Authors: Stefano Longo, Emiliano Cè, Nicholas Toninelli, Fabio Esposito, Giuseppe Coratella
Categories: Review Article, Elongation, Flexibility, Force, Passive Lengthening, PNF
Source: Sports Medicine - Open
Authors: Stefano Longo, Emiliano Cè, Nicholas Toninelli, Fabio Esposito, Giuseppe Coratella
Muscle stretching comprises various modalities that differ in methodological aspects and provide distinct acute and long-term effects on maximal range of motion (ROMmax) and muscle strength. This narrative review aims i) describe each stretching modality in detail, including current approaches to quantifying intensity and volume; ii) examine how variations in these parameters influence acute and chronic adaptations in ROMmax and strength; and iii) provide practical guidance by highlighting each method’s advantages, limitations, and effectiveness in achieving flexibility improvements with minimal impact on strength. The reviewed modalities include passive or active static stretching, dynamic or ballistic stretching, and proprioceptive neuromuscular facilitation (PNF). Intensity is commonly measured as stretch-induced discomfort (static), movement velocity or frequency (dynamic/ballistic), or contraction effort (PNF). Volume is quantified as total time at a given %ROMmax (static), number of repetitions (dynamic/ballistic), or contraction-relaxation cycles (PNF). Passive static stretching is effective for increasing ROMmax but may transiently reduce maximal isometric strength, particularly with higher intensities and durations. Dynamic stretching improves ROMmax without affecting strength, while ballistic stretching carries a greater risk of strength impairment. PNF techniques enhance ROMmax across intensity levels, and modified protocols may reduce associated discomfort. Long-term stretching interventions improve flexibility without negatively impacting strength, supporting their strategic use in training. Static stretching may even produce small strength gains when sustained for at least eight weeks. While all modalities acutely enhance ROMmax, chronic effects vary depending on protocol design. Selecting an appropriate stretching technique should align with specific performance goals, particularly in sports requiring extreme ROMmax or when combined with resistance training. Practical considerations such as safety, control, and participant familiarity are essential for effective long-term program implementation.
Muscle stretching is widely used in both training and rehabilitation to enhance maximal joint range of motion (ROMmax) and reduce joint stiffness [1–4]. Extensive literature reports stretch-induced increases in ROMmax following both acute [2, 3] and chronic (> 2 weeks) stretch training interventions [3]. These effects stem from various neural and mechanical mechanisms [5, 6], many of which also affect acute force production in both the stretched [2, 7–9] and non-stretched, opposite muscle groups [10, 11].
Despite the considerable examination of the acute and long-term stretch-induced ROMmax changes and related mechanisms, the literature lacks a clear and comprehensive analysis of the impact of stretching practice with the aim of increasing ROMmax without negatively affecting muscle strength. First, typically three main stretching modalities have been described and reported in the i) static (active or passive); ii) dynamic and ballistic; and iii) proprioceptive neuromuscular facilitation (PNF) stretching [12]. Second, the variables characterizing the overall stretching dose that can be used to prescribe a stretching routine, i.e., intensity and volume, are delineated differently depending on the modality used. Manipulating these variables could aid in adjusting the overall stretching dose within a single session or throughout a training protocol to maximize the ROMmax gains and minimize the possible acute decrements in muscle strength.
Unlike resistance and endurance training, where the effects of the manipulation of the independent variables that quantify the training dose have been established [13–15], an exploration of how to manipulate both stretching intensity and volume has not been performed, especially considering the multiple stretching modalities. Indeed, a very recent manuscript has provided overall recommendations based on a panel of experts without analyzing the factor-by-factor effects within each stretching modality [16]. Therefore, the present review aimed i) provide a detailed description of each stretching modality and outline the methods available to quantify stretching intensity and volume; ii) investigate how variations in stretching intensity and volume affect both acute and long-term outcomes in terms of ROMmax and muscle strength, based on within-modality comparisons in healthy individuals; and iii) provide practical applications with advantages and disadvantages related to each method, and analysing the acute and chronic effectiveness of different stretching techniques in enhancing ROMmax and their potential impact on muscle strength to assist practitioners to make an informed decision in practice. Given the multitude of aspects inherent to muscle stretching, to address the research question we based our assumptions on systematic or narrative reviews and meta-analyses to summarize each single aspect. When these were not available, we have tried to extrapolate the indications from original studies that examined the factor.
We conducted a search of the electronic databases PubMed, Scopus, and Google Scholar for relevant articles up to January 31st, 2024. An additional update to identify any new publications was performed on September 15th, 2024. The keywords used “muscle stretching” OR “stretching” AND “passive” OR “active” OR “static” OR “dynamic” OR “ballistic” OR “PNF” OR “proprioceptive neuromuscular facilitation” AND “acute effects” OR “long term effects” AND “range of movement” OR “range of motion” OR “ROM” OR “strength” OR “force”. Only articles published in English were selected and their bibliography was checked for further potentially eligible studies. Additionally, only manuscripts examining the effects of muscle stretching alone and not combined with other exercises were included. Manuscripts involving young (< 18 yrs), older individuals (> 65 yrs), or populations with pathological conditions were excluded.
The methodological quality of the included reviews and meta-analyses was assessed using the AMSTAR 2 checklist (A Measurement Tool to Assess Systematic Reviews 2) [17]. Reviews were classified and scored by two reviewers (EC and GC), and if classification remained unclear, a third reviewer was included in the discussion (SL). The 16 items of the checklist were answered with a ‘yes’ equalling 1 point, ‘no’ equalling 0 points, and “partial yes” equalling 0.5 points (where the item included this answer) [17]. Reviews were classified as high (> 80% items satisfied), moderate (40 to 80% items satisfied), or low quality (< 40% items satisfied). The results of the AMSTAR 2 are shown in Table 1. Of the 29 systematic reviews and meta-analyses retrieved, 12 were classified as high, 14 as moderate and 3 as low.Table 1Overall results of the Assessing the Methodological Quality of Systematic Reviews 2 (AMSTAR 2) checklistGreen square = yes; orange square = partly; red square = no. A High rating indicates that more than 80% of the item was fulfilled, Moderate indicates 40% to 80%, and Low indicates less than 40%
This section describes each stretching modality together with the quantification of intensity and volume usually reported in the literature (Fig. 1). Lastly, practical applications can be found, indicating how each stretching modality should be incorporated into practice.Fig. 1Schematic representation of the stretching load (intensity and volume) determination for static, dynamic, ballistic, and proprioceptive neuromuscular facilitation (PNF) modalities. CR: Contraction-Relaxation Method. AC: Antagonist Contraction Method. CRAC: Contraction-Relaxation Antagonist Contraction Method
Static stretching involves rotating one or more joints to stretch the muscle–tendon unit and maintaining the final position reached for a specified duration and intensity [12]. Such joint rotation can be achieved either actively or passively. In active stretching, the movement involves a concentric contraction of the opposite muscle groups to those being stretched, allowing the joint to reach and maintain its end range of motion (ROM) [1]. For example, in active hamstring static stretching the process entails the concentric contractions of the hip flexors and knee extensors to achieve the end ROM and subsequently maintain the final position [12]. Consequently, the degree of rotation exerted on the joint predominantly depends on the antagonist muscle force capacity (particularly at short muscle length) to overcome the agonist muscle or nerve resistance to stretch [12]. Conversely, passive stretching requires an external force to rotate the joint up to the end ROM without any active effort from the subject [9]. The external force is usually provided by an operator, gravity, or a device/machine; thus, the amount of rotation imposed on the joint can vary accordingly and can be much greater than during active stretching [12]. Overall, the stretching process depends on the interplay between the external force inducing the joint rotation that surpasses the passive resistance generated by the muscle–tendon complex [18]. As a result, the ROMmax achievable during active static stretching may be lower than during passive static stretching, albeit active static stretching might be still useful to increase ROMmax [12].
Passive static stretching can be proposed as constant-angle or constant-passive resistive force modalities [19–21]. Constant-angle entails maintaining the same end ROM, resulting in a gradual decrement in perceived discomfort, while constant-passive resistive force involves a progressive change of the ending position towards a more elongated muscle while maintaining the same level of passive resistive force which might result in a maintenance of the perceived discomfort throughout the stretching process.
To quantify the static stretching dose within a single session, factors such as intensity and volume should be considered. Intensity is commonly quantified as a percentage of the maximum point of discomfort at the end ROM, adjusting the joint position accordingly [22–24] or based on a validated stretching intensity scale [25]. Particularly, point of discomfort can be defined as the moment during a passive or active muscle elongation at which the individual perceives a distinct, non-painful sensation of tightness, tension, or mild discomfort, signalling the end range of tolerable stretch before pain or potential tissue damage may occur [26]. This concept is commonly used to define the threshold for static stretching protocols, particularly those aiming to avoid nociceptive responses while still promoting flexibility gains [26]. Furthermore, the maintenance of a constant angle or a constant passive resistive force can be used, with the former implying no change in the end ROM irrespective of the discomfort and the latter a possible progressive joint rotation depending on the capacity to tolerate the stretching discomfort due to the increasing external applied force necessary to maintain the passive resisted force constant [21]. However, while the percentage of the point of discomfort perceived during stretching may serve as a practical indicator of intensity, alternative and potentially more objective parameters, such as passive resistive torque or assessments of muscle–tendon stiffness, could also be considered to quantify stretching intensity. The passive stretching volume for each exercise can be quantified as the total duration of the stretching protocols, which depends on the duration of the single stretching bout and the number of bouts. Hence, a similar volume may derive from different strategies, such as continuous or intermittent protocols [1, 27–29]. For example, a total volume of 200-s stretching exercise can be obtained by a single 1 × 200-s set or 5 × 40-s sets. It should be noted that volume quantification refers to the single exercise and not necessarily to each single muscle group, which may undergo multiple stretching exercises within a single session.
Static stretching presents both advantages and drawbacks. Passive static stretching involves an external force applied to achieve joint rotation without the subject's voluntary effort [9, 12]. This makes it particularly suitable for individuals with reduced motor control, muscle weakness, or post-injury conditions where active muscle contraction is not feasible or advisable. Moreover, the end ROM is clearly identifiable and can be easily adjusted based on the participant’s characteristics and stretch tolerance [20, 21]. On the other hand, the magnitude of joint rotation during passive static stretching is heavily influenced by the type and consistency of the external force applied (e.g., operator, gravity, mechanical devices) [12], possibly affecting the reproducibility and standardization of stretching protocols, especially in unsupervised or self-administered contexts. When performed using constant-passive resistive force techniques, passive static stretching may maintain a high level of perceived discomfort throughout the stretch [19–21]. This could limit compliance, especially in novice, elderly, or clinical populations, and may require careful modulation of stretching intensity using validated scales or subjective thresholds [4, 22, 24]. Lastly, static stretching protocols may be perceived as quite long, possibly decreasing adherence. However, static stretching can be done with a training partner, thus increasing the overall motivation to adhere to passive static stretching protocols.
Dynamic and ballistic stretching rely on the application of greater dynamic forces applied by the agonist muscle overcoming the passive forces from the antagonist, stretched muscle [2]. While often used interchangeably, they differ in technique [2]. Dynamic stretching comprises cyclic mobilizations of the soft tissues up to end ROM with controlled movement velocity through the active joint range of motion, with repeated cyclic muscle loading, i.e., tension associated with the end ROM and unloading, i.e., relaxation to the mid ROM [2, 4, 12, 30–32]. For example, to dynamically stretch the lower-limb posterior muscles, one may swing the leg forward, maintaining an extended knee position due to the action of both hip flexors and knee extensors up to the end ROM. Conversely, ballistic stretching involves cyclic mobilizations of the soft tissue characterized by a bouncing-like action at the end ROM [2, 12]. In the case of the leg swing, significantly increasing movement velocity at the beginning of the swing phase would induce a bounce-like effect on the involved limb. It should be noted that both dynamic and ballistic stretching can be performed using either open or closed kinetic chain exercises. However, while dynamic controlled movements are suitable for both types of exercises, bouncing ballistic movements are very likely performed using open kinetic chain exercises. Lastly, both dynamic and ballistic movements can be performed against gravity, with the end ROM being limited by the compliance of the stretched soft tissues, the resistance of the involved joints, and the strength generated by the muscle groups opposite to those under stretch [2].
The intensity of dynamic and ballistic stretching is typically quantified based on movement velocity or frequency, as well as the ROM achieved [2]. In dynamic stretching, the execution velocity can vary widely, allowing for controlled movement through the desired range; in contrast, ballistic stretching involves rapid acceleration, which generates momentum and produces a rebound effect [12]. In both modalities, the movement excursion can be either submaximal or maximal [12]. Alternatively, the time required to complete the full ROM may be recorded for each movement, e.g., longer time corresponding to slower movements, even though this could be good theoretically but unlikely to be feasible in practice. This leads to quantifying volume by the number of cyclic actions per set rather than by the duration of each cyclic action and the number of sets for each exercise. Additionally, it should be noted that a muscle group can undergo multiple stretching exercises, so the quantification above should be made for each exercise.
Both dynamic and ballistic stretching present advantages and disadvantages. Dynamic stretching may mimic sports performance skills and can be incorporated into specific training routines. Moreover, muscle temperature increases during dynamic actions, making them suitable for warm-up routines [2, 33, 34]. Ballistic actions may offer a higher achievable end ROM compared to dynamic-only stretching [35]. However, movement velocity may sometimes exceed the participant's ability to control the movement and the maximal extensibility of the muscle–tendon complex (particularly with ballistic movements), posing a theoretical risk of overstretching injuries [36]. Indeed, the rapid muscle elongation characteristic of ballistic stretching can elicit a strong activation of the myotatic (stretch) reflex [37]. This reflexive contraction can counteract the intended stretch, reducing its effectiveness and potentially increasing muscle tension. That said, no direct evidence is available, and practitioners should just bear this possibility in mind.
PNF involves the alternating sequence of an active contraction of the target muscles followed by a passive joint rotation in the opposite direction, performed by an external operator or with the aid of a support [38–40]. While the fundamental principle remains consistent, there are several variations of PNF, among which the most recognized in the literature are the contraction-relaxation (CR) and its variants, the antagonist contraction (AC), and the contraction-relaxation antagonist-contraction (CRAC) methods [12, 38–43]. Typically, CR involves an initial passive joint rotation phase up to the end ROM, followed by an isometric contraction of the target muscles and a post-isometric relaxation phase, after which the joint is passively rotated to a new end ROM [44]. A variant of CR includes a concentric contraction of the target muscles against resistance provided by an operator/support [40]. Alternatively, a modified CR version has been proposed, wherein the isometric contraction of the target muscle is performed in the anatomical position [45]. Regarding AC, the movement begins passive joint rotation phase up to the end ROM, followed by a contraction of the antagonist muscle while rotating the joint to a new ROM [40]. The CRAC method merges CR and AC, so it begins with a passive joint rotation phase up to the end ROM, followed by an isometric contraction phase of the target muscles (as in CR); subsequently, an isometric contraction of the antagonist muscles occurs, while further passive joint rotation is performed to a new end ROM [12, 38–43].
To quantify the intensity, the force generated during the different PNF modalities should be considered. Particularly, for CR and modified CR, the intensity should refer to the force generated during the active isometric contraction of the stretched muscle [12, 38–42]. Regarding AC, the intensity should refer to the force generated during the active isometric contraction of the antagonist muscles [40]. Regarding CRAC, the intensity should refer to both the force generated during the active isometric contraction of the stretched muscles and the active isometric contraction of the antagonist muscles [12, 38–43]. To quantify the stretching volume for a single exercise, each repetition can be manipulated through the duration of the initial passive joint rotation phase, the duration of the contraction phase and the duration of the relaxation phase(s), along with the total number of contraction-relaxation phases performed, irrespective of the PNF variations [12, 38–43]. As with the other stretching modalities, this refers to a single exercise, while multiple exercises can be performed for a given muscle group.
PNF presents both strengths and weaknesses. Its advantages include the ability to easily manipulate the intensity and its suitability for individuals with limited mobility. Disadvantages include the limited preparatory effect on dynamic skills and the requirement for an external operator/support for execution. Additionally, performing an isometric contraction at long muscle length may cause discomfort [45]. A modified version has been proposed in which the isometric contraction is performed at a shorter muscle length to alleviate discomfort [45]. Conversely, the CRAC modality entails an active contraction of the antagonist muscles at a short muscle length, which may induce muscle cramp and lead to exercise cessation [46].
While passive static stretching has been extensively studied in the literature, active static stretching has been described as a modality, but not actually largely investigated [12]. We were able to identify only one study that directly compared passive static stretching and active static stretching [47]. The study reported a similar significant increment in 20-m sprint time among a group of rugby players following a stretching protocol targeting the gluteal, hamstrings, quadriceps, adductors, hip flexors, gastrocnemii, and solei [47]. The protocols were matched for stretch duration (20 s per muscle) and stretch intensity (mild point of discomfort) [47]. However, the study did not report data on changes in ROMmax for either stretching modality [47]. Consequently, we focused on the factors that can be modulated during passive static stretching.
A review by Behm and Chaouachi [4] described the impact of the intensity on ROMmax and strength, suggesting that stretching to the point of 100% discomfort may increase ROMmax but impairs strength, jump performance, muscle activation, and balance ability. However, studies using submaximal stretching intensities i.e., below 100% of the point of discomfort, yielded conflicting results. A recent systematic review by Bryant et al. [48] analysed the impact of static stretching intensity, expressed as a percentage of discomfort on ROMmax and strength outcomes. Eighteen studies were included, with 16 examining ROMmax and four investigating muscle strength. The findings indicated that static stretching increased ROMmax, but the influence of intensity on ROMmax changes was inconclusive [48]. Some studies suggested that higher stretching intensities led to greater ROMmax increases, possibly due to reduced reflex activity [49–51]. However, methodological differences and variations in defining stretching intensity thresholds contributed to heterogeneity. Only a limited number of studies explored the effects of different stretching intensities on muscle strength, with some indicating that high-intensity (100% or above point of discomfort) may decrease strength, and others that low-intensity (about 75–80% of point of discomfort) may not affect muscle strength [22, 52]. Although further research is necessary to draw definitive conclusions and examine factors such as the muscle group and the participant backgrounds, stretching intensity may not be decisive for increasing ROMmax, provided that a minimum stimulus is given [53]. Indeed, when lower intensities have been used such as 50% of the point of discomfort, some authors found ROMmax increases [53] while others did not [54]. Therefore, we cautiously suggest increasing the minimum low-intensity threshold to 75–80% of the point of discomfort to effectively increase ROMmax acutely. Such an intensity could be warranted to both increase ROMmax and avoid impairments in strength, irrespective of the other factors examined below. Notably, the point of discomfort is a subjective assessment of the stretching intensity. Therefore, practitioners should be accustomed before starting to use it, so as to increase their ability to distinguish different intensities. An anchorage to the maximum point of discomfort is therefore needed, and we remind that this should be done for every subject involved in the stretching session.
Static stretching intensity could also be monitored by manipulating the angle at end ROM. In the case of the constant-angle approach, the joint position at the end ROM remains unchanged throughout the stretching duration, leading to a progressive reduction in the discomfort perception. Conversely, when applying the constant-force approach, the angle at the end ROM is modified throughout the stretching duration, resulting in a maintenance of the discomfort perception rather than the constant-angle condition. No review or meta-analysis was found on this topic, and few studies compared the constant-angle vs. constant-force methods on ROMmax [54–56]. Two studies found a greater stretch-induced increase in ROMmax after constant-force compared to constant-angle [54, 55], while Herda et al. [55] observed similar results in the stretch-induced increase in ROMmax. Regarding the strength outcomes, there was no clear influence of the constant-angle and constant-force on the muscle strength of the stretched muscles. Indeed, Konrad et al. [56] reported no stretch-induced changes in maximum voluntary contraction, while Herda et al. [55] found similar decreases in maximum voluntary contraction after both constant-angle and constant-force modalities. Therefore, additional studies are required to clarify these contrasting results.
Regarding the overall bout duration, studies have shown similar findings that a total duration exceeding 60 s increases ROMmax while impairing muscle strength [2, 27, 29, 57–60]. In contrast, maintaining a total duration below 60 s induces gains in ROMmax, albeit to a lower extent than durations exceeding 60 s, without affecting muscle strength [2, 27, 29, 57–60]. In this regard, a recent meta-analysis added further and more detailed information about the role of passive static stretching duration on the strength decrements in adults and revisited previous caveats inherent to the passive static stretch-induced strength loss [61]. In the first instance, the authors found that passive static stretching does not reduce overall maximal strength (effect size [ES] = -0.06) [61]. More information was also provided about the strength testing modalities, distinguishing that maximum isometric strength is overall reduced (ES = -0.21), while this is not the case for dynamic strength (ES = 0.09) and performance (ES = 0.08). Static stretching is usually performed on a single muscle group, thus affecting isometric single-joint strength rather than complex dynamic movements, in which many other muscles may play a primary role [61]. When examining further the relationship between the passive static stretching duration and the reduction in isometric strength, bout durations below 60 s do not negatively influence maximal isometric strength (ES =—0.07), while reductions are visible above 60 s (ES = -0.48) [61]. Regarding the total duration, i.e., volume, the authors found that maximal isometric strength does not decrease when total duration is below 480 s (ES = -0.14), while decreases were observed when total duration exceeds 480 s (ES =—0.46) [61]. Therefore, both the strength modality and the bout duration appear to play a role in the magnitude of the passive static stretch-induced decrements in strength, and a possible dose/response effect may be observed considering the total duration of the passive static stretching and the reductions in isometric strength.
Since the overall passive static stretching duration may result from single or multiple stretching bouts, researchers have examined possible differences between continuous and intermittent protocols. No review or meta-analysis is available in this regard, so the results from the studies retrieved should be interpreted with caution. Some authors investigated the changes in ROMmax and muscle strength following a continuous single 5-min versus 5 × 1-min intermittent protocol [62]. In this study, ROMmax increased more after the continuous protocol, while the intermittent protocol led to a greater reduction in strength [62]. In contrast, a more recent study reported no difference in ROMmax increments after a continuous (1 × 90 s) versus intermittent (3 × 30 s) protocol, while muscle strength was not assessed [63]. Another study found that ROMmax increased more after a 4 × 30-s intermittent than a 1 × 120-s continuous protocol, with no information on muscle strength [64]. Additionally, a further study examined ROMmax changes after a continuous (1 × 180 s) or intermittent (6 × 30 s) protocol in novice and stretch-accustomed participants [65]. The results indicated that ROMmax increased in both groups after the intermittent modality, whereas the continuous protocol was effective only in stretch-accustomed participants [65]. Collectively, the acute effects of continuous versus intermittent protocols on ROMmax do not allow a clear indication of one of the two, while continuous protocols are suggested to minimize strength loss.
An important point must be raised when discussing the passive static stretch-induced acute reductions in maximal isometric strength. While these have been generically described as acute effects, such impairments have been overall investigated immediately after the stretching protocols, so a more in-depth examination of the strength reduction time course may provide further information. In this regard, the relevant studies are much fewer, and reviews or meta-analyses are not available to describe clearly the role of intensity and duration, albeit some useful indications can be extrapolated. The very first studies that investigated the time course of the strength reduction observed a full recovery after 60 min [66], 15 min [67] and 10 min [68] following very long passive static stretching protocols, respectively for a total duration of 30 min [66], 60 min [67] and 20 min [68]. Since these durations do not appear feasible in practice (except for gymnasts, ballet dancers, and other sports that require extreme flexibility), subsequent studies investigated more ecological protocols. For example, maximal strength was not affected by 2 min of passive static stretching, while 4 min and 8 min reduced force up to 10 min after [69]. Other authors found a full recovery after 10 min following 225 s of passive static stretching [9, 10], while impairments in strength up to 120 min were observed following 270 s [70]. Interestingly, maximal isometric strength was impaired after an intermittent 5 × 1 min protocol up to 30 min, while a continuous 1 × 5 min protocol led to a recovery in strength after 15 min [62]. Overall, it appears that the relationship between the passive static stretching duration and the reduction in strength is not linear, but it should be noted that all passive static stretching protocols were conducted at maximal intensity, so we do not know whether or not intensity may somewhat affect the strength recovery time course. However, the strength reduction time course seems to recover earlier than the changes in ROMmax [8], which implies that the acute benefits associated with passive static stretching are possibly superior to the negative sides.
The acute increase in ROMmax after passive static stretching is due to potential concomitant neural and mechanical mechanisms, whose occurrence is mainly related to the stretching intensity and duration [71]. The main neural mechanisms may involve a muscle spindle dysfacilitation, a pre-synaptic inhibition (e.g., reduced H-reflex amplitude) [58, 72], with other sensory mechanisms associated such as an increase in stretch tolerance possibly ascribable both to a change in the afferent input from the nociceptive nerve endings and mechanoreceptors, and to a higher participants’ willingness to tolerate larger mechanical stress [21, 73]. Interestingly, pressure or pain-mediated alterations in descending monoaminergic drive were also indicated as further mechanisms acting on the ROMmax increase [18].
From a mechanical perspective, several main mechanisms have been hypothesized to explain the passive stretch-induced increments in ROMmax. Primarily, a decrease in muscle stiffness is often reported [21], which could be due to potential alterations in titin filaments [74, 75], fiber-based connective tissues [76], and intracellular amorphous material [77]. Moreover, a reduction in stiffness of the connective tissue surrounding the muscle (particularly the perimysium) could also contribute to the stretch-induced muscle stiffness decrease [78, 79]. Further factors to be considered are a possible stretch-induced increase in fascicle elongation due to alterations in intra-fascicular and inter-fascicular structures [74–76, 80], as well as in intermuscular myofascial connectivity [76] and in fascicle rotation [18, 81, 82]. More in specifically, fascicle rotation and muscle gearing greater than 1, meaning that fascicle rotation decouples fascicle and muscle stretch, could allow muscles to stretch with less fascicle strain due to rotation effects, theoretically contributing to the increased ROMmax [83–85]. Despite tendon stiffness being an important contributor to limiting the muscle–tendon unit elongation during the joint rotation changes in its mechanical properties after acute stretching interventions are still unclear [18, 82]. The mechanisms behind the static stretching-induced acute changes in ROMmax are summarized in Fig. 2.Fig. 2Schematic representation of the physiological mechanisms behind the acute and long-term increase in maximal joint range of motion (ROMmax) for static, dynamic and ballistic, and proprioceptive neuromuscular facilitation (PNF) modalities. MTU: muscle-tendon unit
Incorporating a passive static stretching routine into daily or weekly training should consider the acute aspects examined above. Given that passive static stretching acutely increases ROMmax across a wide range of intensities and volumes, it may be advisable to conduct passive static stretching at 75–80% of the point of discomfort to minimize the risk of strength impairment. This can be achieved through various total durations, using either continuous or intermittent protocols, with individual bouts lasting less than 60 s and a total duration below 480 s [59]. When passive static stretching is performed after a training/competition or alone in a separate session, practitioners may consider using maximum intensities and exceeding the aforementioned duration thresholds to magnify the dose–response relationship. Interestingly, while passive static stretching appears to have small but significant detrimental effects on maximal isometric strength, it appears to have no impact on explosive and dynamic tasks [61]. Moreover, the decrements in maximal isometric strength are transitory and tend to be restored early unless extreme stretching protocols are conducted [8]. Consequently, one may decide to administer passive static stretching even before dynamic sports actions, although caution is needed when exceeding high intensities and durations [3]. Indeed, when preparing for high-intensity activities, neuromuscular performance is often prioritised over gains in ROMmax, and passive static stretching should be inserted with caution, taking into consideration the possible transient side effects associated with the strength loss. Alternatively, practitioners can perform other exercises or drills, such as skill-related or task-specific movements until this effect is nil [8]. That said, all the aforementioned considerations become less relevant when the primary goal is to increase ROMmax, and the potential reductions in muscle strength are negligible or not of primary concern. For example, when the training session does not involve high-intensity strength efforts, or when the stretching bout is followed by recreational or daily activities, prioritizing the most effective strategy to increase ROMmax becomes more important than concerns about potential side effects.
The long-term (duration > 2 weeks) effects of static stretching on ROMmax and muscle strength were summarized in a recent meta-analysis, which also showed the effects of many moderating factors [3], while another recent meta-analysis focused only on the effects of static stretching on muscle strength [86]. Before proceeding with the examination, we would like to emphasize that Arntz and colleagues identified a small number of studies for which the modality was labelled as active static stretching [3]. However, upon further scrutiny of each study, we found that some stretching protocols were passive, as they involved the application of an external force such as gravity, elastic bands, or the intervention of other muscles besides the antagonists [87–97]. Additionally, one further protocol included dynamic actions, as indicated by the title [98], while another study involved a PNF protocol [99]. Moreover, Thomas and colleagues in another meta-analysis identified and categorized five stretching passive stretching, active stretching, static stretching (when passive or active was not specified), ballistic stretching, and PNF [100]. However, it is worth noticing that studies categorized under "active stretching" primarily investigated passive stretching, as indicated in the original study methods descriptions [101–105]. Additionally, the "static" method without further specification encompassed studies that conducted passive stretching [18, 26, 81, 106–108], while another study also investigated a combination of passive and active stretching [109]. Consequently, long-term studies investigating the effects of active static stretching are still lacking and cannot be considered here. This also calls for a clear taxonomy of the stretching protocols warranted to avoid misunderstanding.
Arntz and colleagues provide indications that stretching intensity, assessed in this case as perceived pain and ranked as none, moderate, and severe pain, does not inherently affect the effectiveness of static stretching in increasing ROMmax [3]. Indeed, stretching with no pain may increase ROMmax by a small-to-large extent, moderate pain by a moderate-to-large extent, and severe pain by a small-to-large extent [3]. Although "pain" does not perfectly match the definition of the point of discomfort and different instructions may lead to different ROMmax [110], it substantially reflects its magnitude; thus, the results of Arntz and colleagues can be practically useful [3]. Additionally, it should be noted that ROMmax may vary depending on the velocity at which the joint rotation is performed, and whether the movement is passive or active [35]. As to the influence of stretching training on strength, trivial-to-small increments were reported independently of the pain perception [3]. Additionally, it seems that these increments were mediated by sex, age, and the training status of the participants, with female or sedentary participants gaining more strength from the stretching training period [3].
Regarding the stretching duration, a deeper analysis should be made based on the indications of Arntz et al. [3]. The number of repetitions per exercise, the time spent under stretching per session, and the total time spent under stretching for the entire protocol appear to proportionally increase ROMmax [3], emphasizing the importance of stretching volume in long-term ROMmax gains. The meta-analysis also offers suggestions regarding the continuous versus intermittent stretching modality, which appears equally effective in increasing ROMmax, with intermittent more advantageous for increasing strength [3]. However, it should be noted that the authors did not specify whether this was valid for equalized static stretching durations, and no direct comparison was found in the literature [3]. Still examining the effects of static stretching training on strength gains, small increments (ES = 0.45) have been observed overall when the total duration is equal to or exceeds 15 min, while a tendency to small strength gains (ES = 0.21) was also reported for durations below 15 min [86]. Lastly, the chronic effects on jumps and sprints were trivially positive [111].
Bearing in mind the great heterogeneity in stretching protocols and data collection methods, potential mechanisms for the chronic increase in ROMmax could include both neural adaptations and mechanical modifications of the muscle–tendon units and passive joint structural properties that increase the stretch tolerance. Regarding the specific neural mechanisms involved in the stretch-induced increase in ROMmax, their role appears not well established. A review highlighted that most studies on long-term stretching training have reported reductions in H-reflex amplitude [72]. However, neural changes in tonic reflex activity and motoneuron excitability have not been shown consistently, possibly because of different static stretching volume and intensity [81, 97, 112, 113]. Nevertheless, it should be noted that ROMmax may also be due to increased subjective tolerance rather than changes in muscle activity [81].
Regarding the mechanical mechanisms, stretch-induced increases in ROMmax have been observed both with and without changes in passive resistive force [6]. Hence, these gains can be generally attributed to a greater tolerance to stretch or pain (i.e., sensory theory) [73], indicating that the individual can tolerate more passive tension after the intervention without a change in passive tension for a given length [6, 73, 81, 112, 114]. Additionally, a stretch-induced decrease in joint resistance to stretch could be due to alterations in muscle–tendon unit mechanical characteristics (i.e., mechanical theory) [73]. For example, a rightward shift in the passive torque–angle curve, a reduction in passive torque at standardized joint angles, or a reduction in passive joint stiffness has been observed [6, 97, 115]. The structural adaptations in fascicle length are still controversial, as different studies showed either greater resting fascicle length [116, 117], possibly contributing to ROMmax increase, or no structural change after stretching intervention [81, 118, 119]. In contrast, tendon stiffness seems not to be affected by long-term passive static stretching, thus resulting in a negligible contribution to long-term ROMmax increments [6, 86, 111]. The mechanisms behind the static stretching-induced long-term changes in ROMmax are summarized in Fig. 2.
Long-term passive static stretching intervention can be administered in multiple ways. First, practitioners may consider a wide range of stretching intensities, possibly adapting the choice based on the acute effects and how they may impact the overall training session [3]. Second, fixing a certain amount of total stretching duration for each cycle seems important, although this can be freely divided into a wide range of sessions, considering each session's purposes. Third, intermittent stretching may be preferable for increasing strength compared to continuous stretching, while both can be used interchangeably for increasing ROMmax. Interestingly, static stretching may be more effective in older adults compared to young adults for increasing strength, while it is equally effective in increasing ROMmax regardless of age [3]. Lastly, it is important to note that long-term static stretching is not detrimental to strength. Figure 3 summarizes the main recommendations for incorporating static stretching, considering both the acute and long-term effects on ROMmax and strength.Fig. 3Schematic representation of the acute (upper panel) and long-term (lower panel) effects on maximal joint range of motion (ROMmax) and strength of static, dynamic/ballistic, and proprioceptive neuromuscular facilitation (PNF) modalities
The acute effects of dynamic and ballistic stretching on ROMmax and muscle strength were synthesized in a systematic review [2] This review also allows for distinguishing the acute effects of dynamic and ballistic stretching training based on the manipulation of movement velocity, amplitude, and duration [2]
Regarding the effects of movement velocity as an indicator of intensity on ROMmax, the review did not identify any relevant studies [2]. Subsequently, one study compared two different velocities, assessed as two different movement cadences of dynamic stretching, i.e., 4 × 10 dorsiflexions at 100% and 50% of the maximum frequency [120] The results indicated that both velocities were similarly effective in increasing ROMmax at the end of the protocol [120]. Regarding muscle strength, the effects appear to be positively correlated with the movement velocity, as summarized by Opplert [2]. Only one study directly compared jumping height following dynamic stretching of the main muscle groups involved in jumps [33]. Specifically, the movements were performed at a cadence of 100 or 50 per minute, with greater increases in jump height observed following the 100-cadence [33]. Although other studies did not directly compare fast versus slow movement velocity, the overall trend suggests that fast movements tend to increase muscle strength acutely, while moderate or slow velocity could lead to less favourable outcomes [2]
As to the movement amplitude as a complementary indicator of intensity, the review did not identify any relevant study that directly compared different amplitudes in dynamic stretching and their effects on ROMmax and muscle strength [2]. To the best of our knowledge, no further study aiming at this purpose was found. Nonetheless, the authors suggested that ballistic stretching might have negative or neutral effects on muscle force, while dynamic stretching appears to be more beneficial [2].
When considering the volume of dynamic or ballistic stretching, the primary variables are the number of movement repetitions per set and the total number of sets. Although no reviews or meta-analyses were found on this topic, direct comparisons have shown that a single set of dynamic stretching does not affect ROMmax, whereas four and seven sets result in similar increases in ROMmax [120, 121]. These findings suggest that the dose–response relationship may plateau after a certain number of total repetitions. Muscle strength appears to be dependent on the total stretching volume if fatigability remains minimal [2]. This recommendation is supported by two studies that observed increases in muscle strength after 6 min of dynamic stretching [122] and reductions in muscle strength after a duration of 16 min [123]. It is important to note that a single set typically lasts between 30 to 90 s [2].
Performing open or closed kinetic chain exercises appears to have an impact, especially on strength. For example, a brief (90 s) bout of dynamic open kinetic chain hamstring stretching increased hamstring ROMmax [124]. However, this was associated with a trade-off, since a reduced eccentric hamstring strength was observed [124]. Conversely, another study found no change in strength and performance [125]. Regarding closed kinetic chain dynamic exercises, in the same aforementioned study the gains in ROMmax were observed accompanied by increases in hamstring eccentric strength [124]. These findings cautiously suggest that dynamic closed-chain exercise may increase ROMmax without impairing strength. Ballistic stretching is almost always performed through open kinetic chain exercises, since ballistic actions typically require a free limb, meaning closed-chain ballistic exercises are uncommon. In the absence of data, ballistic should be thought of as open kinetic chain exercises.
The main neural mechanisms potentially contributing to the acute static stretch-induced increase in ROMmax (e.g., muscle spindle dysfacilitation and pre-synaptic inhibition) seem to play a minor role after dynamic stretching since the time spent at the end ROM is generally lower [2, 8]. In contrast, an active contraction of the opposite muscle group to the stretched muscles may activate another neural mechanism potentially contributing to the stretch-induced increase in ROMmax, i.e., the reciprocal inhibition [31]. Basically, the contraction of the opposite muscle group and the consequent elongation of the stretched muscles could inhibit the α-motoneurons through inhibitory output from the collateral branches of the Ia afferent fibres, leading the stretched muscles to additional relaxation and consequent elongation [2, 11, 31, 126]. Moreover, some sensory mechanisms involved in stretch tolerance could also play a role in explaining the increase in ROMmax after dynamic and ballistic stretching [127, 128].
Regarding the mechanical mechanisms, the active and repeated contractions of the muscle group opposite to the one being stretched may increase local temperature, thereby reducing the muscle’s viscous resistance and enhancing the tissue’s extensibility [4, 129]. Moreover, changes in the viscoelastic properties of the muscle–tendon unit have been hypothesized to account for the ROMmax enhancements, such as the reduction in muscle stiffness possibly due to a collagen fiber rearrangement induced by the repeated muscle stretching cycles [130]. This may lead to a progressive increase in the angular displacement over the cycles observed during dynamic or ballistic stretching [131, 132]. However, the reduction in muscle–tendon unit stiffness induced by dynamic or ballistic stretching has not been consistently observed, suggesting further mechanisms to be explored [131, 132]. The mechanisms behind the dynamic and ballistic stretching-induced acute changes in ROMmax are summarized in Fig. 2.
Increases in ROMmax can be achieved with both fast and slow movements and with a low-to-high volume of dynamic stretching. However, it should be noted that the beneficial effects of acute dynamic stretching on muscle strength may be influenced by the protocol length due to the possible onset of muscle fatigability. That said, dynamic stretching can be included in the warm-up or performed prior to high-intensity strength training sessions, as it does not negatively affect muscle strength [60]. In addition, closed kinetic chain exercises showed positive effect on strength, and should be preferred to open chain exercises when suitable, since both increase ROMmax. In contrast, although ballistic stretching may lead to a greater end-range ROMmax during the movement execution due to its less controlled nature and associated rebound, it may have detrimental effects on muscle strength and should therefore be applied with caution, especially considering the difficulty it poses for inexperienced individuals [133].
Two systematic reviews on long-term studies on dynamic or ballistic stretching have indicated an increase in ROMmax without any observed impairment in muscle strength [119, 130]. However, there is a lack of systematic investigations that directly compare the manipulation of movement velocity, amplitude, or duration. Further studies examining this topic are warranted.
There are very few long-term studies investigating the possible mechanical mechanisms underpinning the dynamic and ballistic ROMmax increases, and we did not find any study that investigated the neural mechanisms. As to the mechanical mechanisms, one study reported a reduction in active tendon stiffness and passive muscle–tendon unit stiffness [129], while another study observed negligible changes [127]. However, the authors of both studies have suggested that the further sensory mechanisms involved in augmented ROMmax may be related to an increased stretch tolerance [127, 129], albeit further longitudinal studies are warranted to clarify the long-term dynamic and ballistic stretch-induced increases in ROMmax. The mechanisms behind the dynamic and ballistic stretching-induced long-term changes in ROMmax are summarized in Fig. 2.
Based on the sparse literature about the possible manipulation of intensity and volume in dynamic and ballistic stretching, we currently lack stronger indications for the practical applications of these variables. Practitioners may possibly refer to the indications provided for the acute changes, so that each dynamic or ballistic stretching session can be included in practice. Figure 3 summarizes the main recommendations for incorporating dynamic and ballistic stretching, considering both the acute and long-term effects on ROMmax and strength.
The effectiveness of various PNF protocols in acutely increasing ROMmax has been highlighted in several systematic reviews [39, 40, 134]. Specifically, the intensity of the target muscle contraction ranged from 20 to 100% of maximum strength and has been found equally effective in promoting acute ROMmax gains [39, 40, 135]. Similarly, variations in the duration of the contraction of the target muscle ranged from 3 to 15 s and did not significantly impact the ROMmax increments among different protocols [39, 40]. Importantly, the number of contraction-relaxation phases did not affect the gains in ROMmax, with performing one cycle being similarly effective as up to four phases [39, 40]. For the CRAC method, only maximal antagonist muscle contraction intensity and maintained for 3 s has been studied [40], which incidentally corresponds to the AC method, so alternatives to both protocols have not been provided to date. While the effects of PNF protocols on muscle strength have been reported, a detailed description of the moderating factors is not as comprehensive as for ROMmax [39], and thus a more accurate methodological investigation is needed. Generally, PNF protocols appear to acutely decrease maximum strength or performance during high-intensity exercises, such as during sprinting, plyometrics, cutting, weight-lifting, or vertical jump height and power, while improving kinematic parameters in sub-maximal efforts, such as stride rate and length in jogging [39].
The PNF methods overall share some common mechanisms with both the passive static and the dynamic stretching modalities that may lead to acutely increased ROMmax. Four main neural mechanisms have been hypothesized to explain the gains in ROMmax after PNF: i) the stress relaxation; ii) the reciprocal inhibition; iii) the autogenic inhibition; and iv) the gate control theory [39, 40, 136]. The stress relaxation and the reciprocal inhibition have been described in the Static Stretching and Dynamic and Ballistic Stretching sections, respectively, so the other two mechanisms will be examined now. The autogenic inhibition is mediated by the Golgi tendon organs (GTOs), which inhibit the motoneuron by decreasing their excitability when the tendon undergoes high-level tension [137]. This process involves the activation of Ib afferent fibers in the GTOs, which send signals to the spinal cord triggering the inhibitory interneurons that decrease a-motoneuron excitability, reducing the motor drive [137]. The consequent relaxation of the stretched muscle is a key component of all PNF techniques underlying the increases in ROMmax [40]. This process could possibly exploit the viscoelastic properties of the muscle–tendon unit, reducing its tension and facilitating muscle elongation, hence enhancing ROMmax [40]. However, the whole theory remains uncertain. Indeed, while GTOs play a meaningful role in muscle fiber inhibition, their activation and the duration of the inhibition post-contraction during PNF are questionable [137]. The gate control theory describes that when two stimuli such as pain and pressure activate their receptors simultaneously, the pressure signal is delivered faster than the pain signal, since it is carried by larger myelinated fibers [138, 139]. This results in the inhibition of pain signals in the dorsal horn of the spine, thereby possibly contributing to increased ROMmax [39, 40].
From a mechanical point of view, a reduction in both muscle and tendon stiffness has been reported to contribute to the acute PNF-induced increases in ROMmax [8, 38]. Accordingly, the voluntary contractions exerted during PNF would change the viscoelastic properties of the tendon tissue, allowing a greater muscle–tendon unit elongation [140]. However, no correlation between changes in stiffness and ROMmax was detected [140]. Therefore, more studies are needed to clarify the mechanical factors influencing the acute increments in ROMmax induced by PNF stretching. The mechanisms behind the PNF-induced acute changes in ROMmax are summarized in Fig. 2.
PNF protocols show the ability to enhance ROMmax using a broad range of contraction intensities and durations of the target muscle. Therefore, the selection of the most appropriate protocol should be based on the specific context, considering that low contraction intensities and short durations are equally effective as more demanding protocols. To address any potential discomfort arising from the isometric contraction phase in a lengthened position of the target muscles, Kay and colleagues [45] proposed a modified CR version where the contraction is performed in the anatomical position. Comparable results in terms of ROMmax gains, neurological, and mechanical changes were reported between the classical and modified CR methods. When utilizing the CRAC modality, the only guidelines refer to the antagonist contraction performed maximally and maintained for 3 s. Here the risk of cramp should be considered, especially if the maximum antagonist muscle contraction is performed at a short muscle length.
A meta-analysis examined the long-term effects of PNF on ROMmax and included studies that directly compared different PNF protocols [44]. Across these studies, various agonist contraction intensities ranged from 25 to 100% of maximum strength, i.e., CR method and variants, while the antagonist’s contraction was consistently performed at 100%, i.e., AC and CRAC methods [44]. Interestingly, the PNF training-induced effects on ROMmax gains appeared to be independent of the target muscle contraction intensity [44]. Regarding duration, the most reported combinations included up to 6 s for the target muscle contraction, up to 5 s for the relaxation phase, and maintenance of the passive joint position from 10 to 30 s [44].
The effects of PNF on muscle strength were summarized in a single meta-analysis [39]. The authors emphasized that no impairment in muscle strength occurs after long-term PNF training, with contraction intensities ranging from 20 to 100% of maximum force, contraction times from 5 to 10 s, and relaxation times up to 10 s [39]. It may be worth considering that performing an active contraction could potentially serve as an additional stimulus for strength improvement. However, it should be noted that the contraction intensity of antagonist muscles during the AC and CRAC methods was not examined as a moderating factor [39], and thus further guidance cannot be provided.
The long-term mechanisms leading to increased ROMmax following PNF interventions have not been systematically investigated. Reflex inhibition has been proposed as a possible neural mechanism, albeit collective observations suggest inconsistency [72]. The mechanical mechanisms may be related to decreased muscle and tendon stiffness [141], although other authors did not observe any change [142]. Alternatively, the long-term gains in ROMmax may be due to an augmented stretch tolerance [6, 127]. More studies exploring mechanisms are needed. The mechanisms behind the PNF-induced long-term changes in ROMmax are summarized in Fig. 2.
PNF may be performed using maximal or sub-maximal contractions to effectively increase ROMmax when using the CR technique and its variations, with the latter being preferable to avoid possible discomfort in people not accustomed. However, when using CRAC only maximum antagonist muscle contraction intensity has been identified, and it is recommended to adhere to these procedures. These protocols can be safely incorporated into long-term training without any adverse effects on muscle strength. Figure 3 summarizes the main recommendations for incorporating PNF, considering both the acute and long-term effects on ROMmax and strength.
The present review has some acknowledged limitations. First, we focused on the effects of muscle stretching on ROMmax and strength, and we did not consider any mechanistic repercussion when comparing different intensities and volumes or stretching modalities. A more in-depth approach would consider the diversity of the stimuli that characterize each modality and is warranted in the future. Second, we unified strength outcomes, and we did not always differentiate into separate strength expressions, possibly failing to provide specific information. Third, we focused only on healthy adults, and the effects of each stretching modality and variable may differ in clinical populations.
The present narrative review offers a methodological description of each stretching modality, together with a methodical quantification of the main independent parameters that can be manipulated to quantify the intensity and the volume of each stretching modality. Additionally, the manipulation of intensity and volume as moderating factors within each stretching modality has been reviewed, and the acute and long-term effects on ROMmax and strength have been summarized, providing practical applications.
As a final remark, the effects of different stretching modalities can be modulated by altering stretching intensity and volume. However, intensity and volume are stretching modality-dependent constructs, as their quantification varies according to the specific characteristics of each stretching technique. Such a complexity hinders between-modality stretching comparison, since the combination of modality-specific intensity and volume may result in countless variations. Consequently, future studies should address this point with a more meticulous approach to the intensity and volume for each stretching modality.