Simultaneous Control of Natural and Extra Degrees-of-Freedom by Isometric Force and EMG Null Space Activation

Myoelectric signals allow to control prostheses and exoskeletons intuitively and effectively by estimating movement intention from the activation of multiple muscles. However, not all muscle activation patterns generate movements, because of the redundancy of the musculoskeletal system. Therefore, such “null space” activations could be used to control extra degrees-of-freedom while simultaneously performing a task. Here, we tested the feasibility of this approach by instructing participants to match the position and orientation of an ellipsoidal target by displacing and rotating an ellipsoidal cursor through the generation of isometric force and electromyographic null space activation. Participants were able to perform the task and their performance improved with practice. However, there was a large variability across participants in their ability to hold the cursor within the target. These results support the feasibility of null space control and suggest that task difficulty must be optimized according to the individual control ability.