Muscle samples were obtained bilaterally from the vastus lateralis of 107 young healthy male soccer players with a hypotrophic quadriceps muscle after 4 to 6 weeks (wk) plaster cast immobilization due to knee ligament injuries. They were rehabilitated with intensive muscle training of the hypotrophic muscle conducted three times per wk for four wk's. Muscle samples were obtained from each leg, before and after the training period. Portions of the specimen were analyzed for total succinate dehydrogenase (SDH) and phosphofructokinase (PFK). Sections of the muscle were stained for myosin adenosine triphosphatase (ATPase) for area measurements of type I and type II fibres and fibre distribution, and NADH-tetrazolium reductase (NADHTR), and a-glycerophosphate dehydrogenase (aGPD) for microphotometric measurement of single fibre oxidative and glycolic potential, respectively. SDH increased from 2.50 to 3.16 (per micro-mol/g/min) (p < 0.001) in the trained leg and was unchanged in the control leg (3.18 per micro-mol/g/min). PFK was unchanged in both legs. Type I fibre distribution was unchanged in both legs, but a significant lower value was demonstrated in the trained (immobilized) leg (37.3 versus 43.2%, p < 0.01). No changes were observed in absolute areas of type I and II fibres in the two legs, however before training type I fibres were smaller in the trained (immobilized) leg compared to the control leg (48.7 versus 52.1 x 10(2) micro-m2, p < 0.05). The oxidative potential in type I fibres increased during training, whereas the oxidative potential in type II fibres as well as glycolytic potential in both fibre types were unchanged. It is concluded that both quantitative and qualitative changes in the muscle may contribute to increased work performance after training of hypotrophic muscle.

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