Differences in muscle morphology assessed by ultrasound at various muscle regions and their impact on voluntary and involuntary force production.

The primary aim of this study was to investigate how measurements from different regions along the rectus femoris (RF) and vastus lateralis (VL) influence muscle morphology, including muscle thickness (MT), muscle stiffness, and muscle quality. An exploratory aim was to examine whether an association exists between voluntary and involuntary force and muscle morphology across the same regions. In one session, participants (n = 13) underwent ultrasound imaging (US), followed by knee extension maximal isometric voluntary contractions and evoked contractions. US recordings (at rest) and testing were conducted while participants were seated at 90º knee flexion (dominant leg) on an isokinetic dynamometer. Muscle morphology was recorded at proximal, medial, and distal regions of RF and VL. During maximum contractions, participants were instructed to exert maximal effort as fast and as forcefully as possible for 5 s, while evoked contractions were performed via femoral nerve stimulation. A one-way repeated measures ANOVA was used for the main aim, while Spearman bivariate correlations were used for the exploratory aim. The primary findings showed that the RF and VL muscles were significantly larger in the medial region (P ≤ 0.023), with no significant differences in muscle quality or stiffness within the same muscle. Additionally, a significant overall relationship was observed between muscle quality and the rate of force development in both muscles (P ≤ 0.037). In conclusion, muscle size varies across the length of the VL and RF muscles, with no changes in muscle quality or stiffness. Furthermore, muscle quality demonstrates a significant association with rate of force development.