A warm-up routine is common practice for preparation for physical exercise. The conventional warm-up generally consists of submaximal aerobic exercise, upper- and lower-extremity static stretching (SS), followed by a rehearsal of the skill about to be performed (40). The inclusion of static stretching is thought to enhance performance and reduce the chance of sustaining an injury (36), but there is no conclusive evidence to suggest that static stretching prior to performance reduces injuries (28,37). There is also no evidence that pre-exercise static stretching prevents or reduces the delayed onset of muscle soreness (17,18). On the contrary, Smith et al. (33) even reported that static stretching produced higher levels of perceived pain than dynamic stretching.
Moreover, static stretching can have a negative effect on strength (3,8) and jump and sprint performance (14,39,41), which are important physical attributes required in a team sport such as soccer. The loss of strength and explosive power has previously been hypothesized to be a result of a decrease in stiffness of the musculotendinous unit (2,22), thus reducing the ability to rapidly generate force during muscle contractions (20). However, more recently it has been demonstrated that the in vivo properties of the tendon are unaltered through static stretching (26). Another possible site where weakness could occur is in changes in a component of the neuromuscular system, such as the myotatic reflex (36), and reductions in the H reflex and M wave have been observed following static stretching (2). The loss of strength and explosive power following static stretching can remain from as short a time as 15 minutes up to 2 hours (4,15,29). This large variation in the duration of the effects of static stretching may be related to the volume of stretching over time, the length of time each stretch is held, or both. Although studies investigating static stretching on performance tend to require each stretch to be held for 30 seconds (12,13,21), pre-event stretches are commonly held between 10 and 15 seconds, raising questions regarding the ecological validity of the research (14).
Bradley et al. (4) reported vertical jump performance had returned to control values 15 minutes after static stretching. Indeed, it might not be advisable for an athlete to wait 15 minutes prior to performance because several physiological changes during this time may have a negative effect on anaerobic performance. For instance, quadriceps muscle temperature may decrease significantly during a 15-minute interval and consequently impair sprint performance (25). This is important because an increase in core and muscle temperature or muscle blood flow may increase the speed of nerve impulses and increases the speed of a contraction, thus allowing rapid and forceful muscle contractions (31).
In contrast to static stretching, dynamic stretching has proved to significantly improve vertical jump, short sprint, and agility performance (11,13,21). Dynamic stretches elevate “core” body temperature; mimic specific movements, preparing the performer optimally for competition (23); and enhance motor unit excitability, creating an improved ability for power production (13). The latter phenomenon is known as postactivation potentiation (PAP). With the progressive intensity of dynamic stretching, a greater effect of the PAP mechanism may be possible as a result of the recruitment of fast-twitch and slow-twitch muscle fibers (16). It has also been suggested that trained individuals with higher strength ability may be better able to benefit from PAP (7,10), suggesting the importance of the preconditioned state for the degree of the benefits from acute PAP manipulation.
Faigenbaum et al. (13) found that when teenage athletes continually wore a weighted vest of 2% body mass during a dynamic stretch protocol, jump performance was significantly enhanced compared to a dynamic stretch protocol without a weighted vest, which was thought to be related to recruitment of additional motor units. A weighted vest of 6% body mass did not improve performance, suggesting the overall volume of resistance during the warm-up may have resulted in fatigue. Indeed, when a weighted vest of 10% body mass was only worn for the last 4 of the 12 dynamic exercises, jump performance significantly improved compared to only dynamic and static stretching alone (38). It is evident from these findings that to benefit from the acute affects of PAP on performance, factors such as the volume and intensity of resistance along with recovery prior to performance must be taken into account. It is possible that adding a heavy load to 1 dynamic exercise only will be sufficient to induce PAP, but this has not been investigated so far.
In elite youth soccer there is an extensive increase in the volume and intensity of training as a player crosses over from the schoolboy program to becoming an academy apprentice. Monitoring the volume and intensity of training during this time requires careful attention, and acute PAP manipulation following a warm-up may be an alternative method in preparation for chronic PAP manipulation (complex training) and provide an enhanced preparation prior to a session with a conditioning emphasis, such as sprinting and/or plyometrics. To date there is no research on the effects of different warm-up protocols in elite youth soccer, the relationship between PAP and fatigue, or identification of an optimal recovery time following acute PAP manipulation prior to anaerobic performance. It was hypothesized that a dynamic warm-up that includes resistance exercise will enhance anaerobic performance activities such as sprinting and jumping compared to a warm-up consisting of only static or dynamic stretching in elite youth soccer players.
Experimental Approach to the Problem
A randomized, within-subject design was used to compare the acute effects of 3 warm-up protocols on anaerobic performance in elite youth soccer players. Each warm-up protocol started with a 5-minute low-intensity jog followed by 10 minutes of static stretching, dynamic stretching, or dynamic stretching followed by 8 front squats + 20% body mass. Immediately following and at 3 and 6 minutes after the warm-up protocol subjects performed the performance tests. As a measure of functional performance, countermovement vertical jumps and 10- and 20-m sprints were performed. To minimize the effects of fatigue, testing procedures were conducted 48 hours following a match or high-intensity training session. To ensure no confounding effects of training, the subject's individual strength and conditioning program was put on a maintenance scheme.
Twenty-two elite youth soccer players participated in this study. However, 2 subjects were unable to complete the study because of injury, and only the data of the 20 subjects who completed the study were included. Subject's age, stature, and body mass were 17.2 ± 1.2 years, 178.2 ± 5.9 cm, 74.5 ± 7.7 kg (mean ± SD), respectively. After full explanation of the rationale of the research, procedures and potential risks, informed consent was obtained from both parents/guardians and subjects before any testing. The procedures were approved by the Research Ethical Committee of Manchester Metropolitan University.
The study was performed in February through March 2007 when the subjects were in the maintenance phase of their training program. All subjects were familiar with the stretch protocols, resistance exercise, and performance tests. Nevertheless, a group session prior to testing was included to recap on techniques to the resistance exercise, testing procedures, and the different modes of stretching. Information sheets were handed out to all subjects to allow them to continually prepare until the day of testing. The warm-up protocols were randomized and conducted on separate days. All study procedures took place in the soccer club's indoor facility between 9:30 and 10:30 am. This time slot ensured the weekly coaching program was not interrupted and prevented the possible bias related to circadian rhythms (1). Subjects performed the countermovement jump test first, recovered for 20 seconds, and then performed the 20-m sprint test with a timing gate at 10 and 20 m to obtain both the 10- and 20-m sprint performance. The 20-second recovery time allowed for resynthesis of the phosphocreatine stores and full recovery before the next anaerobic performance measure (5). This procedure was performed immediately after and at 3 and 6 minutes following each warm-up protocol. Unfortunately, there was no pre-warm-up performance included in the study because of the Academy Directors instructions. Study procedures were constantly supervised by a qualified strength and conditioning coach.
The static stretch (SS) protocol consisted of 5 minutes of low-intensity jogging followed by 10 minutes of static stretching emphasizing the lower-extremity muscle groups (gastrocnemius, quadriceps, hip flexors, adductors, hamstrings, and gluteals). The static stretches used were described by Thompsen et al. (38). The technique of static stretching required the subjects to slowly take up the stretch of the muscle to the point of tension and mild discomfort and hold for a period of 15 seconds. This is the time commonly used by athletes in preparing for competition (14). Each muscle group was stretched twice, and alternating between each leg gave adequate recovery before the next stretch repetition.
The dynamic stretch (DS) protocol consisted of 5 minutes of low-intensity jogging followed by 10 minutes of dynamic stretching emphasizing the same muscle groups included in the SS protocol. The intensity of the dynamic movements progressed from moderate to high intensity in nature. The dynamic stretches used were described by Thompsen et al. (38) and are thought to prepare soccer players because they mimic functional movements found in a competitive match (23). The technique of the dynamic stretching required the subject to maintain an upright posture. Subjects perform each dynamic exercise twice over a distance of 20 yards interspersed with a walk back to the start position.
The dynamic stretch plus resistance exercise (DSR) protocol consisted of 5 minutes of low-intensity jogging followed by 10 minutes of dynamic stretching as used in the DS protocol. This was immediately followed by 8 front squats while holding 2 dumbbells at shoulder height with a combined weight of 20% body mass (range 12-18 kg). Subjects were required to drive upward as fast as possible during the concentric phase of the squat exercise, exploiting the rate of force development, an important factor in explosive strength capabilities (30). Correct technique from picking the dumbbells up, to performing the front squat, to placing the dumbbells back on the floor was under constant supervision.
Power and speed were measured by the countermovement jump test and 10- and 20-m sprint tests, respectively. The countermovement jump test was performed on a Newtest Electronic Jump Mat (Newtest, Oulu, Finland). The countermovement jump test has been shown to be the most valid field-based test when measured by means of contact mat and digital timer for the estimation of explosive power of the lower limbs (19,24). Power was determined by the measurement of flight time between lift-off of the feet from the mat to landing of the feet back on the mat, taking into account body mass. The jump height to the nearest centimeter was automatically calculated. For the countermovement jump test, subjects were required to stand at the center of the jump mat with feet approximately hip-width apart. With hands on hips, subjects were then asked to squat until thighs were parallel with the ground and then immediately jump upward. On leaving the jump mat, subjects had to keep hands on hips and keep legs straight at all times, even on landing. Flexion of the hips and knees prior to landing would increase flight time and limit the accuracy and reliability of the results. Subjects were required to wear athletic trainers.
For the 10- and 20-m sprint test, time was measured using the Newtest Timing Gates. The 20-m sprint test is an appropriate distance because this is the mean sprint distance covered in a competitive soccer match (9). All subjects performed a standing start with dominant foot to the front, 0.7 m from start line. Subjects were required to wear soccer boots. Time was recorded to the nearest 0.01 seconds.
A repeated-measures analysis of variance (ANOVA) was used to analyze the difference between performance measures at a particular time (within-subject variable: immediately following and at 3 or 6 minutes) for all warm-up protocols (between-subject variable: SS, DS, DSR). Bonferroni corrected post hoc comparisons were used to locate differences between warm-up protocols if significant main effects for warm-up or time were found. The dependent variables included countermovement jump height and 10- and 20-m sprint time. Statistical significance was set at p ≤ 0.05. SPSS (version 11.5, SPSS, Inc, Chicago, Illinois, USA) was used to analyze performance measure data.
Mean scores and standard deviations for the performance measures following each warm-up protocol are presented in Figures 1 through 3. Averages and ranges are given in Table 1.
A significant main effect was found for warm-up protocol, highlighting a greater jump performance after the DSR and DS protocol compared with the SS protocol at 0, 3, and 6 minutes: F (2,38) = 53.3, 89.7, 104.2, respectively, p < 0.05. Bonferroni post hoc multiple comparisons revealed an improved jump performance for DSR compared with the DS protocol at 3 and 6 minutes (mean difference = −2.7, −2.3, respectively, p < 0.05). There was a significant interaction between warm-up protocol and time (3 and 6 minutes) on jump height performance: F (2,38) = 77.4, p < 0.05. The interaction was apparent as a significantly improved jumping height at 3 and 6 minutes after OSR, while there was no significant effect of time for the countermovement jump test after the SS and DS protocol, indicating that the jump performance was similar immediately after and 3 and 6 minutes after the protocols (Figure 1).
A significant main effect for 10-m sprint times was found highlighting a better performance after the DS and DSR protocol compared with the SS protocol at 0, 3, and 6 minutes: F (2,38) = 13.7, 18, and 21.1, respectively, and p < 0.05 (Figure 2). Also, a significant main effect for 20-m sprint times was found highlighting a better performance after the DS and DSR protocol compared with the SS protocol at 0, 3, and 6 minutes: F (2,38) = 11.3, 14.6, and 11.4, respectively, and p < 0.05 (Figure 3). There was no significant effect of time for 10- and 20-m sprint times after the SS, DS, and DSR protocols (Figures 2 and 3).
The main finding of the present study was that a warm-up protocol consisting of dynamic stretching followed by 8 squats with an additional load of 20% body mass resulted in a superior countermovement jump performance compared to a dynamic or static stretch protocol alone. For sprint performance, dynamic stretching was superior to static stretching but, in contrast to jump performance, the inclusion of resistance exercise had no additional beneficial effect on sprint performance. These data indicate that a dynamic warm-up protocol is preferable to a static protocol to enhance anaerobic performance in an elite youth team sport.
The inclusion of resistance exercise following dynamic stretching enhanced countermovement jump performance at 3 and 6 minutes when compared with a warm-up protocol consisting of dynamic or static stretching alone. In line with our observation it has been reported that continually wearing a weighted vest for the entire (2% body mass) or for the last 4 exercises (10% body mass) of a dynamic warm-up improved jump performance more than a warm-up consisting of only dynamic or static stretching (13,38). Inclusion of resistance in a dynamic warm-up, however, did not produce a superior sprint performance, as was also observed by others (13). Previously, it has been reported that a dynamic warm-up resulted in a superior 10-m sprint performance than from a static warm-up (12). Here we show that the 20-m sprint performance was also better after a dynamic rather than static warm-up, and the effect of the warm-up was significant even 6 minutes after the completion of the warm-up.
One of the possible mechanisms behind the enhanced jumping and sprinting performance after a dynamic-style warm-up is PAP. Indeed, it has been shown that activation of a muscle may cause an enhanced performance for some time after the cessation of the activation (30). PAP may be a result of increased phosphorylation of myosin light chains, increasing the calcium sensitivity of the myofilaments (30). Also, an increase in muscle temperature and muscle blood flow as a result of dynamic stretching may induce a more forceful and quicker muscle contraction by increasing the speed of nerve impulses (31) and the force-generating capacity of muscle cells (35). Because PAP also occurs after static exercise (16), one may argue that it does not explain the superior performance after dynamic in comparison to static stretching. However, fast motor units exhibit more PAP than slow motor units (16), and because more motor units are recruited during dynamic than static contractions (34) and hence also more fast motor units, the PAP may be larger after a dynamic than a static warm-up. Furthermore, the additional recruitment of fast motor units when resistance exercise is added to a warm-up (13,38) may augment PAP and further enhance performance.
If this suggestion is true, then a dynamic warm-up including 8 countermovement jumps with an additional load of 20% body mass may take advantage of the stretch-shortening cycle where high-threshold fast-twitch motor units have been reported to be recruited prior to slow-twitch, low-threshold motor units (27). This would add to the fact that this is also a specific movement preparing the body neurologically for the countermovement jump test (6). In soccer, movements such as acceleration and jumping are usually preceded by an activity that involves the stretch-shortening cycle (32), thus increasing the specificity of the countermovement jump exercise. Sprinting, however, requires the development of power in the horizontal plane, so a plyometric exercise such as bounding, may translate to enhancing sprint performance.
Although specificity of exercise selection is important, the varying degree between jump and sprint performance for the dynamic plus resistance warm-up in the current study may be more related to PAP. PAP may only be present during the first few muscle contractions and fades during ongoing activity. Therefore, jump performance may benefit more because just 1 maximal contraction of the muscle is required compared to sprint performance, which requires a series of muscle contractions. In our observations this suggestion may be supported with the lower jump performance after dynamic stretching in comparison with dynamic stretching with the inclusion of resistance, while there was no difference in sprint performance between these 2 warm-up protocols. Furthermore, biomechanical requirements and muscle fiber recruitment patterns differ for acceleration and maximal sprinting (32,42), thus sprinting may require a greater coordinated effort of agonists and antagonist muscle groups compared to an in-place vertical jump.
Another unique aspect of the present study was to investigate the interaction between PAP and fatigue. Jump performance was superior at 3 and 6 minutes after than immediately following the dynamic plus resistance warm-up. Previously, it was found that jumping performance was not enhanced 2 minutes after a dynamic stretch protocol while wearing a weighted vest of 6% body weight (13). The authors suggest that fatigue may still be present and that the standard 2-minute recovery as used in previous studies (12,13,21,38) prior to performance may not be sufficient, so recovery time may need to be longer. This may be so and would fit our observations, but the load may just have been so high that it induced fatigue because wearing a vest of 2% body weight did enhance performance 2 minutes after the warm-up (13). Achieving optimal PAP is a catch-22; we want high-intensity exercise, to recruit the fast motor units that exhibit the largest PAP, yet such exercise may induce a larger degree of fatigue. In addition, PAP declines during prolonged recovery (30). Chiu et al. (7), however, found that fatigue was present within the first 5 minutes following an acute heavy-resistance exercise stimulus, whereas PAP remained for more than 18 minutes. This was also evident in the current study because the inclusion of resistance in a dynamic warm-up enhanced jump performance compared to only a dynamic warm-up, but a 3-minute recovery was required before jump ability was superior, which was still evident at 6 minutes. To provide a more detailed description for optimal recovery, it may be appropriate to analyze 1 performance measure at a time. Also, the activation of the PAP mechanism can occur following any high-intensity exercise (30), and 1 performance test may have a subsequent effect on another different test.
A true indication of performance enhancement following the 3 warm-up protocols used in the study could not be determined because pre-warm-up measurements were not included in the study. Although a limitation of the study, the results do highlight the impact each of the protocols has on subsequent anaerobic performance.
Although static stretching is a method of conditioning that contributes to the overall physical development of an elite youth soccer player, especially during the adolescent growth spurt, increasing evidence suggests it may not be the method of choice prior to anaerobic activities such as sprinting and jumping. Dynamic stretching, however, may be a more preferred warm-up method for elite youth soccer players and seems to initiate a greater explosive strength ability compared to a warm-up that includes static stretching. The inclusion of resistance in a warm-up further enhances anaerobic performance; however, the findings of this study and previous research suggest that acute PAP manipulation has its greatest effect on 1-time explosive exercise such as jumping. From a practical point of view, acute PAP manipulation may provide an enhanced preparation prior to a session with a conditioning emphasis, such as plyometrics. An understanding of the relationship between fatigue and the PAP mechanism is vital if optimal recovery is to be prescribed prior to anaerobic performances. Although a new area of research, trial and error may highlight various responses of acute PAP manipulation. Investigating various warm-up structures including resistance exercise in a team sport, without affecting the weekly coaching program, may be beneficial for performance, as shown in this study.
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