Hypohydration effects on skeletal muscle performance and metabolism: a 31P-MRS study

The purpose of this study was to determinewhether hypohydration reduces skeletal muscle endurance and whetherincreased H⁺ andP_i might contribute to performancedegradation. Ten physically active volunteers (age 21-40 yr)performed supine single-leg, knee-extension exercise to exhaustion in a1.5-T whole body magnetic resonance spectroscopy (MRS) system wheneuhydrated and when hypohydrated (4% body wt).³¹P spectra were collected at arate of one per second at rest, exercise, and recovery, and weregrouped and averaged to represent 10-s intervals. The desired hydrationlevel was achieved by having the subjects perform 2-3 h ofexercise in a warm room (40°C dry bulb, 20% relative humidity)with or without fluid replacement 3-8 h before the experiment.Time to fatigue was reduced (P < 0.05) by 15% when the subjects were hypohydrated 213 ± 12 vs. 251 ± 15 (SE) s]. Muscle strength was generally notaffected by hypohydration. Muscle pH andP_i/-ATP ratio were similarduring exercise and at exhaustion, regardless of hydration state. The time constants for phosphocreatine recovery were also similar betweentrials. In summary, moderate hypohydration reduces muscle endurance,and neither H⁺ norP_i concentration appears to berelated to these reductions.