Comparison of enzyme activities among single muscle fibres within defined motor units.

1. Muscle fibres from single motor units of rat extensor digitorum longus were depleted of their glycogen by electrical stimulation and identified by the periodic acid-Schiff stain after treatment in a medium that selectively enhanced glycogen content in the non-depleted fibres. 2. Malate dehydrogenase (MDH) and fructose-1,6-diphosphatase (FDPase) activities were studied quantitatively in single dissected fibres of individual motor units and in fibres selected randomly from the same muscle. 3. In contrast to the large variability of MDH and FDPase in muscle fibres taken randomly, the muscle fibres from the same motor units had similar enzyme activities. 4. The resistance to fatigue of the motor units correlated well with the capacity of aerobic oxidative metabolism, as judged by the activity of MDH in the muscle fibres.