Onion Skin or Common Drive?

A motor unit (MU), which consists of a single motoneuron (MN) and the muscle fibers it innervates, is an essential element of the motor control system. The knowledge in this field, collected over decades, is based on research conducted on both animal and human model systems. Experiments on animals allow direct measurement of the MN characteristics and the contractile properties of the muscle fibers it innervates. Thus, all essential information on the properties of the basic elements of the motor system was obtained from animal studies. In contrast, experiments performed on human subjects, which for obvious reasons rely on indirect methods, study intact MUs in their physiological environment during voluntary contractions. Both types of model systems are complementary, because each system collects information that is difficult or impossible to obtain in the other model system. Not all results from animal studies can be verified in human experiments, but we see no reason to think that the basic principles of motor control are different in animals and human subjects. The control of muscle force involves two essential mechanisms: MU recruitment and rate coding. Earlier studies in motor control provide detailed information on both mechanisms. These studies show that MNs are recruited in an orderly fashion from smallest to largest, as seen in animal muscles (Henneman, 1957; Henneman et al., 1974), and in human muscles (Milner-Brown et al., 1973a,b). For an in depth review on the different aspects of orderly recruitment of MN, see Bawa et al. (2014). In cats, motor units were classified by Burke et al. (1973) on the basis of their twitch contraction time into: (i) slow (S), innervated by the smallest MNs, (ii) fast fatigable (FF), innervated by largest MNs, and (iii) fast resistant to fatigue (FR), controlled by MNs of intermediate size. Slow MUs, recruited at the lowest force levels, are practically not fatigable and may function for several hours, while FF MUs are recruited at the highest force levels for short amounts of time. In human muscles, the results concerning MU recruitment order were inconclusive. In studies by Macefield et al. (1996) and Bigland-Ritchie et al. (1998), no correlation between MU size and contraction speed was found, whereas Milner-Brown et al. (1973a) reported that the larger, higher-threshold MUs tend to have shorter contraction times than the smaller, lower-threshold ones. Since the former studies did not measure recruitment threshold, we may assume that human MUs recruited close …