Author(s): NISHIZAWA, N., MAEO, S.1, KOBAYASHI, Y.1, KINOSHITA, M.1, ICHINOSE, H.1, EIHARA, Y.1, ARAI, H.1, FOLLAND, J.P.2, KANEHISA, H.3, ISAKA, T.1, Institution: RITSUMEIKAN UNIVERSITY , Country: JAPAN, Abstract-ID: 1906

INTRODUCTION:
Sprint performance is related to the muscle size and strength of lower-limb muscles such as the hip flexors and knee extensors. However, little is known about how training-induced changes in muscle size and strength of these muscle groups contribute to sprint performance enhancement. Thus, this study compared the effects of hip flexion (HF) versus knee extension (KE) training on muscle size, muscle strength and sprint performance. We also examined whether training-induced changes in sprint performance were associated with changes in muscle size and strength.
METHODS:
Forty-five healthy untrained adults completed 12 weeks of either HF (n = 23) or KE (n = 22) training. Participants trained each leg unilaterally at 70% of one-repetition maximum (1RM) for 5 sets of 10 repetitions per session, twice weekly. Before and after the intervention, HF 1RM (HF-1RM), KE 1RM (KE-1RM), MRI-derived volumes of 13 individual muscles (including hip flexors, knee extensors and adductors) and 60-m sprint time were assessed. Training effects were evaluated using linear mixed models. Repeated-measures correlations examined associations between changes in sprint time and changes in muscle volumes and each 1RM, and stepwise multiple regression analyses were conducted, separately for each group.
RESULTS:
Increases in hip flexor volume were significantly greater following HF training (+12.7%) than KE training (+6.9%), whereas increases in knee extensor volume were greater following KE training (+4.1%) than HF training (+1.0%) (P ≤ 0.004). Similarly, HF-1RM increased more after HF training (+112.3%) than KE training (+18.9%), and KE-1RM increased more after KE training (+46.5%) than HF training (+24.5%) (P < 0.001). Sprint time showed a significant main effect of time (P = 0.005) with no group by time interaction (P = 0.515), indicating similar reductions in sprint time in both groups (HF: −1.5% vs. KE: −0.9%). Within-group analyses revealed that in the HF group, changes in sprint time were significantly correlated with changes in HF-1RM, KE-1RM, hip flexor volumes, knee extensor volumes and hip adductor volumes (r = −0.462 to −0.654, P ≤ 0.023). Among these correlated variables, rectus femoris hypertrophy emerged as a significant predictor for sprint time improvement in stepwise regression (R2 = 0.222, P = 0.001). In contrast, no significant associations were observed in the KE group, although changes in rectus femoris (r = −0.518) and tensor fasciae latae (r = −0.557) volumes showed trends toward associations with changes in sprint time (both P = 0.074).
CONCLUSION:
Both HF and KE training induced different task-specific increases in muscle size and strength, whilst similarly improving sprint performance. However, associations between muscle adaptations and sprint performance enhancement were evident primarily in the HF group, suggesting that training-induced changes in muscles contributing to HF may be particularly relevant for sprint performance enhancement.