Forearm metabolic asymmetry detected by 31P-NMR during submaximal exercise

This study evaluated the relationship of skeletal muscle energy metabolism to forearm blood flow and muscle mass in the dominant (D) and nondominant (ND) forearms of normal subjects. 31P-Magnetic resonance spectroscopy was used to determine intracellular pH and the ratio of inorganic phosphate to phosphocreatine (Pi/PCr), an index of energy metabolism. Forearm blood flow and muscle mass were measured by venous occlusion plethysmography and magnetic resonance imaging, respectively. Metabolic measurements and flow were determined at rest and during submaximal exercise in both forearms. After a warm-up period, six normal right-handed male subjects performed 7.5 min of wrist flexion exercise in the magnet (1 contraction every 5 s), first with the ND forearm and then with the D forearm, at 23, 46, and 69 J/min. At rest, there were no differences between forearms in Pi/PCr or pH. However, at each work load the D forearm demonstrated significantly lower Pi/PCr and higher pH than the ND forearm. Blood flow was not significantly different between the forearms at rest or during exercise. Because these subjects were not engaged in unilateral arm training, we conclude that 1) Pi/PCr is lower and pH is higher in the D compared with the ND forearm in normal subjects during submaximal exercise, 2) these differences are independent of muscle mass and blood flow, and 3) the cumulative effect of long-term, low-level daily activity provides an adequate training stimulus for muscular metabolic adaptations.