Measurements of in vivo skeletal muscle oxidative capacity are lower following sustained isometric compared with dynamic contractions

Human skeletal muscle oxidative capacity can be quantified non-invasively using 31-phosphorus magnetic resonance spectroscopy (P-31-MRS) to measure the rate constant of phosphocreatine (PCr) recovery (k(PCr)) following contractions. In the quadricep muscles, several studies have quantified k(PCr) following 24-30 s of sustained maximal voluntary isometric contraction (MVIC). This approach has the advantage of simplicity but is potentially problematic because sustained MVICs inhibit perfusion, which may limit muscle oxygen availability or increase the intracellular metabolic perturbation, and thus affect kPCr. Alternatively, dynamic contractions allow reperfusion between contractions, which may avoid limitations in oxygen delivery. To determine whether dynamic contraction protocols elicit greater kPCr than sustained MVIC protocols, we used a cross-sectional design to compare quadriceps kPCr in 22 young and 11 older healthy adults following 24 s of maximal voluntary: (1) sustained MVIC and (2) dynamic (MVDC; 120.. s-1, 1 every 2 s) contractions. Muscle kPCr was similar to 20% lower following the MVIC protocol compared with the MVDC protocol (p = 0.001), though this was less evident in older adults (p = 0.073). Changes in skeletal muscle pH (p = 0.001) and PME accumulation (p = 0.001) were greater following the sustained MVIC protocol, and pH (p = 0.001) and PME (p = 0.001) recovery were slower. These results demonstrate that (i) a brief, sustained MVIC yields a lower value for skeletal muscle oxidative capacity than an MVDC protocol of similar duration and (ii) this difference may not be consistent across populations (e.g., young vs. old). Thus, the potential effect of contraction protocol on comparisons of kPCr in different study groups requires careful consideration in the future.