Objectives: To determine the changes of muscle conditions of lower leg after stretch shortening cycle exercises by MRI. Subjects and methods: This study included 20 healthy male adult volunteers. After conducting control MRI, each subject performed 3 sets of exercise loading, each set consisting of 100 repetitions of rebound jumping on one foot. MRIs were performed immediately after exercise loading (0 h), 6, 30, and 168 h later. After constructing T1/T2/Fractional Anisotropy (FA)/Apparent Diffusion Coefficient (ADC) maps, the changes of three skeletal muscles of the leg (the tibialis anterior [TA], soleus [SOL], and gastrocnemius [GA]) were quantitatively evaluated in each map at each time point. Results: The T1 and T2 values were prolonged after exercise loading, and there was a delay in the recovery of T1 at 6 and 30 h after exercise loading, as compared to those of T2 values over time. The ADC values were elevated in all three muscles immediately after exercise loading, then recovered more slowly than T1 and T2, and still had not returned to baseline 168 h after exercise loading. The FA value decreased in all three muscles after exercise loading, with the greatest decrease occurring immediately after exercise loading. As with the ADC values, the FA values were slow to recover from the decrease, and had not returned to baseline levels 168 h post-loading. Conclusion: The delay of T1 value recovery suggested that the T1 value may reflect the muscle condition like fatigue and damage. Changes in the ADC and FA values over time suggested that structural changes such as minute muscular injuries can be detected by diffusion-weighted MRI. Meanwhile, the changes observed in the T1 and T2 values suggested that the measured relaxation time data reflected not only the water volume in the muscle, but also the muscle condition after exercise loading.
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