Recruitment order of motor units on voluntary contraction: changes induced by proprioceptive afferent activity.

Knowledge of the mechanisms regulating the recruitment of individual motoneurones is essential for the understanding of normal versus pathological movements. These mechanisms have been extensively studied in animal experiments; in the decerebrate cat, for example, it has been found that the most important factor in determining the order of recruitment of motor units in reflex activity is the size of the motoneurones (Henneman, Somjen, and Carpenter, 1965a, b; Somjen, Carpenter, and Henneman, 1965). But very little is known about their recruitment by voluntary effort in the intact human organism. It has been established that the sequence of recruitment during voluntary muscle contraction varies (Seyffarth, 1940; Harrison and Mortensen, 1962), and different theories have been proposed to explain this. It has been found that different units are engaged in different muscular functions (Seyffarth, 1940). It has been proposed that a direct selective cerebral control over single anterior horn cells exists (Basmajian, 1963). The position of the extremity under study during initiation of a voluntary contraction has also been shown to play a role (Wagman, Pierce, and Burger, 1965). In a previous paper (Ashworth, Grimby, and Kugelberg, 1967) it was shown that motor units are usually recruited in the same order, whether they are activated by voluntary effort or through proprioceptive or nociceptive reflex arcs. It could, however, also be established that the pattern of motor unit recruitment changed in the course of an experiment so that the unit first recruited by one of these three routes of activation alternated with a couple of other units of nearby thresholds. The present paper gives an account of a series of experiments designed to find out the cause of these variations, and particularly the role played by proprioceptive facilitation and the gamma loop (Leksell, 1945). METHODS