The demonstration of the size principle in humans using macro electromyography and precision decomposition.

We set out to study the relationship between a motor unit's size and firing rates and its recruitment threshold and recruitment order. The data were collected from the first dorsal interosseous muscle of 11 normal subjects and analyzed using the precision decomposition and macro electromyography techniques. Our study showed that the recruitment order of a motor unit varies directly with its recruitment threshold (P<0.00005) and that there is a progressive increase in the macro potential size of successively recruited motor units (P=0.002). The firing rates of motor units vary inversely with their recruitment order (P=0.006), the smaller, earlier recruited units consistently reaching higher firing rates than the larger, later recruited units. This study conforms the existence of a size principle of motor unit recruitment in humans and reveals the interactions between a motor unit's size and its firing rate properties.