Effect of Soleus Muscle Shear Modulus on Deep Squatting Motion at Increased Ankle Dorsiflexion.

Introduction Deep squats are important functional movements in many Asian cultures, and the ability to perform this movement is a key indicator of lower-limb function. Ankle joint range of motion, particularly dorsiflexion, plays an important role in deep squats. This study aimed to investigate the relationship between muscle stiffness in the triceps surae (medial and lateral heads of the gastrocnemius and the soleus) and the ability to perform deep squatting motions, as measured by the shear modulus. Methods A total of 45 healthy participants were recruited for this study, including 24 males (53.3%) and 21 females (46.7%). We measured the stiffness (shear modulus) of the gastrocnemius and soleus muscles at various ankle dorsiflexion angles (0-40°) in the flexed knee position. The participants were divided into a "possible group" (those able to perform a deep squat with heels on the floor) and an "impossible group" (those unable to perform the deep squat with heels on the floor). The shear modulus values of the medial and lateral heads of the gastrocnemius and the soleus muscle were compared between the two groups. Results The shear modulus of the soleus muscle at 40° of ankle dorsiflexion was significantly lower in the impossible group (14.09 ± 6.79 kPa) than in the possible group (9.37 ± 5.55 kPa, p < 0.05). The intraclass correlation coefficient (ICC) values for the muscle measurements ranged from 0.75 to 0.89, indicating good reproducibility. Conclusion This study showed that soleus muscle stiffness at 40° ankle dorsiflexion differed between individuals who can and cannot perform deep squats. This suggests that insufficient elongation of the soleus muscle may hinder the achievement of the dorsiflexion angle necessary for deep squats.