Relevance of muscle fibre type to fatigue in short intense and prolonged exercise in man.

It has been suggested that the histological and histochemical features of human muscle are important in determining performance capacity. The relationship between muscle fibre types (Type I, slow twitch fibres; Type II, fast twitch fibres) and performance on standardized tests has been studied in subjects accustomed to physical exercise, and related to their patterns of lactate metabolism, expressed as the onset of blood lactate accumulation (OBLA). This variable was found to be the best predictor of endurance capacity of the variables studied. It is suggested that in healthy male subjects muscle lactate is crucial in short, intense forms of exercise (the higher the lactate formation, the better the performance) and also in prolonged, "endurance' forms of exercise (the later the onset of lactate formation, the higher the sustainable exercise intensity). In subjects with a high proportion of fast twitch fibres, more lactate will be formed at the same exercise intensity. This is advantageous for short intense exercise but impairs endurance performance. The deleterious effects induced by glycogen depletion were studied and found to be most pronounced in subjects rich in fast twitch (glycogen-dependent) fibres. Indications were also obtained that muscular performance is regulated in different ways in males and females. In women an inverse relationship was found between fast twitch fibres and muscle power, and between fatigue and lactate concentration, whereas direct relations were found in men.