Muscle pain after exercise is linked with an inorganic phosphate increase as shown by 31P NMR.

31P nuclear magnetic resonance (NMR) spectra were obtained from the forearm muscles of 5 subjects before and after performing a muscle stretching (eccentric) exercise routine. Spectra collected before and immediately after exercise showed normal resting phosphorylated metabolite levels and unchanged intracellular pH (pHi). Measurements made on the day following exercise, when muscular pain was apparent, revealed an elevated inorganic phosphate level. No significant changes in other metabolites or pHi were detected. This study gives the first indication of biochemical change following a form of exercise that is associated with considerable muscle pain and damage. The findings may help in understanding pathological processes resulting in pain and damage in muscle.