Effects of static stretching programs performed at different volume-equated weekly frequencies on passive properties of muscle–tendon unit

Abstract

Whether static stretching (SS) frequency has an effect on increasing the range of motion (ROM) and decreasing muscle stiffness remains unclear. Therefore, this study aimed to investigate the effects of two 6-week SS programs performed with different frequencies but generally the same duration of stretching on the passive properties of the medial gastrocnemius muscle–tendon unit. The study participants comprised 24 male volunteers randomly assigned to either the one-time/week group or the three-times/week group, performing 6 min of SS once per week and 2 min of SS thrice per week, respectively. The dorsiflexion ROM (DF ROM) and muscle stiffness of the medial gastrocnemius during passive ankle dorsiflexion were assessed using a dynamometer and ultrasonography before and after 6 weeks of SS programs. The results show that the DF ROM was increased and muscle stiffness was decreased significantly in the three-times/week group (P < 0.01 and P < 0.01, respectively), whereas no significant changes were observed in DF ROM and muscle stiffness in the one-time per week group (P = 0.25 and P = 0.32, respectively). These results suggest that a high-frequency SS program is more effective than a low-frequency SS program in increasing ROM and decreasing muscle stiffness.

Introduction

Static stretching (SS) is widely used to increase the range of motion (ROM) and/or decrease muscle stiffness in both clinical and athletic settings. Previous studies have reported that a long-term SS program increased ROM (Medeiros and Martini, 2018, Thomas et al., 2018) and decreased muscle stiffness (Ichihashi et al., 2016, Nakamura et al., 2012, Nakamura et al., 2017), although some studies have reported no changes in muscle stiffness after a long-term SS program (Freitas et al., 2018, Konrad and Tilp, 2014). Furthermore, other studies have pointed out that a poor ROM (Backman and Danielson, 2011, Witvrouw et al., 2001) and increased muscle stiffness (Pickering Rodriguez et al., 2017, Watsford et al., 2010) may lead to sports injuries. Moreover, an SS program was effective in preventing muscle strain (McHugh and Cosgrave, 2010). Therefore, a regular SS training program is considered as an effective method for the prevention of sports injuries and their recurrence.

Factors determining the effects of an SS program on ROM are the duration, intensity, and frequency of SS. For example, one study has reported that SS three to five times per week is more effective in improving ROM than that once per week among participants with limited hamstring extensibility (Marques et al., 2009). Moreover, a recent review article on the effects of an SS program on ROM has concluded that a weekly frequency is positively associated with ROM (Thomas et al., 2018). However, interestingly, this review article focused on the total stretching duration per week (i.e., stretching duration * set * frequency) and concluded that the total stretching duration per week is an important factor for increasing ROM (Thomas et al., 2018). Therefore, we assumed that an increase in ROM by an SS program is affected by increasing the frequency of stretching because the total stretching duration per week is increased due to an increased frequency. Whether stretching frequency is an important factor for increase in ROM should be clarified by comparing the effects of SS at different frequencies under an equivalent total stretching duration.

The effects of an SS program on muscle stiffness have been previously investigated, and it has been reported that the stiffness of the medial gastrocnemius (MG) and hamstring muscles was decreased after a 4-week SS program (Ichihashi et al., 2016, Nakamura et al., 2012). The effects of an SS program comprising 2 min of stretching thrice per week on the stiffness of MG have been investigated, demonstrating that an SS program duration of more than 3 weeks is needed to decrease muscle stiffness (Nakamura et al., 2017). Considering the findings reported by Nakamura et al. (2017), an SS of at least 6 min per week could decrease muscle stiffness. Based on the previous results, we assumed that the importance of stretching frequency could be clarified by comparing the effects of SS programs at different frequencies under the 6-min duration per week to investigate the effects of frequency in decreasing muscle stiffness.

As described above, increased stretching frequency is an important factor for increasing the ROM and decreasing muscle stiffness. However, because the total stretching duration has been increased due to increased stretching frequency, increased ROM and decreased muscle stiffness could be involved with not only increased stretching frequency but also increased total stretching duration. Although some studies have investigated the effect of SS programs using different stretching frequencies and total stretching duration (Marques et al., 2009, Thomas et al., 2018), no study has investigated the effects of SS programs at different frequencies under an equivalent total stretching duration on ROM and muscle stiffness. Therefore, this study aimed to compare the effects of two 6-week SS programs with different frequencies but the same total stretching duration on the passive properties of MG (i.e., ROM and muscle stiffness). In this study, we hypothesized that stretching performed with a high frequency is more effective in improving the DF ROM and decreasing gastrocnemius muscle stiffness than stretching performed with a lower frequency. The knowledge about whether stretching frequency can affect an increase in ROM or decrease in muscle stiffness would provide important information for a coach or rehabilitation staff when prescribing SS programs.