Energetics of isometric exercise in man.

In isometric contractions of the plantar flexors (5-40% of maximal tension, Tmax), VO2 is linearly related to the force exerted, averaging 2 ml/min-kg of tension. At tension levels above 5% Tmax the lactic acid contribution to the overall energy demand is constant at about 20%. Therefore, up to at least 40% Tmax,-muscle blood flow is not totally occluded, though it is impaired even at low force levels. Below 5% Tmax no lactic acid (LA) accumulates in blood. The energy required for the development of the tension is linearly related to the force exerted up to 33% Tmax, thereafter increasing disproportionately. In the transition from rest to static contractions of the plantar and forearm flexors (30 and 40% Tmax, respectively) VO2 increases initially to 200% of the controls, leveling off later at 150%. During recovery, VO2 increases up to 200% of the initial resting level, due to the payment of a large O2 debt, decreasing then with a t 1/2 of about 30 s. The glycolytic component is relatively more important in isometric contractions of the forearm than of the plantar flexors. No LA accumulates in static contractions of the plantar flexors of 5-10 s duration interrupted by equal pauses.