Muscle fiber composition, jumping performance and rate of force development adaptations induced by different power training volumes in females.

Aim of the study was to investigate the effect of three different eccentric-only power training volumes on muscle fiber type composition and power performance. Twenty-nine females were assigned into three groups and performed 10 weeks of either 3 (Low Volume), 6 (Moderate Volume) or 9 (High Volume) sets/session of four fast velocity eccentric-only half-squats against 70% of concentric 1RM, followed by 3 maximum countermovement jumps (CMJ) 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 to 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 IIx percentages, fiber CSA, %CSA and the changes in performance (r: -0.787 - 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 bullets * Low but not high volume of power training maintains type IIx muscle fibers. * Early rate of force development increases after low or moderate but not after high power training volume. * Training-induced changes in type IIx muscle fiber percentage is related with changes in early rate of force development.