complete certain task, lessening the need for precisely timed muscle contractions.