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TO THE EDITOR: There is little doubt that the cost of locomotion is impaired with age as it is evident that centenarians, due to their exceptional longevity, present unique adaptations in their skeletal muscle efficiency to compensate for their extremely low VO2max. However, the underlying question of whether skeletal muscle efficiency is altered with age is unsettled (4, 6). In fact, both opponents present compelling evidence in support of their opinion, and the reason for this disagreement is likely that both authors are looking at different muscles. Indeed, there is accumulating evidence that age-related alterations in skeletal muscle efficiency vary among muscle group. For instance, a selective atrophy, independent of the fiber type, has been documented in skeletal muscles with age (5). It is therefore likely that a similar phenomenon occurs for muscle energetics properties. The suggestion that ATP cost of contraction is improved with age (6) is based on examinations of the tibialis anterior. Interestingly, this muscle also exhibits preserved features with age in terms of mitochondrial efficiency (1) and oxidative capacity (3). In contrast, reduced oxidative capacity (3), in conjunction with impaired mitochondrial and contractile efficiency (2), has been documented in the quadriceps, which would, at least partially, explain the increased cost of locomotion with age as this muscle group is a major contributor to force production during these activities. The reasons for the heterogeneous effect of aging on skeletal muscle efficiency remain unclear, but likely stem from differences in fiber type and chronic load associated with locomotion.

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