In humans and other mammals extended bouts of dynamic or isometric exercise result in a failure to produce the force that was possible at the start of the exercise (i.e. fatigue). Experimental results from non-invasive measurements in exercising human subjects clearly indicate that mechanisms located within the muscle are important determinants of fatigue. But it is difficult to follow the dynamics of fatigue in intact human muscle fibres in vivo. Muscle biopsies, NMR recordings and blood samples obtained during exercise have identified factors that potentially affect contractile function. These factors have been subsequently studied in isolated single intact fibres to determine how they affect force production and the likely mechanisms. Overall, these experiments indicate that increased inorganic phosphate and possibly ROS depress contractions resulting in fatigue, whereas increased extracellular [K+] and reduced intracellular pH are less important. This overview uses data from isolated intact single muscle fibres from both rodents and humans to illustrate and explain the mechanisms underlying the changes in intracellular calcium and the decreased force production seen during various types of exhaustive exercise.