Evidence from five\digit grasping studies indicates that grip forces exerted by pairs of digits tend to be synchronized. mean phase difference was then computed on the non\random distributions. We found that the number of significant phase\difference distributions increased markedly with increasing synchronization strength from 18% for no synchrony to 65% and 82% for modest and strong synchrony conditions, respectively. Importantly, most of the mean angles clustered at very small phase difference values (0 to 10), indicating a strong tendency for forces to be exerted in a synchronous fashion. These results suggest that motor unit synchronization could play a significant functional role in the coordination of grip forces. pre\synaptic inputs to the motoneurons (Kirkwood 1979). It should be noted that most motor unit studies have focused on within\muscle motor unit synchrony, i.e., pairs of motor units belonging to the same muscle. However, the above evidence from multi\digit grasping studies prompts questions that must be addressed by studying the behavior of motor units belonging to different muscles. Although several studies have reported across\muscle synchronization (Bremner et al. 1991a, 1991b, 1991c; Gibbs et al. 1995; Huesler et al. 2000; Hockensmith and Fuglevand 2000), this phenomenon deserves further investigation. In particular, what needs to be determined is the functional consequences of across\muscle synchronization. The purpose of the present investigation was to examine the extent to which across\muscle motor unit synchronization can affect the relationship between muscle forces. To address this issue, we used a motor unit model to simulate force produced by two muscles using three physiological levels of motor unit synchrony across the two muscles. In one condition, motor units in the two muscles discharged independently of one another. In the other two conditions, the timing of randomly selected motor unit discharges in one muscle was adjusted to impose low or high levels of synchrony with motor units in the other muscle. The results of the present investigation indicate that synchrony among motor units in different muscles can account for a large part of coordinated force fluctuations across digits during gripping tasks. Preliminary accounts of these results have been published as an abstract (Fuglevand and Santello 2002). Methods Motor unit model Isometric forces developed concurrently in two muscles were simulated using a motor unit model (for details, see Fuglevand et al. 1993). Each muscle consisted of 120 motor units and the properties of Rabbit polyclonal to GNRH the motor units 4707-32-8 supplier in the two muscles were the same. Motor unit twitches were modeled as the impulse response of a critically damped 2nd order system (Fig. 1). Each motor unit was assigned a unique twitch amplitude and twitch contraction time. The distribution of motor units based on twitch amplitude was skewed such that many motor units had small twitch forces and relatively few motor units had large twitch forces. Forces were scaled relative to the twitch force of the weakest motor unit and twitch forces ranged from 1.0 to 100.0 arbitrary force units. Contraction times were assigned as an inverse function of twitch amplitude and ranged from 30 ms for the strongest unit to 90 ms for the weakest unit (Fig. 1). Fig. 1 Twitch properties of simulated motor units based on the model of Fuglevand et al. (1993). The twitch force of each motor unit was simulated as the impulse response of a critically damped 2nd order system ((Fuglevand et al. 1993). Maximum discharge rates were inversely related to recruitment threshold and varied 4707-32-8 supplier from 25 imp/s for the highest threshold unit to 35 imp/s for the lowest threshold unit. To emulate the stochastic nature of motor neuron activity, the discharge times of individual motor units predicted from the above equation were then adjusted to simulate a Gaussian random process with a coefficient of variation (standard deviation/mean 100) in the interdischarge intervals of 20%. Prior to imposition of synchrony (see below), each motor unit discharged independently of every other motor unit and successive 4707-32-8 supplier interdischarge intervals were uncorrelated within a motor unit. Motor unit force was modeled as a sigmoid function of discharge.