Changes in muscle proton transverse relaxation times and acidosis during exercise and recovery

We studied changes in muscle proton ( 1H) 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 ( 31P-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 1H 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 31P-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