Phosphocreatine Onset and Recovery Kinetics in Aging Skeletal Muscle

Abstract

2438 Significant metabolic adaptations have been observed in human skeletal muscle within 5–7 days of endurance training. These studies, on young subjects, typically involved cycle ergometry at moderate exercise intensities and the use of muscfle biopsies. PURPOSE: To characterize the effects of age on metabolic adaptation to training we used 31P magnetic resonance spectroscopy (MRS) to examine metabolic changes in a small muscle mass during a period of short-term high intensity endurance training in an aged population. METHODS: We monitored the kinetics of phosphocreatine (PCr) breakdown at the onset of exercise, PCr recovery kinetics, and the levels of PCr depletion in the calf muscle during plantar flexion exercise over a 6-day training period. Each day, 6 healthy males, age 70 ± 2.7 yrs, performed 6 sets of 4 min plantar flexion exercise at 1 Hz. Initial training load was set at 80% of maximum work rate (WR max), with the load increased by 1 Watt each day if the subject could complete the previous days work without failure. Metabolic adaptations were monitored on days 0, 3, and 6 during exercise at 50% of initial WR max. RESULTS: WRmax increased by an average of 32.3 ± 4.4%. Time constants for PCr breakdown at the onset of exercise decreased from 58.1 ± 8.6s to 35.9 ± 5.7s (p < 0.05) pre- to post-training. PCr depletion was spared by 17.0 ± 4.3% (p < 0.05) after 6 days of training. PCr recovery time was unchanged. CONCLUSION: These metabolic adaptations suggest that aging skeletal muscle maintains the plasticity to adapt rapidly to endurance exercise training. The faster PCr onset kinetics indicate a more rapid metabolic adjustment to steady-state exercise which in part may account for the sparing of PCr for a given work rate post-training. The lack of decrease in PCr recovery rate suggests that these metabolic changes are independent of a change in skeletal muscle oxidative capacity. Supported by NIH HL-17731, AHA 9960064Y, TRDRP 10KT-0335, and NIH RR 14785