Jumping Exercise Restores Stretching-Induced Power Loss in Healthy Adults

Th e purpose of this study was to examine the acute eff ects of jumping exercise (JE) immediately aft er diff erent stretching protocols on fl exibility and power in healthy adults. Th is study was conducted with a balanced crossover design. Th irteen healthy males (25.4±3.46 years old) voluntarily participated in this study. All participants randomly completed four trials, including three diff erent stretching protocols; 1) static stretching (SS), 2) dynamic stretching (DS), 3) proprioceptive neuromuscular facilitation stretching (PNFS), and 4) a non-stretching control (NS) followed by the JE with seven-day intervals between tests. JE was composed of three sets of fi ve tuck jumps. Flexibility was determined by the ability to perform a straight leg raise (SLR) and power by vertical jump performance (VJP). Both SLR and VJP were measured at four time points; 1) baseline, 2) post-jogging, 3) post-stretching, and 4) post-JE; 4 × 4 repeated measures analysis of variances were applied. Th ere were signifi cant interaction eff ects on SLR (F=8.935, p<.001) and VJP (F=3.965, p=.009). Th e SLR score increased in all stretching protocols except the NS protocol post-stretching and postJE. Aft er stretching, the VJP score decreased in the NS (-2.6%), SS (-3.6%), and PNFS (-4.4%) protocols but maintained a positive score for the DS (1.8%) protocol. However, the VJP score recovered to the previous value in the SS (3.2%) and PNFS (6.5%) protocols aft er the jumping exercise. Th e present study suggests that jumping exercise immediately aft er SS and PNFS protocols could be an effi cient program for restoring stretching-induced power loss in healthy adults.


Introduction
Th e importance of warm-up prior to main exercise and sports events has been widely recognized for preventing injuries and optimizing exercise performance (Woods, Bishop, & Jones, 2007). Th e warm-up programme generally consists of light aerobic activity, stretching, and sport-specifi c movements for 15-20 minutes (Woods et al., 2007). Light aerobic activities, such as jogging and cycling, have been known to increase body temperature and blood circulation, which leads to improved exercise performance, such as fl exibility, strength, and power (Bishop, 2003;Young & Behm, 2002).
Stretching exercises are commonly applied following light intensity aerobic activities to increase fl exibility and decrease injuries (Hartig & Henderson, 1999). Various stretching protocols, such as static stretching (SS), dynamic stretching (DS), and proprioceptive neuromuscular facilitation stretching (PNFS), have been introduced as pre-exercise stretching protocols. SS is a common stretching technique that serves as a warm-up programme, and this technique has been known to improve range of motion and decrease muscle soreness (Andersen, 2005). Th e DS has become a preferred choice in the athletic community in recent years, because this technique has been shown to improve performances in power (Franco, Signorelli, Trajano, Costa, & de Oliveira, 2012), sprints (Fletcher & Jones, 2004), and strength (Sekir, Arabaci, Akova, & Kadagan, 2010) despite musculotendinous unit (MTU) stiff ness being decreased (Herda et al., 2013). Th e PNFS is widely applied in a clinical environment to enhance both active and passive ranges of motion with the ultimate goal being to optimize muscular performance (Bradley, Olsen, & Portas, 2007). Th e PNFS protocol is not commonly recommended immediately prior to explosive athletic movement because it could diminish jump performance and muscle strength (Bradley et al., 2007;Marek, Cramer, Fincher, & Massey, 2005). However, this protocol provides great benefi ts for those who participate in exercises that require great fl exibility, such as gymnastics. Many studies on stretching and exercise performance have been conducted, but the outcomes are still controversial depending on the duration and type of stretching protocols (Bradley et al., 2007;Fletcher & Jones, 2004;Franco et al., 2012;Marek et al., 2005;Sekir et al., 2010), the performer's baseline status Donti, Tsolakis, & Bogdanis, 2014), and gender diff erences (Donti et al., 2014).
As a fi nal component of warm-up programmes, potentiating exercise is applied with specifi c forms related to upcoming sports events or activities. Th e potentiating exercise focuses on the intensity of activities that include various explosive movements such as sprinting, jumping, and throwing (Till & Cooke, 2009;Tillin & Bishop, 2009). It has been known to facilitate a high degree of central nervous stimulation; thus, the recruitments of fast twitch motor units are enhanced (Hamada, Sale, MacDougall, & Tarnopolsky, 2000;Hodgson, Docherty, & Robbins, 2005). In particular, plyometric type jumping exercise (JE) is oft en used as a form of potentiating exercise. However, it is not well understood how plyometric jumping exercise infl uences fl exibility and power performance when combined with diff erent stretching protocols. A previous study reported that three sets of fi ve jumping exercises aft er static stretching restored counter-movement jump (CMJ) in international fencing athletes (Tsolakis, Bogdanis, 2012). However, Donti et al. (2014) reported that one set of fi ve tuck jumps did not improve power performance in elite gymnasts. Another study also reported that jumping exercise immediately aft er DS did not improve vertical jump performance (Turki et al., 2011).
As mentioned previously, the performance outcomes, including strength and power, following diff erent stretching are well understood in the previous studies Bishop, 2003), but it is unclear the how the jumping exercise aff ects performance when combined with diff erent types of stretching (i.e., SS, DS, and PNFS) even though warm-up programmes commonly include stretching and explosive movements. In this study, we have specifi cally selected a jumping exercise (tuck jump) as a potentiating exercise because strength and power in lower limbs play an important role in most sports events. In addition, the tuck jump does not require specifi c techniques, which enable it to be applied for general populations, such as healthy adults. Th us far, most studies regarding stretching and performance predominantly have used athletes as subjects (Donti et al., 2014;Tsolakis & Bogdanis, 2012;Turki et al., 2011). However, it is important to know that athletes are a unique group in comparison to the general population, because their body (i.e., physiological, functional) and mind (psychological) respond diff erently to exercise or warm-up (Dehkordi, 2001;Koch et al., 2003). We believe the importance of warm-up programmes should be emphasised in healthy adults as well as athletes as the number of participants who exercise has increased among healthy adults. Th is study would provide practical information to healthy adults, which enable the application of a warm-up programme before exercise or a sport event. Th erefore, the purpose of this study was to examine the acute eff ects of jumping exercise immediately aft er diff erent stretching protocols on fl exibility and power in healthy adults. We hypothesize that jumping exercise immediately aft er diff erent stretching protocols will enhance fl exibility and power performance in healthy adults.

Participants
Participants were recruited through advertisements at the university. Seventeen healthy collegiate males voluntarily participated in this study. No subjects engaged in any stretching-related exercise (i.e. yoga, Pilates), and had no skeletomuscular injuries in the previous two to three years. During the study period, four subjects dropped out due to personal reasons. Th erefore, thirteen healthy males (25.4±3.46 years, 171.7±6.97 cm, 77.0±12.28 kg) completed the study. Th e study procedures, including the potential risk factors, were explained to the participants. Written informed consent was obtained from the participants prior to testing. Th is study was approved by the Institutional Review Board of Texas A&M University-San Antonio. p pp p y y y y FIGURE 1 Study procedure Note. * SLR; straight leg raise, VJP; vertical jump performance; * Stretching protocols: three diff erent stretching protocols (SS, DS, PNFS) and NS control were applied in 7-day intervals; * Potentiating exercise: three sets of fi ve tuck jumps.

Study design
Th is study was conducted in a balanced crossover design ( Figure 1). Height and weight were measured via the use of a wall-mounted stadiometer (Stadi-O-Meter®, Rockton, USA) and a digital scale (SECA, Hamburg, Germany), respectively. Prior to stretching, participants performed fi ve minutes of jogging on a treadmill (6.4 km/hour). Th ree diff erent stretching protocols (SS, DS, and PNFS) combined with JE were randomly applied with a seven-day interval between tests. Non-stretching combined with JE served as the control group. Straight leg raise (SLR) and VJP were measured at four time points; baseline, post-jogging, post-stretching, and post-JE with three minutes of recovery time between the measurements.

Stretching protocols
Th ree stretching (SS, DS, and PNFS) protocols were specially targeted towards lower limb muscles (calf, hamstrings, quadriceps, and gluteus maximus). Each stretching protocol was applied to a single muscle group of 30 seconds with mild discomfort for a total of fi ve minutes. For NS control, participants sat in a chair for fi ve minutes. Th e stretching protocols were modifi ed based on previous studies (Donti et al., 2014;Franco et al., 2012). Th e components of stretching techniques are described in Table 1.

Jumping exercise programme
Aft er each stretching protocol was completed, participants performed three sets of fi ve tuck jumps with 30-second intervals between sets (Donti et al., 2014). Subjects stood facing a wall further than arms-length away. Next, subjects leaned into the wall and placed their hands on the wall for stability. Subjects placed their right foot forward while leaving their left foot back. While leaning into the wall, subjects' calf muscles are stretched until the point of slight discomfort. Position was repeated for the opposite side.

Hamstring
Subjects sat on a mat with left leg fully extended. Next, the right leg fl exed at the knee and the foot was placed alongside the medial aspect of the knee of the left leg. The subjects then bent forward, keeping the back straight, while grabbing the dorsifl exed left foot until the hamstring stretched to the point of slight discomfort. Position was repeated for the opposite side.

Quadriceps
Subjects stood facing a wall, placing their left hand on the wall for stability. The right knee was fl exed so that the right hand could hold the right ankle and pull toward to hip until slight discomfort in the quadriceps. Position is repeated for the opposite side.

Gluteus maximus
Subjects stood facing a wall and fl exed the right hip. Next, the knee was fl exed so that the subject could hold the right ankle. While holding the right ankle, the knee was pulled towards the chest until slight discomfort in the glutes. Position is repeated for the opposite side.

Calf
Walking with dorsifl exion to plantar fl exion: Subjects stood and raised one foot with the knee fully extended. Then, subjects dorsifl ex the ankle joint intentionally so that the toe was pointing upward. Then, subjects plantar fl ex the ankle joint intentionally so that the toe was pointing downward. Both positions of fl exion should cause slight discomfort.

Hamstring
Frankenstein walks: Subjects fl exed at the hip while intentionally keeping the knee fully extended. Leg was then fl exed at the hip as high as possible until slight discomfort was felt in the hamstring. Inch worms: Subjects started in the push-up position. Keeping the knees extended, subjects walked feet forward towards their hands. Once slight discomfort is felt, subjects walked hands forward, keeping knees extended, back to start position.

Quadriceps
Heel-ups: subject kicked heels towards buttocks while moving forward High knee up to chest: subject running forward with high knee up

Gluteus maximus
Walking lunges: Subjects took one large step forward with either the right or left foot. Next, with both arms out in front, the subject rotated their upper bodies, keeping arms horizontal to the ground. Exercise performance testing SLR and VJP tests were selected for the measurements of fl exibility and power performance. Th e SLR was measured with a goniometer (Baseline stainless steel goniometers, USA). Participants lay in a supine position on a medical bed with their backs fl at to prevent possible pelvic rotation. Th en participants raised their dominant leg as far as possible while maintaining the knee fully extended with ankle joint in a dorsifl exion position. One lever of the goniometer was marked on the lateral midline of the pelvis, while the pivot was placed on the lateral aspect of the hip joint, at the greater trochanter. Th e opposite leg was fi rmly held down to prevent fl exion at the hip joint. Participants performed two-trials, and the highest score was recorded. Th e intra-class correlation coeffi cient (ICC) for SLR was 0.98. Th e VJP test was measured as a marker of power performance. Participants fi rst raised their right arm on the measuring bar with a fully extended elbow, which marked an initial point. Th en, they were instructed to jump with their maximal eff ort as high as possible. Th e jump height was calculated from maximal jump height minus initial point. Each participant performed two trials, and the highest score was recorded. Th e ICC for VJP was 0.93.

Statistical analysis
SPSS (version 24.00, SPSS Inc., Chicago, Illinois) was used for statistical analysis. All data were presented as mean and standard deviations. Th e percentage change of scores was also calculated for all measures. 4 × 4 repeated measures analysis of variances (ANOVA) were applied to analyse the changes of SLR and VJP between (a) diff erent stretching protocols and (b) time sequences. If any signifi cant interactions or main eff ects were detected, repeated measure ANOVAs with a Bonferroni post hoc test was applied. One-way ANOVAs were applied to analyse the percentage changes of SLR and VJP between the diff erent protocols at the post-jogging, the-post-stretching, and the post-JE. Partial eta squared (η p 2 ) was used to classify the eff ect size. Th e reliability estimated for the best score in SLR and VJP was determined by calculating the intra-class correlation coefficient (ICC) (Wood, & Zhu, 2006). Th e level of statistical signifi cance was set at p<.05.

Flexibility
Th ere was a signifi cant interaction eff ect (protocol × time) on the straight leg raise (F=8.935, p<.001, η p 2 =.427). Th ere were also signifi cant eff ects for time (F=61.789, p<.001, η p 2 =.837) and trial (F=17.739, p<.001, η p 2 =.596). Th e Bonferroni post hoc test showed that the SLR score signifi cantly increased in all trials aft er jogging. Although the SLR score increased aft er SS, DS, and PNFS, this score did not change aft er NS. Jumping exercise immediately aft er all stretching protocols did not provide additional benefi t in fl exibility. Overall, jumping exercise immediately aft er three stretching protocols increased SLR from baseline. Figure 2 describes the changes in fl exibility.

Power
A signifi cant interaction eff ect (protocol × time) on VJP was observed (F=3.965, p=.009, η p 2 =.248). Th ere was also a signifi cant time eff ect (F=20.403, p<.001, η p 2 =.630). Th e Bonferroni post hoc test revealed that the VJP score signifi cantly increased in all trials aft er jogging. However, the VJP score decreased aft er the NS, SS, and PNFS protocols, although it did not change aft er DS. Aft er the jumping exercise, the VJP score was only restored post-jogging in the SS and PNFS protocols, whereas it did not change in the NS and DS protocols. Overall, the jumping exercise immediately aft er DS and PNFS signifi cantly improved the VJP from baseline. Figure 3 represents the changes of VJP.

Discussion
Th is study was aimed at investigating the acute eff ects of jumping exercise immediately aft er diff erent stretching protocols on fl exibility and power in healthy adults. Th e main fi ndings are as follows: 1) jumping exercise immediately aft er the DS and PNFS protocols improves fl exibility and power from the baseline; 2) jumping exercise restores the static and PNF stretching-induced power loss in healthy adults.
Th e fl exibility and power increased in all groups aft er jogging in the present study. It is known that increasing body temperature through light intensity activities provides physiological benefi ts (Bishop, 2003;Young & Behm, 2002). With an increase in body temperature, blood fl ow increases through vasodilation; thus, more oxygen might be supplied to working muscle as well as increase nerve transmission (Bishop, 2003). Although this study did not directly measure body temperature, we assume that increased body temperature through jogging may improve fl exibility and power performance.
In the current study, the percentage increase in SLR from jogging was greater in SS (9.4%), DS (4.9%) and PNFS (11.9%) trials than NS (-0.1%) trial at post-stretching. Even though there were no statistical diff erences in SLR among the three stretching protocols, the PNFS protocol showed the greatest improvement in SLR. PNFS is known as the most eff ective method for increasing range of motion in joints and fl exibility (Konrad, Gad, & Tilp, 2015). Th is contract-relax stretching method may have an impact on autogenic inhibition, especially the Golgi tendon organ. Increasing tension during the contraction phase may increase antagonist muscle activity while the function of the Golgi tendon organ decreases during the relaxation phase; therefore, the joint range of motion increased (Konrad et al., 2015). Th e DS protocol showed the lowest improvement in SLR among three stretching protocols. A previous study reported that DS is not as eff ective in increasing fl exibility compared to SS and PNFS .
In the present study, VJP signifi cantly decreased in the NS (-2.6%), SS (-3.6%), PNFS (-4.4%) trials but did not change in the DS (1.8%) trial post-stretching. Non-dynamic stretching, such as static and PNF stretching, has been known to reduce power performance   (2005) demonstrated that three sets of 30 seconds static stretching decreased jump performance (-5.6%). Another study also reported that four reps of 30 seconds SS and PNFS (5 sec contract and 25 sec relaxation phase) for fi ve minutes, decreased vertical jump performance (SS; 4%, PNFS; 5.1%). Th e mechanism underlying these results demonstrated that prolonged static stretching might inhibit the neural drive and asynchronies of muscle activity (Power, Behm, Cahill, Carroll, & Young, 2004). Th e stretch-induced impairment of the length-tension relationship may be another factor that limits further motor unit recruitments . However, the VJP score was maintained aft er the DS trial in the present study. We assume that DS may have a diff erent role in power performance compared with other stretching protocols. DS involves many active movements, which induce physiological changes such as increased heart rate, body temperature, and altering other metabolic factors (Fletcher, 2010). Even though a previous study speculated that MTU stiff ness is decreased aft er DS (Herda et al., 2013), which is similarly shown in other stretching protocols (Ryan et al., 2014), the physiological benefi ts may outweigh the stretching-induced power loss. Th is study confi rmed that fi ve minutes of static and PNF stretching alone reduce VJP while DS did not aff ect the improved VJP induced by jogging.
A plyometric-type jumping exercise aft er stretching is commonly applied as a potentiating exercise to promote muscle activity. In the present study, jumping exercise restored the VJP in the SS (3.2%) and PNFS (6.5%) protocols whereas this score did not signifi cantly change in the NS and DS protocols. A previous study reported that the combination warm-up (run+stretch+jumps) programme showed the highest score in jumping performance compared to running alone or a run+stretch warm-up programme (Young & Behm, 2003). Another study reported that 45 seconds of static stretching decreased CMJ (5.5%), but 3 sets × 5 tuck jumps immediately aft er stretching restored the power performance in international fencing athletes (Tsolakis & Bogdanis, 2012). Donti et al. (2014) conducted diff erent volumes of jumping exercise. Th e study reported that 3 sets × 5 tuck jumps aft er 30 sec static stretching enhanced CMJ, but one set of fi ve times tuck jump did not improve CMJ in elite gymnasts.
Th ere are various possible reasons that additional jumping exercise aft er stretching provides benefi t to power performance. Th e fi rst theory is that various explosive-movements may increase central nerve stimulation, which involves the Hoff mann Refl ex (H-refl ex), resulting in greater fast twitch motor unit recruitments (Hodgson et al., 2005). Th e second theory involves phosphorylation, which produces more ATP, resulting in greater muscle activation at the structure level of skeletal muscle (Rixon, Lamont, & Bemben, 2007). It has also been proposed that explosive movements recruit more fast twitch muscle fi bres that lead to improved power performance aft er JE (Hamada et al., 2000). However, the additional jumping exercises aft er DS do not provide positive benefi ts over VJP. Behm, Button, Barbour, Butt, & Young (2004) pointed out the importance of balance between post-activation potentiation exercise and fatigue. A previous study supported our result that dynamic stretching alone improved muscular performance, but an additional 3 × 3 times of tuck jumps did not provide the benefi ts on VJP (Turki et al., 2011). Th e current study suggests that jumping exercise (5 times × 3 set) aft er DS may not provide additional benefi t over VJP, while undertaking this programme immediately aft er SS and PNFS may restore the stretching-induced power loss.
Th e present study suggests that jumping exercise immediately aft er SS and PNFS protocols could be an efficient programme for restoring stretching-induced power loss in healthy adults. However, jumping exercise aft er DS did not provide additional benefi t to power performance.