Cerebral Metabolism is Influenced by Muscle Ischaemia During Exercise in Humans

Maximal exercise reduces the cerebral metabolic ratio (O2/(glucose + 1/2 lactate)) to < 4 from a resting value close to 6, and only part of this decrease is explained by the 'intent' to exercise. This study evaluated whether sensory stimulation of brain by muscle ischaemia would reduce the cerebral metabolic ratio. In 10 healthy human subjects the cerebral arterial-venous differences (a-v differences) for O2, glucose and lactate were assessed before, during and after three bouts of 10 min cycling with equal workload: (1) control exercise at light intensity, (2) exercise that elicited a high rating of perceived exertion due to a 100 mmHg thigh cuff, and (3) exercise followed by 5 min of post-exercise muscle ischaemia that increased blood pressure by approximately 20 %. Control exercise did not significantly affect the a-v differences. However, during the recovery from exercise with thigh cuffs the cerebral metabolic ratio decreased from a resting value of 5.4 +/- 0.2 to 4.0 +/- 0.4 (mean +/- S.E.M.; P < 0.05) as a discrete lactate efflux from the brain at rest shifted to a slight uptake. Also, following post-exercise muscle ischaemia, the cerebral metabolic ratio decreased to 4.5 +/- 0.3 (P < 0.05). The results support the hypothesis that during exercise, cerebral metabolism is influenced both by the mental effort to exercise and by sensory input from skeletal muscles.