Different effects of exercise and edema on T2 relaxation in skeletal muscle.

The hypothesis that increased muscle T2 after exercise is caused by increased extracellular fluid volume was tested by comparing the effects of exercise versus external leg negative pressure on muscle T2 relaxation in normal human subjects. T2 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. T2 relaxation in anterior leg muscle before exercise was characterized by a single component with mean T2 = 29.3 +/- 0.7 (SE, n = 5). After ankle dorsiflexion exercise, this single component broadened, and mean T2 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 T2 observed during exercise.