Acute effects of static and dynamic stretching exercises on lower extremity isokinetic strength in taekwondo athletes

1. Introduction

The main component in a traditional warm-up is submaximal aerobic running, which is used to increase the body temperature approximately 1–2 ∘ C. Another essential component in a warm-up is static stretching (SS) [1]. Pre-exercise SS is widely believed to reduce the risk of injury and enhance performance [2].

However, recent studies reported that pre-exercise SS decreased the maximal isometric force production, jump height, sprint performance, balance, agility, and movement time compared to dynamic stretching (DS) [2, 3, 4]. Negative effects of SS are attributed to mechanical factors (e.g., alterations in muscle tendon unit stiffness) and neural alterations such as changes in reflex sensitivity and decreased motor unit activation as evidenced by reductions in EMG output [2, 5].

Because of the negative performance effects of SS prior to muscular strength and power tasks, several researchers have suggested the use of DS exercise [2, 6, 8]. Yamaguchi et al. [8] reported that the improvement in muscle performance following DS might be associated with two factors: an increase in muscle temperature and the occurrence of post-activation potentiation (PAP). PAP involves the phosphorylation of myosin regulatory light chains, which improve actin–myosin interaction. This interaction is caused by the voluntary contraction of the muscle antagonist to target the stretched muscle [9].

The explosive strength at the beginning and during the motion is extremely important in Taekwondo. Strength may decrease the decision-making duration of the opponent as the speed increases, and it may also increase the possibility of making mistakes [10]. For this reason, some researchers pointed out to a stretching protocol that will not cause decreases in the strength performance. Strength and condition trainers have adopted dynamic and active stretching protocols to prepare the athletes for competitions [11, 12]. Kim et al. [13] argued that the measurement of extensor and flexor strength in taekwondo athletes is an important indicator in monitoring training effects.

Contradictory results regarding the effects of SS and DS on isokinetic strength were also reported. Some researchers argued that SS has detrimental effects on isokinetic strength [14, 15, 16, 17], whereas others reported no strength isokinetic deficits after SS [18, 19, 20].

Therefore, the current study primarily aimed to examine and compare the acute effects of SS and DS on the knee and ankle flexor and extensor concentric (CON) isokinetic PM at 60 and 180 ∘ /s in well-trained male taekwondo athletes. Thus, we hypothesized that SS would lead to stretching-induced PM deficits of the knee and ankle joints at both velocities when compared to DS.

2. Methods

3. Results

No significant differences ( $p>$ 0.05) were observed among the stretching exercises with respect to either joint or velocity (Tables 1 and 2).

The main findings point out no stretching-induced strength deficit by SS exercises while the DS exercises do not cause a higher strength increase compared to their SS counterparts on the knee and ankle flexors and extensors of well-trained taekwondo athletes. In this respect the results of this study should be examined in the light of Kapo et al.’s findings [22] who have reported superior strength effect of DS compared with proprioceptive neuromuscular facilitation in 50 male athletes from different sports disciplines, such as karate, taekwondo, box, football, and athletic sprint.

In contrast with our study, Sekir et al. [23] reported that SS significantly decreased quadriceps and hamstring muscle strength in eccentric (ECC) and CON modes at same velocities in well-trained track and field athletes. On the other hand, similar to the results of our study, Egan et al. [20] found that SS did not adversely affect the CON strength over a velocity spectrum during any of the post-stretching intervals in National Collegiate Athletic Association Division I women’s basketball players.

Regarding recreational athletes, researchers reported that SS exercises have generally no stretching-induced PM deficits in the knee muscles [5, 15, 18, 19]. Ayala et al. [18] reported that short pre-exercise SS and DS of the lower limb stretching routine did not elicit stretching-induced deficit or improvements on the knee flexor and knee extensor isokinetic CON and ECC strength and power at different velocities (60 ∘ /s, 180 ∘ /s, and 240 ∘ /s). However, Marek et al. [16] and Miyahara et al. [14] reported different results than the previously mentioned studies.

As mentioned, two factors have been proposed to explain the stretching-induced strength deficit [20]: (a) mechanical factors associated with decreased musculotendinous stiffness that may alter the length-tension relationship and (b) neural factors caused by decreased muscle activation.

We found that the majority of the studies involved the effects of different stretching exercises on the isokinetic strength of knee muscles. In this regard, the results of the current study also suggest that differences in isokinetic ankle flexion and extension PM after NS, SS, and DS are not significant. According to Kim et al. [24] competitive taekwondo athletes use kicking more often than punching during the competition. Additionally, the World Taekwondo Federation competition rules indicates that to score a point, the kick must be powerful [24]. Thus kick flexion and extension strength are very important for achieving successful results in taekwondo [24, 25].

In conjunction with previous studies, disparate findings on the effects of SS and DS on isokinetic strength exist. Their causes remain unknown [26]; however, they could be related to the duration and intensity of SS exercise, subjects’ training or competition experience level [20, 27], and isokinetic test speeds [28]. According to Ayala et al. [18] the overall SS duration per isolate muscle group of ⩽ 60–90 s may have no stretching-induced alterations in strength and power during CON and ECC isokinetic muscle actions.

Based on previous studies [20, 23, 29] well-trained athletes who are accustomed to static or dynamic movement actions as in taekwondo may be less susceptible to the stretching-induced strength deficit. These results should only be considered in male taekwondo athletes who can perform SS and DS prior to training and competition without subsequent strength deficits.

5. Conclusion

In sports events in which slight positive increases and differences in force and performance affect the result of the competition, it is recommended that the stretching protocols applied in warm-up are performed in accordance with the requirements of relevant sports event. All considered we recommend that future studies are conducted by considering the mechanical and neuromuscular factors that include proper set numbers and stretching durations to achieve positive increases in strength in Taekwondo athletes. However, whether this conclusion may be extended to other sporting events requires further research.