Studying metabolic regulation in human muscle

Phosphorus magnetic resonance spectroscopy, despite some limitations, is a valuable non-invasive window on muscle metabolism in vivo, particularly oxi dative ATP synthesis. A number of experiments have shown this to be dominat ed by closed-loop feedback mechanisms: a well-known model posits regulation by ADP, but there are others, difficult to distinguish experimentally. Mor eover 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 o xygenation), and improvement of the conceptual tools appropriate to such st udies, where data are limited and steady-state assumptions frequently inapp licable.