HYPOHYDRATION EFFECTS ON SKELETAL-MUSCLE PERFORMANCE AND METABOLISM -A P-31-MRS STUDY

The purpose of this study was to determine whether hypohydration reduc es skeletal muscle endurance and whether increased H+ and P-i might co ntribute to performance degradation. Ten physically active volunteers (age 21-40 yr) performed supine single-leg, knee-extension exercise to exhaustion in a 1.5-T whole body magnetic resonance spectroscopy (MRS ) system when euhydrated and when hypohydrated (4% body wt). P-31 spec tra were collected at a rate of one per second at rest, exercise, and recovery, and were grouped and averaged to represent 10-s intervals. T he desired hydration level was achieved by having the subjects perform 2-3 h of exercise in a warm room (40 degrees C dry bulb, 20% relative humidity) with or without fluid replacement 3-8 h before the experime nt. Time to fatigue was reduced (P < 0.05) by 15% when the subjects we re hypohydrated [213 +/- 12 vs. 251 +/- 15 (SE) s]. Muscle strength wa s generally not affected by hypohydration. Muscle pH and P-i/beta-ATP ratio were similar during exercise and at exhaustion, regardless of hy dration state. The time constants for phosphocreatine recovery were al so similar between trials. In summary, moderate hypohydration reduces muscle endurance, and neither H+ nor P-i concentration appears to be r elated to these reductions.