The capability to load-dependent muscle fiber type specific adaptations demonstrates the enormous plasticity of human skeletal muscle. The cellular specialization enables remodeling of the muscle's structural makeup by changing their phenotype. Competitive road cycling is still dominated by an extensive endurance training that favors slow twitch Type I muscle fibers (MFT I). So far the importance of exercise-induced stimuli on fast twitch Type IIA & IIX muscle fibers (MFT IIA & MFT IIX) in endurance sports has been neglected in the literature. The aim of this study was to investigate the stress dependent phenotype response (MFT I, IIA, IIX) in junior male competitive cyclists in training (TP) and competition period (CP). Ten junior male cyclists participated in this study (17.3 ± 0.5 yrs;BMI: 20.9 ± 1.7 kg/m²). They underwent an incremental cycle ergometer test with spirometry and a muscle biopsy at the beginning and end of the CP (feb and oct). The subjects showed a significant increase of V^·O₂ peak (64.9 ±5.2 vs. 70.0 ±5.5 ml•kg^-1•min^-1; p < 0.05), but no changes in the max Power Output [Watt, W] (346.4 ±33.42 vs. 331.6 ±33.01 W;5.0 ±0.32 vs. 4.7 ±0.28 W•kg^-1). The energy consumption increased more at the same performance rate (0.059 ± 0.001 kcal/min/W vs. 0.063 ± 0.002 kcal/min/W; p < 0.001). Muscle samples were obtained from the vastus lateralis muscle and fibers were classified as MFT I, IIA or IIX by using histochemical myofibrillar ATPase staining. A slow to fast shift of the fibers could be observed determining the phenotypes (MFT I: -6%; MFT IIA: +3.7%; MFT IIX: +2.3%). In conclusion, switching from the classical, high-volume extensive endurance training in TP to the CP resulted in a load-dependent slow to fast phenotype shift. The very homogenous adaptation of the Type IIX fibers was a striking phenomenon. The increased energy turnover may be induced by the metabolic rate of the modulated fiber types indicates the decrease of cycling efficiency.