Dependence of muscle and deep fascia stiffness on the contraction levels of the quadriceps: An in vivo supersonic shear-imaging study.

In the present study we investigated muscle contraction-driven changes in deep fascia mechanical property, to reveal mechanical interactions between them. Fourteen males (22-37 yr) performed isometric knee extension at 20, 40, 60% of the maximal voluntary contraction (MVC). During each contraction and at rest, shear wave velocities (SWV) of the rectus femoris (RF) and vastus lateralis (VL) and fascia lata were measured in both longitudinal and transverse ultrasound transducer directions relative to the thigh. Surface electromyogram was recorded from RF and VL and root mean square (RMS) values were determined. The slopes of the linear correlations between normalized SWV (%SWV: relative to rest) and RMS (%RMSEMG : relative to MVC) were calculated for different sites and directions. Both muscles and fascia lata became stiffer as the muscle activation level increased to comparable degrees, with the slopes of those changes being 4-9 times higher in the longitudinal than transverse direction. The fascia lata showed lower slopes than those of muscles in the longitudinal direction while in the transverse direction neither parts showed significant differences. These results suggest that the force produced by the muscles partly transmitted to the fascia lata in the longitudinal and transverse directions, causing anisotropic changes in musculofascial entity.