Different effects of exercise and edema on T 2 relaxation in skeletal muscle

The hypothesis that increased muscle T 2 after exercise is caused by increased extracellular fluid volume was tested by comparing the effects of exercise versus external leg negative pressure on muscle T 2 relaxation in normal human subjects. T 2 in lower leg muscles was measured by echo‐planar imaging at 63 echo times from 24 to 272 ms, and the relaxation spectrum was calculated by using a non‐negative least squares algorithm. T 2 relaxation in anterior leg muscle before exercise was characterized by a single component with mean T 2 = 29.3 ± 0.7 (SE, n = 5). After ankle dorsiflexion exercise, this single component broadened, and mean T 2 increased to 38.3 ± 0.7 ms. In contrast, after leg negative pressure, which increased the total leg muscle cross‐sectional area by 21% (range 12–32% n = 6), there was a variable appearance of much slower‐relaxing components (60–500 ms). The results suggest that increased extracellular fluid can account for only a minor portion of the increase in muscle T 2 observed during exercise.