Does oxidative capacity affect energy cost? An in vivo MR investigation of skeletal muscle energetics

Abstract

Investigations of training effects on exercise energy cost have yielded conflicting results. The purpose of the present study was to compare quadriceps energy cost and oxidative capacity between endurance-trained and sedentary subjects during a heavy dynamic knee extension exercise. We quantified the rates of ATP turnover from oxidative and anaerobic pathways with (31)P-MRS, and we measured simultaneously pulmonary oxygen uptake in order to assess both total ATP production [i.e., energy cost (EC)] and O(2) consumption (O(2) cost) scaled to power output. Seven sedentary (SED) and seven endurance-trained (TRA) subjects performed a dynamic standardized rest-exercise-recovery protocol at an exercise intensity corresponding to 35% of maximal voluntary contraction. We showed that during a dynamic heavy exercise, the O(2) cost and EC were similar in the SED and endurance-trained groups. For a given EC, endurance-trained subjects exhibited a higher relative mitochondrial contribution to ATP production at the muscle level (84 +/- 12% in TRA and 57 +/- 12% in SED; P < 0.01) whereas the anaerobic contribution was reduced (18 +/- 12% in TRA and 44 +/- 11% in SED; P < 0.01). Our results obtained in vivo illustrate that on the one hand the beneficial effects of endurance training are not related to any reduction in EC or O(2) cost and on the other hand that this similar EC was linked to a change regarding the contribution of anaerobic and oxidative processes to energy production, i.e., a greater aerobic energy contribution associated with a concomitant reduction of the anaerobic energy supply.

Muscle energetics changes throughout maturation: a quantitative31P-MRS analysis

“…Although no corresponding results are available in children for the purpose of comparison, the results we obtained in adults are consistent with previous studies with similar end-of-exercise acidosis (61) and similar exercise conditions (45). Given that similar acceleration of pH recovery kinetics has been reported in highly endurance-trained athletes compared with lesstrained subjects (25,43), this result provides additional support for an enhanced oxidative capacity before puberty.…”

“…The relative contributions of oxidative and anaerobic processes are tightly linked to exercise modality, i.e., intensity and duration, and individual capacity, e.g., fiber-type composition and training status (43). While differences among protocols might explain the various results reported so far, these issues also underline the importance of exercise standardization to properly compare subjects with obvious different muscle capacities.…”

Comparative determination of energy production rates and mitochondrial function using different 31P MRS quantitative methods in sedentary and trained subjects

“…In the present study, we found that both the initial EC of muscle contraction and EC calculated over the entire exercise period were not influenced by the training status, regardless of the method used. These findings strongly support previous results (4,5), and confirm that unaltered EC is not related to any bias in the calculation method. It also contradicts the hypothesis of an altered efficiency caused by an increased aerobic fitness (51).…”

“…Based on a computer simulation (Fig. 7), we illustrated that the rapid decrease in [PCr] at the onset of exercise potentially leads to a large reduction in DG ATP , which is further exacerbated by the transient alkalosis commonly observed during knee extension exercise at this intensity (4). As a result, for a given power output, more ATP would be required as long as [PCr] does not reach a steady state.…”

“…Skeletal muscle bioenergetics has been largely investigated noninvasively using 31 P MRS. On the basis of high-energy phosphate metabolites and pH measurements, a large number of publications have been devoted to the quantitative investigation of muscle energetics in patients with muscle diseases (1)(2)(3) and of the metabolic effects of training (4)(5)(6).…”