825 Introduction Skeletal muscle is a dynamic and plastic tissue that responds rapidly to changes in frequency and duration of use (Musacchia et al., 1988). An increase in muscle activity builds and maintains muscle mass, while inactivity results in tissue degeneration. In mammals, prolonged immobilisation or unloading of skeletal muscle results in muscle ‘wasting’ or muscle disuse atrophy (Booth, 1982). Muscle disuse atrophy is characterised by a reduction in muscle fibre cross-sectional area due to the combined effects of increased catabolism and decreased anabolism of skeletal muscle protein and an imbalance of muscle biochemistry (Kandarian and Stevenson, 2002). In experimental models of hindlimb immobilisation and unloading the effects of muscle disuse atrophy on muscle morphology and muscle mechanics are variable and depend upon muscle type and fibre type (Ariano et al., 1973; Booth and Kelso, 1973; Edgerton et al., 2002; Peter et al., 1972; Trappe et al., 2004; Witzmann et al., 1982). Most muscles are composed of a heterogenous mixture of fibre types, although there are exceptions. For example, the mammalian soleus muscle is almost completely slow twitch (Ariano et al., 1973). During normal activity, oxidative muscles tend to be used frequently, for extended periods of time and at low intensities. In comparison, fast-twitch glycolytic muscles are used infrequently, for short periods and at high intensity. When oxidative muscles become inactive the scope of the change in the level of activity is much greater than that experienced by glycolytic, fast-twitch muscles (Hudson and Franklin, 2002b). In general, slow-twitch (oxidative) fibres are more vulnerable to muscle disuse atrophy than fast-twitch (glycolytic) fibres (Edgerton et al., 2002; Maier et al., 1973; Nordstrom et al., 1995; Tomanek and Lund, 1974). Prolonged disuse alters muscle fibre type characteristics, with oxidative fibres acquiring the structural, biochemical and Prolonged immobilisation or unloading of skeletal muscle causes muscle disuse atrophy, which is characterised by a reduction in muscle cross-sectional area and compromised locomotory function. Animals that enter seasonal dormancy, such as hibernators and aestivators, provide an interesting model for investigating atrophy associated with disuse. Previous research on the amphibian aestivator Cyclorana alboguttata (Günther 1867) demonstrated an absence of muscle disuse atrophy after 3 months of aestivation, as measured by gastrocnemius muscle contractile properties and locomotor performance. In this study, we aimed to investigate the effect of aestivation on iliofibularis and sartorius muscle morphology and contractile function of C. alboguttata over a longer, more ecologically relevant time- frame of 9 months. We found that whole muscle mass, muscle cross-sectional area, fibre number and proportions of fibre types remained unchanged after prolonged disuse. There was a significant reduction in iliofibularis fibre cross-sectional area (declined by 36% for oxidative fibre area and 39% for glycolytic fibre area) and sartorius fibre density (declined by 44%). Prolonged aestivation had little effect on the isometric properties of the skeletal muscle of C. alboguttata. There was a significant reduction in the isometric contraction times of the relatively slow-twitch iliofibularis muscle, suggesting that the muscle was becoming slower after 9 months of aestivation (time to peak twitch increased by 25%, time from peak twitch to half relaxation increased by 34% and time from last stimulus to half tetanus relation increased by 20%). However, the results of the work-loop analysis clearly demonstrate that, despite changes to muscle morphology and isometric kinetics, the overall contractile performance and power output levels of muscles from 9-month aestivating C. alboguttata are maintained at control levels. Key words: aestivation, muscle disuse atrophy, fibre, morphology, contractile properties, locomotion, work loop, Cyclorana alboguttata. Summary The Journal of Experimental Biology 210, 825-835 Published by The Company of Biologists 2007 doi:10.1242/jeb.02711 Getting the jump on skeletal muscle disuse atrophy: preservation of contractile performance in aestivating Cyclorana alboguttata (Günther 1867) Beth L. Symonds 1 , Rob S. James 2 and Craig E. Franklin 1, * 1 School of Integrative Biology, The University of Queensland, St Lucia, Queensland 4072, Australia and 2 Department of Physiology and Sport Science, Coventry University, James Starley Building, Priory Street, Coventry, CV1 5FB, UK *Author for correspondence (e-mail: c.franklin@uq.edu.au) Accepted 9 January 2007 THE฀JOURNAL฀OF฀EXPERIMENTAL฀BIOLOGY