Role of across-muscle motor unit synchrony for the coordination of forces

Abstract

Evidence from five-digit grasping studies indicates that grip forces exerted by pairs of digits tend to be synchronized. It has been suggested that motor unit synchronization might be a mechanism responsible for constraining the temporal relationships between grip forces. To evaluate this possibility and quantify the effect of motor unit synchrony on force relationships, 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. Fast Fourier transform analysis was performed to compute the phase differences between forces from 0.5 to 17 Hz. We used circular statistics to assess whether the phase differences at each frequency were randomly or non-randomly distributed (Rayleigh test). The 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.

Limits to the Control of the Human Thumb and Fingers in Flexion and Extension

“…They include the divergence of corticospinal axons to reach more than one motoneuron pool (e.g., Cheney 1978, 1980;Shinoda et al 1981; for review, see Lemon 2008) and the major overlapping and intermingling of cortical output zones to individual upper limb muscles documented in nonhuman primates (e.g., Landgren et al 1962;Rathelot and Strick 2006;Schieber and Hibbard 1993). As a result of such factors, there is short-term synchronization of motor unit discharge of functionally linked hand muscles or across muscle compartments of the multitendoned extrinsic hand muscles (Hockensmith et al 2005;Johnston et al 2005;Nordstrom et al 1992;Reilly et al 2004;Santello and Fuglevand 2004;Winges and Santello 2004). There is also evidence for coherence between cortical activity and forearm EMG (e.g., Baker et al 1997;Brown 2000;Conway et al 1995;Gross et al 2000;Salenius and Hari 2003) and coherent forces across different digits during voluntary tasks (e.g., Kilbreath et al 2002;Li 2002;Rearick and Santello 2002;Santello and Soechting 2000;Yu et al 2007).…”

Muscle-Pair Specific Distribution and Grip-Type Modulation of Neural Common Input to Extrinsic Digit Flexors

“…We addressed this issue by studying an object-hold task requiring fine coordination of digit forces. We focused on the most basic unit of force control (i.e., single motor units) and the phenomenon of common synaptic input to motor neuron pools of hand muscles as this mechanism might play an important role in the coordination of the digits (Reilly and Schieber 2003;Reilly et al 2004;Santello and Fuglevand 2004).…”

Sensitivity of EMG-EMG coherence to detect the common oscillatory drive to hand muscles in young and older adults

“…The coupling of individual motor units, in particular, is traditionally measured with coherence analysis derived from the discharge times of a pair of concurrently active motor units, termed motor unit coherence (Farmer et al 1993;Kakuda et al 1999;Kilner et al 2002). This approach may be problematic, however, as pairwise correlations between the discharges of two neurons may significantly underestimate correlated activity across entire populations of neurons (Negro and Farina 2011;Santello and Fuglevand 2004;Schneidman et al 2006). Because surface electromyograms (EMGs) reflect activity from many motor units, coherence measures derived from pairs of surface EMGs recorded over synergist muscles (i.e., EMG-EMG coherence) have been used to provide a more representative measure of rhythmic activity across muscles Farmer et al 2007;Kilner et al 1999).…”