Figure 9.

Schematic representation of the results and suggested neural connectivity of voluntary motor commands to motor neuron pools. A, Previous studies that grouped the vastii or hand muscles based on global EMG signals have found strong coherence between the two muscles, indicating a dominant common input across synergistic muscles (Figure 1). After we removed those motor units that shared both motor unit modes (green line showing without common units), the correlation between the two pools of motor units was significantly reduced, as indicated in the coherence graph. This indicates that the coherence found in previous studies is mainly attributable to those motor units that shared two distinct sources of common synaptic. B, Visual representation of our current findings. Factor analysis suggested that there are at least two sources of common synaptic input (motor unit modes) to motor neurons that innervate each pair of synergistic muscles. Most motor neurons, but not all of them, innervating each vastus muscle receive common input from a unique source (green and violet lines), but some motor neurons receive inputs from the source directed to the other muscle (dashed green and violet lines, top graph), and some receive inputs from both sources.