Aim: The study aimed to quantitatively clarify the effects of several weeks of static stretching on the flexibility of the rectus femoris muscle using shear wave elastography. Material and methods: Fifteen healthy men (age: 26.4 ± 2.2 years) were instructed to perform 5 min of voluntary static stretching of their right rectus femoris muscles five times a week for four weeks. The participants adjusted their stretching intensity to a point immediately before experiencing discomfort or pain. The Young’s modulus of the rectus femoris muscle and the knee-flexion range of motion were measured as indicators of flexibility. The Young’s modulus was measured using shear wave elastography. Measurements were performed at baseline, as well as at two and four weeks after the stretching program started. A generalized linear mixed model was used to assess the change in the Young’s modulus after the stretching program and the effects of the Young’s modulus on the knee-flexion range of motion. Results: The Young’s modulus of the rectus femoris muscle decreased after two and four weeks of stretching compared with the baseline (p = 0.0004 and p <0.0001, respectively). The Young’s modulus of the rectus femoris muscle and the fourweek duration of stretching affected the knee-flexion range of motion (p = 0.0242 and 0.0016, respectively). Conclusions: Shear wave elastography quantitatively revealed that several weeks of static stretching increased the flexibility of the rectus femoris muscle in healthy men. A four-week static stretching regimen reduced the Young’s modulus of the rectus femoris muscle and increased the knee-flexion range of motion.

Young’s modulus; healthy volunteer; shear wave elastography; static stretching; rectus femoris muscle