The influence of recruitment order and temperature on muscle contraction with special reference to motor unit fatigue.

Although recruitment in man often proceeds from the slowest to fastest motor units, a variety of stimuli can alter this order. To study the effect of recruitment order on isometric performance, the tetanic tension of individual fast and slow twitch motor units was measured during fatiguing isometric contractions of the medial gastrocnemius muscle of the cat with recruitment proceeding from either the slowest to fastest or fastest to slowest motor units throughout the contractions. The muscles were stimulated electrically through the ventral roots of the spinal cord. Fatiguing isometric tensions were examined at 10, 20, 40, 70, or 100% of the initial strength of the muscles at 28 and 38 degrees C. The results of these experiments showed that for contractions at low isometric tensions (20 and 40% of the muscles' strength), especially at the lower temperature, the endurance was longer when recruitment proceeded from the slowest to fastest units. The reason for this appeared to be linked to the more rapid rate of fall of tension over time in fast as opposed to slow twitch motor units.