Performance and fibre characteristics of human skeletal muscle during short sprint training and detraining on a cycle ergometer.

The ergometric effect of sprint training and detraining was studied in relation to muscle fibre changes in seven students trained during 9 weeks on a cycle ergometer. Before and after training and after 7-week detraining, they performed a force-velocity test on a friction-loaded cycle ergometer. On these three occasions, muscle samples were taken from vastus lateralis muscle at rest for histochemical analysis. The training-induced shift of the force-velocity relationship was such that the increase in maximal velocity (vmax) was greatest against high braking forces (FB) with unchanged vmax with no load. This was associated with higher maximal power output (28%) and peak force (16%). The increased maximal mean power output to reach a maximal velocity during a short sprint was obtained against a 23% higher optimal FB (FB,Wmax). At the same time, a considerable hypertrophy in fast twitch b (FTb) fibres was observed. All these changes were maintained after detraining. The training-induced changes in vmax reached against FB1Wmax(vm2Wmax) allowed us to produce evidence for two particular sub-groups in which inverse fibre conversions were observed. In subgroup A, the lowered post-training vm,Wmax was associated with a decrease in both FTa and FTb fibres. Conversely, the vm,Wmax, increase in subgroup B was associated with a higher percentage of FT fibres as the result of increased FTa fibres and decreased FTb fibres. Thus, the fibre hypertrophy associated with a unidirectional fibre translation [FTb-->FTa-->slow twitch (ST)] toward fibres with a high thermodynamic efficiency would result mainly in increased force qualities, whereas the bidirectional fibre translation (ST-->FTa<--FTb) would allow enhancement of both force and velocity properties.