Transition between reciprocal activation and co-contraction during wrist posture control

In daily life we perform numerous tasks which require position control against disturbances. We can counter these disturbances by opposing forces using reciprocal activation of muscles, or attenuate their effect by increasing the joint impedance using muscle co-contraction. Increasing impedance is the only possible strategy to minimize the effect of high frequency disturbances, while for low frequency disturbances either reciprocal activation or co-contraction (and hence increased impedance) may be used. We analyze here how the selection between reciprocal activation and co-contraction depends on the frequency and amplitude of a disturbance and how and the control strategy is modified. Our results indicate that with increase of disturbance frequency the reciprocal activation decreases and is gradually replaced by co-contraction. Co- contraction increases with the magnitude and frequency of reciprocal activation even when impedance increase does not directly help the task, suggesting that co-contraction is a pre- requisite for reciprocal activation control.