Fig. 3.

Directional activity tuning plots for whole muscles as measured with surface EMG. BI activity is broadly tuned and can be fit by a single cosine function. Data from PD can be best fit by two cosine functions with nearly opposite peaks (two circles with two arrows). Partially because of the sampling bias inherent in multiunit surface EMG, it is not clear exactly how these tuning curves correspond to the tuning of the individual motor units. All data are from subject B.