Progressão do volume de treinamento de força: efeitos sobre a força muscular e as adaptações contráteis de fibras musculares isoladas em indivíduos treinados

Abstract

Introduction: Training volume is a major variable manipulated in resistance training (RT) prescription, but it remains unclear whether substantial increases in weekly volume promote greater strength gains in previously resistance-trained individuals. Moreover, volume dose–response studies have predominantly assessed voluntary strength, and whether different magnitudes of volume progression elicit distinct adaptations directly at the muscle-fiber level remains unknown. Objective: To compare the effects of individualized 20% (VOL20) and 120% (VOL120) increases in habitual weekly RT volume on maximal dynamic strength, the contractile properties of isolated muscle fibers, and muscle fiber cross-sectional area (fCSA) in previously resistance-trained individuals. Methods: Eighteen resistance-trained participants (11 men and 7 women), aged 18 - 35 years, were included in a randomized, unilateral, within-subject design. Each lower limb was allocated to VOL20 or VOL120. The intervention lasted eight weeks and consisted of two weekly sessions of unilateral 45° leg press and knee extension performed to concentric failure within a 9-12 repetition-maximum range. Maximal dynamic strength was assessed using the one-repetition maximum (1RM) test. Vastus lateralis biopsies were obtained before and after the intervention to determine maximal isometric force of isolated Ca²⁺-activated muscle fibers (P₀), force normalized to fiber diameter (P₀/D), specific force (P₀/fCSA), and histologically determined fCSA. Primary outcomes were analyzed using linear mixed models, with the protocol × time interaction as the effect of interest. Comparisons between percentage changes in 1RM and muscle-fiber contractile properties, as well as analyses stratified by fiber type, were performed exploratorily. Results: Weekly set volume during the intervention was 18.23 ± 3.18 sets in VOL20 and 33.47 ± 5.82 sets in VOL120. 1RM increased in both the 45° leg press and knee extension (both p < 0.001), with no protocol × time interactions (p = 0.888 and p = 0.330, respectively). P₀ and P₀/D also increased over time (both p < 0.001), without protocol × time interactions (p = 0.448 and p = 0.096, respectively). P₀/fCSA increased over time (p = 0.010), also without a protocol × time interaction (p = 0.114). No significant changes in histological fCSA were observed. In the exploratory analyses, voluntary strength gains did not consistently exceed the adaptations observed directly in isolated muscle fibers. Conclusion: Eight weeks of RT concurrently increased maximal dynamic strength and the intrinsic force-generating capacity of isolated muscle fibers in previously resistance-trained individuals. Despite the large contrast between the individualized volume progressions, the 120% increase did not provide evidence of superior adaptations compared with the 20% increase. These findings further demonstrate that functional adaptations at the muscle-fiber level can occur in resistance-trained individuals without detectable increases in histological fCSA.