Stretching activities before exercise are believed to prepare the musculo-skeletal system for physical activity and sport events by improving joint range of motion, thus promoting improved performance and reducing the relative risk of injury. 1 Consequently, athletes, coaches and sport practitioners regularly include stretching exercises in both training programs and in pre-event warm-up activities. 2

However, recent evidence has questioned the traditional hypothesis that supported the practice of pre-exercise stretching as a measure to increase sport performance. 3,4 In this sense, it has been shown that a bout of static stretching may temporarily reduce strength performance, in relation to force and power production, when it is performed prior to events. 3,4 It was shown that pre-exercise static stretching might acutely compromise a muscle's ability to produce strength either isometrically 5,6 or isokinetically 7-11 (mainly under concentric actions) for the knee joint measured throughout a single-joint isokinetic testing protocol. Explanations for this so-called stretching-induced strength deficit include: (a) alterations in the mechanical components of skeletal muscle contraction 7,8 ; (b) decreases in muscle activation 5,12,13 ; or (c) a combination of both mechanical and neural factors. 7 In contrast, some evidence exists indicating that dynamic stretching exercises may induce improvement in isometric and isokinetic strength and power performance. 11,14,15 Although the exact mechanisms by which dynamic stretching may improve strength performance are not well known, previous studies have suggested that a dynamic stretching exercise might exert positive effects on muscular performance by an elevation of muscular temperature, 16 or post-activation potentiation 14,15 caused by voluntary contractions of the antagonist of the target muscle.

These effects have implications for athletes involved in activities that require maximal strength and power production, such as rugby and football, and have led some researchers to recommend that pre-exercise static stretching should be omitted or replaced by dynamic stretching during warm-ups prior to strenuous exercise and/or sport events. However, when the body of literature regarding the acute effects of pre-exercise stretching on strength and power production is carefully scrutinized, some important limitations are noted, which may question the applicability of the last recommendation in the physical training context. For instance, most of the studies that have investigated the acute effects of static stretching on strength and power have designed protocols which use overall stretch durations on a single muscle group (quadriceps, gastrocnemius and hamstrings mainly), ranging from 90 s to 60 min. 5,12-17 These single muscle group and long stretching protocols are not representative of typical warm-ups used by athletes and recreationally active people to prepare for exercise or competition. 18 Furthermore, very few studies have carried out direct comparisons between stretching protocols with consistent stretch doses (overall and single stretching duration) and different stretch techniques (i.e. static vs dynamic stretching) on concentric and/or eccentric maximal isokinetic strength and power output to elucidate the optimal pre-participation protocol for sport activities. 11,19 Additionally, although there are studies indicating improved muscle strength performance following dynamic stretching, 14,15 it is not known how dynamic stretching affects strength and power during isokinetic quadriceps and hamstring actions in concentric and eccentric modes.

Therefore, the main purpose of the current study was to examine and compare the acute effects of short duration static and dynamic lower-limb stretching routines with consistent stretching parameters (duration, intensity, number of exercises, repetitions) on the knee flexor and extensor peak torque and mean power during maximal concentric and eccentric muscle actions in recreationally athletes.

Tables 1 and 2 present the mean and standard deviation for peak torque and power in each experimental session (k = 3) for knee extension and knee flexion in both concentric and eccentric muscle actions respectively.

As presented in Tables 3 and 4, there were no a clear main effects (p > 0.05; trivial effect with a probability of 75–95%; d < 0.4) on concentric and eccentric knee flexion and extension peak torque and power between paired treatments. However, there were possible positive effects (d > 0.15; positive effect with a probability of 75–95%;) of dynamic stretching on some peak torque and power variables (see Tables 3 and 4) when they were compared with the static stretching treatment.

The primary findings of the present study indicate that short and contextualized lower limb static and dynamic stretching routines have no a stretching-induce strength and power deficit or improvement effects on concentric and eccentric knee flexion and extension isokinetic movements at three different speeds (60, 180 and 240 °/s) in recreationally athletes.

Our findings are not consistent with several recent studies, 11,12,17-27 although not all, 23,28,29 that has indicated that a bout of static stretching may cause transient decreases in isolated muscle strength. A possible explanation for these conflicting results could be attributed to the different static stretch durations used in these studies. Generally, in those studies that have reported static stretching-induced strength and power deficits, a single muscle group was statically stretched for between 90 s and 60 min. 5,12-17 Contrarily, our study in conjunction with some studies that have shown no static stretching-induced strength and power deficits have used a lower overall stretch duration ranging from 30 to 90 s. 23,28,29 Therefore, it would appear that there is a dose-dependent threshold of static stretching necessary to reflect any statistically detectable change in isokinetic strength and power. This hypothesis has been recently confirmed by some studies which have examined and carried out direct comparisons between the acute effects of stretching routines with different overall stretch doses and consistent stretching parameters (technique intensity, exercise positions and muscle stretched). 23,28,30–32 For example, Zakas et al. 23 after examining the effects of two different overall durations (45 s and 300 s) of acute static stretching on isokinetic peak torque production in pubescent soccer players reported that stretching caused a significant decrease in strength performance (5–12%) when the stretch duration was 300 s, while a stretch duration per isolate muscle of 45 s did not alter the mechanism of force production. In addition, Murphy et al. 30 found that a bout of 6 × 6 s of static stretching for the hamstring was enough to improve hip flexion ROM for 30 minutes without cause impairments on jump height and reaction time. Consequently, overall static stretch duration per isolate muscle group ≤60–90 s may have no stretching-induced alterations in strength and power during concentric and eccentric isokinetic muscle actions.

The results of the current study also suggest that there were no significant differences in isokinetic strength and power performance after dynamic stretching compared with control condition. These findings are not consistent with previous studies that have reported increased strength and/or power after a bout of dynamic stretching. 11,33,34 A possible explanation for the discrepancy between the results of the current study, that showed no dynamic stretching-induced improvements on isokinetic strength and power; in contrast with the results reported by previous studies may be due to the different stretch duration used. For example, Sekir et al. 11 designed a dynamic stretching protocol with an overall stretch duration per muscle (quadriceps and hamstrings) of 60 s (4 × 15 dynamic movements) and Manoel et al. 34 carried out 3 repetitions of 30 s dynamic stretches, while the current study stretched the major muscle groups of the lower limb (psoas, quadriceps, hamstrings, gluteus and adductors) using a overall stretch duration of 30 s per muscle group (2 × 15 dynamic movements). Perhaps, as occur with static stretching, the dynamic stretching-induced enhancement of muscular performance phenomenon may be governed by a dose–response relationship, where the shorter volumes (<30 s) do not affect muscle performance and longer duration may facilitate performance (>60–90 s). 4 However, future studies are necessary to test this hypothesis.

Another important issue regarding the pre-exercise stretching routine design is the stretch technique used. When static and dynamic stretching treatments were compared, the results of the current study showed that dynamic stretching reported slightly higher scores than static stretching (d > 0.15; percentage change ranged form 1.2 to 14.7) for most of the strength and power variables. Therefore, this finding supports the recent claims that suggest that dynamic stretching is preferable to static stretching as part of a warm-up designed to prepare for physical activity due to the possible enhancement of muscular performance 11,33–37 ; and the similar acute increases in static flexibility as static stretching. 38,39

Another important clinical question is whether the effects of stretching of knee flexor and extensor muscle groups, which are closely related to the actual demands of sport on strength performance, elicit a similar response, in order to make evidence-based recommendations. The results of the current study and the findings reported by Sekir et al. 11 have demonstrated that knee flexor and extensor muscles respond in the same way to static and dynamic stretching.

Two different methodological aspects of the current study should be highlighted because they might make the results more valid than previous studies. The first aspect is the design of the stretching protocol used. The current study used a multiple-muscle stretching protocol (in which participants stretched the major lower-limb muscles) instead of the widely used single-muscle protocol (in which participants stretched only the muscle studied). 15-27 The rationale for using a multiple-muscle stretching protocol was because an acute bout of static stretching may reduce muscle activation via peripheral (autogenic inhibition of the Golgi tendon reflex, mechanoreceptor and nociceptor afferent inhibition) and central nervous system (supraspinal fatigue) mechanisms. 5,8,12 In this sense, Avela et al. 5 and Cramer et al. 8 found that an acute bout of static stretching caused a decrease in muscle activation not only in the stretched muscle but also in the un-stretched contralateral muscle (via central nervous system mechanism). However, the degree of contribution of each mechanism (peripheral and central) on the reduction in muscle activation is still unclear. Therefore, effects of stretching before exercise and sport events should be investigated using multiple-muscle stretching protocols that reflect the stretching stimuli that athletes and recreationally active people usually apply both to the peripheral and central nervous system during a typical warm-up to make evidence-based recommendations. The second methodological aspect that should be underlined is related to the isokinetic testing position used. Studies that have investigated the effects of stretching on isokinetic strength, with the goal of making recommendations to design warm-up protocols that allow athletes improving performance and reducing the risk of lower limb musculoskeletal injury, have typically reported data obtained from participants tested in a seated position. However, rarely are field and court sport athletes active with those kinematics (e.g., the hip flexed at 80–110°). 40,41 Most lower limb injuries occur while athletes engage in some running activity where the hip angle is reported to typically be approximately 10–20° to the vertical with foot plant occurring directly inferior to the torso and not with a hip flexion angle of 80–110°. 41 Thus, it could be argued that isokinetic screening where the hip angle is more similar to when executing real-world sporting tasks would be more ecologically valid than using other traditional methods. 22,40 Based on the last statement, the current study selected a prone position with hip flexed 10–20°, which replicates the hip position and knee flexor and extensor muscle length-tension relationships that occur during running/sprinting. 22,40 Although the standing position appears to be the most ecologic valid testing position, it was not used because of technical issues (the bench of the dynamometer could not adapted to this position). However, it is possible that if the same hip flexion is used in both standing and prone positions, the stretch-tension relationship of the knee flexors and extensors will not likely differ and the relative contribution of the active contractile components of the muscles to overall force production would not change. Future studies are necessary to test this hypothesis.

Although the current study is the first that has designed and examined the acute effects of a short and sport contextualized static and dynamic pre-exercise lower limb stretching routine with consistent stretch parameters on several isokinetic concentric and eccentric strength parameters (peak torque and power) in a large sample size of recreationally athletes, some limitations should be noted. The first limitation is that this study did not directly evaluate changes in the range of motion or changes in resistance and tolerance to stretch due to the experimental stretching treatments. Therefore it is not known whether the stretching interventions were actually effective in increasing flexibility or in decreasing muscle stiffness, although previous studies from our laboratory that have used identical stretching doses have reported increases in flexibility. 38 Another potential limitation of the current study is the population used. Although this investigation used 49 participants, much higher than previous studies, the participants were homogenous based on age and physical status, which could limit the external validity of the results.

Therefore, the results of the present study indicate that short pre-exercise static and dynamic lower-limb stretching routines do not elicit stretching-induce deficits or improvements on knee flexor and knee extensor isokinetic concentric and eccentric strength and power. However, there is some evidence from our findings, in conjunction with similar previous studies, that dynamic stretching is preferable to static stretching as part of a warm-up designed to prepare for physical activity due to the possible enhancement of muscular performance.

The authors have no conflicts of interest to declare.