CHANGES IN MUSCLE PROTON TRANSVERSE RELAXATION-TIMES AND ACIDOSIS DURING EXERCISE AND RECOVERY

We studied changes in muscle proton (H-1) transverse relaxation times (T-2) by magnetic resonance imaging during exercise and compared these changes with alterations in muscle metabolism measured by phosphorus-31 magnetic resonance spectroscopy (P-31-MRS). Eleven subjects completed two trials of intermittent incremental forearm wrist flexion exercise requiring 30 contractions/min for 5 min, 7 min of recovery between stages, and 5-N load increments/stage. Between stages of the first trial, T-2 images of muscle H-1 were obtained. Muscle T-2 increased from 27.3 +/- 1.1 (SD) ms at rest to 35.8 +/- 3.6 ms after volitional fatigue (P < 0.05), whereas less active wrist extensor muscle T-2 remained unchanged (26.8 +/- 0.9 to 28.8 +/- 1.6 ms; P > 0.05). After localizing the predominant muscle recruited from the T-2 images, subjects completed an identical trial at least 1 wk later but involving surface coil P-31-MRS of the T-2-enhanced muscle to measure the H+ concentration ([H+]). Intramuscular [H+] of T-2-enhancing muscle increased from 1.1 +/- 0.1 X 10(-7) M at rest to 4.1 +/- 2.0 x 10(-7) M after volitional fatigue. Both muscle T-2 and intramuscular [H+] increased in a bimodal manner, with T-2 increasing before muscle [H+] (P < 0.05). The correlation coefficient between the percent change in T-2 and muscle [H+] during exercise was +0.74 (range 0.48-0.98; P < 0.05) and +0.47 during recovery. After 12 min of recovery, muscle [H+] decreased to 1.4 + 0.3 x 10(-7) M (P < 0.05), and T-2 remained close to postexercise values (32.2 +/- 3.1 ms, P > 0.05). The data indicate that 1) the T-2 increases during increases in exercise intensity are nonlinear, 2) the T-2 increases during exercise are significantly correlated with increases in [H+], and 3) the slow recovery of T-2 compared with [H+] indicates that [H+] has a minor contribution to the recovery in T-2.