Sport performances are affected by the strength or muscular power output. Therefore, athletes and coaches have to select optimal warm-up before sport activities to improve muscular performance (40,41). Generally, preactivity warm-up contains a component of low-intensity activity followed by a stretching component.
Many coaches suggest static stretching (SS) before unit training or competitions, despite having little knowledge on the exact effects of this type of stretching (34). This is based on the notion that stretching improves performance, prevents injuries, and increases flexibility. According to recent studies, scientific evidence is yet to prove that SS prevents injuries or improves performance (34). In fact, many authors report that SS can have negative effects on athletic performance such as a decrease in maximal force production (13,15,27,32), jump height (6,24,42), and sprint speed (18,25) and also lead to an increase in reaction time and impairing balance (5). Research suggests that the effects of stretching on performance depends on the type of stretching conducted (2,3,19,20). It has recently been found that for athletes performing activities, which include basic biomotor abilities (such as strength, speed, and endurance), functional dynamic stretching (DS) is more successful than SS during warm up (4). Theoretically, DS and SS have the same positive effects on flexibility, although DS may improve explosive power performance (23,24,31).
Other various findings have shown that the proprioceptive neuromuscular facilitation (PNF) technique, which includes isometric contraction, can improve range of motion (ROM) by creating autogenic inhibition (26,35). However, effects of PNF on muscle strength are contradictory. Marek et al. (27) found that after PNF stretching, quadriceps muscle performance decreased during isokinetic exercises. Conversely, another study that examined effects of PNF on hamstring torque found that increasing hamstring flexibility was an effective method to improve hamstring strength both concentrically and eccentrically (38).
Literature in the area of stretching techniques is plentiful, however, once again, there is little consensus on which stretching technique (or combination of techniques) is the most effective (33). Studies show that SS decreases (6,32), whereas DS increases muscle power (12,17,39). The first hypotheses of this study was to determine if vertical jump (VJ) height would increase after BS treatment, which is a type of DS. However, it has not been examined whether combining BS with PNF techniques enhances muscle power and flexibility. Recent studies show that BS improves VJ height; however, the effects of PNF on strength is not conclusive (27,38). One way of improving strength might therefore be combining PNF and BS, which is our second hypotheses to determine if BS followed after PNF would increase VJ height. There is little evidence showing a negative effect of SS after PNF techniques on muscle power and flexibility. However, because of the negative effects of SS and PNF on strength, a decrease in VJ height is expected. Third hypotheses of the study was to conclude that VJ height obtained as a result of SS followed after PNF will be lower than BS followed after PNF.
The aim of this study was therefore to evaluate 3 different flexibility (BS, PNF + BS, and PNF + SS) techniques on VJ performance and to determine the appropriate stretching method in warm-up routine to assist in the planning of training techniques.
Experimental Approach to the Problem
A cross-sectional, within-group study design was used to evaluate the acute effects of stretching on explosive force production during warm-up. To evaluate the effects of 3 different stretching protocols on muscular performance, all the subjects underwent a familiarization session. Range of motion of each subject was evaluated using the sit and reach test. After a minimum of 48 hours after the evaluation of subject's flexibility, test protocols were applied. Each subject attended the laboratory on 1 occasion at the same time of the day (10 AM–12 PM, within 2 hours) during off-season to prevent subjects from being affected by factors of readiness level of central nervous system, heat, humidity, etc. In test protocols, all subjects performed aerobic warm-up (AW) followed by BS then PNF + BS, and finally PNF + SS treatments, respectively on the same day. Participants were tested by countermovement jump (CMJ) after AW and every stretching. The protocol designed to test the effects of the second treatment on the first and the third treatment on the second. Therefore, balance randomization has not been done.
One hundred male athletes (age = 22.8 ± 3.4 years; height = 1.79 ± 0.1 m; weight = 76.9 ± 11.1 kg; BMI = 23.7 ± 2.5 kg·m−2) volunteered to participate in this study. All subjects were healthy, physically active and participated in a variety of activities (volleyball, football, handball, etc.). All subjects were sport science and physical education students. The subjects reported having been physically active for approximately 10–15 hours per week. The participants had approximately 10 years of athletic background. Inclusion criteria were as follows: (a) subjects were physically active at least 2 years before the experiment, (b) free from any lower-limp musculoskeletal injuries, (c) did not use alcohol or any tobacco-related substances, and (d) refrained from any unhealthy dietary habits and did not use of performance enhancing drugs. All procedures were approved by the Ege University Ethics Committee, and participants were provided with an informed consent after the study was approved. A full explanation of study design was given to all of the participants before the test. Subjects were instructed to refrain from vigorous physical activity for 48 hours before the testing session. Subjects were also instructed to ingest a light meal and fluids before the experiment (Table 1).
Before the experimental procedures, all the subjects performed a familiarization session at least 48 hours before the testing day. During the familiarization session, subjects completed a 5-minute jogging session on a treadmill and then 3 different stretching techniques (BS, PNF + BS, PNF + SS). In addition to this, subjects were familiarized to the sit and reach test.
Sit and Reach Test
Participants pressed against the testing board while sitting on the mat. Knees were extended, and then the participants were instructed to reach and hold as far as possible along the board, without bouncing.
Countermovement Jump Test
Countermovement jump was performed for measurement of the explosive force and reuse of elastic energy during inversion of eccentric to concentric movement (28). Subjects were asked to stand on both feet on the Newtest (Newtest 2000; Newtest Oy, Oulu, Finland) and lower their body towards the ground by moving into flexion position at the trunk and lower extremity while extending upper extremity. Then subjects were asked to jump as high as possible while extending their legs. The jump was repeated 3 times.
Subjects were required to attend an orientation session in which they were familiarized with the testing procedures. Two days after familiarization, subjects returned for testing. The procedure consisted of 4 stages, these included: AW, ballistic stretching (BS), progressive neuromuscular facilitation along with ballistic stretching (PNF + BS), and progressive neuromuscular facilitation along with static stretching (PNF + SS). In all stretching techniques, lumbar extensor, gluteus maximus, and hamstring muscles were stretched with a single stretching exercise. Each stretching technique was applied for 4 sets bilaterally. All stages were completed in 1 day for each participant. According to the test protocol, subjects completed a 5-minute warm-up on treadmill. Then subjects used 3 stretching techniques in order of BS, PNF + BS, and PNF + SS. After the warm-up and between each treatment, subjects rested for 2 minutes. In CMJ conditions, 3 maximal VJ were performed after warm-up and at each stage, as mentioned above. Each jump was repeated 3 times, with a 1-minute rest between each repetition. The best CMJ was chosen for the evaluation.
In BS technique, each subjects had to bounce once per second for 5 seconds. In PNF + BS technique, slow reversal-hold-relax (SRHR) technique was used as PNF. Slow reversal-hold-relax procedure includes 5-second isotonic contraction of the antagonist muscle, followed by a 5-second isometric contraction of the antagonist, 3 seconds of voluntary relaxation, followed by a 5-second isotonic contraction of agonist muscle followed by 3 seconds of voluntary relaxation. Right after the last relaxation of SRHR, 5 seconds of BS was carried out.
In the PNF + SS treatment, SRHR procedure was used exactly the way it was used in PNF + BS technique. After the PNF technique, SS was carried out with the assistance of the instructor for 30 seconds in the mild uncomfortable stretching position.
After normality was assured by a Kolmogorov-Smirnov test and Q-Q plots were assessed for linearity, data were analyzed using parametric tests. Data were divided into 2 different classifications. One was done according to initial flexibility scores, and then the resulted values were divided within themselves in 3 groups (33.33% percentiles; low flexibility group [F1] [flexibility between 0 and 21 cm, age: 23.06 ± 2.71, BMI: 24.01 ± 2.09, athletic background: 11.03 ± 4.03], moderate flexibility group [F2] [flexibility between 22 and 30 cm, age: 22.63 ± 3.29, BMI: 24.18 ± 2.80, athletic background: 10.06 ± 4.03], and high flexibility group [F3] [flexibility between 31 and 45 cm, age: 22.28 ± 4.05, BMI: 22.89 ± 2.43, athletic background: 9.43 ± 4.04]). Second classification was done according to prejump height (33.33% percentiles; poor prejump performance group [PJ1] [flexibility: 22.45 ± 7.74, age: 22.51 ± 2.34, BMI: 24.36 ± 2.66, athletic background: 10.69 ± 3.98], moderate prejump performance group [PJ2] [flexibility: 25.25 ± 8.01, age: 23.71 ± 3.85, BMI: 23.70 ± 2.49, athletic background: 10.47 ± 4.16], and high prejump performance group [PJ3] [flexibility: 27.8 ± 9.44, age: 22.1 ± 3.62, BMI: 23.1 ± 2.22, athletic background: 9.55 ± 4.08]). Statistical program (SPSS 17.0) was used for statistical analysis. Values of p ≤ 0.05 were considered statistically significant. Data are presented as mean ± SD. Repeated measure analysis of variance (ANOVA) was used to determine an intraclass correlation conducted for CMJ after each of the 3 treatment (BS, PNF + BS, and PNF + SS) in each classification. Bonferroni and LSD post hoc test was applied to assess the difference between the baseline and post-BS, post-PNF + BS, and post-PNS + SS.
The assumption of variance homogeneity related with classified groups was tested by using Levene Test. In the case of variance of homogeneity, ANOVA test was applied followed by post hoc Bonferroni and LSD. When homogeneity of variance was violated, Welch Test was applied and Games-Howell test was used for the post hoc. Since flexibility groups had variance homogeneity, ANOVA test was applied and then, post hoc Bonferroni and LSD was used. Because of a violation of homogeneity for prejump performance groups, Welch Test was applied along with post hoc Games-Howell Test.
For the prejump performance scores, there were significant differences when comparing all flexibility groups (p ≤ 0.005, p = 0.0038). Vertical jump performances of low flexibility group (F1) were lower than high flexibility group (F3). There were no significant differences among VJ performances of all flexibility groups after BS, PNF + BS, and PNF + SS (p > 0.05).
Among VJ performances of all groups classified according to prejump performance, there were significant differences after each intervention (p ≤ 0.05, p = 0.000). Significant differences (p ≤ 0.05) were found in repeated measures of flexibility groups. Proprioceptive neuromuscular facilitation + BS decreased VJ height, whereas BS increased it in F1 group. In F1, F2, and F3 groups, PNF + SS decreased VJ height compared with BS and PNF + BS. Proprioceptive neuromuscular facilitation + SS decreased VJ height compared with PNF + BS in F3 group (p ≤ 0.05).
There was a significant increase in VJ height in the BS condition compared with prejump performance in F2 groups (p ≤ 0.05) but not in F3 groups. There also was a significant reduction in VJ height in the PNF + BS compared with the prejump performance in F3 group (p ≤ 0.05) (Table 2; Figure 1). Significant differences (p ≤ 0.05) were found in repeated measures of all prejump performance groups.
In the poor prejump group (PJ1), moderate prejump group (PJ2) and high prejump group (PJ3), there was a significant reduction (p ≤ 0.05) in VJ height in the PNF + SS compared with the BS (p ≤ 0.05). In PJ 1 and PJ 2 groups, PNF + BS decreased VJ height significantly compared with BS (p ≤ 0.05). Ballistic stretching increased VJ height when compared with the prejump performance in PJ1 group (p ≤ 0.05). In PJ2 and PJ3 groups, there was a significant reduction (p ≤ 0.05) in VJ height in the PNF + SS compared with the prejump performance (Table 3; Figure 2).
Significant differences were found in repeated measures of the whole group (p ≤ 0.05). Proprioceptive neuromuscular facilitation + SS decreased VJ performance compared with the prejump performance, BS, and PNF + BS. There was also decrease in the VJ performance in PNF + BS compared with the BS. Ballistic stretching increased VJ performance compared with the prejump performance (Table 4; Figure 3).
This study aimed to determine the appropriate stretching technique used in warm-up period of power-required sport activities. Acute effects of different lower-limp stretching techniques on explosive force production were compared. Vertical jump scores were used as a measure of significance.
Initial flexibility scores proved to be significant in determining whether an individual's flexibility changed the effect that stretching had on VJ scores. However, Unick and et al. found no effect of flexibility on VJ performance (36).
Ballistic stretching improved VJ height significantly in subjects having low and moderate flexibility. Ballistic stretching, by producing more intense action potentials, increased the number of firing motor units as a reaction to abrupt stretching of muscle spindle, which may lead to myotatic reflex for individuals having low flexibility. This positive neurological effect of BS may have increased performance.
Guissard et al. (22) reported that BS caused facilitation of the stretch reflex, which is mediated by the facilitator influences of muscle spindles type I and II receptors on homonymous alpha motor neuron excitability. This study supports our findings. Unick et al. did not find significant differences after BS on VJ performance (36). Likewise, Bradley et al. (8) found that BS has no negative effect of VJ performance. However, Nelson et al. (29) showed that BS decreases maximum strength by 8%. Although different studies present various results, in accordance with our first hypothesis, BS is recommended for athletes with poor flexibility to increase their jump height performance.
After BS, vertical jumping performance was not affected in the high-flexibility group, whereas performance decreased after PNF + BS, which contradicts with our second hypothesis; that shows PNF had a depressing effect on power production in this group.
Previous studies show that PNF is more effective in increasing ROM compared with passive stretching; maximal voluntary contraction before stretching causes more increase in the length of muscle (22). This might be the mechanical reasons for the increase in ROM. This can also be explained by neurological mechanisms such as autogenic and reciprocal inhibition. Rapid decrease in musculotendinous stiffness is observed as a result of inhibition of motor neuron pool after maximal voluntary contraction (21,43). Probable decrease of muscle endurance and strength is also associated with neurological and mechanical factors mentioned above.
In all flexibility groups, the intervention of PNF + SS decreased VJ height compared with the PNF + BS intervention, which shows that SS has an important contribution to the effects of PNF. This supports our third hypothesis. According to the test results, it is not recommended to practice PNF + SS because when this stretching technique was used by the athletes immediately before an explosive athletic movement, their power production decreased.
Decrease in performance after SS is explained by a combination of mechanical and neurological factors (9). Mechanically, SS results in a longer and more compliant muscle-tendon unit (37). Contractile elements must then contract more rapidly and over a greater distance to “pick-up the slack” resulting in a reduced peak torque and a slower rate of force development (18,37). Neurologically, SS seems to decrease motor unit activation (1,7,27,32).
It can be expected that an athlete having high flexibility also has high jumping performance and are affected less by the inhibition created by SS. Flexibility training using SS may increase excitability threshold of golgi tendon organ. As in athletes having high flexibility tendon tension doesn't increase too much even at the end of ROM, it could be expected that strong inverse myotatic reflex is not generated and the following jumping performance will not be affected much. However, the decrease in performance was maximum in high-flexibility group in this study. It could be explained by mechanical factors created by SS in the muscle in condition of increased ROM.
Positive neurological effects of BS seemed to improve the VJ heights of athletes with poor prejumping performance, accompanied by low flexibility. However, increase in performance after BS was not observed after PNF + BS in poor prejumping performance group, showing that PNF negates the positive effects of BS.
Previous studies have shown that PNF stretching significantly decreases acute maximal performances, which supports our results (7,11–13,16). However, some other studies have conflicting results showing that PNF improves or does not affect VJ performance (10,30). In moderate and high prejumping performance group, VJ performance decreased maximally when SS is applied in combination with PNF, which could be related with the negative neurological effects of PNF and SS. Our findings are supported by other studies, which indicate that PNF and SS cause acute decreases in maximal performance (7,11–13,16). However, Manoel and et al. (26) found that SS and PNF do not decrease knee extension power in women. In a similar study, Cramer et al. (14) found out that SS did not result in a performance decrease. When all these are taken into consideration, for athletes with poor jump performance, it would be efficient to use BS in the warm-up period. They can also use PNF + BS or PNF + SS; however, they would not benefit from either of them because they do not affect performance in VJ. But with athletes who have high jumping performance, we do not recommend the usage of PNF + SS because it decreases performance. Although it only increases performance slightly, athletes might also benefit from BS.
In the whole group, BS treatment increased VJ height. However, VJ height decreased in PNF + BS treatment compared with the BS treatment. Vertical jump height decreased also in PNF + SS treatment compared with PNF + BS treatment. Our study therefore shows that BS has positive effects on lower extremity explosive power production, whereas PNF and SS methods affected it negatively. For the conclusion, our study clearly shows that to improve strength, BS can be used during the warm-up period.
Based on the results of this study, it can be suggested that PNF + SS combination must be avoided as part of a pre-event warm-up session. In addition to this, athletes especially who have low flexibility and power-production capacity are advised to incorporate BS in the warm-up period of training and competitions to increase their power-production capacities. However, additional investigation is required to find appropriate stretching methods to improve talents of athlete having high flexibility and power-production capacities.
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Keywords:Copyright © 2014 by the National Strength & Conditioning Association.
ballistic stretching; proprioceptive neuromuscular facilitation stretching; static stretching; vertical jump