Abnormal spatial patterns of elbow muscle activation in hemiparetic human subjects

The patterns of electromyographic (EMG) activity in spastic-paretic and contralateral elbow muscles of 10 hemiparetic human subjects were compared during a sequence of graded voluntary isometric contractions against 4 different-sized loads. These loads were orientated successively at 8 different angles over a 360 degree range, referenced to a plane at the wrist orthogonal to the long axis of the forearm. Comparisons of EMG activity recorded from normal and paretic limbs revealed that there were marked differences in the torque angles which evoked significant EMG activity, in the angular range of EMG, in the angle of peak EMG, and in the scaling of EMG magnitude with increasing isometric loads. In severely impaired limbs, there was a marked shift in both the peak EMG angle and the angular domain of EMG activity for both biceps and triceps muscle groups, away from the normal elbow flexion-extension axis towards external humeral rotation and shoulder girdle elevation. The extent of the disturbance in the spatial patterns of EMG activity was closely correlated with the clinical severity of the spastic-paretic disability, which was quantified using a functional scale patterned after that described by Fugl Meyer et al. (1975). The observed patterns of EMG activity in paretic flexor muscles do not conform with established synergistic patterns, such as might be released by excitation of the flexor reflex in a normal limb. Possible origins for the anomalous EMG patterns are discussed.