Although several previous studies have investigated the acute effects of SS and FR interventions, most studies have adopted longer durations of SS and FR than those used in most common sports settings. Therefore, to enhance ecological validity in this study, we examined the effects of short-term SS and FR interventions and the combined effects of SS and FR in terms of typical sports and training applications. To the best of our knowledge, this is the first crossover design study to examine the effects of short-term SS, FR, and the combination of SS and FR. The results showed small to moderate magnitude ROM, moderate to large magnitude PPT significant increases, and moderate magnitude tissue hardness significant decreases in all conditions. The MVC-ISO torque demonstrated a significant small-magnitude impairment in the 60-s SS and 30-s SS conditions. As we hypothesized, the short-term FR and 30-s SS+30-s FR conditions increased ROM without decreasing muscle strength. The SS-only condition decreased MVC-ISO while ROM increased. This result supports the previous study by Behm et al (Behm et al., 2016; Behm et al., 2021). For these reasons, athletes and coaches should avoid SS-only warm-up. As shown in Table 1, knee flexion ROM significantly increased in all conditions in this study, which is consistent with the previous studies examining the effects of short-term SS (Sato et al., 2020; Takeuchi and Nakamura, 2020) and FR interventions (Nakamura et al., 2021a; Sullivan et al., 2013). Previous studies have implicated that changes in stretch tolerance could contribute to the increase in ROM immediately after SS (Chaouachi et al., 2017; Weppler and Magnusson, 2010) and FR intervention (Nakamura et al., 2021a; Nakamura et al., 2021b). Although the detailed mechanism of the increase in ROM is unknown in this study, changes in stretch tolerance could be involved. Interestingly, prior studies have reported that 30-s FR on the ankle plantar flexors did not increase ROM (Nakamura et al., 2021b), but less than 20-s FR on the hamstrings did increase ROM (Sullivan et al., 2013). We investigated the effect of FR on the knee extensors, and 30-s FR intervention significantly increased ROM. Such short-duration interventions of SS or FR are unlikely to provide sufficient stress to promote muscle or myofascial morphological modifications (Behm et al., 2016; Behm and Chaouachi, 2011; Behm et al., 2021; Behm and Wilke, 2019; Konrad and Tilp, 2020). Similarly, SS and FR-induced neurophysiological responses such as muscle spindle disaffiliation need more prolonged interventions, while cutaneous afferent and Hoffman (H) reflex inhibition and another stretch, pressure, or force-sensitive reflex effects can diminish within seconds of their application (Behm et al., 2021). Also, a previous study showed that there was no significant change in the shear elastic modulus of the knee extensor after 60-s SS intervention (Takeuchi et al., 2021). Hence, these results suggest that short-term SS and FR intervention increases ROM primarily through changes in stretch tolerance, but the effects of short-term SS and FR intervention might vary by muscle. Interestingly, increasing SS or FR duration from 30-s to 60-s or combining short-term SS and FR interventions did not induce a further increase in ROM. Therefore, considering time efficiency, a 30-s SS or FR intervention was recommended as a warm-up routine to increase ROM. In addition, PPT was significantly increased in all conditions in this study. As described above, SS and FR could alter stretch tolerance (Chaouachi et al., 2017; Nakamura et al., 2021b; Weppler and Magnusson, 2010). The detailed mechanism of the significant increase in PPT in this study is unknown, but it may involve changes in pain perception due to SS and FR interventions. Behm and colleagues (Behm et al., 2016; Behm and Chaouachi, 2011) have suggested that hypoalgesic responses could arise from the Gate Control (Melzack and Wall, 1965) and the diffuse noxious inhibitory control (Mense, 2000) theories of pain inhibition. Therefore, it is possible that SS and FR interventions could increase PPT as well as change in stretch tolerance. Also, the results suggest that even short-term SS and FR interventions can alter pain perception. In addition, tissue hardness significantly reduced in all conditions in this study. Previous studies showed the muscle stiffness of medial gastrocnemius muscle significantly decreased immediately after 120-s SS (Nakamura et al., 2014; Nakamura et al., 2015; Nakamura et al., 2021c). Also, the previous study showed no significant change in shear modulus of knee extensor after 60-s SS intervention (Takeuchi et al., 2021). Hotfiel et al. (2017) also showed increased tissue perfusion and decreased tissue stiffness immediately after FR intervention (Hotfiel et al., 2017). Tissue hardness could reflect changes in subcutaneous tissues, aponeurosis, and muscles, and could be a different index from shear modulus and muscle stiffness. In the present study, tissue hardness decreased in all conditions, suggesting that similar effects can be obtained in short-term SS and FR interventions. The results of this study showed that MVC-ISO torque was significantly decreased (small magnitude effect size) in both 60-s SS and 30-s SS conditions, while no significant change was observed in the 30-s SS+30-s FR, 60-s FR, and 30-s FR conditions. Previous studies showed that FR interventions have at least no adverse effects on performance (Cheatham et al., 2015; Wiewelhove et al., 2019). The results of this study extended these findings and showed that short-term FR interventions of 30-s or 60-s have no negative effects on muscle strength. Conversely, previous studies concluded that longer than 60-s SS interventions could induce impairments in muscle strength and performance, such as stretching-induced force deficits (Behm et al., 2016; Behm and Chaouachi, 2011; Kay and Blazevich, 2012). Possible mechanisms of stretching-induced force deficits might include changes in sustained or persistent inward currents to motoneurons (Behm et al., 2021) and decreased efficiency of force transmission to skeletal muscles due to decreased stiffness of muscle-tendon units (Huijing, 1999). Behm and Chaouachi (2011) also concluded that short stretching times during the warm-up might not negatively affect subsequent performance, especially in highly trained populations, if the total stretching time per muscle is 30-s or less (Behm and Chaouachi, 2011). However, the results of this study showed that the MVC-ISO torque significantly decreased after 30-s SS intervention. Subjects in this study were active male university students but not consistently highly trained. Whereas MVC-ISO was significantly decreased in the 60-s SS and 30-s SS conditions, no significant change in MVC-ISO torque was observed in the 30-s SS+30-s FR condition, in which 30-s SS intervention was followed by 30-s FR intervention. The previous study suggested that FR intervention after SS intervention restores the excitability of motoneurons produced by SS intervention, resulting in recovery from decrease in muscle strength (Nakamura et al., 2023). Therefore, it is possible that no change in MVC-ISO was observed in 30-s SS+30-s FR condition in this study. Interestingly, MVC-CON torque did not significantly change under all conditions. As described above, FR has no adverse effect on performance (Cheatham et al., 2015; Wiewelhove et al., 2019), in accordance with the lack of impairments MVC-CON and MVC-ISO in the present study. Additionally, Sato et al. (2020) showed no significant change in MVC-CON torque after 20-s of SS for the ankle plantar flexors. Also, the previous study showed that MVC-CON is less likely to cause a decrease in strength after SS intervention than MVC-ISO (Behm et al., 2016). Thus, our results suggested that short-term SS has no adverse effect on the isokinetic contraction torque. In practical implications, as a pre-exercise warm-up, short-term, i.e., 30-s FR intervention is recommended when the goal is to increase ROM while maintaining muscle strength. Similarly, a short-term FR intervention after a short-term SS can eliminate the effect of strength impairments. Also, we recommended that athletes and coaches should avoid warm-ups employing only SS, since decrements may accompany increases in ROM in muscle strength. There were some limitations of this study. In this study, we investigated only the acute effect of SS and FR intervention, and the sustained effect is unknown. The subjects of this study were not athletes; therefore, future studies are needed to investigate the short-term FR and SS effects on athletes, including the sustained effects, to establish an effective warm-up routine. Knee flexion ROM was performed by a physical therapist, and measurements were taken while the hip joint was in the neutral position. However, no equipment or measurement method was used to objectively prove that the hip joint was in the neutral position. The tissue hardness measurements in this study were evaluated from above the skin, so it is unclear whether the obtained tissue hardness data reflect changes in subcutaneous tissues, tendon membranes, or muscles. There were no significant differences in all PRE variables among the five conditions, but PPT and tissue hardness measurements were not so highly reliable. Therefore, it is necessary to clarify the data by shear wave elastography in future studies. |