Quantitative evaluation of the effects of several weeks of static stretching on the flexibility of the rectus femoris using shear wave elastography: a before–after study

Aim: The study aimed to quantitatively clarify the effects of several weeks of static stretching on the flex- ibility 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 in- tensity to a point immediately before experiencing discomfort or pain. The Young’s modulus of the rec- tus 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 base- line, 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 ef- fects 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 four- week 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 in- creased 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.