Specific contributions of various muscle fibre types to human muscle performance: an in vitro study

Human skeletal muscle fibres can be divided in five groups: 1, 1-2A, 2A, 2A -2B and 2B, by using myosin heavy chain (MHC) isoforms as molecular markers . This study aimed to define the contribution of each fibre type to the con tractile performance of human muscles. Single fibre segments were dissected from bioptic samples of vastus lateralis and chemically skinned. Force-vel ocity properties, including isometric tension (P-0), maximal shortening vel ocity (V-max), maximum power output (W-max) and the velocity at which W-max is reached (V-opt), were determined at maximum calcium activation. Among t hese parameters W-max showed the largest range of variation: about nine tim es between 2B and slow fibres. V-opt also showed large (about four times) a nd significant variations between fibre types. Force development at submaxi mum calcium activation was studied and force-pCa curves were obtained for e ach fibre type. Calcium sensitivity was greater in 2B than in 2A and in slo w fibres. The slope of the force-pCa curve was greater in fast than in slow fibres. At the end of the experiment the MHC isoform composition of each f ibre segment was determined by gel electrophoresis. The functional properti es of each fibre type are discussed in the light of the motor unit recruitm ent mechanism to understand their possible physiological role. (C) 1999 Els evier Science Ltd. All rights reserved.