The Acute Effect of Proprioceptive Neuromuscular Facilitation in Explosive Force and Jump Resistance of Volleyball Players

The inclusion of stretching during warming up is a common practice within the training sessions of professional and recreational athletes [1,2]. Historically, this practice was maintained since it enforced possible effects on improved performance and injury prevention [3-5]. However, more recent studies have challenged these ideas, demonstrating that stretching before the main part of the training session does not prevent exercise-related injuries, and besides, depending on its type, intensity and volume, it can promote temporary decreases in neuromuscular performance and reduction of performance [2,6,7].


Introduction
The inclusion of stretching during warming up is a common practice within the training sessions of professional and recreational athletes [1,2]. Historically, this practice was maintained since it enforced possible effects on improved performance and injury prevention [3][4][5]. However, more recent studies have challenged these ideas, demonstrating that stretching before the main part of the training session does not prevent exercise-related injuries, and besides, depending on its type, intensity and volume, it can promote temporary decreases in neuromuscular performance and reduction of performance [2,6,7].
In relation to the reduction in performance, the effect of several stretching methods has been verified in the manifestations of the force [6]. Among the types of stretching, the static is the most studied and the one that presents greater consensus among the results about the performance. In contrast, the Proprioceptive Neuromuscular Facilitation (PNF) still presents gaps regarding its effect on performance. These manifestations, the explosive force, especially of the lower limbs, is one of the most important factors that determine the physical condition of the athlete and their performance [8], especially when this ability is inherent to the sport, that can be affected by stretching. Another important indicator of the athlete's physical performance is muscular endurance that can also be affected by stretching protocols performed during the warming up phase of a training session [9,10].
The volleyball is a sport that requires constant jumps [5,11] requiring that players have the ability to produce explosive force of lower limbs as well as have the ability to maintain the performance of these jumps (jump resistance) during a match. Especially in a real competitive game situation, the performance of numerous jumps within a match [12] demonstrates the need to maintain a high level of explosive force in this sport mode. Knowing the importance of maintaining these manifestations of force for success in the sport, it is essential that coaches have knowledge of factors that can impair the performance of the athletes.
Some studies have shown that stretching can negatively influence the physical performance of athletes by generating decreased in muscle strength [13]. Thus, the effect of stretching on performance has been widely discussed [1]. Among the types

Abstract
Stretching during warm-up has been widely discussed for exerting a possible influence on performance. Thus, the purpose of this study was to verify the influence of the Proprioceptive Neuromuscular Facilitation (PNF) in the explosive force (EF) and the jump resistance (JR) of athletes. Twelve male volleyball players (21 ± 4 years; 178 ± 0cm; 75.82 ± 11.02kg; 23 ± 2.87kg/m2) underwent two PNF protocols: a) Protocol 1: four repetitions with 5-second maximum voluntary isometric contraction followed by 30 seconds of passive stretching and b) Protocol 2: four repetitions with 5-second maximum voluntary isometric contraction followed by 60 seconds of passive stretching. For the assessment of EF and JR the height of the countermovement vertical jump was considered measured through the Jump System Pro. The Student t test was used to compare the heights of the jumps between pre and post stretching moments for the two analyzed variables. EF (Protocols 1 and 2 t (7) = 1.462, p = 0.261, and t (7) = 1.402, p = 0.302, respectively) and JR (Protocols 1 and 2, t (7) = 1.542, p = 0.242, and t (7) = 0542, p = 0.712, respectively) did not differ significantly (p>0.05) after both stretching protocols. The results of this study show that the stretching protocols used did not acutely affect the performance of EF and JR of volleyball athletes.  of stretching, the static is the one that presents the most studies and that show agreeable data that discourages its conduction before strength and power activities [1,2]. The Proprioceptive Neuromuscular Facilitation (PNF) is a fairly common type of stretching in the athletic scenario [14], however, unlike static stretching, it presents limited evidence about its effect on the manifestations of force [1,2].
The few studies that sought to verify the influence of PNF in performance show very divergent results [9]. Specifically, regarding the explosive force measured from the performance in vertical jumps, there are studies that showed negative effect [2,13] and others which did not find performance reduction after PNF protocol [15,16]. Regarding the muscle endurance, Franco and Colleagues [10] and Barroso and Colleagues [9] evaluated muscle resistance through the number of repetitions performed in the exercises of bench press and leg press, respectively, and found reductions of the number of repetitions after PNF, showing that such type of stretching can negatively influence muscle resistance. However, to our knowledge, no study analyzed the effect of PNF on jump resistance (JR).
Based on this controversy regarding the acute effect of stretching on performance and, therefore, considering that the PNF is a common type of stretching however without agreement results in the literature regarding performance, the aim of this work was to analyze the acute effect of PNF on the explosive force (EF) and jump resistance (JR) of volleyball athletes. Considering the significant body of evidence that shows the negative effect of passive stretching in performance and knowing that it composes the elongation of the PNF type, our initial hypothesis is that stretching applied in this study exerts negative influence on the performance of explosive force and muscular endurance, the reduction of performance dependent on the protocol used.

Countermovement Vertical Jump
For the evaluation of the EF and JR, the countermovement jump was used. In addition to its practicality and specificity with volleyball, the countermovement jump is a widely used method for evaluating the explosive force of lower limbs [5,11,13]. The

Stretching Protocols
The PNF stretching method by contract-relax technique [17] was used for the posterior musculature of the lower limbs (hamstring and glutes) in the exercise of unilateral bending of hips with knee in extension. In this method, the agonist musculature was passively stretched to a point of discomfort so then the subject performs a maximum voluntary isometric contraction of 5 seconds. So, the physiotherapist would passively stretch the target musculature for 30 or 60 seconds. Four sets were held, separated by 10 seconds of rest, for each leg in a non-alternating form. In Protocol 1 (P1), a 30-second passive stretching was adopted and in Protocol 2 (P2) a 60-second passive stretching was adopted, both of which were already used in previous studies [2,5,11,18,19]. The total stretching time in Protocols 1 and 2 was 280 and 520 seconds, respectively.
To ensure a consistent stretching, the exercises were performed by experienced physiotherapists in the PNF technique.   Rhea [20], considering "trivial < 0.35", "small, 0.35-0.80", "moderate, 0.80-1.5" and "large > 1.5". The power test was used to observe the power of the sample in each motor tests.

Results
No differences were identified between the values of the explosive force before and after stretching in Protocols 1 and 2 t (7) = 1.462, p = 0.261, and t (7) = 1.402, p = 0.302, respectively. Similarly, there were also no significant differences for muscular resistance located before and after stretching in Protocols 1 and 2, t (7) = 1.542, p = 0.242, and t (7) = 0542, p = 0.712, respectively. Although there were no statistically significant differences, protocols 1 and 2 caused percentage decreases in the explosive force of -2.4% and -3.3%, respectively. As well as in localized muscular resistance, with deficits of -2.3% and -0.9% in Protocols 1 and 2, respectively.

Discussion
The objective of this study was to analyze the acute effect of two PNF protocols in the explosive force and vertical jump resistance of athletes. A reduction of 2.4 and 3.3% was found in the explosive force after PNF Protocol 1 and 2, respectively. In JR, it was found a reduction of 2.3 and 0.9% after PNF protocol 1 and 2, respectively. On the other hand, other authors suggest that PNF protocols should be avoided as an integral part of the warm up conducted prior to explosive force activities [2,13]. Church and Colleagues  The differences in the manipulations of the aforementioned variables of stretching may cause differences in the effect of stretching in activities involving the stretch-shortening cycle [21].
Moreover, studies were carried out with subjects of different profiles ranging from experienced jumping activities athletes to non-athlete subjects. This profile difference can present different responses in activities requiring rapid force [22]. It is also assumed that another possible explanation for the differences in performance found after stretching is by the number of stretched muscles. The vertical jump is a complex movement involving the major participation of three muscle groups: hamstring, femoral quadriceps and plantar flexors [2] and between the researches there were differences between the stretched muscles. In the present research only, the hip extenders were stretched. In the other studies, it was used more than one stretching exercise that contemplated two to four muscular groups, between plantar flexors, hamstring, femoral quadriceps, gluteus and adductor of the hip.
Considering that the stretching protocols used in this study did not adversely influence the performance of the vertical jump, it is possible to achieve it prior to the training of EF and JR. Although some studies present restrictions regarding doing stretching exercises before explosive force activities, such as jumps and sprints, it seems that the total volume and the amount of muscles involved are more important to consider, since studies with low volume or few muscles involved did not show deleterious effect on performance.
The new finding of this study is that the PNF protocols used did not reduce the vertical jump resistance. Muscular resistance is an important factor for the performance of many sporting activities [10], but few studies have investigated the influence of PNF in this manifestation of force [9] and, to our knowledge, no other work evaluated the effect of PNF on muscular endurance of athletes by Differently, this study did not find a reduction in JR after PNF protocols. The divergence between the results can be explained by the influence of methodological differences between studies as the form of evaluation of muscular resistance (resistance exercises and vertical jump), the evaluated body segment (upper and lower segment) as well as by difference between the technique and the volume of the stretching employed. More research needs to be carried out in order to better clarify the effect of PNF on muscular resistance in its various forms of evaluation taking into account the specificity of the sport modality. However, due to the evidences available until this moment, one suggestion for coaches and professional who plan exercises bouts for athletes is use few muscles per session to stretch, trying to decrease the possibility of deleterious effect, whereas has more evidences showing that stretching a lot of muscles by a long period prior to high intensity jump activities in sports could cause decrease of performance [23][24][25][26].

Conclusion
The study showed that the PNF protocols used did not generate significant negative effects on jump performance, not interfering neither explosive force nor jump resistance of volleyball athletes. In this way, the stretching protocols used in this study can be included during the warming up of athletes without causing damage to the performance of the vertical jump.