Static stretching (SS) is commonly used as a part of a warm-up routine to increase flexibility and prevent sports-related injuries (Takeuchi et al., 2019). To evaluate changes in flexibility, range of motion (ROM), passive torque and muscle-tendon unit stiffness are often measured (Young et al., 2013). Previous review studies have reported that SS increases ROM (Behm et al., 2016; Radford et al., 2006). Alteration of ROM after SS is attributed to changes in muscle-tendon unit stiffness (Konrad et al., 2016; Mizuno, 2017; Morse et al., 2008; Ryan et al., 2009) and tolerance for stretching (Brusco et al., 2019; Magnusson et al., 1996; Nakamura et al., 2013). Muscle-tendon unit stiffness is defined as the value of the slope of the torque-angle curve during passive joint movement (Magnusson et al., 1996). Previous reports have shown that too much stiffness may lead to various lower body injuries including soft-tissue, joint and bone injuries, occurring in non-contact situations (Ekstrand and Gillquist, 1983; Pickering et al., 2017; Watsford et al., 2010). The relationship between relatively high stiffness and the incidence of sports-related injuries is due to a diminished cushioning effect from soft-tissues, resulting in greater stress (Butler et al., 2003; Grimston et al., 1991; Hennig and Lafortune, 1991). Therefore, it is important to reduce muscle-tendon unit stiffness to prevent sports-related injuries.
The effects of SS on ROM and muscle-tendon unit stiffness are affected by the duration of the stretch (Bandy et al., 1997; Matsuo et al., 2013; Ryan et al., 2009). ROM is increased immediately after SS (Boyce and Brosky, 2008; Butler et al., 2016; Fletcher and Monte-Colombo, 2010; Sato et al., 2020), while muscle-tendon unit stiffness is decreased after more than 180 seconds of SS in the hamstrings (Matsuo et al., 2013; Nakamura et al., 2019). Previous studies reported that conditioning coaches use SS for approximately 20 seconds as a part of a warm-up routine (Simenz et al., 2005; Takeuchi et al., 2019). It is reported that SS for 20 seconds in the hamstrings increased ROM, but muscle-tendon unit stiffness did not change (Matsuo et al., 2013). Therefore, it is possible that SS used in a warmup routine cannot decrease muscle-tendon unit stiffness, with the result that the purpose of the prevention of sports-related injuries cannot be achieved. However, it is very difficult to use SS for more than 180 seconds in a warm-up routine, because the time of sports practice is very limited for many athletes. Therefore, it is necessary to develop a method of SS that can decrease muscle-tendon unit stiffness in 20 seconds.
The intensity of SS is one of the factors that influences the effects of SS (Kataura et al., 2017). Kataura et al. (Kataura et al., 2017) reported that the intensity of SS was negatively correlated with the relative change of passive stiffness of the hamstrings. Therefore, it is possible that high intensity SS for 20 seconds could decrease muscle-tendon unit stiffness. Thus, the purpose of the present study was to examine the effects of high intensity SS for 20 seconds on flexibility (ROM, passive torque, muscle-tendon unit stiffness) and strength (peak torque, knee angle at peak torque) in the hamstrings.
Thirteen healthy men (mean [+ or -] SD; 21.2 [+ or -] 0.4 years, 174.0 [+ or -] 0.06 m, 65.2 [+ or -] 10.5 kg) and four healthy women (mean [+ or -] SD; 21.3 [+ or -] 0.5 years, 164.0 [+ or -] 0.08 m, 55.3 [+ or -] 8.1 kg) were recruited. Participants who regularly performed any flexibility and strength training or who had a history of lower limb pathology were excluded. All participants were informed of the requirements and risks associated with their involvement in this study and signed a written informed consent document. The study was performed in accordance with the Declaration of Helsinki (1964). The Ethics Committee of Kobe International University approved the study (approved No. G2019-094).
The participants visited the laboratory three times, with an interval of one week between each visit. They underwent three different intensities of SS in right hamstrings, in random order. Before and after each SS, all measurements except the numerical rating scale (NRS) were taken. NRS was taken during SS, immediately after SS, and 24 hours after SS. All experiments were completed in the same room, in which the temperature was maintained at 25 degrees C.
An isokinetic dynamometer machine (CYBEX NORM, Humac, California, USA) was used in the present study. This study used a sitting position in which the hip joint was flexed, which has been shown to efficiently stretch the hamstrings (Kataura et al., 2017). The participants were seated on a chair with the seat tilted maximally, and a wedge-shaped cushion was inserted between the trunk and the backrest, which set the angle between the seat and the back at approximately 60 degrees. The chest, pelvis, and right thigh were stabilized with straps. The right knee joint was aligned with the axis of the rotation of the isokinetic dynamometer machine. The lever arm attachment was placed just proximal to the malleolus medialis and stabilized with straps. In the present study, reported knee angles were measured using the isokinetic dynamometer machine. A 90-degree angle between the lever arm and floor was defined as 0 degrees of knee flexion/extension.
Range of motion, passive torque, and muscle-tendon unit stiffness
ROM, passive torque and muscle-tendon unit stiffness were calculated by using the isokinetic dynamometer machine in the same fashion as a previous study (Kataura et al., 2017). The knee was passively extended at 5 degrees/second from 0 degrees to the maximally tolerable angle without pain. A previous study reported that the velocity does not cause stretch reflex during passive joint movement (Christopher I Morse, 2011). ROM was defined as the maximal knee extension angle from 0 degrees, and the passive torque was defined as the torque at the maximal knee extension angle. Muscle-tendon unit stiffness was defined as the values of the slope of the regression line that was calculated from the torque-angle relationship using the least-squares method (Magnusson et al., 1996). Muscle-tendon unit stiffness was calculated from the same knee extension angle range before and after SS. The calculated knee extension angle range was defined as the angle from the 50% maximum knee extension angle to the maximum knee extension angle measured before SS. However, if the maximum knee extension angle measured after SS was smaller than that before SS, the muscle-tendon unit stiffness before and after SS was calculated from the 50% maximum knee extension angle to the maximum knee extension angle measured after SS (Kataura et al., 2017). The value of ROM before SS was used for the intensity of SS to normalize to percentages.
Peak torque and knee angle during maximum voluntary isokinetic concentric contraction
The peak torque of knee flexion during maximum voluntary isokinetic concentric contraction at 60 degrees/second was...