We studied the effects of jumping training in 15 mature female Fischer 344 rats (12 monthold). The animals were divided into two groups: sedentary (S; n = 7) or jumping (J; n = 8)group. The animals in the J group were trained with vertical jumps (40 cm high, 100repetitions/day, 5 days/week) for 8 weeks. After 8-weeks of training, the plantaris muscle(PLA) was isolated, and crude myosin was extracted. Myosin heary chain (MyHC) isoformcomposition was analyzed by sodium dodecyl sulfate polyaclylamidegel electrophoresis.The final body weights of each group were not significantly different. PLA weight andrelative PLA weight of the J group were significantly higher than those of the S group. Whileno statistical difference was found in the compositions of the MyHC I, IId and IIa isoforms ineither group. However, the value of the type IIb MyHC isoform in the J group was relativelylower than that in the S group.These results indicated that the MyHC isoform composition of the PLA muscle changed withjumping training. We conclude that jumping training during the middle stage of maturationaffected the muscle contractile protein phenotype.