Strenuous physical activity results in depressed muscle function characterised by diminished force production, reduced speed and slowed relaxation. Studies of isolated muscle fibres have shown reduced force to be due to a combination of impaired cross-bridge function, reduced Ca(2+)-sensitivity and decreased Ca(2+)-release. The first two changes are caused by an accumulation of inorganic phosphate and a lowering of pH as a result of increased energy metabolism; the cause of the reduced Ca(2+)-release is unclear. Reduced speed is due to lowered pH and possibly to an increase in ADP (adenosine diphosphate). Slowed relaxation appears to be due to metabolically induced changes both in Ca(2+)-reuptake and cross-bridge function.