WINGES, S. A.*; SANTELLO, M.; Arizona State University: Control of whole hand grasping: A role for motor unit synchronization?

Previous work on multi-digit grasping has shown that normal forces exerted by pairs of digits tend to be synchronized across a variety of task conditions. We have suggested that motor unit (MU) synchrony might play a significant role in the coordination of force exerted by pairs of digits. In the present study we examined the extent to which MU synchrony occurs in extrinsic hand muscles during five-digit grasping. Single MU potentials were recorded by intramuscular needle electrodes from a thumb muscle (Flexor Pollicis Longus; FPL) and four compartments of the Flexor Digitorum Profundus (FDP2, 3, 4, and 5). EMG activity was recorded while subjects (n = 5) held a lightweight object vertically. Normal forces exerted by each digit were also recorded. MU synchronization was quantified using the common input strength index (CIS; Nordstrom et al., 1992). We found typical force sharing patterns between digits as previously observed in 5-digit grasping tasks. The thumb and little finger produced the largest and smallest mean force, 2.6�1.4 N and 0.53� 0.22 N, respectively. Preliminary EMG data from 188 pairs of motor units yielded CIS values ranging from 0 to 0.92 across all pairs of muscles. From this pool, 48% of the MU pairs had a CIS value above 0.3, indicating moderate MU synchrony, the mean CIS value at or above the moderate synchrony level being 0.44 (�0.12). The strongest synchronization was observed between the thumb and index finger (FPL and FDP2) with a mean CIS of 0.49(�0.14), while the lowest synchronization was observed between the thumb and little finger (FPL and FDP5) with a mean CIS of .20(�.16). Further testing on larger samples of motor units will allow us to further address the extent to which synchronization strength differs between digits pairs. The functional consequences of MU synchrony are currently being investigated (Fuglevand and Santello, 2002).