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The Effects of Stretching Interventions on Strength Performance:
A Critical Review of Literature
By Jerry Coy
Introduction
Warming-up and stretching before any physical activity is crucial in order to avoid injury, improve flexibility and performance. At least that was the mantra from physical education teachers, coaches, and aerobics instructors for several decades. One problem with that principle; it was never validated with scientific evidence. Resent research about stretching has reevaluated the conventional wisdom and now there is a 180-degree paradigm shift. Stretching is applying tensile force to lengthen muscles and connective tissues in order to increase range of motion and flexibility. Strength and flexibility are two, but equally important, components of physical fitness. (LaRoche, Lussier, & Roy, 2008) has found a negative correlation between acute stretching and a voluntary muscle force. (Fowles, Sale, & MacDougall, 2000) found a phenomenon called “stretching-induced force deficit” which is a temporary reduction of force production following acute stretching of the muscle. There are two main hypothesis to explain the mechanism behind the phenomenon; (1) Neurological factors, the activation of the Golgi tendon organs (a proprioceptive stretch receptor) inhibits the α motor neuron and (2) Mechanical factors with the viscoelastic and plastic properties of musculotendinous units (Fowles, Sale, & MacDougall, 2000) . There are several stretching techniques including static, dynamic, ballistic, passive, active, and PNF. There are also several ways to apply a stretching intervention: before or after competition, duration or quantity of stretching, and acute or chronic stretching. Although recent research suggests that acute pre-competition stretching may inhibit strength performance, incorporating other stretching interventions, like a post-event chronic stretching routine such as, dynamic or PNF stretching, to increase flexibility can be employed to minimize the force deficit or even increase strength performance.
Acute Static StretchingSeveral recent studies have shown that acute stretching does inhibit strength performance; however, some of these studies used an exaggerated quantity of stretching intervention and just looked at acute static stretching (Fowles, Sale, & MacDougall, 2000; Kokkonen, Nelson, & Cornwell, 1998; Nelson & Kokkonen, 2001; Nelson, Kokkonen, & Eldredge, 2005) . In the study by Fowles, Sale, & MacDougall, (2000), they found that 30 minutes of stretching does reduce total isometric peak torque in plantarflexors. They admitted that the exaggerated 30-minute stretching protocol is a lot more stretching than most athletes would normally use. In response to Fowles’ study, a study to determine a dose-response of stretching found that with longer the stretch duration, there is more inhibition of peak torque (Ryan et al., 2008). They studied various stretching durations (2, 4, & 8 minutes) and found significant decreases in torque in the 4 and 8-minute stretches but no significant change in the control or 2 minute duration. Where Fowles, et al. (2000) found a 28% decrease in torque after a 30-minute stretch, a 20-minute stretch only had a 10% decrease, 7% after a 10-minute stretch and insignificant changes in 1-4 minute stretch durations (Ryan et al., 2008). Like Fowlers, another study tested static stretching and how it affects the one repetition maximum test (1RM). This study tested 20-minute passive stretching of the lower extremities verses control non-stretching. They found significant decreases in strength with the 1RM in knee flexion and extension (7.3% & 8.1% respectively) (Kokkonen, Nelson, & Cornwell, 1998). Along with Kokkomen, et al. (1998), Rosenbaum & Henning also observed a significant 5% decrease in isometric peak torque following 3 minutes of static stretching of the triceps, which was accompanied by an increase in muscular compliance (Rosenbaum & Henning, 1995).
Like the previously mention studies (Fowles, Sale, & MacDougall, 2000; Kokkonen, Nelson, & Cornwell, 1998; Nelson & Kokkonen, 2001; Nelson, Kokkonen, & Eldredge, 2005), this other study tested acute static stretching but unlike the other studies, they tested on subject who have been incorporating stretching into their training routine for 10 weeks of stretching to see if there is any adaptation to stretching and the stretching-induced force deficit phenomenon. (Nelson, Kokkonen, & Eldredge, 2005) The subjects had engaged in 30 minute supervised stretching for at least 10 weeks prior to the study. On the day of testing the stretching group did 20 minutes of stretching prior to strength testing of knee extension and flexion 1RM. They too found significant decreases in strength after stretching compared to non-stretching group.
Twenty minutes of stretching is still an exaggerated duration. Unlike the previous three studies, (Winke, Jones, Berger, & Yates, 2010) looks at moderate intensity of static stretching and torque performance. The stretching protocol consisted of 3 repetitions of 30-second static stretches with 15-second rest intervals for a total of 3-minutes per leg. Even though they did find a significant increase in flexibility by 10.5%, they did not find a significant decrease in either eccentric or concentric leg torque. This is evidence that if the stretching intervention is not too intense, it may not cause significant loss of strength performance.
Dynamic Stretching
Although there is sufficient evidence to support the acute stretch-induced force deficit with intense static stretches, dynamic stretches may be better for strength performance. (Nelson & Kokkonen, 2001) studied ballistic stretching protocols but also exaggerated the intervention to 20 minutes of stretching. This very intense stretching also found a significant decrease in 1RM knee extension and flexion strength. In contrast to the high intensity stretching, (Beedle, Rytter, Healy, & Ward, 2008) looked at more moderate intensity of both static and dynamic protocols. They did not find a significant decrease in strength (leg press and bench press) with a more moderate intensity of either protocol when compared to the no stretch control. When you directly compare the static stretching with dynamic style, evidence shows that dynamic does not decrease strength (Sekir, Arabaci, Akova, & Kadagan, 2010) Subjects were randomly assigned, on different days, either dynamic, static or no stretch (control) protocols then tested for isokinetic strength as well as electromyography (EMG). They found significant decrease in strength performance of quadriceps and hamstrings with static stretching; in contrast with the dynamic stretch protocol, they found significant increase in strength performance.
PNF Stretching
Another type of stretching yet to be examined is proprioceptive neuromuscular facilitated (PNF) stretching. PNF combines active contraction with passive static stretching. PNF is usually performed in a clinical environment by a certified trainer or licensed massage therapist. PNF stretching did not inhibit isokinetic force production in Higgs & Winter, (2009) research even though they achieved significant gains in flexibility. Like Higgs & Winter (2009), (Molacek, Conley, Evetovich, & Hinnerichs, 2010) also studied chronic PNF protocol except they studied both high volume and low volume PNF stretching and compared to no stretching control. Even though they hypothesized that high-volume would have significant negative effect on 1RM, the results demonstrated an absence of negative effects. The evidence shows that PNF stretching when performed by a certified therapist will not produce the stretching induced force deficit (Worrell, 1994). One hypothesis to the mechanism why PNF does not induce a force deficit because of the Golgi tendon organs are temporarily inhibited by the α-motor neuron during the isometric contraction phase of the PNF (BRADLEY, 2007). The study by (Worrell, 1994) showed no significant gains in flexibility, but significant increases in eccentric peak torque with PNF at 15 sessions with 20-minute sessions over 3 consecutive weeks. Unlike Worrel et al. (1994), another study by Handel et al. (1997) found significant increases in flexibility (≤ 6.3%). Also knee flexor and extensor muscle eccentric peak torque increased (18.2% and 23.0%, respectively), knee flexor concentric peak torque increased too (9.4%) and knee flexor isometric peak torque increased as well (11.3%) (Handel, Horstmann, & Dickhuth, 1997).
Chronic Stretching
So far, all of the above studies have looked at acute stretching when the stretching intervention was within 1-2 hours prior to the performance testing. Although the stretching-induced force deficit phenomenon last up to 2 hours after the stretching, after that period, the stretching improves gains in strength. Evidence demonstrates that chronic stretching can stimulate gains in strength via hypertrophy (Kokkonen, Nelson, Tarawhiti, Buckingham, & Winchester, 2010; Rubini, 2007). Placing a tension on muscle can induce Z-line ruptures, which will increase protein synthesis and growth factor production (Goldspink & Harridge, 2003; Rubini, 2007)
According to research by Kokkonen et al. (2010), chronic stretch protocols enhanced gains in strength and hypertrophy and there is a synergistic influence of stretching on strength gains. Static stretching for 2 days per week (Tuesday and Thursday) with a progressive weight resistance-training program on alternate 3 days per week (Monday, Wednesday, & Friday) was compared to resistance training alone without stretching. The group that incorporated a chronic static stretching program had significant greater gains in 1RM strength. (Kokkonen, Nelson, Tarawhiti, Buckingham, & Winchester, 2010) . It is also suggested to stretch after the athletic performance to avoid the acute stretch-induced force deficit that follows acute stretching (Stone, 2006) . Stone, (2006) also recommends an aerobic warm-up to be accompanied with any stretching protocol to reduce the force deficit.
Conclusion
The evidence strongly supports the hypothesis of the acute static stretch-induced force deficit phenomenon; however, flexibility is just as an important component of fitness as strength Stretching is the way to increase flexibility and should not be sacrificed to maximize strength performance. Evidence also shows there is a dose response and a positive correlation to the dose and the deficit phenomenon. Exaggerated duration of stretching shown to decrease strength performance probably should be avoided within two hours prior to athletic performance. Chronic stretching training combined with resistance training has also been shown to boost strength performance due to the hypertrophy adaptations to the micro ruptures in z-lines similar to resistance training. Gains in strength via hypertrophy are observed after 3 weeks of just flexibility training, without any resistance strength training (Rubini, 2007) Therefore, evidence suggests that, although stretching exercises have a negative acute effect on strength, chronic stretching may not be negatively affected. As the muscles adapt to other stimulus, doesn’t it make sense that a chronic training over time can stimulate the body to adapt to flexibility training. Although there is research to support adaption to chronic stretching, the body of research is low. There needs to be more research on chronic, as well as, PNF stretching. The mechanisms that cause the stretch-induced force deficit Phenomenon is not fully understood and should be studied more; however, the neurological hypothesis fits best because of the EMG readings in the (Kokkonen, Nelson, Tarawhiti, Buckingham, & Winchester, 2010) study show evidence to support that hypothesis. Flexibility training is a very important resource in the therapy domain. Knowing how and when to stretch, one can achieve balance between strength and flexibility attributes of performance.
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