The mechanisms underpinning the chronic gains from SS exercise-related flexibility remain poorly defined. Two major mechanisms have been proposed in the literature. The first is related to sensory perception (i.e., sensory theory). Sensory theory indicates that chronic exposure to stretching results in an increased stretch tolerance [20,28,29], probably due to a modification of the subjective perception of discomfort [20,28,30], which seems to be caused by changes at the level of the nociceptive endings [22]. The second is called “mechanical theory”, which suggests that stretching exercises change the muscle-tendon unit’s (MTU) mechanical properties, namely through decreases in tissue stiffness or geometry (i.e., the addition of sarcomeres in series), or both [28,31]. In this sense, Blazevich, et al. [32] examined muscle-tendon lengthening and fascicle elongation and rotation during maximal plantar flexor stretches, in young healthy male participants aged 20 years. The stretching task consisted of ankle rotation from a 30° plantar flexion toward a dorsiflexion at 2°/s using an isokinetic dynamometer until the maximum tolerable stretch limit was achieved. The findings show that the maximal stretch was achieved through significant muscle (14.9%) and tendon (8.4%) lengthening. Furthermore, the findings indicate that participants with a greater range of motion displayed a lesser resistance to stretching and a greater passive joint moment (i.e., greater stretch tolerance) compared with those who were less flexible [32]. In addition, the same authors reveal that the more flexible participants displayed greater fascicle rotation during stretches than their less flexible counterparts [32]. Moreover, there is evidence that three weeks of twice daily SS exercises of the plantar flexors in young healthy males aged 18 years increased maximum tolerable passive joint moment (i.e., stretch tolerance) as well as muscle and fascicle lengthening [33].

It should be noted though that the two aforementioned mechanisms potentially underpinning chronic SS exercise-related flexibility adaptation are not conclusive. Freitas, Mendes, Le Sant, Andrade, Nordez and Milanovic [20] conducted a systematic review with a meta-analysis on the chronic effects of different types of stretching exercises (i.e., static, dynamic, and proprioceptive neuromuscular facilitation) on the MTU’s structural properties. The aggregated data from the 26 studies indicate that three to eight weeks of stretching increases the stretch tolerance, but appears not to markedly affect the MTU’s mechanical properties [20]. However, the same authors [20] failed to account for potential moderator variables, such as the type (e.g., static vs. dynamic) and the applied load of stretching (e.g., time under stretching), meaning that further studies are needed. Overall, if the goal is to improve flexibility, SS represents the most effective approach.

The chronic effects of SS exercises on muscle strength and power were addressed in several original studies. For example, Hunter and Marshall [38] revealed increased jump height after chronic SS exercises (∆1.3%, compared to a non-stretching control −0.3%) in physically active males aged 24 years. Additionally, 15 sessions with 20 min per session over three weeks of SS exercises showed an increase in eccentric peak torque at 60°/s and 120°/s (∆8.5% and 13.5%, respectively), with an 11.2% increase in concentric peak torque at 120°/s of knee flexors in healthy active young adults [16]. Furthermore, Warneke, Konrad, Keiner, Zech, Nakamura, Hillebrecht and Behm [23] reveal significant improvement in maximal isometric strength of plantar flexors after six weeks of SS exercises performed on a daily basis in healthy young adults. Another recent study showed similar results reporting that long-lasting (one hour) SS daily for six weeks resulted in substantial muscle strength improvements of the stretched leg (∆16.8%) with no significant changes in the non-stretched leg or in the control group [39]. Furthermore, the chronic effects of SS exercises on muscle strength and power have been the topic of a recent systematic review with a meta-analysis [13]. Data aggregated from 41 original studies indicate that, regardless of age and sex, chronic SS exercises benefit muscle strength and power, though trivial to small in magnitude (muscle power: standardised mean difference [SMD] = 0.18; 95% confidence interval [CI] = 0.12 to 0.25; muscle strength; SMD = 0.21; 95% CI = 0.10 to 0.33) [13]. Interestingly, in addition to the 95% CI, the authors report a commonly neglected, but informative, statistical variable, which is the 95% prediction interval (PI). In fact, the 95% PI accounts for the uncertainty of the effects expected in similar future studies [40,41,42]. The results show that most of the 95% PI for muscle strength was above zero indicating that chronic SS exercises may produce a positive effect in most of the upcoming studies. As to muscle power, both ends of the 95% PI are above zero implying that 95% of future investigations will show beneficial effects of chronic SS exercises [13]. These outcomes indicate that muscle strength and power will most likely benefit from chronic SS exercises. The outcome of the study from Arntz, Markov, Behm, Behrens, Negra, Nakamura, Moran and Chaabene [13], related to muscle strength, was supported by a recently published study [14]. More particularly, the authors meta-analyzed the literature on the chronic effect of stretching on muscle strength in healthy individuals; they reveal a small positive effect of long-term SS training on muscle strength [14].

The mechanisms under which chronic SS exercises improve muscle strength and power are still inconclusive. One speculation is that chronic SS exercises appear to stimulate muscle growth and, therefore, hypertrophy [43,44]. For instance, 12 weeks of SS exercises with five weekly sessions have been shown to induce larger enhancements in gastrocnemius cross-sectional area (CSA) and fascicle length of the stretched leg, as well as larger one-leg countermovement jump performance, compared to the control leg in adolescent female volleyball players [19]. In their narrative review, Nunes, Schoenfeld, Nakamura, Ribeiro, Cunha and Cyrino [16] suggest that low-intensity stretching appears not to trigger any change in muscle size and architecture, but high-intensity stretching does. However, this assumption is rather speculative and should therefore be investigated in future studies. Another potential theory discussed in the literature pertains to the mechanical properties of the MTU. Earlier studies report an increased MTU compliance following chronic SS exercises [45,46]. This could improve activities involving the stretch–shortening cycle (e.g., jumping, rebound bench press, and jogging) through better use of elastic energy [47,48,49,50]. However, it should also be noted that other studies did not report any change in the mechanical properties of the MTU after chronic SS exercises [20,30,51], leaving this research open for much debate in future studies.

To sum up, there is compelling evidence that chronic SS exercises generate positive effects on muscle strength and power, irrespective of age and sex (Figure 1). It is worth noting that SS exercises could particularly constitute a useful alternative for those who cannot afford a gym membership for RT and for injured individuals who cannot move their injured limb dynamically. In addition, the practice of RT is usually associated with a certain level of discomfort due to the stress and strain that the exercises induce. Consequently, SS could represent a more relaxing mode of training for those who cannot afford such discomfort. Therefore, flexibility should not be omitted as a standard component of physical fitness, as it contributes to muscle strength and power, both of which are crucial for maintaining/improving functional capacities and promoting health [34,35,36,52].

The chronic effect of SS exercises on muscle hypertrophy is an emerging topic. Promising evidence indicates that SS training exercises could favor muscle hypertrophy whether performed as a single mode [15,24,44] or combined with RT exercises [17,18,54]. More specifically, the results of the study of Panidi, Bogdanis, Terzis, Donti, Konrad, Gaspari and Donti [15] show that 12 weeks of single-mode SS training induced significant improvements in the gastrocnemius cross-sectional area of the stretched leg with no observed changes in the contralateral control leg in adolescent female volleyball players. In the same context, Simpson, Kim, Bourcet, Jones and Jakobi [44] studied the effects of six weeks of single-mode SS training on the muscle architecture of the gastrocnemius in male healthy young adults. The results indicate that there is an improved gastrocnemius muscle thickness and a greater fascicle length increase in the lateral compared to the medial gastrocnemius after training [44]. More recently, Warneke, Zech, Wagner, Konrad, Nakamura, Keiner, Schoenfeld and Behm [24] examined the effect of six weeks of daily SS training of plantar flexors on muscle thickness of the medial and lateral gastrocnemius in healthy physically active male and female participants. The same authors report a 4% to 14% increase in muscle thickness following training, regardless of sex [24].

On the other hand, performing SS exercises between the RT sets appears to be a promising training approach leading to a greater muscle hypertrophy than the RT alone. This is what a recent study by Van Every, Coleman, Rosa, Zambrano, Plotkin, Torres, Mercado, De Souza, Alto and Oberlin [17] demonstrates. In fact, the authors revealed that a combined loaded inter-set SS with RT improves muscle thickness of the soleus better than the traditional RT alone. The same authors further revealed a modest advantage of a combined inter-set loaded SS with an RT over an RT alone on the isometric strength of the plantar flexors [17]. Evangelista, De Souza, Moreira, Alonso, Teixeira, Wadhi, Rauch, Bocalini, Pereira and Greve [54] investigated the effect of eight weeks of single-mode RT versus RT with integrated SS exercises between the sets on biceps brachii, triceps brachii, rectus femoris, and vastus lateralis hypertrophy in sedentary healthy adults. The results indicate similar muscle thickness gains of biceps brachii, triceps brachii, and rectus femoris with greater gains in vastus lateralis muscle thickness following the combined RT and SS exercises compared to the RT alone [54]. The authors conclude that integrating the SS exercises with RT seems to have a greater benefit on muscle hypertrophy compared with the RT alone. Furthermore, Schoenfeld, Wackerhage and De Souza [18] postulate that integrating SS between the RT sets provides an additional muscle growth stimulus without increasing the duration of the training session and could, therefore, be considered a better option compared to the RT alone [18].

Overall, there is emerging evidence that single-mode SS training contributes to muscle hypertrophy (Figure 1). Additionally, recent studies indicate an advantage of combining the RT with the SS exercises integrated between the sets compared to the RT alone (Figure 2). However, more research is warranted to substantiate the existing outcomes.

There are indications in the literature that increasing flexibility could mitigate the incidence and risk of injury [7,55,56,57]. For instance, Cross and Worrell [58] studied the effects of SS training on the incidence of lower extremity musculotendinous strains in young football players. The same authors report that the incorporation of SS exercises could lead to the decreased incidence of musculotendinous strains. Findings from a meta-analysis of the literature [59] indicate that poor flexibility in the lateral flexion and a restricted range of motion in the hamstrings can contribute to the development of low back pain, regardless of age and sex. Additionally, there is an indication that reduced trunk flexibility in children is associated with lumbar stress fracture [60]. Moreover, it is well-known that SS exercises decrease MTU stiffness [45,46]. This increase in the muscle–tendon compliance, according to Witvrouw, et al. [61], is needed for activities conducted in the stretch–shortening cycle to effectively store and release a high amount of elastic energy. The same authors suggest that when the level of MTU compliance is insufficient, there is a risk that the demands in energy absorption and liberation rapidly surpass the capacity of the MTU, which once it has initially occurred, could lead to a higher risk of injury [61]. Other studies support the fact that increasing MTU compliance might allow for more efficient use of elastic energy during activities involving the stretch–shortening cycle (e.g., jumping, rebound bench press, jogging) [47,48,49,50]. Woods, Bishop and Jones [7] introduced the “non-injury zone” (NIZ) theory. The NIZ stands for the range of motion through which a given muscle can freely move without any risk of injury. Any movement beyond the NIZ would lead to an injury of the respective muscle. In this sense, if the length of the muscle increases due to stretching, this would widen the NIZ. This expansion in the NIZ would allow a greater range of motion through which the muscle can freely move without exposing it to a greater risk of injury [7]. However, this is hypothetical and yet to be empirically proven. Recently, Behm, Kay, Trajano, Alizadeh and Blazevich [57] synthesized the available literature related to the effects of stretching on injury prevention and concluded that chronic stretching, particularly SS, has the potential to mitigate the incidence of musculotendinous injury. This particularly refers to running-based sports [57]. According to the same authors, the beneficial effects of SS exercises on the reduction in the incidence of injury seem to be due to the reduced MTU stiffness (i.e., an increased MTU compliance) or longer muscle lengths (an altered force–length relationship), among other factors [57].