Although skeletal muscle work efficiency probably plays a key role in limiting mobility of the elderly, the physiological mechanisms responsible for this diminished function remain incompletely understood. Thus, in the quadriceps of young (n=9) and old (n=10) subjects, we measured the cost of muscle contraction (ATP cost) with 31P-magnetic resonance spectroscopy (31P-MRS) during (i) maximal intermittent contractions to elicit a metabolic demand from both cross-bridge cycling and ion pumping and (ii) a continuous maximal contraction to predominantly tax cross-bridge cycling. The ATP cost of the intermittent contractions was significantly greater in the old (0.30±0.22 mM·min−1·N·m−1) compared with the young (0.13±0.03 mM·min−1·N·m−1, P<0.05). In contrast, at the end of the continuous contraction protocol, the ATP cost in the old (0.10±0.07 mM·min−1·N·m−1) was not different from the young (0.06±0.02 mM·min−1·N·m−1, P=0.2). In addition, the ATP cost of the intermittent contractions correlated significantly with the single leg peak power of the knee-extensors assessed during incremental dynamic exercise (r=−0.55; P<0.05). Overall, this study reveals an age-related increase in the ATP cost of contraction, probably mediated by an excessive energy demand from ion pumping, which probably contributes to both the decline in muscle efficiency and functional capacity associated with aging.

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