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Thus, two independent measures reveal that a drop in mitochondrial-coupling efficiency in muscle GPX4 accompanies the reduced exercise efficiency of these elderly subjects. This study evaluated the contribution of inefficient metabolic conversion to the decline in exercise efficiency with age. Both muscle contraction and mitochondrial coupling have been suggested to become less efficient with age and to contribute to the reduced efficiency of exercise in the elderly (Woo et al. 2006) and higher oxidative cost of walking (Martin et al. 1992; Malatesta et al. 2003; Ortega & Farley, 2007). Figure 1 provides a scheme of the fluxes from O2 uptake to muscle work output that comprise the key energy conversion steps involved in exercise. Distinguishing which steps decline is key to understanding why exercise becomes inefficient Osimertinib and how to intervene to reverse this factor in the functional disability that accompanies old age. Here we used direct measurements of energy output to identify the energy conversion step(s) that become inefficient. The aerobic performance test provided a measure of the efficiency of exercise, while direct measures of energetic capacity (Pmax and ATPmax) and mitochondrial content (Vv[mt,f]) were used to identify the site of metabolic inefficiency in contractile coupling and/or mitochondrial coupling. These measurements were paired with a thermodynamic analysis to quantify the contribution of each step to the reduced exercise efficiency that comes with age. Given that both contractile (due to differing biomechanics) and mitochondrial efficiency could vary between maximal and submaximal determinations, we also provide a check on the validity of combining metabolic capacity measurements to evaluate the inefficiencies during submaximal exercise (Fig. 5). The evidence for lower exercise efficiency with age is shown in Fig. 2 by a higher O2 uptake at a given leg power output on the cycle ergometer in representative elderly versus adult see more subjects. This higher O2 uptake per leg power output is in agreement with the substantially greater O2 cost of walking reported for elderly subjects (Martin et al. 1992; Malatesta et al. 2003; Ortega & Farley, 2007). The ��O2 uptake per unit change in power output in the adults yielded a delta efficiency value (?D= 0.27; Fig. 3) that is typical for a wide range of studies of healthy adults (e.g. 0.29; Whipp & Wasserman, 1969). In contrast, the higher O2 uptake per power output in the elderly subjects yielded significantly lower delta efficiency values (?D= 0.23), which was also found during treadmill walking in elderly subjects (Woo et al. 2006). Other studies have found an unchanged submaximal to change in power output ratio with age, indicative of a stable delta efficiency (Murias et al. 2010).