Background Little is known about the value of 31P-magnetic resonance spectroscopy (31P-MRS) in in vivo assessment of exhaustive exercise-induced injury in skeletal muscle.We aimed to evaluate the value of a 31P-MRS study using the quadriceps femoris after a single bout of acute exhaustive swimming in rats,and the correlation between 31P-MRS and histological changes.Methods Sixty male Sprague-Dawley rats were randomly assigned to control,half-exhaustive,and exhaustive exercise groups.31P-MRS of the quadriceps femoris of the right lower limb was performed immediately after swimming exercise to detect Pi,PCr,and β-ATP.The Pi/PCr,Pi/β-ATP,PCr/β-ATP,and PCr/(PCr+Pi) were calculated and pH measured.Areas under the receiver operating characteristic curve (AUCs) were calculated to evaluate the diagnostic potential of 31P-MRS in identifying and distinguishing the three groups.HE staining,electron microscopy and desmin immunostaining after imaging of the muscle were used as a reference standard.The correlation between 31P-MRS and the mean absorbance (A value) of desmin staining were analyzed with the Pearson correlation test.Results Pi,PCr,Pi/PCr,and PCr/(PCr+Pi) showed statistically significant intergroup differences (P＜0.05).AUCs of Pi,PCr,Pi/PCr,and PCr/(PCr+Pi) were 0.905,0.848,0.930,and 0.930 for the control and half-exhaustive groups,while sensitivity and specificity were 90％/85％,95％/55％,95％/80％,and 90％/85％,respectively.The AUCs of Pi,PCr,Pi/PCr and PCr/(PCr+Pi) were 0.995,0.980,1.000,and 1.000 for the control and exhaustive groups,while sensitivity and specificity were 95％/90％,100％/90％,100％/95％,and 100％/95％,respectively.The AUCs of Pi,PCr,Pi/PCr,and PCr/(PCr+Pi) were 0.735,0.865,0.903,and 0.903 for the half-exhaustive and exhaustive groups,while sensitivity and specificity were 80％/60％,90％/75％,95％/65％,and 95％/70％,respectively.In the half-exhaustive group,some muscle fibers exhibited edema in HE staining,and the unclear Z-discs and the mitochondria with vacuolar degeneration under electron microscopy.Compared with the half-exhaustive group,muscle fiber edema was increased in the exhaustive group,and the Z-discs were broken and the mitochondria exhibited marked vacuolar degeneration under electron microscopy.There were significant difference in A values of desmin staining in the right vastus lateralis among the control,half-exhaustive,and exhaustive groups with 0.58±0.06,0.30±0.04,and 0.21±0.02,respectively (P＜0.05).Histological examination also showed injury-induced changes in the vastus lateralis among the different intensities groups.Statistically a moderate correlation between 31P-MRS and desmin was observed,the correlation coefficients of Pi,PCr,Pi/PCr,and PCr/(PCr+Pi) were-0.706,0.709,-0.726,and 0.791,respectively (P＜0.01).Conclusions 31P-MRS can effectively reflect the changes in energy metabolism in the skeletal muscle after a single bout of acute exhaustive swimming in rats.Based on the significant correlation between 31P-MRS parameters and histological changes,the changes of Pi,PCr,Pi/PCr,and PCr/(PCr+Pi) can indirectly reflect the degree of exercise-induced injury.