Purpose: To assess early changes in muscle function and hypertrophy, measured as increases in muscle cross-sectional areas (CSAs) and total volume, over a 4 weeks inertial resistance training (RT) program.Methods: Ten young RT-naive volunteers (age 23.4 ± 4.1 years) underwent 10 training sessions (2–3 per week) consisting of five sets of 10 flywheel squats (moment of inertia 900 kg⋅cm2). Magnetic resonance imaging (MRI) scans of both thighs were performed before (PRE), and after 2 (IN) and 4 (POST) weeks of training to compute individual muscle volumes and regional CSAs. Scans were performed after ≥96 h of recovery after training sessions, to avoid any influence of acute muscle swelling. PRE and POST regional muscle activation was assessed using muscle functional MRI (mfMRI) scans. Concentric (CON) and eccentric (ECC) squat force and power, as well as maximal voluntary isometric contraction force (MVIC) of knee extensors and flexors, were measured in every training session.Results: Significant quadriceps hypertrophy was detected during (IN: 5.5% ± 1.9%) and after (POST: 8.6% ± 3.6%) the training program. Increases in squat force (CON: 32% ± 15%, ECC: 31 ± 15%) and power (CON: 51% ± 30%, ECC: 48% ± 27%) were observed over the training program. Knee extensor MVIC significantly increased 28% ± 17% after training, but no changes were seen in knee flexor MVIC. No correlation was found between regional muscular activation in the first session and the % of increase in regional CSAs (r = -0.043, P = 0.164).Conclusion: This study reports the earliest onset of whole-muscle hypertrophy documented to date. The process initiates early and continues in response to RT, contributing to initial increases in force. The results call into question the reliability of mfMRI as a tool for predicting the potential hypertrophic effects of a given strengthening exercise.

This study determined the acute changes in spatio-temporal and impulse variables when wearable resistance (WR) of 2% body mass was attached distally to the thighs during 50 m maximal sprint-running. Fifteen sub-elite male sprinters performed sprints with and without WR over 50 m of in-ground force platforms in a randomised order. A paired t-test was used to determine statistical differences (p < .05), with effect sizes (ES) calculated between conditions over steps: 1-4, 5-14, and 15-23. WR resulted in small increased 10 and 50 m sprint times (1.0%, ES = 0.31, 0.9%, ES = 0.44, respectively, p > .05) compared to the unloaded sprint condition. For spatio-temporal variables, the WR condition resulted in moderate ES changes in step frequency (−2.8%, ES = −0.53, steps 5-14, p > .05), and contact time (2.5%, ES = 0.57, steps 5-14, and 3.2%, ES = 0.51, average of 23 steps, p > .05), while step length was unaffected during all step phases of the sprint (ES = 0.02-0.07, p > .05). Regarding kinetics, during steps 5-14, WR resulted in a moderate decrease (−4.8%, ES = −0.73, p < .05) in net anteriorposterior impulses and a moderate decrease in vertical stiffness (−5.7%, ES = −0.57, p > .05). For athletes seeking to overload step frequency and develop anterior-posterior impulse during mid to late accelerated sprinting, WR enables the application of a sprint-specific form of resistance training to be completed without decreasing step length.

The purpose of this study was to determine and compare thigh muscle volumes (MVs), and sprint mechanical properties and performance between male and female national-level sprinters. We also studied possible relationships between thigh MVs and sprint performance. Nine male and eight female national-level sprinters participated in the study. T1-weighted magnetic resonance images of the thighs were obtained to determine MVs of quadriceps, hamstrings and adductors. Sprint performance was measured as the time to cover 40 and 80 m. Instantaneous sprint velocity was measured by radar to obtain theoretical maximum force (F0), theoretical maximum velocity (V0) and maximum power (Pmax). When MVs were normalized by height–mass, males showed larger hamstrings (13.5%, ES = 1.26, P < 0.05) compared with females, while quadriceps and adductors showed no statistically significant differences. Males were extremely faster than females in 40 m (14%, ES = 6.68, P < 0.001) and in 80 m (15%, ES = 5.01, P < 0.001. Males also showed increased sprint mechanical properties, with larger F0 (19%, ES = 1.98, P < 0.01), much larger Pmax (46%, ES = 3.76, P < 0.001), and extremely larger V0 (23%, ES = 6.97, P < 0.001). With the pooled data, hamstring and adductor MVs correlated strongly (r = -0.685, P < 0.01) and moderately (r = -0.530, P < 0.05), respectively, with sprint performance; while quadriceps showed no association. The sex-stratified analysis showed weaker associations compared with pooled data, most likely due to small sample size. In conclusion, males were faster than females and showed larger MVs, especially in hamstrings. Moreover, regarding the thigh muscles, hamstrings MV seems the most related with sprint performance as previously proposed.