Relationship between rectus femoris stiffness, muscle activity, and force production during graded isometric...

Surface electromyography (sEMG) is widely used to assess muscle activation; however, practical limitations may restrict its routine clinical use. Handheld myotonometry, which may provide complementary information on the mechanical behavior of active muscles, was used to examine the association between rectus femoris (RF) muscle stiffness, sEMG amplitude, and isometric knee extension force across graded contraction intensities. Fifteen healthy adults performed isometric knee extensions at 0%, 10%, 20%, 30%, 50%, 70%, and 100% of their maximum voluntary contraction (MVC). RF stiffness, sEMG root mean square amplitude, and knee extension force were recorded at each contraction level. RF stiffness increased with contraction intensity and showed a strong positive association with the sEMG amplitude (R² = 0.706). Stiffness was also associated with the knee extension force (R² = 0.561).   Although the EMG amplitude increased with contraction intensity, no significant difference was observed between 70% and 100% MVC, whereas stiffness continued to differentiate between these higher contraction levels. RF muscle stiffness was strongly associated with sEMG amplitude and knee extension force during graded-isometric contractions. These findings suggest that handheld myotonometry may provide a practical complementary measure of the muscle mechanical response associated with the activation level under standardized conditions.