Postural activation of the human medial gastrocnemius muscle: are the muscle units spatially localised?

Non-technical summary In the medial gastrocnemius muscle of cats, the longitudinal size (3-4 cm) of the territory of motor units is large (similar to 60%) relative to the muscle length. In the human medial gastrocnemius, the size of motor unit territories is unknown. By comparing intramuscular and surface electromyograms we show that, when subjects stand at ease, the motor units active in the medial gastrocnemius have small territories. They extend no longer than 4 cm and no less than 1 cm along the longitudinal axis (similar to 25 cm long) of the muscle. Physiologically, the small territories of motor units give the nervous system a mechanism for the independent activation of sub-volumes of the medial gastrocnemius muscle.In cat medial gastrocnemius (MG), fibres supplied by individual motoneurones (muscle units) distribute extensively along the muscle longitudinal axis. In the human MG, the size of motor unit territory is unknown. It is uncertain if the absolute size of muscle unit territory or the size relative to the whole muscle is most comparable with the cat. By comparing intramuscular and surface electromyograms we tested whether muscle units extend narrowly or widely along the human MG muscle. Due to the pennation of the MG, if individual motoneurones supply fibres scattered along the muscle, then action potentials of single motor units are expected to appear sparsely on the surface of the skin. In nine healthy subjects, pairs of wire electrodes were inserted in three locations along the MG muscle (MG(60%), MG(75%) and MG(90%)). A longitudinal array of 16 surface electrodes was positioned alongside the intramuscular electrodes. While subjects stood quietly, 55 motor units were identified, of which, significantly more units were detected in the most distal sites. The surface action potentials had maximum amplitude at 4.40 +/- 1.67 (mean +/- s.d.), 8.02 +/- 2.16 and 11.63 +/- 2.09 cm (P < 0.001) from the most proximal surface electrode, for motor units in the MG(60%), MG(75%) and MG(90%) locations, respectively. Single motor unit potentials were recorded by five consecutive surface electrodes, at most, indicating that muscle units extend shortly along the MG longitudinal axis. It is concluded that relative to the whole muscle, and compared with the cat, muscle units in human MG are localised. The localisation of muscle units might have implications for the regional control of muscle activity.