Influence of muscle blood flow on fatigue during intermittent human hand-grip exercise and recovery.

1. The influence of muscle blood flow on fatigue and recovery was studied in the forearm muscles of eight male subject performing a powerful isometric hand-grip exercise. The exercise was performed with the exercising forearm normally perfused and, on a separate occasion, with its blood flow occluded with a sphygmomanometer cuff. 2. In the no cuff condition, peak force declined to an initial plateau at 40-50% of the maximal voluntary grip force (MVC). When perfusion was occluded, the force decline was similar during the first minute of exercise, then force fell rapidly to exhaustion. 3. In a separate experiment to investigate the mechanisms underlying the plateau in force loss, occlusion of blood flow during the force plateau phase resulted in a rapid decline in force to exhaustion. 4. Recovery of peak force after the cuff exercise was significantly greater during the initial 3.5 min of recovery than after no-cuff exercise. After this time, recovery was similar for both conditions. 5. Muscle blood flow occlusion during intermittent exercise profoundly reduces endurance without prolonging recovery. Recovery time may depend on the duration and energy cost of the exercise rather than on the degree of force loss. 6. The present study suggests that the fall in muscle force induced by a continuous MVC is a combination of profound short-term fatigue in anaerobic muscle fibres due to the consumption of their short-term energy supplies, plus a decline in force production by aerobic muscle fibres that is the consequence of hypoxia. Thus, MVC may not be a good model of fatigue occurring under submaximal conditions, as hypoxia of type I fibres is unlikely to occur under physiological conditions in which muscle contractions are usually intermittent.