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Magnetic resonance (MR) imaging studies of exercising leg muscles were performed to compare the changes in MR transverse relaxation times (T2) that result from exercise of the anterior tibialis (AT) and extensor digitorum/hallicus longus (E) in the anterior compartment of the lower leg with those T2 changes in the medial and lateral gastrocnemius (G) in the posterior compartment. Spin-echo MR images were obtained at 1.5 Tesla before and during the first 14 min of recovery from dynamic exercise. In order to normalize the exercise, workloads for each subject were set at 25% of the measured maximum voluntary contraction (MVC) of the anterior and posterior compartments. In separate exercise sessions, a nonmagnetic, pneumatic exercise apparatus was employed for either dorsiflexion or plantarflexion against a fixed constant resistance for two different exercise durations (1 min 45 s or 5 min). Transaxial MR images (TR = 1000 ms, TE = 30, 60, 90, 120 ms, 128 x 256 matrix, 1.5 cm slice) were used to calculate T2 values. Although subjects performed approximately 7-fold more work (P < or = 0.001, dorsiflexion vs plantarflexion) during plantarflexion than during dorsiflexion at both exercise duration's, the exercise induced T2, while being greater than those at rest (P < or = 0.001), were not significantly different in the different compartments. We conclude that, when exercised at the same workload (25% of MVC), these two muscles produce T2 changes that are not significantly different from each other.