Authors: Shahab Alizadeh1, Abdolhamid Daneshjoo2, Ali Zahiri1, Saman Hadjizadeh Anvar1, Reza Goudini1, Jared P Hicks1, Andreas Konrad1,3,4, David George Behm1
Institutions: 1School of Human Kinetics and Recreation, Memorial University of Newfoundland, St. John’s Newfoundland and Labrador, Canada
2 Department of Sport Injuries, Physical Education and Sport Sciences Faculty, Shahid Bahonar University, Kerman, Iran.
3 Institute of Human Movement Science, Sport and Health, Graz University, Graz, Austria
4 Associate Professorship of Biomechanics in Sports, Technical University of Munich, Munich, Germany
Stretching has been considered an essential component of warm-ups, fitness, and health . However, over the last 25 years it has been often reported that static stretching used as part of warm-up can result in performance (e.g., strength, power, speed, balance) deficits [1-5]. However, a number of reviews have shown that if static stretching of less 60-s per muscle group is employed into a warm-up involving dynamic activities, performance impairments should be trivial [1-5]. It is universally agreed that acute and chronic (training) stretching increases joint range of motion (ROM) in healthy populations [1, 6-8]. However, recent research has demonstrated that stretch training is not the only way to improve ROM.
Recent commentaries [9, 10] suggest that stretch training benefits such as improvements in flexibility, balance, cardiovascular measures, decreases in pain, decreased injury incidence, may be also be induced by other types of training exercises (e.g., resistance training). Nuzzo  suggested in a commentary that resistance training produced similar ROM increases as stretching. A recent meta-analysis  examined 11 studies and reported no statistically significant difference between stretch and resistance training, but a small magnitude effect size in favour of stretching. There are several reports of substantial ROM increases following eccentric [12-14] and traditional (both concentric and eccentric contractions) resistance training with increases in ROM [15-21]. However, research would be valuable to determine whether increases in ROM with stretching and resistance training are similar in magnitude and whether these ROM improvements are differentially affected by sex, trained state of the individual or the frequency and duration [22, 23] of resistance training.
Since the only published meta-analysis examined 11 studies , we conducted a meta-analysis of 55 studies (222 measures of subjects averaging 23.9±6.3 years (range: 8.1 – 78.8 years)) to provide a more comprehensive picture. A random-effect meta-analysis was conducted by using the Comprehensive Meta-Analysis (CMA) software according to the suggestions of Borenstein .
The major findings of this meta-analysis were that resistance training either with free weights, machines, or Pilates significantly improved joint ROM by a moderate magnitude. However, there was no significant ROM improvement with calisthenic (body mass) type of resistance training. However, these calisthenic results should be interpreted with caution since this analysis was based on only four measures. In addition, the extent of ROM improvements with resistance training were similar to stretch training or the combination of resistance training + stretch training vs. stretch training alone. Another interesting finding was that “untrained and sedentary” individuals showed significant and larger magnitude ROM enhancements versus “trained or active people”. There were no significant differences between sex or contraction type (e.g., concentric vs eccentric) and there were no differential effects based on age, training duration, or frequency. Although all joints showed moderate to large magnitude ROM increases with RT, there were no significant differences between the joints. Although some joints have a substantially greater ROM than others (e.g., hip flexion > dorsiflexion) , we found that the relative resistance training-induced increases were similar.
Why would resistance training be as effective as stretching to improve ROM? It is suggested that the only difference between resistance training and dynamic stretching is that both actions involve controlled movement through the active joint ROM [2, 3, 25], but resistance training adds an external load. Free weights and machines (including Pilates) resistance training allow the joints to reach their endpoint ROM or the point of maximum discomfort at a controlled pace. In contrast, calisthenics does not always permit a full ROM such as with push-ups, which are limited by chest circumference and the floor or ground. Dynamic stretching in some studies has been reported to produce similar [26, 27] as well as greater [28, 29] acute increases in ROM as static stretching. Thus, it is possible that the full or nearly full ROM used with resistance training is more important for increasing ROM than the external load.
It has also been suggested that neural adaptations may contribute to improved flexibility [1-4]. There have been reports with static stretch training (3 and 6 weeks) of reductions in Ia (excitatory) afferent feedback from muscle spindles, which could reduce reflex-induced contractions inducing a more relaxed muscle (disfacilitation) [30, 31]. However, dynamic stretching and resistance training would tend to excite rather than disfacilitate muscle spindle activity and thus would be an unlikely chronic training-induced mechanism for increased ROM. Golgi tendon organ (GTO) inhibition is more likely to occur with large amplitude stretches  and higher muscle tension, however GTO inhibition tends to subside almost immediately (60-100 ms post-stretching) after the stimulus discontinues , so it is also an unlikely candidate for chronic dynamic stretching or resistance training mechanisms. Recurrent or Renshaw cell inhibition is more prevalent with acute dynamic rather than static contractions  and thus can help stabilize motoneuron discharge variability, and motor unit synchronization . However, there is no research to confirm whether any of these possible acute neural responses lead to chronic resistance training adaptations for enhanced ROM.
Morphologically (i.e., muscle and tendon structure), there is some evidence for dynamic, ballistic stretch training to decrease tendon stiffness . There are also reports of acute dynamic stretch-induced decreases in passive resistive torque  and muscle stiffness [7, 37] suggesting a more compliant musculotendinous unit. However, a six week ballistic stretch training program did not detect any significant change in muscle morphology . Magnusson and colleagues  contend that in response to loading, tendon metabolic activity is relatively high and can undergo significant length changes allowing the tendon to adapt to changing demands (i.e., changes in force, length, compliance). Furthermore, repeated loading of the tendon with stretching can shift the stress-strain curve to promote an elevated elastic modulus .
In contrast, a review by Thomas et al.  indicated that the loading of a tendon with resistance training increases its stiffness by modifying elastic properties versus morphological adaptations such as increased cross-sectional area. This increase in tendon stiffness was not dependent on muscle contraction type, trained state, or age. The review also summarized that there are reports of both increases and decreases in muscle tissue stiffness with resistance training and thus there is a lack of clarity regarding the effects of resistance training on muscle stiffness. Hence, it is unknown whether the increases and possible increases in tendon and muscle stiffness respectively with resistance training are counterbalanced by increased compliance with dynamic stretching.
There is strong evidence that stretching will increase stretch (pain) tolerance (sensory theory) [42, 43]. The discomfort with resistance training would contribute to this increase in pain (stretch) tolerance permitting the individual to push beyond prior limits of discomfort. Hence, the mechanisms for increasing ROM with dynamic stretching with load (resistance training) would likely be related to musculotendinous unit changes in stiffness and compliance as well as augmented stretch tolerance. As there were no significant differences between sex or contraction type and there were no effects of age, training duration, or frequency, these ROM increases and mechanisms may be similar across different populations and training parameters.
“Untrained and sedentary individuals” displayed a significantly higher magnitude of ROM change compared to “trained or active people”. This difference is likely related to the baseline level of flexibility. Trained individuals would have already experienced an increased ROM due to their prior training. Hence, their scope of training-induced ROM increases would be smaller compared to previously untrained individuals [1, 44]. However, the trained individuals still experienced significant ROM improvements albeit to a lesser degree than the untrained.
In conclusion, since resistance training with external loads can improve ROM, additional stretching before or after resistance training may not be necessary to enhance flexibility. Based on the present studies and the literature, both stretching and resistance training can improve ROM, improve strength [1, 45, 46], and decrease musculotendinous injury incidence . When circumstances dictate (i.e., time restrictions), flexibility training benefits can be incorporated into resistance training workouts, however, stretch training should still be advocated as a fitness and training component. For example, resistance training would not be suitable as a component of a warm-up prior to competition and thus stretching would play an important role in certain activities. Stretching is also used as a form of relaxation for many practitioners, for which resistance training may not be as appropriate.
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