Authors: David G Behm1, Andreas Konrad2 and Konstantin Warneke3
Institutions: 1 School of Human Kinetic and Recreation, Memorial University of Newfoundland, St. John’s, Newfoundland and Labrador, Canada, A1C 5S7. 2 Institute of Human Movement Science, Sport and Health, Graz University, Graz, Austria. 3 Institute of Sport Science, University of Klagenfurt, Universitätsstraße 65-67 9020 Klagenfurt, Austria
Corresponding Author: David G Behm, School of Human Kinetic and Recreation, Memorial University of Newfoundland, St. John’s, Newfoundland and Labrador, Canada, A1C 5S7. 709-864-3408
Running Title: Resistance and Stretch Training Similarities
It has been well known for millennia (e.g., legendary Milo of Croton progressing from lifting a calf everyday as it grew to a mature bull) that resistance training can increase muscle strength. It has also been known for millennia that stretch training can improve flexibility (ancient Asian yoga and martial art exercise training) (5). Training used to be relatively black and white or polarized with stretching to improve flexibility, resistance training to increase strength, prolonged running, cycling, swimming, or other similar moderate intensity, continuous activities for aerobic conditioning. However recent developments have clouded and confused the purity of the traditional training routines.
Improvements in aerobic capacity can now be achieved with minimal time commitments and interval rather than continuous training. High intensity interval training (HIIT) has demonstrated that interval training of even just 4-6 repetitions of 30-seconds will enhance aerobic capacity (14, 15). Resistance training can not only increase strength, power and hypertrophy but can also enhance range of motion (ROM) to a similar extent as static stretching (2). Other interventions such as foam rolling (21) and instrument assisted soft tissue mobilization (16) can also improve ROM. Stretch training can not only improve ROM but recent reviews (3) and original research (35-38, 41) have reported that stretch training can also increase muscle strength and hypertrophy. Hence, appropriate exercise prescriptions for promoting strength and flexibility may be intertwined to a greater degree than prescribed in the past. The objective of this review is to highlight the complementary aspects of stretching and resistance training for flexibility and strength enhancement while also commenting on other techniques such as foam rolling.
There has been a paradigm shift away from static stretching as a component of a warm-up, as acute bouts (single sessions) of prolonged (>60-seconds per muscle group) static stretching without the inclusion of dynamic activities within the pre-activity warm-up have been reported to impair subsequent strength, power, balance, and reaction time (8, 9, 12, 17). However, it is ubiquitously known that acute and chronic (training) static stretching can increase ROM (5, 8-10). A recent meta-analysis quantified the magnitude of stretch training adaptations. Konrad et al. (18) analyzed 77 studies demonstrating that stretch training provided a moderate magnitude ROM improvement. Their review also demonstrated that proprioceptive neuromuscular facilitation (PNF) and static stretching produced almost twofold greater ROM gains than ballistic or dynamic stretch training. Furthermore, females showed higher ROM gains than males. Somewhat surprisingly, neither stretch volume, intensity, nor frequency were significant training moderators for increasing ROM.
A single bout of static stretching not only improves ROM of the stretched muscle or joint but can induce moderate magnitude enhancements in the ROM of other non-stretched muscles especially with stretch durations exceeding 240-seconds (6). While chronic static stretch training has shown to be effective to increase ROM of the contralateral homologous (same) muscle (26, 28), unilateral stretch training has not increased non-stretched heterologous (different) muscles ROM (i.e., stretching upper limbs and testing lower limbs ROM) (23).
Foam rolling can also improve ROM. Another recent meta-analysis by Konrad et al. (21) reported a moderate magnitude ROM increase with foam rolling training with greater improvements with > 4 weeks of foam roll training. They reported no significant difference in ROM improvements between chronic rolling and stretching. In addition, foam rolling increased joint ROM when applied to hamstrings and quadriceps, but no improvements in ankle dorsiflexion with foam rolling on the triceps surae.
Acute increases have also been reported to be similar to stretching as meta-analyses by both Konrad et al. (20) and Wilke et al. (43) revealed no significant ROM differences between a single acute session of stretching and foam rolling exercise. However, the Konrad meta-analysis only found three effect sizes to be eligible and hence, caution has to be taken not to overemphasize their results. If the objective is to acutely or chronically enhance ROM, current evidence shows that either foam rolling or stretching can be considered as similarly effective.
In fact, just pretending to roll (dynamic activity) can increase ROM. Warneke et al. (34) had individuals foam roll their hamstrings for four repetitions of 45-seconds each or place their thighs on a padded roller and move back and forth with the same motion as with foam rolling (no pressure on the tissue was applied by the foam roller = sham rolling). Both interventions provided similar, significant, moderate to large magnitude increases in passive hamstrings and ankle dorsiflexion ROM. Thus, dynamic warm up effects (sham foam rolling) were as effective as foam rolling suggesting there were no additive effects of foam rolling and sham rolling movements for ROM increases.
While it has been commonly prescribed and is a common ritual for many individuals to stretch prior to resistance training, recent commentaries (27) and meta-analyses (1, 2) suggest this habit may not be necessary for enhancing ROM. An initial meta-analysis of 11 studies by Afonso et al. (1) revealed no significant ROM differences between strength and stretch training. More recently, Alizadeh et al. (2) analysed 55 studies and reported a moderate magnitude ROM improvement with resistance training (free weights, machines, Pilates) but no significant increase with body weight calisthenic type resistance training. There was also no additive effect of combining resistance training and stretch training as there was no significantly greater improvement than with stretching alone. Whilst there was no sex, age, training duration, contraction type, or training frequency differences, they did find that “untrained and sedentary” individuals had significantly higher magnitude ROM increases than “trained or active people”. They concluded that since resistance training with external loads was a similar action as dynamic stretching, stretching before or after resistance training may not be essential to improve further improve flexibility.
Mechanisms for the increase in flexibility
One of the major mechanisms purported to improve ROM with the aforementioned interventions is an increase in pain or stretch threshold (5, 25). This hypothesis is especially strengthened by the observation of non-local ROM improvements as these non-stretched muscles have not been subjected to any physical stressors that would induce morphological changes (6, 7). However, as static stretching and resistance training would induce longitudinal stresses on the musculotendinous tissue, morphological adaptations such as reductions in passive and active muscle and tendon stiffness, viscoelastic properties, alterations in fascicle length, pennation angle and rotation may contribute (5, 11, 29, 32) to improved ROM.
In summary, while stretching is steeped in the tradition of increasing ROM, recently other activities (i.e., resistance training, foam rolling, instrument assisted soft tissue mobilization, dynamic movement, among others) have also proven to be effective or similar to static stretching for enhancing flexibility. Hence, static stretching may not always be a pre-requisite activity for improving ROM if resistance training and other activities are consistently performed.
In a similar vein, while resistance training has been the bastion for inducing strength improvements, it may not be the only training avenue. A meta-analysis of 41 studies by Arntz et al. (3) reported trivial-to-small positive effects of chronic static stretching on muscle strength and power. There were greater benefits apparent with sedentary versus recreationally active participants, in study samples with higher proportions of females, and older participants. One might counter that just the action of standing on one leg, for example, and maintaining balance for a quadriceps stretch would have led to the increase in muscle strength in the more inactive population as most studies performed the exercise while standing. Greater benefits were also reported with a higher number of repetitions per stretching exercise and session, suggesting a potential dose response relationship pointing towards longer time under tension to be more effective.
Accordingly, while 5 minutes of daily stretching for four muscle groups showed only small, significant strength increases of up to 5% (44), 10 minutes of daily plantar flexors static stretching for 6 weeks induced trivial to small but significant increases in isometric strength and squat jump height, but no change in countermovement and drop jump performance, thus counteracting inactivity-related performance decreases in young and healthy participants (37). Similarly, an upper body static stretching program of the pectorals for 15 minutes per day for 8 weeks produced large magnitude bench press strength increases (42). Additionally enhancing the stretching time up to 2 hours daily maximized the effects (36). A series of chronic stretching studies had active individuals use an orthotic device while seated, to stretch the plantar flexors daily for one hour for 6 weeks (38, 41) finding moderate to large magnitude improvements in muscle strength (up to 22.9%) and hypertrophy (up to 16%).
Interestingly, the authors were not able to detect a significant difference between 1 hour of daily stretching and the commonly used resistance training regimen of 3 times per week with 5×12 repetitions for six weeks, suggesting very high volume stretching to be a sufficient alternative if willing to perform 7 hours of stretching per week (40). Hence, individuals who abstain from exercise due to the intensity and discomfort of resistance training may be able to augment their strength and hypertrophy with passive static stretching. Furthermore, injured individuals who cannot sustain higher intensity contractions may benefit from these stretch-induced adaptations. However, resistance training is more efficient as more muscle groups can be trained in a shorter time interval.
Whereas, acute prolonged static stretching without additional dynamic activities may impair subsequent performance, foam rolling does not seem to acutely impair performance. A meta-analysis by Konrad et al. (22) revealed non-significant trivial (positive) effects on subsequent performance, which did not differ from the effects of static or dynamic stretching. Subgroup analyses did show more favourable results after specifically foam rolling the quadriceps, when foam rolling was applied for longer than 60 seconds, or when the foam rolling included vibration. With chronic foam rolling training, Konrad et al. (19) reported no significant performance changes with foam rolling training that exceeded two weeks. Their subgroup analyses did not find any changes with age groups, weeks of training, or total volume of foam rolling.
Foam rolling can be used as an activity to train or prepare (warm-up component) the core (trunk) muscles for activity. If foam rolling is applied to the hamstrings, quadriceps, or calf muscles, a position related to a plank (rolling the quadriceps) or reverse (supine) plank (rolling the hamstrings and calves) position must be taken. A previous study showed that the muscle activity (i.e., electromyography) of the core muscles with a plank or reverse plank position is similar to that with quadriceps and hamstrings foam rolling, respectively (45).
Mechanisms for performance changes
While the underlying mechanisms for improving muscle strength with moderate to high intensity resistance training are often attributed to neural (e.g., increased motor unit recruitment, rate coding, synchronization, improved motor learning or control, decreased motor unit thresholds) and morphological (e.g., muscle hypertrophy) (4, 13, 24, 30, 31) adaptations, it may be difficult to comprehend how passive static stretching could also induce improvements in muscle strength and hypertrophy. Whereas there are very few stretching studies examining muscle strength and hypertrophy mechanisms in humans, based on the related literature (33), Warneke et al. (39) postulated that the stretch-induced mechanical tension played the most significant role. This prolonged passive tension could produce some degree of muscle damage stimulating anabolic signaling to initiate an increase in muscle protein synthesis.
The recent literature demonstrates there are a diverse variety of ways to improve flexibility (e.g., resistance training, foam rolling, instrument assisted soft tissue mobilization and others) and thus there is not a need for a sole reliance on static stretching to the exclusivity of other activities to achieve greater flexibility. Similarly, while resistance training may be an effective and efficient means of promoting muscle strength and hypertrophy, there may be opportunities when chronic static stretching can play a role.
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