Individual variation in contractile cost and recovery in a human skeletal muscle.

Abstract

This study determined the variation among individuals in ATP use during contraction and ATP synthesis after stimulation in a human limb muscle. Muscle energetics were evaluated using a metabolic stress test that separates ATP utilization from synthesis by 31P NMR spectroscopy. Epicutaneous supramaximal twitch stimulation (1 Hz) of the median and ulnar nerves was applied in combination with ischemia of the ringer and wrist flexors in eight normal subjects. The linear creatine phosphate (PCr) breakdown during ischemic stimulation defmed ATP use (APCr per twitch or -P/twitch) and was highly reproducible as shown by the relative standard deviation [(standard deviation/mean) x 100] of 11% in three repeated measures. The time constant of the monoexponential PCr change during aerobic recovery represented ATP synthesis rate and also showed a low relative standard deviation (9%). Individuals were found to differ significantly in both mean -P/twitch (PCr breakdown rates, 0.29-0.45% PCr per sec or % PCr per twitch; ANOVA, P < 0.001) and in mean recovery time constants (41-74 sec; ANOVA, P < 0.001). This range of -P/twitch corresponded with the range of fiber types reported for a flexor muscle. In addition, -P/twitch was negatively correlated with a metabolite marker of slow-twitch fiber composition (Pj/ATP). The nearly 2-fold range of recovery time constants agreed with the range of mitochondrial volume densities found in human muscle biopsies. These results indicate that both components involved in the muscle energy balance-xidative capacity and contractile costs-vary among individuals in human muscle and can be measured noninvasively by 31P NMR.Muscle has the ability to perform a wide range of functions that depend on the metabolic and contractile properties of the underlying fibers and the recruitment pattern of the motor units. Most muscles contain a mixture of fiber types displaying a continuum of contractile speeds and metabolic capacities ranging from the slow-twitch oxidative type I fibers to the fast-twitch glycolytic type II fibers (1). Animal studies have shown that fibers differ in the contractile cost per twitch (2, 3). In addition, the rate of ATP resynthesis also varies among fibers depending on the oxidative phosphorylation capacity. The lower cost per twitch and higher oxidative capacity of slow-twitch fibers allow higher sustained twitch rates than fast-twitch fibers (3, 4). Thus, both contractile and metabolic properties are critical to sustained muscle performance.Human muscle has also been found to have a range of fiber types based on studies of cadaveric muscle and fresh muscle biopsies. Recent 31P NMR studies have suggested that metabolic differences exist among fibers within human muscle (5,6) and between muscles of individuals at distinct training levels (7,8). However, it is not clear whether the fibers also differ in contractile costs in humans. In animal studies,The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marke...

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