Reduced muscle activation during exercise related to brain oxygenation and metabolism in humans

Maximal exercise may be limited by central fatigue defined as an inability of the central nervous system to fully recruit the involved muscles. This study evaluated whether a reduction in the cerebral oxygen-to-carbohydrate index (OCI) and in the cerebral mitochondrial oxygen tension (P-mito,P-O2) relate to the ability to generate a maximal voluntary contraction and to the transcranial magnetic stimulated force generation. To determine the role of a reduced OCI and (P-mito,P-O2) in central fatigue, 16 males performed low intensity, maximal intensity and hypoxic cycling exercise. Exercise fatigue was evaluated by ratings of perceived exertion (RPE), arm maximal voluntary force (MVC), and voluntary activation of elbow flexor muscles assessed with transcranial magnetic stimulation. Low intensity exercise did not produce any indication of central fatigue or marked cerebral metabolic deviations. Exercise in hypoxia (F-I,F-O2) 0.10) reduced cerebral oxygen delivery similar to 25% and P-mito,P-O2 decreased 11 +/- 4 mmHg (P < 0.001) together with OCI (6.2 +/- 0.7 to 4.8 +/- 0.5, P < 0.001). RPE increased while MVC and voluntary activation were reduced (P < 0.05). During maximal exercise P-mito,P-O2 declined 8 +/- 4 mmHg (P < 0.05) and OCI to 3.8 +/- 0.5 (P < 0.001). RPE was 18.5, and MVC and voluntary activation were reduced (P < 0.05). We observed no signs of muscular fatigue in the elbow flexors and all control MVCs were similar to resting values. Exhaustive exercise provoked cerebral deoxygenation, metabolic changes and indices of fatigue similar to those observed during exercise in hypoxia indicating that reduced cerebral oxygenation may play a role in the development of central fatigue and may be an exercise capacity limiting factor.