Selective activity of flexor and extensor wrist muscles is reduced in post-stroke patients

Introduction: Loss of selective muscle activation after stroke contributes to poor arm function but is difficult to quantify. The objective of this study was to quantify selective activity of flexor and extensor wrist muscles in post-stroke patients. Methods: 31 Patients in the chronic phase after stroke and 14 matched healthy controls exerted a flexion and extension torque onto a haptic wrist manipulator. EMG of the flexor and the extensor carpi radialis muscles was stratified for equal flexion and extension torques. The Activation Ratio per muscle was determined, i.e. ratio of the difference of EMG activity during flexion and extension over summed EMG activity. A ratio close to one indicates selective activation while a ratio close to zero indicates indifferent activation. Results: Control subjects could exert higher (p<0.001) flexion and extension torques (25.42 Nm and 14.32 Nm) compared to post-stroke subjects (14.45Nm and 6.69Nm). The Activation Ratio’s for flexor and extensor muscle were significantly lower (p<0.001) in post-stroke subjects (ARflex: 0.72 – 0.54, ARext:0.79 – 0.64). Discussion and conclusion: Activation Ratio’s allow for muscle specific determination of selective activity which is advantageous in case of diverging muscle features. In post-stroke patients the loss of selective activity has to be accounted for when explaining and intervening on loss of function. Clinical message: Quantification of muscle specific selective activation in post stroke hemiparesis allows for assessment of contribution of the muscle to co-contraction and functional loss and evaluation of therapeutic options