Contractile properties of normal and dystrophic human skeletal muscles

Abstract

<p>The aim of this study was to provide a better understanding of muscle function in healthy adult subjects (n=46) and in patients with myotonic muscular dystrophy (MMD; n=25) and limb-girdle muscular dystrophy (LGMD; n=20). Evoked and volitional contractions were examined in two opposing muscle groups in the leg, the ankle plantar-flexors (PF) and dorsi-flexors (DF). The following contractile properties were investigated under isometric conditions: (1) twitch torque, (2) twitch speed, (3) twitch potentiation, (4) voluntary muscle strength, (5) extent of motor unit activation during maximum voluntary effort and (6) muscle fatigue. In both control and dystrophic populations, striking contractile differences were observed between the PF and DF muscle groups. The DF muscles differed from the PF muscles in demonstrating smaller twitches, briefer contraction and half-relaxation times, marked twitch potentiation, more complete motor unit activation, and greater susceptibility to fatigue. Muscle fatigue was found to be caused by peripheral, rather than central failure; it occurred at the level of excitation-contraction coupling and/or the contractile machinery.</p> <p>Comparisons of muscle contractile properties between patients with muscular dystrophy (MMD and LGMD) and their respective matched controls have disclosed that in any patient at a given stage, the dystrophic process may sometimes spare a muscle group while destroying another, regardless of their functions and fibre-type compositions. Furthermore, both groups of dystrophic patients showed normal twitch potentiation and muscle fatigue behaviour.</p> <p>Apart from providing comprehensive information on normal and dystrophic skeletal muscles, this study constitutes a non-invasive and quantitative method for monitoring the time course of human muscular dystrophy and for assessing the benefits of any future therapy.</p>