Phosphorus magnetic resonance spectroscopy, despite some limitations, is a valuable non-invasive window on muscle metabolism in vivo, particularly oxidative ATP synthesis. A number of experiments have shown this to be dominated by closed-loop feedback mechanisms: a well-known model posits regulation by ADP, but there are others, difficult to distinguish experimentally. Moreover the contribution of open-loop control mechanisms (‘feed forward’ or ‘parallel activation’) in vivo remains controversial. Progress will require more precise data, better integrated with other measurements (e.g. muscle oxygenation), and improvement of the conceptual tools appropriate to such studies, where data are limited and steady-state assumptions frequently inapplicable.

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