Human skeletal muscle fibres: molecular and functional diversity

Contractile and energetic properties of human skeletal muscle have been stu died for many years in vivo in the body. It has been, however, difficult to identify the specific role of muscle fibres in modulating muscle performan ce, Recently it has become possible to dissect short segments of single hum an muscle fibres from biopsy samples and make them work in nearly physiolog ic conditions in vitro. At the same time, the development of molecular biol ogy has provided a wealth of information on muscle proteins and their genes and new techniques have allowed analysis of the protein isoform compositio n of the same fibre segments used for functional studies. In this way the h istological identification of three main human muscle fibre types (I, IIA a nd IIX, previously called IIB) has been followed by a precise description o f molecular composition and functional and biochemical properties. It has b ecome apparent that the expression of different protein isoforms and theref ore the existence of distinct muscle fibre phenotypes is one of the main de terminants of the muscle performance in vivo. The present review will first describe the mechanisms through which molecular diversity is generated and how fibre types can be identified on the basis of structural and functiona l characteristics. Then the molecular and functional diversity will be exam ined with regard to (1) the myofibrillar apparatus; (2) the sarcolemma and the sarcoplasmic reticulum; and (3) the metabolic systems devoted to produc ing ATP. The last section of the review will discuss the advantage that fib re diversity can offer in optimizing muscle contractile performance. (C) 20 00 Elsevier Science Ltd. All rights reserved.