Within-and Between-Muscle Heterogeneity in Quadriceps Thickness Changes Following Maximal Concentric Isokinetic Exercise

The purpose of this study was to examine early post-exercise changes in quadriceps muscle thickness after a protocol of consecutive isokinetic concentric leg extension muscle actions and determine whether these changes are muscle- or site-specific. Fifteen young adults (14 males, 1 female) completed 50 maximal unilateral concentric leg extensions at 120°·s⁻¹. Vastus lateralis (VL) and vastus medialis (VM) thickness was assessed using ultrasound at 40%, 60%, and 80% of femur length pre-exercise, immediately post, and at 5-, 10-, 15-, and 30-min. Absolute thickness (mm) and relative change from baseline (ΔThickness, %) were analyzed with linear mixed-effects models. Muscle thickness increased immediately post-exercise and remained elevated at 5-min post-exercise, then declined toward baseline by 30-min. Peak ΔThickness occurred most often immediately post-exercise (71.4%), less frequently at 5 min (19.0%) and 10 min (9.5%). VM presented greater muscle thickness relative change than VL at post-exercise (17.5% vs 12.9%, g=0.59) and 5-min (15.1% vs 11.8%, g=0.43). Additionally, VL showed larger relative changes distally than the other measurement sites (g=0.39-0.50, p<0.05), whereas VM was more uniform across sites (g≤0.20). Individual-level patterns also varied, with the distal region most frequently contributing to VL thickness change at both post-exercise and 5-min assessments (53%), whereas VM showed less consistent regional dominance. Acute responses of quadriceps muscle thickness to exercise peak immediately post-exercise, but they differ within and between quadriceps components.