12-Week Resistance Training and Detraining: Hormonal, Structural and Functional Adaptations to Two Protocols in Male Runners Running title: Adaptation to resistance training and detraining

Purpose This study investigated structural, functional, and hormonal adaptations following 12 weeks of resistance training (RT) using two loading protocols, and the effects of four weeks of detraining, in male runners. Methods Forty-five healthy men (age: 21 ± 1.7 years; body mass: 67.5 ± 4.35 kg; BMI: 22 ± 1.03 kg/m²) were randomly assigned to high-load RT (seven exercises, three sets at 70–85% 1RM to failure), low-load RT (seven exercises, three sets at 30–45% 1RM to failure), or control groups (n = 15 each). Blood samples were collected at baseline, one hour and 24 hours after training completion, and 24 hours after detraining. Results Both training groups showed significant increases in serum testosterone (p < 0.001), IGF-1 (p < 0.001), cortisol (p < 0.001), T/C ratio (p < 0.001), and muscle thickness of the biceps brachii and vastus lateralis (p < 0.001) compared with baseline. The high-load RT group demonstrated significantly greater improvements than the low-load and control groups (p ≤ 0.05). After four weeks of detraining, most RT-induced adaptations were significantly reduced or completely lost (p ≤ 0.05). Conclusions Low-load RT (30–50% 1RM) elicited meaningful physiological and structural adaptations in male runners; however, these changes were smaller than those observed following high-load RT (70–85% 1RM). Furthermore, a four-week detraining period substantially reversed the training-induced benefits, underscoring the importance of training continuity to maintain physiological and performance adaptations.