FIG. 2.

Normal forces, electromyographic (EMG), motor-unit synchrony, and coherence during object hold. A: normal forces exerted by the thumb and middle finger (Tx and Mx, respectively) and EMGs from the thumb muscle (FPL) and the middle finger compartment of FDP (FDP3) during a 2-digit object hold (subject 3). The data shown are from a smaller recording period than the entire 4-min duration of the object-hold trial. B: motor units discriminated from each raw EMG trace and their respective instantaneous discharge rates. C: the action potential of a single motor unit discriminated from each EMG channel. The number of spikes for the motor units shown was 1,400 and 1,743 for FPL and FDP3, respectively. D: cumulative sum (CUSUM) of the events of the cross-correlogram and the cross-correlogram (top and bottom trace, respectively) for the same 2 motor units. Vertical solid lines denote the ±20 ms time period relative to 0. Vertical dotted lines indicate 10 and 90% of the CUSUM value within ±20 ms used to define the width of the cross-correlogram peak above chance level (horizontal line, bottom trace). The number of events below chance level is calculated from the corresponding gray area. E: coherence estimate for this pair of motor units. The values on the y axis are the magnitudes of the coherence across the frequency range of interest (x axis; bin width = 0.78 Hz). Horizontal dotted line indicates the 95% confidence limit above which the magnitude of the coherence is defined as statistically significant.