The aim of the present study is to compare the shear modulus of the quadriceps femoris during voluntary and NMES-evoked isometric knee extension.
Twenty healthy participants performed both voluntary and NMES-evoked isometric knee extensions at 5%, 10%, and 20% of their maximal voluntary contraction (MVC), with each task performed twice. A hand-held dynamometer was positioned on the distal lower leg to measure isometric knee extension force. For the VC tasks, participants received real-time feedback on knee extension force via a screen to guide their efforts. During NMES tasks, 5 × 9 cm self-adhesive electrodes were attached to the participants' thighs, delivering a biphasic pulse at 30 Hz with a pulse width of 300 µs. The current intensity was adjusted by the researcher based on the force feedback, and the researcher ensured no pain or discomfort was experienced by the participants. The shear modulus of the rectus femoris (RF), vastus intermedius (VI), and vastus lateralis (VL) were measured in both resting and contracted states using SWE. A linear array probe was used and placed along longitudinal axis of the muscle.
Two-way ANOVA revealed no significant interaction between contraction type (VC or NMES) and contraction intensity for the shear modulus of RF and VL. However, a significant main effect of contraction intensity was observed, with post hoc tests indicating the following pattern: resting 5% 10% 20% MVC. In contrast, a significant interaction was found for VI, where the shear modulus during NMES-evoked contractions at 5%, 10%, and 20% MVC was significantly lower than during VC. No significant differences between contraction intensities were observed for VI.
For RF and VL, similar shear modulus increases were observed with increasing contraction intensity for both VC and NMES. However, VI exhibited significantly lower shear modulus values during NMES compared to VC, indicating insufficient contraction during NMES. These findings demonstrate distinct differences in muscle activation between voluntary and NMES-evoked contractions, particularly between superficial muscles (RF and VL) and the deeper muscle (VI). SWE can effectively evaluate the SMM of individual muscles, including deep muscles, and can assess muscle contractions evoked by NMES.
This study highlights the distinct characteristics of NMES-evoked muscle contractions from the perspective of SMM, offering deeper insights into the application of NMES as a therapeutic modality in physical therapy.
neuromuscular electrical stimulation
quadriceps femoris
