Noninvasive Monitoring of Training Induced Muscle Adaptation with 31P-MRS: Fibre Type Shifts Correlate with Metabolic Changes

Purpose. To evaluate training induced metabolic changes noninvasively with P-31 magnetic resonance spectroscopy (P-31-MRS) for measuring muscle fibre type adaptation. Methods. Eleven volunteers underwent a 24-week training, consisting of speed-strength, endurance, and detraining (each 8 weeks). Prior to and following each training period, needle biopsies and P-31-MRS of the resting gastrocnemius muscle were performed. Fibre type distribution was analyzed histologically and tested for correlation with the ratios of high energy phosphates ([PCr]/[P-i], [PCr]/[beta ATP] and [PCr + P-i]/[beta ATP]). The correlation between the changes of the P-31-MRS parameters during training and the resulting changes in fibre composition were also analysed. Results. We observed an increased type-II-fibre proportion after speed-strength and detraining. After endurance training the percentage of fast-twitch fibres was reduced. The progression of the [PCr]/[P-i]-ratio was similar to that of the fast-twitch fibres during the training. We found a correlation between the type-II-fibre proportion and [PCr]/[P-i] (r = 0.70, P < 0.01) or [PCr]/[beta ATP] (r = 0.69, P < 0.01); the correlations between its changes (delta) and the fibre-shift were significant as well (delta[PCr]/[P-i] r = 0.66, delta[PCr]/[beta ATP] r = 0.55, P < 0.01). Conclusion. Shifts in fibre type composition and high energy phosphate metabolite content covary in human gastrocnemius muscle. Therefore P-31-MRS might be a feasible method for noninvasive monitoring of exercise-induced fibre type transformation.