Characterization of muscle belly elastic properties during passive stretching using transient elastography

Passive muscle stretching can be used in vivo to assess the viscoelastic properties of the entire musculo-articular complex, but does not allow the specific determination of the muscle or tendon viscoelasticity. In this respect, the local muscle hardness (LMH) of the gastrocnemius medialis (GM) belly was measured during a passive ankle stretching of 10 subjects using transient elastography. A Biodex isokinetic dynamometer was used to stretch ankle plantar flexors, to measure ankle angle, and the passive torque developed by the ankle joint in resistance to the stretch. Results show that the LMH increased during the stretching protocol, with an averaged ratio between maximal LMH and minimal LMH of 2.62±0.46. Furthermore, LMH–passive torque relationships were nicely fitted using a linear model with mean correlation coefficients (R2) of 0.828±0.099. A good reproducibility was found for the maximal passive torque (ICC=0.976, SEM=2.9 N m, CV=5.5%) and the y-intercept of the LMH–passive torque relationship (ICC=0.893, SEM=105 Pa, CV=7.8%). However, the reproducibility was low for the slope of this relationship (ICC=0.631, SEM=10.35 m−2, CV=60.4%). The y-intercept of the LMH–passive torque relationship was not significantly changed after 10 min of static stretching. This result confirms the finding of a previous study indicating that changes in passive torque following static stretching could be explained by an acute increase in muscle length without any changes in musculo-articular intrinsic mechanical properties.