RESUMEN
This study aimed to investigate the effect of 3 different eccentric-only power training volumes on muscle fiber type composition and power performance. Twenty-nine females were assigned into 3 groups and performed 10 weeks of either 3 (low volume), 6 (moderate volume), or 9 (high volume) sets/session of 4 fast-velocity eccentric-only half-squats against 70% of concentric 1-repetition maximum (1RM), followed by 3 maximum countermovement jumps (CMJs) after each set. Half-squat 1RM, CMJ height/power, maximum isometric force, rate of force development (RFD) and muscle fiber cross-sectional area (CSA) were increased in all groups (p = 0.001). Low-volume training induced higher increases in CMJ height/power and early RFD, compared with the moderate- and high-volume training programs (p < 0.001). Significant reductions in type IIx muscle fiber percentages and %CSAs were found after moderate- and high-volume training, with concomitant increases in type IIa fibers (p = 0.001). Significant correlations were found between the changes in type IIa and type IIx percentages, fiber CSA, %CSA, and the changes in performance (r: -0.787 to 0.792; p < 0.05). These results suggest that relatively large eccentric power training volumes may result in detrimental neuromuscular adaptations, minimal changes in early RFD, and a reduction of type IIx muscle fiber percentage. Novelty Low but not high volume of power training maintains type IIx muscle fibers. Early rate of force development increases after a low- or moderate-power training volume, but not after a high-power training volume. Training-induced changes in type IIx muscle fiber percentage is related with changes in early rate of force development.