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 contractile performance of human muscles. Single fibre segments were dissected from bioptic samples of vastus lateralis and chemically skinned. Force-velocity properties, including isometric tension (P0), maximal shortening velocity (Vmax), maximum power output (Wmax) and the velocity at which Wmax is reached (Vopt), were determined at maximum calcium activation. Among these parameters Wmax showed the largest range of variation: about nine times between 2B and slow fibres. Vopt also showed large (about four times) and significant variations between fibre types. Force development at submaximum calcium activation was studied and force-pCa curves were obtained for each fibre type. Calcium sensitivity was greater in 2B than in 2A and in slow 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 fibre segment was determined by gel electrophoresis. The functional properties of each fibre type are discussed in the light of the motor unit recruitment mechanism to understand their possible physiological role.