The redundancy present within the musculoskeletal system may ofer a non-invasive source of
signals for movement augmentation, where the set of muscle activations that do not produce force/
torque (muscle-to-force null-space) could be controlled simultaneously to the natural limbs. Here, we
investigated the viability of extracting movement augmentation control signals from the muscles of
the wrist complex. Our study assessed (i) if controlled variation of the muscle activation patterns in
the wrist joint’s null-space is possible; and (ii) whether force and null-space cursor targets could be
reached concurrently. During the null-space target reaching condition, participants used muscle-to force null-space muscle activation to move their cursor towards a displayed target while minimising
the exerted force as visualised through the cursor’s size. Initial targets were positioned to require
natural co-contraction in the null-space and if participants showed a consistent ability to reach for
their current target, they would rotate 5◦
incrementally to generate muscle activation patterns further
away from their natural co-contraction. In contrast, during the concurrent target reaching condition
participants were required to match a target position and size, where their cursor position was instead
controlled by their exerted fexion–extension and radial-ulnar deviation, while its size was changed by
their natural co-contraction magnitude. The results collected from 10 participants suggest that while
there was variation in each participant’s co-contraction behaviour, most did not possess the ability
to control this variation for muscle-to-force null-space virtual reaching. In contrast, participants did
show a direction and target size dependent ability to vary isometric force and co-contraction activity
concurrently. Our results indicate the limitations of using the muscle-to-force null-space activity of
joints with a low level of redundancy as a possible command signal for movement augmentation.