Normalized metabolic stress for³¹P-MR spectroscopy studies of human skeletal muscle: MVC vs. muscle volume

Abstract

A critical requirement of submaximal exercise tests is the comparability of workload and associated metabolic stress between subjects. In this study, 31P-magnetic resonance spectroscopy was used to estimate metabolic strain in the soleus muscle during dynamic, submaximal plantar flexion in which target torque was 10 and 15% of a maximal voluntary contraction (MVC). In 10 healthy, normally active adults, (PCr + Pi)/PCr, where PCr is phosphocreatine, was highly correlated with power output normalized to the volume of muscle in the plantar flexor compartment (r = 0.89, P < 0.001). The same variable was also correlated, although less strongly (r = 0.78, P < 0.001), with power normalized to plantar flexor cross-sectional area. These findings suggest that comparable levels of metabolic strain can be obtained in subjects of different size when the power output, or stress, for dynamic plantar flexion is selected as a function of plantar flexor muscle volume. In contrast, selecting power output as a function of MVC resulted in a positive linear relationship between (PCr + Pi)/PCr and the torque produced, indicating that metabolic strain was increasing rather than achieving constancy as a function of MVC. These findings provide new insight into the design of dynamic muscle contraction protocols aimed at detecting metabolic differences between subjects of different body size but having similar blood flow capacity and mitochondrial volume per unit of muscle.

Reliability of 31P-magnetic resonance spectroscopy during an exhaustive incremental exercise test in children

“…This test does have limitations, however. The metabolic perturbation during exercise is highly correlated to muscle size and therefore quantification of either muscle volume or cross-sectional area using magnetic resonance imaging is essential to normalise work output (Fowler et al 1997), especially when metabolic comparisons are performed between groups heterogeneous in muscle size (child vs. adult). In addition to the previously addressed issue of detection rate, the ITs may not be suitable for the assessment of muscle oxidative capacity in young people who are contraindicated to exercise to exhaustion or find exercise compliance difficult at high work intensities.…”

Skeletal muscle energetics assessed by 31P-NMR in prepubertal girls with a familial predisposition to obesity

“…32 With submaximal steady-state exercise protocols, the rates of ATP synthesis and hydrolysis for muscle contraction are matched, resulting in a constant ATP level in the muscle. 33 This type of exercise protocol offers the advantage of better compliance, a greater specificity to oxidative ATP synthesis, and less influence of fatigue mechanisms, as seen with ischemic protocols. 33 Sapega et al 1 suggest that a significant component of physiologic muscular fatigue after moderate to heavy work is independent of ATP and pH.…”

“…33 This type of exercise protocol offers the advantage of better compliance, a greater specificity to oxidative ATP synthesis, and less influence of fatigue mechanisms, as seen with ischemic protocols. 33 Sapega et al 1 suggest that a significant component of physiologic muscular fatigue after moderate to heavy work is independent of ATP and pH.…”

Evidence that a higher ATP cost of muscular contraction contributes to the lower mechanical efficiency associated with COPD: preliminary findings

“…As the metabolic changes are strongly related to the power output normalized to muscle volume and normalization of power-to-muscle volume results in a similar metabolic stress between individuals of different sizes (8), EC was normalized for muscle volume.…”