The metabolic costs of different types of contractile activity of the human adductor pollicis muscle.

1. The metabolic costs and physiological consequences of shortening contractions of a human muscle working in situ have been compared with those of the muscle maintaining a continuous isometric contraction and when performing repeated brief isometric contractions. 2. After a total 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 the half‐time of relaxation. This was in contrast to the continuous isometric contraction protocol where there was less than 10% loss of force or slowing of relaxation. 3. The ATP cost over the first 5 s of the continuous isometric protocol was 27 mmol (l intracellular water)‐1 while for the shortening and repeated brief isometric protocols the costs were 48 and 46 mmol (l intracellular water)‐1, respectively. 4. The results show that shortening and repeated brief isometric contractions are considerably more energetically demanding, and hence more fatiguing, than sustained isometric contractions.