What is it that forces someone performing a maximal effort to slow his pace or to stop? In a recent issue of The Journal of PhysiologyRasmussen et al. (2010) report results that enlighten somewhat the still poorly understood mechanisms underlying the limits of exercise performance. For about a century exercise physiology has mostly concentrated on muscle, cardio-vascular or pulmonary physiology and still today some maintain that the limits of performance are in one or the other. But what happens when one engages in a maximal physical effort, for instance pedalling on a cycle ergometer until exhaustion? At some stage one cannot keep up the intensity of the effort and one has to a stop the effort or at least decrease its intensity. At that moment it may well be that some system (e.g. cardiac output/oxygen transport) is so heavily taxed that its capacity is being reached. But it is the brain which commands the reduction of the intensity or the end of the effort. Physical effort is the result of recruitment of motor units by the brain and the end of effort is the result of their de-recruitment (Kayser, 2003). What remains elusive is how the brain is led to de-recruit motor units at exhaustion. Rasmussen et al. (2010) have in typical Danish style (i.e. methodologically strong and rather invasive) set out to investigate whether at maximum exercise performance in normoxic and/or hypoxic conditions increased brain metabolic activity may perhaps outstrip oxygen delivery leading to a reduction in motor command. To this end they measured arterial–venous differences of blood gases, glucose and lactate over the brain, by inserting blood sampling lines high into the right internal jugular vein and a brachial artery. Brain blood flow was estimated from Doppler measured velocity in the middle cerebral artery. They asked subjects to cycle for 20 min at a moderate load in normoxia followed, after 10 min rest, by 20 min at the same load in hypoxia (= 0.1), a load chosen at the limit of what could be maintained in hypoxia. After a rest the subjects then did a maximum performance test in normoxia. The brain's ability to fully recruit muscle was evaluated by asking the subjects to perform maximum voluntary contractions of the biceps brachii while superimposing electrical stimulation of the motor points or magnetic stimulation over the motor cortex. The authors found that both in hypoxia and during maximum exercise in normoxia the ability of the brain to recruit additional (non-fatigued) muscle (biceps brachii) was compromised, indicated by a 10% decrease in voluntary activation level as quantified with the superimposed stimulation technique. This was accompanied by a 20% reduction in brain oxygen delivery despite an increase in middle cerebral artery blood velocity. Using estimated tissue oxygen tension () they found correlations with the rate of perceived exertion (Borg scale) and voluntary activation levels. Since acute restoration of oxygen supply is known to reverse the advent of exhaustion of exercise in hypoxia (Kayser et al. 1994; Amann et al. 2007; Subudhi et al. 2008), it thus seems plausible to expect a causal link between brain oxygen supply and locomotor muscle recruitment. Using a-v differences that reflect overall effects, Rasmussen et al. (2010) have approached the brain as whole. But it can be expected that during heavy exercise certain groups of neurons are particularly active and may be exposed to more extreme metabolic conditions. The challenge for future work lies in getting a better view of what happens in certain regions of the brain. Motion artefacts during whole body exercise like cycling still pose enormous problems, but the advent of increasingly sophisticated brain imaging techniques are likely to allow a better insight into what happens in the brain at the moment of exhaustion from intense effort. Some groups are starting to work with multisite brain tissue saturation measurements using near infrared spectroscopy, other groups experiment with EEG. Rasmussen et al. (2010) are from the famous ‘Danish school’, a formidable heritage from the early days of August Krogh and the more recent Copenhagen Muscle Research Centre set up by Bengt Saltin. I expect those Danish brains to come up soon with more good ideas to bring us closer to an understanding of what happens in the brain leading it to decrease muscle recruitment at the point of exhaustion.
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Sep 26, 2024
Todd Cunningham 
Open Access