NORMALIZED METABOLIC STRESS FOR P-31-MR SPECTROSCOPY STUDIES OF HUMANSKELETAL-MUSCLE - MVC VS MUSCLE VOLUME

A critical requirement of submaximal exercise tests is the comparabili ty of workload and associated metabolic stress between subjects. In th is study, P-31-magnetic resonance spec troscopy was used to estimate m etabolic strain in the soleus muscle during dynamic, submaximal planta r flexion in which target torque was 10 and 15% of a maximal voluntary contraction (MVC). In 10 healthy, normally active adults, (PCr + P-i) /PCr, where PCr is phosphocreatine, was highly correlated with power o utput normalized to the volume of muscle in the plantar flexor compart ment (r = 0.89, P < 0.001). The same variable was also correlated, alt hough less strongly (r = 0.78, P < 0.001), with power normalized to pl antar flexor cross-sectional area. These findings suggest that compara ble levels of metabolic strain can be obtained in subjects of differen t size when the power output, or stress, for dynamic plantar flexion i s selected as a function of plantar flexor muscle volume. In contrast, selecting power output as a function of MVC resulted in a positive li near relationship between (PCr + P-i)/PCr and the torque produced, ind icating that metabolic strain was increasing rather than achieving con stancy as a function of MVC. These findings provide new insight into t he design of dynamic muscle contraction protocols aimed at detecting m etabolic differences between subjects of different body size but havin g similar blood flow capacity and mitochondrial volume per unit of mus cle.