Time Course of Changes in Vertical- Jumping Ability After Static Stretching

International Journal of Sports Physiology and Performance, 2007;2:170-181 2007 Human Kinetics, Inc. Time Course of Changes in Vertical- Jumping Ability After Static Stretching Jason Brandenburg, William A. Pitney, Paul E. Luebbers, Arun Veera, and Alicja Czajka Purpose: To examine the acute effects of static stretching on countermovement vertical-jump (CMVJ) ability and monitor the time course of any stretch-induced changes. Methods: Once familiarized, 16 experienced jumpers completed 2 testing sessions in a randomized order. Each session consisted of a general warm-up, a pretreatment CMVJ assessment, a treatment, and multiple posttreatment CMVJ assessments. One treatment included lower-body static stretching, and the second treatment, involving no stretching, was the control. Posttreatment CMVJ measures occurred immediately, 3, 6, 12, and 24 minutes posttreatment. Stretching consisted of 3 static-stretching exercises, with each exercise repeated 3 times and each repetition held for 30 s. Results: Prestretch CMVJ height equaled 47.1 (± 9.7) cm. CMVJ height immediately poststretch was 45.7 (± 9.2) cm, and it remained depressed during the 24-min follow-up period. Pre-no-stretch CMVJ height was 48.4 (± 9.8) cm, whereas immediately post-no-stretch CMVJ height equaled 46.8 (± 9.5) cm, and as in the stretch treatment, post-no-stretch CMVJ height remained lower than pre-no-stretch values. Although there was a significant main effect of time (P =.005), indicating that CMVJ was lower and remained impaired after both treatments, no significant interaction effect (P =.749) was observed. Conclusion: In comparison with the no-activity control, static stretching resulted in similar reductions in CMVJ ability when examined over the same time course, so athletes preparing for CMVJ should avoid periods of inactivity, as well as static stretching. Key Words: warm-up, stretch-induced, muscle activity, jump performance There is a sizeable body of research examining the acute influence of stretching on strength and power performance, with the findings from most, but not all, of these studies indicating a negative impact of stretching on muscle performance. Despite the reasonable consistency of these findings there has been reluctance to recommend discontinuing stretching as part of preevent preparation. 1 Some of this hesitation likely results from the purported physical, and perhaps psychological, benefits preevent stretching might provide a competitor. 2 Alternatively, there are a number Brandenburg, Pitney, Veera, and Czajka are with the Dept of Kinesiology and Physical Education, Northern Illinois University, De Kalb, IL 60115. Luebbers is with the Dept of Health, Physical Education, and Recreation, Emporia State University, Emporia, KS 66801. 170

Effect of Static Stretching on Jumping 171 of uncertainties to address before stretching recommendations can confidently be developed. For example, there are a number of variables such as duration, intensity, and type of stretch that might influence the stretch-induced response. Moreover, the length of time that muscle performance remains impaired is another question that has received little attention. Of the studies examining the acute effects of static stretching on muscle performance, most typically assess muscle performance shortly after stretching, without additional follow-up measures. 3,4 Consequently, the length of time that any stretch-induced impairment in muscle performance persists is not well understood. In addition, the findings from the studies that have included follow-up measures of muscle performance are limited by the stretching protocols employed or the timing of the poststretch measures of muscle performance. For example, Fowles et al 5 observed significant reductions in plantar-flexor force that continued for 60 minutes after termination of the stretch protocol. The static-stretching protocol used in this study consisted of a series of repeated stretches, with the entire stretch duration lasting 30 minutes. Although the total stretch duration incorporated in these studies permitted researchers to better understand the acute effects of stretch on muscle performance and the duration of this response, the practical value of the results is disputable because the total stretch time was not representative of typical preevent stretching routines employed by competitive and recreational athletes. Power et al 6 observed stretch-induced deficits in maximal isometric force of the quadriceps that were still present 120 minutes poststretch. Their subjects, however, had no significant reductions in isometric force immediately and 30 minutes poststretch. The presence of reduced isometric force 60 to 120 minutes poststretch but not within 30 minutes of the end of the static-stretching protocol is perplexing. More to the point, the follow-up measures only occurred at 30-minute intervals. Although this measurement frequency might provide a general timeline outlining the recovery from stretch-induced decreases in muscle performance, much of this time course remains unknown. A better understanding of the time course of stretch-induced changes in muscle performance would be beneficial. From a practical perspective, although there is usually a brief period of time between the stretching component of the warm-up and the start of the competition, it is unlikely that this interval exceeds 30 minutes. A clearer understanding of the time course of recovery after stretch-induced decreases in muscle performance would make it possible to advise performers when to stretch in order to avoid the loss in force while perhaps still experiencing some of the purported benefits stretching provides. Thus, the objectives of this study were to examine the acute effects of static stretching on vertical-jump ability and monitor the time course of any stretch-induced changes in performance. Participants Methods Eighteen participants, 9 men (mean ± SD for height, weight, and age were 183.2 ± 9.1 cm, 84.2 ± 6.5 kg, and 22.3 ± 1.9 years, respectively) and 9 women (mean ± SD for height, weight, and age were 167.6 ± 13.5 cm, 68.5 ± 17.1 kg, and 21.4 ± 1.6 years, respectively), volunteered to participate in this study. All 18 satisfied the

172 Brandenburg et al participation requirements, which included previous jumping experience defined as either current participation in a jumping sport (eg, volleyball, gymnastics) or current use of lower body plyometric exercises in a consistent training program and no current or past injury to the knee or ankle joints or the muscles around these joints. Two of the 18 participants withdrew for reasons not related to this study. Before participating in the study, each subject provided written consent after being informed of the specific experimental protocols. All experimental protocols were approved by the Northern Illinois University human subjects institutional review board. Experimental Design Participants visited the laboratory on 3 different occasions. The first visit served to familiarize them with the jumping and stretching procedures. To accomplish this, subjects performed 15 maximum-effort countermovement vertical jumps with 15 seconds of standing rest between attempts. They were then provided with 5 minutes of seated rest before performing another 5 maximum-effort jumps. During this session participants were also introduced to the 3 stretching exercises to be performed during 1 of the testing sessions. The second and third visits to the laboratory served as the testing sessions. In each testing session subjects first performed a 5-minute warm-up on a bicycle ergometer at a self-selected intensity. 7 Subjects were asked to use a low resistance setting while pedaling at a comfortable cadence and, while doing so, to avoid fatiguing the muscles of the lower body. Once the participants had warmed up, pretreatment vertical-jump performance was assessed. After this, subjects performed the assigned treatment for the session. The treatments were either 9 minutes of lower body static stretching or a control treatment that consisted of 9 minutes of standing. Measures of vertical-jumping ability were reassessed immediately after and 3, 6, 12, and 24 minutes posttreatment. Each subject performed both treatments with the order of the 2 testing sessions being randomized and counterbalanced. A minimum of 24 hours and a maximum of 6 days were permitted between the familiarization and testing sessions. Participants were asked to refrain from any strenuous activity, particularly resistance exercise, involving the muscles of the lower body the day of and the day preceding testing. Static-Stretching Procedures During the stretch treatment subjects performed 3 static-stretching exercises. A standing unilateral plantar-flexor stretch on a slant board was performed. Subjects stood with 1 foot flat on the floor and the other foot, with the assistance of a slant board, placed in dorsiflexion. The subjects were then asked lean forward while maintaining full extension at the knee until mild discomfort was felt in the plantarflexor muscle group. Throughout the stretch subjects were required to maintain foot contact with the slant board. To stretch the knee flexors, a standing unilateral hamstrings stretch was performed. In the standing position, participants placed 1 of their legs on a bench. The standing (nonstretched) leg pointed ahead with the knee either fully extended or slightly flexed. In this position, each participant flexed the trunk toward the elevated

Effect of Static Stretching on Jumping 173 foot while maintaining a straight back, thus isolating the stretch to the knee flexors. During the stretch the knee of the stretched leg remained extended and the ankle remained in a neutral position. Subjects could select from 2 bench heights (60 or 70 cm) to perform this stretch. The bench height, largely based on subject height and flexibility of the hamstrings, was determined in the familiarization session. To stretch the knee extensors, a standing unilateral quadriceps stretch was performed. While standing on 1 leg subjects grasped the ankle of the non-weightbearing leg. Subjects then pulled the ankle toward the buttocks, thus flexing the leg at the knee. During the stretch subjects were instructed to remain erect while keeping the hip of the stretched leg in a neutral position and the knee of the flexed leg under the hip. Each exercise was performed unilaterally, and subjects alternated between stretching the left and right sides. Each stretch was performed for a total of 3 repetitions on each side with each repetition lasting 30 seconds. Thus, each muscle group on each side of the body was placed under stretch for 90 seconds. The total time required to complete the entire stretch treatment was 9 minutes. For each stretch, participants were asked to attain and hold a position of mild discomfort but not pain. They were instructed that mild discomfort meant the stretched muscles should be taut. Once this position was achieved and verbally acknowledged by each subject, the investigator would begin the timer. All stretching exercises were supervised and timed by the investigators. Vertical-Jump Protocols To assess jumping performance, we had subjects perform a bilateral countermovement vertical jump (CMVJ). To commence the jump subjects assumed a standing position with their feet positioned approximately shoulder width apart and hands placed on the hips. When verbally signaled, subjects performed a preparatory dip before jumping upward as high as possible. As a result of the previous jumping experience of the subjects, the rate and depth of the preparatory dip were selfselected. Subjects were required to keep their hands on their hips throughout the entire jump. Jumping performance was assessed pretreatment and immediately, 3, 6, 12, and 24 minutes posttreatment. During each of these assessments of jumping ability, 3 maximal-effort CMVJ repetitions were performed. Fifteen seconds of rest separated repetitions. From the 3 jumps the maximum height, as well as the median height, of the 3 jumps was used in the analysis. In addition, the maximum and median jump values from the pretreatment assessment of jumping ability on both testing days (no stretch and stretch) were used in the reliability analysis. CMVJ height was measured using a jump-timing mat that calculated verticaljump height based on the amount of time the subject spent in the air (Just Jump System, Probotics Inc, Huntsville, Ala). 8 Consequently, each jump was closely observed to ensure that subjects did not alter their jumping technique to manipulate time spent in the air. If jumping technique, particularly after the take-off phase, deviated from the norm (eg, flexion of the hip, flexion of the knee, or dorsiflexion of the ankle) for a particular subject, the jump was discarded and the subject was asked to repeat the attempt. Before each jump, subjects were verbally encouraged to jump as high and as straight up as possible.

174 Brandenburg et al EMG Procedures During each CMVJ the electrical activity of the vastus lateralis and gastrocnemius muscles was recorded from the right leg using pregelled, disposable surface electrodes (Ag-AgCl, 10-mm, Biopac Systems Inc, Goleta, Calif). Specifically, recording electrodes were placed over the distal portion of the muscle belly of the vastus lateralis. To record gastrocnemius-muscle activity, the 2 recording electrodes were placed over on the lateral side of the leg one third of the way down the leg between the head of the fibula and the inferior end of the calcaneus. 4 Each pair of recording electrodes was spaced 2 cm apart. In addition to the recording sites, 2 bony surfaces superficial to the tibia were prepared to serve as ground-electrode sites. 9 Before electrode application, the electrical impedance of the skin at each site was reduced by shaving (if necessary), mildly abrading the area with sandpaper, and then cleansing the area with isopropyl alcohol swabs. To ensure that electrode placement was identical on the 2 testing days, the electrode locations were outlined with a permanent marker. Muscle activity during each jump was recorded with a data-acquisition unit (MP100, Biopac Systems Inc). The sampling rate for the acquisition of muscle activity was set at 1000 Hz. The signal was high-pass-filtered at 10 Hz and lowpass-filtered at 500 Hz. Raw data were then stored on a personal computer for signal processing and analysis. The raw signal was smoothed (every 50 samples) and rectified to get a root-mean-square signal using Acknowledge 3.72 software (Biopac Systems Inc). The maximum amplitude of the root-mean-square signal of the highest jump at each measurement time (group of 3 jumps) was determined and used for analysis. Absolute EMG values for each jump during each testing session were then normalized against the corresponding pretreatment value (eg, root mean square of 3 minutes poststretch root mean square of prestretch jump = normalized value for 3 minutes poststretch). Statistical Analysis To determine whether any differences in jumping ability and muscle activation were evident in response to the 2 treatments, a 2 (stretch and control conditions) by 6 (time: pretreatment and immediately, 3, 6, 12, and 24 minutes posttreatment) analysis of variance (ANOVA) with repeated measures was performed for each dependent variable (maximum CMVJ height, median CMVJ height, normalized EMG). If a difference was present, planned comparisons, using paired-sample t tests, were performed to determine the exact location of the difference. Test retest reliability of each dependent variable, as determined with an intraclass correlation, was determined using a 2-way ANOVA. Intraclass correlation coefficients (single measure) for maximum and median vertical-jump height, as well as maximum root-mean-square amplitude of the quadriceps and gastrocnemius muscles, were.97,.98,.93, and.88, respectively, with no significant differences between the mean values for each treatment (P >.05). With the exception of the planned comparisons, the alpha level for statistical significance was set at P <.05. Alpha level for the planned comparisons was set at P <.03. All statistical analyses were conducted using SPSS 12.0 for Windows (SPSS Inc, Chicago, Ill).

Effect of Static Stretching on Jumping 175 Results Pretreatment and posttreatment maximum CMVJ values for the static-stretching and control treatments are displayed in Figure 1. Although the repeated-measures ANOVA revealed a significant effect of time (F 1,15 = 6.5, P =.005), no interaction effect was observed (F 1,15 = 0.5, P =.749). When the maximum CMVJ height data were collapsed across the 2 treatments, post hoc analysis indicated that the values from each of the posttreatment assessment times were significantly less than pretreatment jump height (P <.05). Furthermore, jump height at 24 minutes poststretch was significantly less than jump height at 12 minutes poststretch (P =.017). A similar trend was observed with the median CMVJ values. Normalized EMG data of the quadriceps and gastrocnemius muscles in response to both treatments are presented in Figures 2 and 3, respectively. Briefly, no differences in quadriceps (interaction effect: F 1,15 = 0.8, P =.593) or gastrocnemius (interaction effect: F 1,15 = 2.0, P =.152) activity were observed between the 2 treatments. Discussion This study set out to (1) determine whether vertical-jump ability was influenced when preceded by a bout of static stretching and (2) monitor the time course of any stretch-induced change in jump performance. When compared with the noactivity control treatment, vertical-jump performance after static stretching did not respond differently. Figure 1 Countermovement-vertical-jump (CMVJ) values (mean ± SD) before and after the static-stretching and control treatments (- - - - = poststretch).

176 Brandenburg et al Figure 2 Normalized EMG (mean ± SD) of the quadriceps before and after the staticstretching and control treatments (- - - - = poststretch). Figure 3 Normalized EMG (mean ± SD) of the gastrocnemius muscles before and after the static-stretching and control treatments (- - - - = poststretch). The absence of any differences between the control and stretch treatments of the present study is consistent with results of other studies examining the influence of stretching on CMVJ performance. Recently, Unick et al 7 examined CMVJ performance in female basketball players and determined that static stretching of the lower limbs, in comparison with a no-stretch control condition, did not negatively affect their jump performance. Likewise, other researchers have found that jump

Effect of Static Stretching on Jumping 177 performance (whether measured by height or velocity) did not differ after either a static-stretching treatment or a no-stretch, control condition. 3,10-12 Our findings differ, however, from those of previous studies that did identify changes in jump performance after static-stretching protocols. Behm et al, 13 Young and Behm, 14 and Wallmann et al, 4 for example, found that vertical-jump performance was significantly impaired when followed by static-stretching procedures, leading us to question what factors might account for these inconsistent findings. Intuitively, one might think that the degree to which static stretching influences subsequent vertical-jump performance might depend on configuration of the stretching protocol. The present study, like the aforementioned studies also failing to observe a difference between the static-stretch and control treatments, used a volume of stretching thought to reflect the practices of competitors. Subjects in the present study performed 3 repetitions of 3 lower limb stretching exercises with each repetition held for 30 seconds. A collective examination of the staticstretching protocols of the other studies finding no stretch-induced decline in jumping ability reveals the use of a 15-second stretch duration, 3 to 4 lower body stretching exercises, and the performance of 3 repetitions of each exercise. 3,7,11,12 Consequently, the brief duration of the static-stretching protocols of the present study, as with these others, might account for the similar results between the stretch and no-stretch conditions. Young and Elliot 15 observed a reduction in drop-jump performance after three 15-second repetitions of a plantar-flexor, as well as quadriceps, stretch. Furthermore, a mean 5.6% reduction in vertical-jump height was noted after three 30-second stretches limited exclusively to the gastrocnemius muscle. 4 Considering the brief stretching protocols used by Young and Elliot 15 and Wallmann et al, 4 the contribution of the brief stretch durations to the similar responses after stretching and no stretching in the present study must be questioned. A closer inspection of the studies that revealed no static-stretch-induced reduction in vertical-jump performance reveals that the participant pool of these studies was composed of experienced jumpers or individuals familiar with lowerbody plyometric training (in comparison with the studies showing a poststretch decrease). For instance, no poststretch reductions in vertical-jumping ability were elicited in female college basketball players 7 or a population of female college athletes that included volleyball and tennis players, as well as sprinters, throwers, and jumpers. 10 Little and Williams 12 and Burkett et al 3 also failed to see a significant static-stretch-induced reduction in vertical-jumping ability in professional soccer players and college football players, respectively. Although the present study did not require subjects to be members of a specific team, the requirements for subjects in this study did include participation in a jumping sport such as volleyball or basketball or participation in plyometric training; indeed, the subjects participated in volleyball, tennis, soccer, gymnastics, cheerleading, rugby, martial arts, and track and field. Thus, it is possible that the training background of the subjects used in the present study contributed to the similar responses to the stretch and control treatments. The mechanisms accounting for this, however, are unclear. The stretch-induced decrements in muscle performance are typically attributed to a change in stiffness of the muscle tendon unit or a reduction in the ability to activate the involved muscle, perhaps through a reduced sensitivity of the

178 Brandenburg et al muscle-spindle reflex. 16 Thus, it would seem that the stretching protocol was ineffective at targeting these mechanisms, or the subjects previous jumping experience or adaptations from the jumping experience minimized the impact of the static stretching on these mechanisms. Similar to the study of Cornwell et al, 17 EMG data from the present study showed no change prestretch to poststretch, as well as no difference with the nostretch treatment. As Cornwell et al 17 concluded, this result implies that the ability to activate the jumping muscles during a countermovement-style jump was not altered after the static-stretching protocol of the present study. Alternatively, it is possible that the stretch duration, and perhaps intensity, was not enough to alter the stiffness of the muscle tendon complex. Previously, Halbertsma et al 18 observed a significant prestretch-to-poststretch increase in hamstring range of motion after ten 30-second static stretches. The increase was attributed to a greater stretch tolerance and not a reduction in muscle stiffness because passive muscle tension at select joint angles remained unchanged before and after stretching. The authors concluded that stretch durations of 30 seconds, as used in the present study, were inadequate to elicit a change in muscle tendon stiffness. Magnusson et al 19 drew similar conclusions with stretch durations of 45 seconds. Because changes in neither range of motion nor muscle stiffness were measured in the present study, however, we can only speculate on any influence or lack of influence that static stretching had on the muscle tendon unit. An alternative explanation for the similar responses during the stretch and no-stretch conditions might be the result of neuromuscular adaptations or strategies already in place as a result of the subjects jumping experience. Avela et al, 20 using a protocol of 10 sets of 10 drop jumps, monitored and compared the reflex sensitivity of sprint athletes with that of high jumpers. As the protocol progressed, reflex sensitivity in the sprinters decreased while remaining constant in the highjump athletes. The researchers conlcuded that the regular high-impact loading performed by the high jumpers might have lead to structural or neural adaptations that prevented the redcution in reflex sensitivity during the extensive drop-jump protocol. If jump-induced neuromuscular adaptations maintained reflex sensitivity over the course of multiple drop jumps, it is possible that similar adaptations in the subjects of the present study minimized any static-stretch-induced changes in neuromuscular performance, thus accounting for the similar results of the stretch and no-stretch treatments. The absence of any differences between the 2 treatments of the present study might be attributed to the nature of the assessment of muscle performance, as well as static-stretch intensity. A CMVJ (as was used in the present study), in comparison with a drop jump, has a longer transition phase (or contact time), thus imposing different neuromuscular demands. 21 Consequently, a CMVJ might not be a sensitive enough measure to detect any stretch-induced impairments in muscle performance. Young and Elliot 15 observed significant poststretch reductions in drop-jump perfromance in subjects with jumping experience. Unick et al, 7 however, used both a CMVJ and a drop jump to examine the influence of static stretching on performance and found that static stretching did not reduce the performance of either jump, suggesting that a CMVJ is as effective as a drop jump in assessing poststretch performance.

Effect of Static Stretching on Jumping 179 Recently, Young et al 21 observed that static-stretch intensity influenced the magnitude of the poststretch deficit. Significant reductions in drop-jump performance followed static stretching taken to a range of motion just short of the pain threshold (intensity of 100%), whereas no change in performance followed static stretching taken to 90% of that point. Likewise, jumping ability was impaired after a stretching protocol incorporating an intensity described as to the pain threshold. 14 Subjects in the present study were asked to attain and hold each stretch at the point of mild discomfort but not pain. Based on the findings of Young et al, 21 the intensity of stretch used in the present study (likely falling between the 100% and 90% intensities described by Young et al 21 ) might have been insufficient to elict an acute impairment in CMVJ performance. Cramer et al 16 and Marek et al, 22 however, observed significant reductions in quadriceps torque using the same static-stretch intensity as that used in the present study. Of interest is the significant time effect suggesting that jump performance was equally impaired after both treatments. The gradual reduction in jump performance observed during the control treatment was unexpected and must be highlighted as a potential limitation. It is possible that this slight and gradual reduction, rather than a stable performance, contributed to the lack of an interaction effect between the 2 conditions. The reductions in performance during the control treatment largely occurred pretreatment to immediately posttreatment and 12 to 24 minutes posttreatment. During the control treatment, subjects performed 9 minutes of standing between the pretreatment and immediately posttreatment jumps. The time of 9 minutes of standing was selected to resemble the stretching protocol, which consisted of 9 minutes of stretching, with each stretch being performed in a standing position. It is plausible that the 9 minutes of inactivity negatively influenced the jumping muscles, thus accounting for the lower posttreatment jump values. In retrospect, the addition of a more active warm-up condition might have better stabilized the posttreatment jump data. The 12- to 24-minute reduction in CMVJ performance might be explained by a similar mechanism. During the poststretch period, subjects, when not jumping, were required to remain seated. It is possible that 24 minutes of seated rest negatively influenced the muscles. Seated rest was selected for ease of control between trials and in some instances was thought to represent how some players enter a game or competition after being on the sidelines or bench. Conclusions and Practical Applications The results of the present study suggest that static stretching, in comparison with no activity, does not acutely alter the jumping ability of subjects with jumping experience or a history of lower-body plyometric training. Neither static stretching nor short periods of inactivity, however, appear to optimally prepare the muscles for explosive efforts such as a CMVJ. In preparation for activities involving stretch-shortening cycle movements, such as a CMVJ, the results from the present study indicate that static stretching and periods of inactivity should be avoided in the short time leading up to performance (eg, the warm-up). Findings from this study also underscore the need for individuals who are required to enter a game after spending time being inactive on

180 Brandenburg et al the sidelines to find and implement strategies to overcome the negative effects of the relative inactivity. Although it is possible that the similar responses between the static-stretching and no-activity treatments is the result of previously established neuromuscular adaptations in the subjects of the present study, future research is needed to specifically address this finding. References 1. Witvrouw E, Mahieu N, Danneels L, McNair P. Stretching and injury prevention: an obscure relationship. Sports Med. 2004;34:443-449. 2. Bishop D. Warm up I: potential mechanisms and the effects of passive warm up on exercise performance. Sports Med. 2003;33:439-454. 3. Burkett, LN, Phillips WT, Ziuraitis J. The best warm-up for the vertical jump in college-age athletic men. J Strength Cond Res. 2005;19:673-676. 4. Wallmann HW, Mercer JA, McWhorter JW. Surface electromyographic assessment of the effect of static stretching of the gastrocnemius on vertical jump performance. J Strength Cond Res. 2005;19:684-688. 5. Fowles JR, Sale DG, MacDougall JD. Reduced strength after passive stretch of the human plantarflexors. J Appl Physiol. 2000;89:1179-1188. 6. Power K, Behm D, Cahill F, Carroll M, Young W. An acute bout of static stretching: effects on force and jumping performance. Med Sci Sports Exerc. 2004;36:1389-1396. 7. Unick J, Kieffer HS, Cheesman W, Feeney A. The acute effects of static and ballistic stretching on vertical jump performance in trained women. J Strength Cond Res. 2005;19:206-212. 8. Scott SL, Docherty D. Acute effects of heavy preloading on vertical and horizontal jump performance. J Strength Cond Res. 2004;18:201-205. 9. French DN, Kraemer WJ, Cooke CB. Changes in dynamic exercise performance following a sequence of preconditioning isometric muscle actions. J Strength Cond Res. 2003;17:678-685. 10. Church JB, Wiggins MS, Moode FM, Crist R. Effect of warm-up and flexibility treatments on vertical jump performance. J Strength Cond Res. 2001;15:332-336. 11. Knudson D, Bennett K, Corn R, Leick D, Smith C. Acute effects of stretching are not evident in the kinematics of the vertical jump. J Strength Cond Res. 2001;15:98-101. 12. Little T, Williams AG. Effects of differential stretching protocols during warm-ups on high-speed motor capacities in professional soccer players. J Strength Cond Res. 2006;20:203-207. 13. Behm DG, Bradbury EE, Haynes AT, Hodder JN, Leonard AM, Paddock NR. Flexibility is not related to stretch-induced deficits in force or power. J Sports Sci Med. 2006;5:33-42. 14. Young WB, Behm DG. Effects of running, static stretching and practice jumps on explosive force production and jumping performance. J Sports Med Phys Fitness. 2003;43:21-27. 15. Young WB, Elliot S. Acute effects of static stretching, proprioceptive neuromuscular facilitation stretching, and maximum voluntary contractions on explosive force production and jumping performance. Res Q Exerc Sport. 2001;72:273-279. 16. Cramer JT, Housh TJ, Johnson GO, Miller JM, Coburn JW, Beck TW. Acute effects of static stretching on peak torque in women. J Strength Cond Res. 2004;18:236-241. 17. Cornwell A, Nelson AG, Sidaway B. Acute effects of stretching on the neuromechanical properties of the triceps surae muscle complex. Eur J Appl Physiol. 2002;86:428-434.

Effect of Static Stretching on Jumping 181 18. Halbertsma JPK, Van Bolhuis AI, Goeken LNH. Sport stretching: effect of passive muscle stiffness of short hamstrings. Arch Phys Med Rehabil. 1996;77:688-692. 19. Magnusson SP, Aagaard P, Nielson JJ. Passive energy return after repeated stretches of the hamstring muscle-tendon unit. Med Sci Sports Exerc. 2000;32:1160-1164. 20. Avela J, Finni J, Komi PV. Excitability of the soleus reflex arc during intensive stretchshortening cycle exercise in two power-trained athlete groups. Eur J Appl Physiol. 2006;97:486-493. 21. Young W, Elias G, Power J. Effects of static stretching volume and intensity on plantar flexor explosive force production and range of motion. J Sports Med Phys Fitness. 2006;46:403-411. 22. Marek SM, Cramer JT, Fincher L, et al. Acute effects of static and proprioceptive neuromuscular facilitation stretching on muscle strength and power output. J Athl Train. 2005;40:94-103.