THE METABOLIC COSTS OF DIFFERENT TYPES OF CONTRACTILE ACTIVITY OF THEHUMAN ADDUCTOR POLLICIS MUSCLE

1. The metabolic costs and physiological consequences of shortening co ntractions of a human muscle working in situ have been compared with t hose of the muscle maintaining a continuous isometric contraction and when performing repeated brief isometric contractions. 2. After a tota l of 10 s stimulation, the shortening and intermittent brief isometric protocols had very similar effects, causing a 30% loss of force and a threefold increase in tile halftime of relaxation. This was in contra st to the continuous isometric contraction protocol where there was le ss than 10% loss of force or slowing of relaxation. 3. The ATP cost ov er the first 5 s of the continuous isometric protocol was 27 mmol (1 i ntracellular water)(-1) while for the shortening and repeated brief is ometric protocols the costs were 48 and 46 mmol (1 intracellular water )(-1), respectively. 4. The results show that shortening and repeated brief isometric contractions are considerably more energetically deman ding, and hence more fatiguing, than sustained isometric contractions.