Abstract
The purpose of the present study was to examine the acute effects of high-intensity jack-knife stretching for 60 s on flexibility of the hamstrings. Twelve healthy participants underwent jack-knife stretching for 60 s (3 repetitions of 20 s stretching with 30 s intervals) at two different intensities based on the point of discomfort (POD and PODmax). To examine any change in flexibility, knee extension range of motion (ROM), passive torque at end ROM, and muscle–tendon unit stiffness were measured before and after stretching. To evaluate hamstrings pain, a numerical rating scale (NRS) was described. The knee extension ROM (p < 0.01) and passive torque at end ROM (p < 0.05) were significantly increased at both intensities. The muscle–tendon unit stiffness was significantly decreased in PODmax intensity (p < 0.01), but there was no change in POD intensity (p = 0.18). The median values of NRS during the stretching were 0 and 6–7 in POD and PODmax intensity, respectively, although it was 0 immediately after the stretching protocol in both intensities. These data suggested that high-intensity jack-knife stretching is an effective and safe method to decrease muscle–tendon unit stiffness of the hamstrings.
Highlights
The purpose of the present study was to examine the acute effects of high-intensity jack-knife stretching for 60 s on flexibility of the hamstrings
The effects of different intensities (POD and PODmax) of jack-knife stretching on the knee extension range of motion (ROM), passive torque at end ROM, muscle–tendon unit stiffness, knee angle during the stretching, and numerical rating scale (NRS) were examined
The results showed that the knee extension ROM and passive torque at end ROM increased regardless of its intensity, the muscle–tendon unit stiffness decreased only after jack-knife stretching at the intensity of PODmax
Summary
The purpose of the present study was to examine the acute effects of high-intensity jack-knife stretching for 60 s on flexibility of the hamstrings. The median values of NRS during the stretching were 0 and 6–7 in POD and PODmax intensity, respectively, it was 0 immediately after the stretching protocol in both intensities These data suggested that high-intensity jack-knife stretching is an effective and safe method to decrease muscle–tendon unit stiffness of the hamstrings. Previous studies performed high-intensity static stretching at the intensity of 120%ROM14 and 120%POD or more[15,16] and reported that high-intensity static stretching effectively decreased the muscle–tendon unit stiffness of the hamstrings even if with a short duration of high-intensity stretching (≤ 20 s). It may be possible to develop a self-stretching technique that can effectively decrease the muscle–tendon unit stiffness of the hamstrings by performing jack-knife stretching at high-intensity. The purpose of the present study was to compare the acute effects of different intensities (POD and PODmax) of jack-knife stretching on the flexibility of the hamstrings. Because the effect of jack-knife stretching on the muscle–tendon unit stiffness is unknown, the effects of normal (POD) and high-intensity (PODmax)
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