Since the first experiment on isolated muscle in 1974, phosphorus-31 nuclear magnetic resonance spectroscopy (³¹P-MRS) has become widely accepted as the “gold standard” method for non-invasive measurements of energy metabolism in exercising muscle. The scope of the present review is related to the metabolic changes occurring in human exercising muscle and to the metabolic effects of training as recorded using ³¹P MRS. ³¹P-MRS investigation of exercising muscle requires by definition the utilisation of dedicated ergometers allowing on the one hand to perform an exercise inside a superconducting magnet and on the other hand accurate measurement of power output.

The specificity of the training stimulus is related to its type which can be classified as resistance or endurance training. MR studies have initially been devoted to the comparative analysis of high-energy phosphate metabolites at rest in order to provide information about changes in fiber type composition after endurance or resistance training while ¹H MR imaging (MRI) gave additional information regarding alteration in muscle volume. Increased in proportion of type II fibers induced by resistance training was associated with an elevated PCr/Pi and PCr/ATP_β ratios whereas opposite effects were reported after endurance training.

Another fruitful approach has been related to changes in ATP producing mechanisms (oxidative vs anaerobic) and muscle energy cost as a result of training. Energy cost was significantly higher in resistance-trained as compared to sedentary and endurance trained subjects. Resistance-trained subjects also exhibited an increased contribution from PCr and glycolysis to ATP production whereas this contribution was significantly decreased in endurance-trained subjects. Rather than an improvement in energy cost, endurance training seems to be favour exercise performance through changes in ATP-producing mechanisms. On the contrary, resistance training effects are clearly related to alteration in fiber type composition.

In this review, we will also consider improvement in mitochondrial function after endurance training and the role of O₂ supply as a limiting factor in endurance-trained and sedentary subjects.