Flexibility training has been shown to improve joint ROM (Behm, 2018; Behm et al., 2016a; Behm and Chaouachi, 2011), muscle performance (Behm, 2018; Medeiros and Lima, 2017), and enhance rehabilitation progress for injured or mobility impaired individuals (Andrews, 2012). Static stretch (SS) training is often recommended for athletes, fitness enthusiasts and recreationally active individuals to decrease musculotendinous injuries, especially with high velocity, explosive and change of direction activities (Behm, 2018; Behm et al., 2016a), while physiotherapists often implement SS for their patients (Decoster, 2009).
While these benefits have been shown with SS training programs that typically involve SS 3-5 days per week for 3-10 weeks (Behm, 2018; Medeiros and Lima, 2017), there are few studies that have employed daily SS. Mahieu et al. (2007; 2009) employed daily stretching for 6-weeks and found 11.5%-20.8% ROM increases with the plantar flexors and dorsiflexors. Few studies have instituted daily, multiple stretch training sessions. Rancour et al. (2009) reported enhanced hip ROM following 4-weeks of twice daily hamstrings SS of 2-minutes duration per leg. Levenez et al. (2013) had participants SS thrice daily for 30 seconds each for 5-weeks and found an improvement in drop jump height. Both Blazevich et al. (2014) and Kubo et al. (2002) did not find any significant change in plantar flexors peak torque following twice daily stretch training for 3-weeks with 4 x 30-s and 5 x 45-s stretch durations respectively. In contrast, Abdel-aziem and Mohamed (2012) incorporated twice daily SS training, 5 days per week for 6-weeks and reported increased plantar flexors eccentric and concentric peak torques. Thomas and colleagues (2018) in their review suggested a time dependency to stretching benefits with greater total durations providing greater ROM improvements. They recommended a minimum duration of 5-minutes of stretching per week performed 5 days per week. None of these studies directly compared single versus multiple daily SS training sessions. It is unknown whether multiple SS training sessions induce greater improvements in ROM, strength and power than single sessions.
Acute stretching sessions have demonstrated nonlocal or global effects of stretching. For example, unilateral hamstrings SS increased the contralateral hamstrings (hip flexion) ROM (Chaouachi et al., 2017). Hip adductors SS or dynamic stretching improved shoulder horizontal abduction ROM whereas shoulder flexion stretches increased hip flexion ROM (Behm et al., 2016b). Wicke et al. (2014) reported significant hips, back and shoulders ROM improvements following hamstrings SS. While many studies have reported SS-induced impairments of the stretched muscle groups (Behm, 2018; Behm et al., 2016a; Behm and Chaouachi, 2011), there is conflicting research regarding the effect of unilateral SS on non-local muscles. There are reports of no significant global (non-local) SS-induced contralateral balance impairments after plantar flexors SS (Lima et al., 2014), no significant deficits with contralateral homologous (quadriceps) isokinetic torque or power (Chaouachi et al., 2017), and no decrements in knee extension isometric maximum voluntary contraction (MVC) force output following contralateral quadriceps and hamstrings SS (Behm, 2019). In contrast, Marchetti et al. (2014) illustrated increased lower body propulsion duration (performance detriment), while da Silva et al. (2015) showed decreased force after shoulder SS and impaired contralateral jump height and impulse after unilateral plantar flexor SS. There are no studies examining global or non-local SS training (chronic rather than acute) effects. Furthermore, the conflicting findings regarding global SS-induced strength impairments suggest that continued investigations are necessary. Such crossover investigations provide insight into the neural versus mechanical mechanisms underlying SS-induced changes in ROM or muscle performance (i.e. strength and power).
Hence, the objective of this study was to investigate the effects of a once versus twice daily, 2-week quadriceps and hamstrings SS program on passive static, active dynamic and ballistic ROM as well as knee extension and flexion MVC and drop jump parameters. A second objective was to examine the effect of such a SS training program on potential crossover effects on the same flexibility and muscle performance measurements.
An "a priori" statistical power analysis (software package, G * Power 22.214.171.124) was conducted based on related studies (Behm et al., 2016b; Behm, 2019; Chaouachi et al., 2017) to achieve an alpha of 0.05 and a power of 0.8. The analysis indicated that between 8-10 participants per group were sufficient to achieve adequate statistical power. The current study consisted of male (n = 12) and female (n = 18) healthy, recreationally active (non-competitive activities on average 2-3 times per week for fitness and health), participants between the ages of 20 and 47 (22.47 [+ or -] 5.1). There were no significant anthropometric or activity level differences between the three groups (Table 1). Participants were recruited using a snowball sampling technique and using controlled randomization were divided into three groups: two SS trained groups (once and twice daily) and a control group, with 10 participants in each group (4 males per group and 6 females per group). Exclusion criteria included pre-existing health conditions, such as musculoskeletal injuries in the past six months or unable to properly perform the stretches involved in the study. Prior to testing, all participants were provided with information regarding the nature of the study, the requirements upon participation in each data collecting session. This study received ethical approval from the institution's Interdisciplinary Committee on Ethics in Human Research (ICEHR: 20190627-HK).
To assess the effect of daily SS frequency over a 2-week training period on hip flexion passive static, active and ballistic ROM, knee extension and flexion MVC force and drop jump performance in both the stretched and un-stretched, contralateral limbs, participants were randomly allocated into a once daily (1 x day), twice daily (2 x day) training, or a control group (Figure 1). Half of the participants in each experimental group were assigned to SS their right hamstrings and quadriceps, while the remaining participants SS the contralateral leg. Participants attended pre-and post-test sessions lasting approximately 30 minutes each. Two pre-tests with 15 minutes recovery were conducted to assess the reliability of the measures. Data recorded during the second pre-test was used for analysis to mitigate the effects of multiple testing sessions on ROM (Behm, et al. 2016a). Pre- and post-tests were separated by a 2-week training or 2-week control (no stretching) period. Prior to commencing the testing protocols each participant was required to provide signed consent, indicate their activity level, record age and anthropometric characteristics, and undergo a 5-minute warm up on an Ergomedic cycle ergometer (Monark Ltd, Sweden) at 70 revolutions per minute and one kilopascal (kPa). Test measures included passive static, active and ballistic hip flexion ROM, unilateral knee extension and flexion MVC force, and a unilateral drop jump test. The order of MVC and drop jump tests were randomized, with ROM measures tested last to avoid any strength or power impairments that might possibly be induced by the passive SS test. The order of the passive static, active and ballistic ROM were randomized.
Stretch training intervention
An extensive familiarization session was conducted prior to testing to ensure the participants were well informed of the testing and stretch training procedures and stretch training intensity. The 2-week training intervention undergone by the stretch training groups consisted of hamstrings and quadriceps SS held for 30-seconds and repeated 3-times per session with 15-seconds rest between repetitions...