Lower-limb strength was examined in 67 of the 75 studies in which strength tests were completed. Overall, similar responses were observed in the knee extensors (–3.7%), knee flexors (–6.3%), and plantar flexors (–5.6%). When these studies were further separated based on the stretch duration (<60 s vs. ≥60 s per muscle group), evidence for a dose-dependent effect of stretch was observed in the knee extensors (<60 s, –2.6%; ≥60 s, –3.8%), knee flexors (<60 s, –4.8%; ≥60 s, –6.4%), and plantar flexors (<60 s, –3.5%; ≥60 s, –5.9%). Taken together, these data are indicative of a dose-dependent effect of stretch, with similar moderate-to-large mean changes calculated for all muscle groups after shorter and longer stretch durations, respectively. However, considering the large 95% CIs in several of the findings (Supplementary Table S4 1 ), some caution should be used when interpreting the mean changes reported, because substantial variability exists among studies.

Fourteen studies incorporating 22 maximal strength–based measures imposed <60 s of SS, with 16 nonsignificant changes, 6 significant reductions, and no significant improvements in performance being reported; collectively, there was a moderate reduction in performance (–2.8%) (Supplementary Table S4 1 ). However, when strength-based studies using ≥60 s were examined, 72 studies incorporating 166 measures with 73 nonsignificant changes and 92 significant reductions were found; only 1 significant improvement in performance was observed. Compared with shorter-duration stretches, the mean 5.1% reduction was greater. Mean changes are clearly greater for strength- than for power–speed-based tasks regardless of duration (although this may be the result of strength-based studies using substantially longer stretch durations), and the dose–response effect remains clear.

Twenty-six studies incorporating 38 power–speed-based measures used <60 s of SS, with 29 nonsignificant changes, 4 significant reductions, and 5 significant improvements in performance; collectively, there was a trivial change in performance (–0.15%) (Supplementary Table S4 1 ). It is interesting to note that although most of the findings were not statistically significant after short-duration stretching, a greater number of significant improvements than reductions were found in jumping ( Murphy et al. 2010 b ), sprint running ( Little and Williams 2006 ), and cycling ( O’Connor et al. 2006 ) performances. Thus, there is no clear effect of short-duration SS on power–speed-based activities, although changes may be observed on a study-by-study (and hence, subject-by-subject) basis. Nonetheless, when 28 power–speed-based studies (44 measures) using ≥60 s of stretching were examined, 27 nonsignificant changes and 17 significant reductions were found, with no study reporting a significant performance improvement. Compared with shorter-duration stretching, the mean reductions were marginally greater (–2.6%) (Supplementary Table S4 1 ). Despite a greater likelihood and magnitude of effect of longer-duration SS, changes are most likely to be small to moderate.

To determine whether SS produced similar performance changes in different performance activities, the findings of the studies were separated into power–speed- or strength-based tasks. Fifty-two studies reported 82 power–speed-based measures (i.e., jumping, sprint running, throwing), with 56 nonsignificant changes, 21 significant reductions, and 5 significant improvements; collectively, there was a small 1.3% reduction in performance. Seventy-six studies reported 188 strength-based measures (i.e., 1-RM, MVC), with 79 nonsignificant changes, 108 significant reductions, and only 1 significant improvement. There was a moderate reduction in performance (–4.8%), which indicates a more substantial effect of SS on strength-based activities. The stretch durations imposed between activity types were considerably longer for strength-based activities (5.1 ± 4.6 min) than for power–speed-based activities (1.5 ± 1.6 min), which may explain the greater mean performance reductions after SS.

The largest systematic review to date ( Kay and Blazevich 2012 ) examined 106 SS studies; our searches found a further 19 studies since 2011 that met our criteria, resulting in 125 studies incorporating 270 maximal performance measures ( Table 1 , Supplementary Table S1 1 and Supplementary Fig. S1 a 1 ) examining the acute effects of SS on performance (e.g., vertical jump height, sprint running time, chest and bench press 1-repetition maximum (1-RM), and maximal voluntary contractions (MVC)). The data revealed 119 significant performance reductions, 145 nonsignificant findings, and 6 significant improvements after SS. Unfortunately, 42 studies failed to adequately report either mean changes (16 nonsignificant and 2 significant; 7% of total findings) or pre- and poststretch means ± SD data (36 significant and 38 nonsignificant; 27% of total findings), which prevented the inclusion of ES for these measures. The weighted estimates of the remaining 178 measures revealed a moderate 3.7% mean performance reduction ( Table 1 ). Thus, although there are some occasions in which large or very large reductions are reported (e.g., Trajano et al. 2014 ), SS generally induces moderate mean (<5%) performance impairments when testing is performed within minutes of stretching. Given the substantial between-study differences in poststretch changes (range, +5% to –20.5%), closer examination of the possible variables that influence the likelihood and magnitude of performance change after SS is required.

Inconsistent results are reported with the use of ballistic or bobbing (bounce through the movement at the end of ROM) movements. Both Bacurau and colleagues (2009) and Nelson and Kokkonen (2001) used 20 min of ballistic stretch activities and reported a 2.2% decrease in leg press 1-RM and a ∼5%–7% decrease in knee flexion and extension 1-RMs, respectively (likely fatigue related). Other studies imposing shorter durations of ballistic stretching or bobbing actions at end ROM have reported no significant effects ( Bradley et al. 2007 ; Samuel et al. 2008 ). Cumulatively, the data show a tendency toward an increase in performance with faster and/or more intense ballistic stretches, but substantial variability exists among studies and with regard to performances in different tests within studies, so a firm conclusion cannot be drawn.

Only 11 studies tested specifically during concentric (16 measures) or eccentric (3 measures) contractions ( Table 2 and Supplementary Table S4 1 ). There was a trivial average 0.4% increase in concentric force or torque (Supplementary Table S4 1 ). The 3 eccentric measures meeting our criteria had extensive variability (Supplementary Table S4 1 ), and thus, the relatively small percentage decrease (–1.2%) is not truly reflective. Hence, the limited data indicate generally inconsequential contraction type–dependent effects of DS on force production.

Force measurements have been performed using isometric or slower, dynamic movements (e.g., leg extensions, squats); thus, the test movement velocity does not always correspond with the DS movement velocity. The data analysis revealed small weighted changes for both strength-based performances (18 measures) and power-based tests (51 measures) ( Table 2 ). When evaluated further, moderate mean improvements of 2.1% were observed for jump performances (34 measures), whereas repetitive actions such as running or sprinting or agility (17 measures) showed a small 1.4% improvement. The lack of movement velocity similarity between the leg press and DS activities may have been a factor, with a trivial (4 measures) mean impairment of –0.23%. This may indicate that part of the positive effect of DS comes from allowing practice at tasks similar to those in the tests.

DS involves the performance of a controlled movement through the ROM of the active joint(s) ( Fletcher 2010 ). For a number of reasons, DS is sometimes considered preferable to SS in the preparation for physical activity. First, there may be a close similarity between the stretching and exercise movement patterns ( Behm and Sale 1993 ). Second, DS activities can elevate core temperature ( Fletcher and Jones 2004 ), which can increase nerve conduction velocity, muscle compliance, and enzymatic cycling, accelerating energy production ( Bishop 2003 ). Third, DS and dynamic activities tend to increase rather than decrease central drive, as may occur with prolonged SS ( Guissard and Duchateau 2006 ; Trajano et al. 2013 ).

PNF stretching

PNF stretching incorporates SS and isometric contractions in a cyclical pattern to enhance joint ROM, with 2 common techniques being contract relax (CR) and contract relax agonist contract (CRAC) (Sharman et al. 2006). The CR method includes an SS phase followed immediately by an intense, isometric contraction of the stretched muscle, with a further additional stretch of the target muscle completed immediately after contraction cessation. On the other hand, the CRAC method requires an additional contraction of the agonist muscle (i.e., opposing the muscle group being stretched) during the stretch, prior to the additional stretching of the target muscle (Sharman et al. 2006). Despite its efficacy in increasing ROM, PNF stretching is rarely used in athletic preactivity routines, possibly because (i) there is normally a requirement for partner assistance, (ii) it may be uncomfortable or painful, and (iii) muscle contractions performed at highly stretched muscle lengths can result in greater cytoskeletal muscle damage (Butterfield and Herzog 2006) and speculatively an increased risk of muscle strain injury (Beaulieu 1981), although no data clearly support this. Notwithstanding these potential limitations, PNF stretching remains an effective practice and its impact on muscular performance is worthy of examination.

Relatively few studies report the effects of PNF stretching, and no comprehensive or meta-analytical review exists that evaluates the effects of PNF stretching. This is surprising because PNF is a highly effective stretching method for ROM gain and includes an SS phase within the protocol and thus may be predicted to influence physical performance. Our search revealed 14 studies reporting the effects of PNF stretching on performance, with 11 using the CR method and 3 using CRAC. Because of the limited number of studies using CRAC, and the differences in methodology across stretching modes, we have reported only on CR stretching. Eleven studies incorporated 23 performance measures (Table 3, Supplementary Table S31, and Supplementary Fig. S1c1) examining the acute effects of CR PNF stretching on maximal muscular strength and power performance. Seventeen nonsignificant findings and 6 significant performance reductions were reported; no studies reported a performance improvement immediately after PNF stretching. Although the majority of studies reported no significant change in performance, our weighted estimate showed a 4.4% mean reduction in performance (Table 3). Thus, although notable performance impairments have been reported, PNF stretching generally induces small-to-moderate changes in performance that may be meaningful only in some clinical or athletic environments.

»View table Table 3.Summary of data from Supplementary Table S3 Summary of data from Supplementary Table S3 1 on proprioceptive neuromuscular facilitation stretching studies.

Dose–response relationship The limited number of studies imposing PNF stretching, coupled with the relatively small range of stretch durations (5–50 s), made an examination of the dose–response relationship impossible. The CR routine was normally repeated 2–5 times, providing an average SS phase of 2.5 ± 2.9 min. Based on our report of the effects of SS using durations >60 s, it may be concluded that the deficit induced by SS (–4.6%) is similar to that induced by PNF stretching (–4.4%). However, 9 of the 11 studies incorporating PNF stretching also compared the results with an SS condition, which enabled a direct comparison of the 2 stretch modes and eliminated stretch duration as a confounding factor. These studies showed that SS had a smaller negative impact (–2.3%) than did PNF stretching (–6.4%), indicating a more substantive effect after PNF. Regardless, the data are indicative of a small-to-large effect of PNF stretching on maximal muscular performance.

Effect of PNF on power–speed tasks Three studies reported 4 vertical jump performances, including squat and countermovement jump heights. One study reported a moderate-to-large and statistically significant reduction (–5.1%) in jump height (Bradley et al. 2007); however, this effect was no longer observed at 15 min after stretch; no significant difference was reported in the remaining 2 studies (Christensen and Nordstrom 2008; Young and Elliott 2001). One study (Bradley et al. 2007) examining 2 jump measures did not report either the mean changes or pre- and poststretch results, which prevented the calculation of ES for these measures. Nonetheless, analysis of the available data revealed a small mean reduction (–1.6%); thus, any impact on jump performance is likely to be trivial to small (Supplementary Table S41).

Effect of PNF on strength tasks Eight PNF studies examined 19 strength-based measures; 16 nonsignificant findings and only 3 significant losses were reported. One study (Reis et al. 2013) did not report percentage changes, which would have enabled weighted estimate calculation, whereas another study (Balle et al. 2015) did not report pre- and poststretching results, which would have enabled ES calculation. Nonetheless, the weighted estimate for the available findings revealed a large mean performance reduction (–5.5%); however, large 95% CIs (Supplementary Table S41) indicate a highly variable impact on muscular strength that may be practically meaningful yet small in comparison to interindividual variability in strength scores.