Introduction

Stretching exercises are commonly performed by apparently healthy people and athletes with the goal to increase or maintain joint range of motion, and improving athletic performance, respectively.1, 2 Moreover, it can be used as part of warming up exercise before sports activities3 and rehabilitation programs.4, 5

There are several types of flexibility training methods, such as the static stretching, which is used to reach the above-mentioned benefits.3 Despite the advantages of stretching, there are several studies which have been indicated that performing stretching before resistance exercises can significantly reduce torque,6, 7 strength performance,8, 9 power output, and agility.10

Regardless the force production mechanism, muscle power is characterized by several tasks performed in sport fields. One of these movements commonly performed during sport training programs and muscle power assessment is the vertical jump (VJ). In relation to concurrent training between flexibility and muscle power, there is still no consensus about the effect of stretching before power training. Bradley et al.11 observed a significant reduction in the VJ performance after passive static stretching (PSS) applied in quadriceps, hamstring, and ankle flexors muscles. However, Sandberg et al.12 found significant improvement in the VJ performance after PSS applied in hip flexors and ankle dorsiflexors muscles when compared to a protocol without stretching. In addition, other studies have been noted that the chronic application of PSS does not seem to have a negative effect on VJ performance.13, 14

Besides these controversial results, to date, there is a lack of evidences of the subsequent VJ performance followed by PSS only adopted for knee flexors (KF) muscles. The KF muscles have an important function during the VJ, because of it synergic role with the quadriceps and gastrocnemius muscles, which is considered responsible for improving the joint stabilization during knee flexion–extension, and also improving the elastic energy storage.15 Additionally, KF plays an important role during jumping tasks in order to assists the anterior cruciate ligament stabilization and also avoid the anterior tibial displacement.16

Furthermore, evidences related with the potential effects of PSS implemented with the goal to improve the VJ performance, may help coaches and conditioning professionals during the prescription of power training for lower limb muscles. Therefore, the purpose of this study was to investigate the acute effect of PSS applied in KF muscles on the VJ performance of recreationally trained men and women. We hypotheses that lower volume of PSS applied in KF muscles may increasing the stretch-shortening cycle (i.e. elastic energy storage) during the eccentric phase, and consequently, improving the VJ performance.

The study was comprised 10 male and 10 female volunteers with previous experience in resistance training. The follow inclusion criteria were adopted: (a) to practice resistance training (RT) for at least three years; (b) to perform jumping tasks and power training with a frequency of three times per week; (c) to perform stretching exercises at least three months. The exclusion criteria were: (a) to respond the PAR-Q questionnaire positively; (b) to present any previous osteomioarticular injuries or surgery in lower limb; (c) to use any ergogenic substance that could influence in the power performance.

All the participants signed a consent form and the study was duly approved by the Castelo Branco University Research Ethics Committee under No. 2012/056 protocol and it was in according to the guidelines of the National Health Council Resolution 466/12 of human experiments. The anthropometric measures (i.e., height and body mass) were measured on a digital scale Filizola TM (Beyond Technology), following the recommendations proposed by the International Society for the Advancement of Kinanthropometry (ISAK).

Before each test session, a standard warm up was adopted. The warm up was applied during 10 min and consisted of jogging, short sprints, and small jumps.17

After the warm up, two experimental protocols were applied in two non-consecutive days with 48 h intervals in a crossover randomized design: (a) traditional protocol (TRAD) – the participants performed the VJ without prior PSS; (b) passive static stretching protocol (PSS) – the researchers applied PSS on the knee flexors immediately followed by the VJ performance.

The PSS applied in the knee flexors was applied following the recommendations of Herda et al.18 The subject was positioned lied down in a supine position, and then the evaluator conducted a flexion of one hip, keeping the knee fully extended until to reach a point of discomfort indicated by the participants. Then, the range of motion of the stretched leg was maintained for 30 s, while the opposite lower limb was kept in a rest position on the ground. The same procedure was performed three times alternating right and left legs (see Figure 1). Due to the great variability of the participant's flexibility, was not possible to adopt a standard hip flexion angle to the stretch.

Figure 1. Passive static stretching protocol applied to the knee flexors muscles.

Vertical jump testing

Firstly, the subject took the standing position, keeping aside a graduated surface and the right shoulder flexion of 180° and elbow in extension to the brand was registered for the initial test. Secondly, the distal phalanx of the middle finger of each subject was covered with chalk to represent the highest point reached on the wall surface. Then, during the downward phase of the VJ, the subjects performed and hold the flexion of the hips and knees, while the pendulum motion of flexion, extension and hyperextension of the shoulders (see Figure 2A). In the ascending phase, the extent of the ankle, knee and hip was carried out and, bending the shoulders in order to jump as high as possible, touching the middle finger in the graduated wall surface (see Figure 2B). Throughout the test, it was not allowed to move his feet before the VJ and the subjects were instructed to perform a full knee extension during the aerial part of the jump. Three attempts were allowed adopting 2-min intervals between them. The greatest value in centimeters between the three attempts was recorded.

Figure 2. Vertical jump protocol. A.) The inicial position of the body before the jump. B.) The end phase of the jump were the participants touched the middle finger in the graduated wall surface. The participants were instructed to use their arms freely during the jump to allow touching the wall.

Statistical analyses

Statistical analysis was performed in SPSS version 20.0 software (Chicago, IL, USA). Statistical analysis was initially performed using the Shapiro–Wilk test of normality and homoscedasticity test (Bartlett criterion). All variables showed normal distribution and homoscedasticity. Descriptive statistics (mean and standard deviation (SD)) were computed and presented for each dependent variable. The intraclass correlation coefficient (ICC = (MSb − MSW)/[MSb + (k − 1) MSW), where MSb = mean-square between, MSw = mean-square within, and k = average group size, was calculated to determine the reproducibility of intersubject for each measure. Paired T test was used to determine significant differences among protocols. The effect size was also computed following the Rhea19 recommendations. The value of p ≤ 0.05 was adopted for all inferential analyses.

The ICC for VJ measures range about 0.91 and 0.93 between both protocols. Table 1 presents the participant's characteristic as age, height, total body mass and body mass index. No differences were noted inter and intragroup for the anthropometric variables (p ≤ 0.05).

The mean values and the standard deviation of the maximum height reached by the volunteers obtained in the VJ are presented in Table 2. In the men group, there was a significant increase in the VJ performance under PSS (13.6%) compared to TRAD protocol (p = 0.001). Similar results were noted under PSS (11.9%) compared to TRAD condition for women group (p = 0.021). The effect size was classified as small for both groups.

The purpose of the current study was to investigate the acute effect of PSS applied in KF muscles on VJ performance between men and women with previous RT experience. The main finding of the current study was the significant increasing in the VJ performance after PSS applied on hamstring muscle compared to the protocol without muscle stretching in both men and women groups. This finding suggests that PSS with lower volume applied only in the KF muscles can improve the VJ performance.

The VJ is considered a multi-joint movement and involves several muscle groups; other muscles besides the knee extensors, such as hip extensors and plantar flexors are also involved directly in its execution. In this sense, Sandberg et al.12 verified the involvement of these other muscles in the VJ performance. Through the application of PSS on hip flexors and ankle extensors, muscles considered antagonists of the hip extensors and plantar flexors respectively, the authors observed a significant improvement in the performance of VJ after PSS protocol compared to the protocol without stretching. However, there are several muscles involved in the VJ task, and our results sought to present the responses induced by implementation of PSS applied only in the hamstring muscles and its influence in the VJ performance.

On the other hand, some studies have observed detrimental effects of PSS on strength and power performance when the stretching was applied in agonist muscles (i.e., target muscle) for both men and women.6, 20 Bradley et al.,11 investigated the effect of PSS, ballistic stretching and Proprioceptive neuromuscular facilitation (PNF) applied in the quadriceps, hamstrings and ankle extensors on the VJ performance immediately after stretching and during intervals of 5, 15, 30, 45 and 60 min. The authors found significant reduction in the performance of VJ up to 15 min after performing the PSS. Despite this controversial finding, some methodological differences, as the muscles involved in the stretching, can justify these distinct evidences. In Bradley et al.,11 beyond the quadriceps muscle was involved, which is considered one of the agonists of the movement, a total of 10 min of five stretching exercises were performed, through two sets of 30 s for both legs, nevertheless in the current study was applied only three sets of 30 s only in the KF.

Additionally, the contrast between methodological variables of prescription, such as the volume and the duration of each stretching set, can influence the acute responses induced for a specific task. These influences was noted in previous studies, as demonstrated by Power et al.,21 who observed reduction in torque and isometric strength of the knee extensors after PSS applied on the quadriceps, hamstrings and extensors of the ankle, during a protocol composed by three sets of 45 s with 15 s interval. However, the authors found no significant difference in the VJ performance. Moreover, Franco et al.8 investigated the effect of different volumes of PSS and PNF applied on the triceps brachii before the performance of bench press exercise. The authors observed which one set of 40 s of PSS, resulted in a significant reduction in number of repetitions performed compared to the protocol without PSS. In this context, the studies which did not indicate deleterious effect of PSS on the VJ performance, often applied relatively low volume of PSS, similar to the procedures adopted in the current study. This evidence suggests which the volume, set, and the muscles involved in the stretching can be a determinant factor to induce a deleterious effect of the muscle power performance.

Biomechanical and neural aspects induced by stretching exercises which affect the VJ should be considered with the goal to better understand the possible strategies which could be implemented to improve its performance. Static stretching involves some mechanical adaptations within muscle, that alter properties of connective tissue elements, which results in a reduction in stiffness and consequently the passive tension.22 These mechanisms involved, likely modify neuromuscular feedbacks that would result in a decrease of activation and muscle force-generating capacity.23 Furthermore, although the major KF function as a joint stabilizer while performing knee flexion–extension,15, 16 it has only a secondary role to boost the body vertically,24 thereby a possible reduction of its performance caused by stretching was enough to result in an increase performance of its antagonist, the knee extensors, as together with the gluteus maximum are the mainly responsible to produce angular acceleration and consequently reach higher heights during jumping.25

Considering that distinct levels of individual's flexibility might result different neuromuscular responses after stretching,26 one of the limitations of the current study is that no baseline levels of flexibility were measured prior the experimental protocols. Additionally, future studies should consider the use of surface electromyograms to evaluate muscle activation pattern during VJ, and investigate the effect of different volumes of PSS on the KF and consequently its performance in the VJ. Thus, the comparison between different muscle groups in response to the application of PSS prior to muscle power tasks should be investigated. Nevertheless, the present study adopted procedures that could be easily implemented in sport fields, and it was verified that the PSS simply applied on the KF improves the VJ performance. In this context, considering that most athletes have been performing the VJ in their RT programs, these findings may help sport professionals during the prescription of stretching exercises and muscle power in the same training session with the goal to improve athletic performance and strength outcomes.

In conclusion, the results of this study suggest that PSS applied on the KF muscles with a total duration of 90 s (3 sets of 30 s) for each member is enough to significantly improve performance in VJ for both women and men with previous experience in RT. These findings indicate that lower volume of PSS can be applied only in the KF muscles prior to the VJ, thereby enhancing muscle performance during this specific movement and consequently optimizing the training outcomes. Therefore, future studies could investigate the effect of varying the volume of the PSS, muscles groups, and whether applied other types of stretching would result in an increasing in jumping performance.

Authors declare that they do not have any conflict of interests.

Received 1 March 2016

Accepted 10 May 2016

Corresponding author. gabriel.andrade.paz@gmail.com