The Acute Effects of Weighted Vest Protocols on 20-Metre Sprint Performance in Youth Soccer Players

This investigation examined the effects of a warmup containing weighted vest (WV) sprints on subsequent 20-metre sprint time relative to a control (C) condition in youth soccer players (n =12, mean ± SD age 16 ± 0.60 years, height 175.17 ± 5.92 cm and body mass 61.85 ± 5.88 kg). The main experimental trials consisted of three WV conditions at 10, 20 and 30% of body mass (WV10, WV20 and WV30) and C. Participants were required to complete one 20-metre sprint with each of WV conditions or without additional mass as part of C prior to a 20-metre sprint at 4-, 8and 12-minutes. A two-way repeated measures ANOVA revealed no significant difference between any of the conditions and rest periods (p = >0.05). The between condition effect sizes for 20-metre sprint times were moderate at 4and 12-minutes post WV10 (d = -0.86 and -1.15, respectively) and 12-minutes post WV20 (d = -0.84) and WV30 (d = -0.80). Moderate effect sizes were also observed at 4-minutes post WV10 (d = -1.04) and WV20 (d = -0.67) for 10-metre sprint times. These findings demonstrate that WV loading has no significant effect on 20-metre sprint time in youth soccer players. However, there is an opportunity for S&C coaches to implement WV warm-ups of no more than 30% body mass to improve 20-metre sprint times. INTRODUCTION The acute effects of warming-up on measures of physical performance have been studied since the 1930s (45). These investigations have typically found the completion of an active warm-up prior to training or competition to have a positive impact on athletic performance (8,20,51). A review by Radkin et al. (39) concluded that there is little evidence to suggest that warming-up is detrimental to performance with post-warm-up improvements noted between <1% and 20% for tasks such vertical jumping, long jump and agility. Furthermore, similar improvements were found in sporting performance, including softball, basketball and golf (39). The traditional warm-up incorporates a brief period of submaximal aerobic activity followed by a series of static and/ or dynamic stretching routines and sport-specific exercise (32,35). In more recent years, the addition of a post-activation performance enhancement (PAPE) complex within a warm-up has grown in popularity amongst strength and conditioning (S&C) coaches (11,26). This method involves performing a conditioning exercise (CE) prior to an explosive movement with similar biomechanical characteristics (15,26,44). To date, a number of studies have examined the acute effects of a heavy resistance CE within a warm-up on sprint performance (42,56). This commonly involves the execution of a multi-joint free weight exercise at loads exceeding 80 to 85% of an Copyright: © 2021 by the authors. Licensee IUSCA, London, UK. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). International Journal of Strength and Conditioning. 2022 individual’s one repetition maximum (1-RM) (44,56). The most frequently used heavy resistance CE is the back squat with findings demonstrating significant improvements in sprint performance over distances of 10 to 40-metres following 1 to 5 repetitions at >85% of 1-RM (p = <0.05) (5,9,29,34,57,58). The ability to attain the accompanying PAPE benefits are, however, thought to be associated with the physical conditioning of the individual (4,37,57) and their level of muscular development (40). These characteristics can be directly influenced by the age of the participant as children possess less voluntary muscle speed, strength and power, even when corrected for age or biological maturation (1,38). Likewise, children and adolescents possess a significantly lower percentage of type IIx muscle fibres when compared with adults (27,28,46) and have a reduced ability to utilise higher-threshold motor units (13,16,19,23,40,43) which are more responsive to heavy resistance exercise (24,54). The combination of these factors may influence the ability of adolescent participants to derive benefits from heavy resistance CE. Therefore, alternative methods of warming-up that may be better tolerated are appealing. Recently, Grimes et al. (22) investigated 20-metre sprint performance 5-, 6and 7-minutes after a sled push at 50 and 100% of body mass amongst male soccer players. Despite no significance in either condition, a moderate effect size at 6and 7-minutes (d = -0.71 ± 0.57 and -0.62 ± 0.65, respectively) in the 50% of body mass condition was observed alongside no decrements in sprint performance following both conditions and all recovery durations. It is likely that the lack of significance may have been as a result of the sled pushing altering participants natural sprint mechanics through removal of the arm action, excessive forward trunk lean and an increase in ground contact times (30). Likewise, the load imposed during the 100% of body mass condition may have been too great, lacking transfer to that of maximal sprint speed and the ability of youth participants to derive benefits from heavy resistance CE (47). The ability to access sleds and sufficient load may also be restrictive to youth-based team sports, hence, alternate means of adding load to dynamic movements may be more applicable. One such method is the use of a weighted vest (WV) CE included within a warm-up (7,50). For example, Faigenbaum et al. (18) found that when adolescent athletes (15.3 ± 1.2 years) continually wore a WV of 2% body mass during a dynamic warm-up, jump performance was significantly improved compared with a dynamic warm-up alone (42.1 ± 5.2 cm vs 37.1 ± 5.1 cm, respectively; p = 0.04). A weighted vest of 6% body mass, however, did not improve performance, suggesting that the additional load may have been too great. Similarly, Reiman et al. (41) also found no advantageous effects of a dynamic warm-up with the addition of a WV at 5% of body mass on subsequent lower body power output. When a weighted vest of 10% body mass was worn only for the last 4 of 12 dynamic exercises, jump performance was significantly improved compared to dynamic movements alone (49). These findings suggest that improvements can be established following heavier WV activity, however, the amount of time participants wear the WV during the warm-up must be carefully considered to ensure fatigue is managed and potentiation is realised (50). In further support, Turner et al. (55) observed significant improvements in 20and 10-metre sprint performance after 3 sets of alternate leg bounds with the addition of a WV at 10% body mass, albeit in adult participants. It is evident from these findings that to benefit from the acute effects of WV drills contained within a warm-up, factors such as the volume and intensity of added mass along with the duration of recovery prior to performance must be well-thought-out. When considering the recovery period following heavy resistance CE, performance is initially impaired, potentiation is then realised, peaks and decreases in an inverted U-shaped fashion (6,52,56). However, when considering a lighter CE, that of a WV, the same level of agreement is yet to be reached with the reporting of equivocal results. Nonetheless, previous investigations have found potentiating effects in numerous physical performance tasks (i.e., vertical jump, long jump, medicine ball toss and 10-yard sprint) 2-minutes post warm-up containing dynamic exercise with a WV at 2% of body mass (18). Similar results have been demonstrated after <60 seconds (33,53) and 3-minutes (12,48,49). It would therefore appear that the recovery duration necessary to observe a potentiated response may be lower than would be required following heavy resistance CE given the significant reduction in system mass loading (21). Despite the aforementioned investigations suggesting that a lighter CE requires less recovery prior to subsequent physical performance, very few have attempted to examine the effects on more functional indices of sporting performance in youth populations, such as sprinting. Hence, further research is warranted to determine an optimal load The Acute Effects of Weighted Vest Protocols on 20-Metre Sprint Performance in Youth Soccer Players 2 Copyright: © 2021 by the authors. Licensee IUSCA, London, UK. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). International Journal of Strength and Conditioning. 2022 Bright, T. E., Hughes, J. D., Handford, M. J., Annis, B., Westwood, C and recovery period following a warm-up containing WV activity prior to sprint performance in youth soccer players. The potential that a dynamic warm-up protocol with the inclusion of a WV could result in enhanced subsequent sprint performance could have significant implications for S&C coaches who work with young soccer players. Accordingly, this study aimed to examine the acute potentiating effects of WV sprints at 10, 20 and 30% of body mass on subsequent 20and 10-metre sprint performance after 4-, 8-, and 12-minutes of rest, relative to C. It was hypothesised that all WV conditions would significantly enhance 20and 10-metre sprint performance in comparison with C.


INTRODUCTION
The acute effects of warming-up on measures of physical performance have been studied since the 1930s (45). These investigations have typically found the completion of an active warm-up prior to training or competition to have a positive impact on athletic performance (8,20,51). A review by Radkin et al. (39) concluded that there is little evidence to suggest that warming-up is detrimental to performance with post-warm-up improvements noted between <1% and 20% for tasks such vertical jumping, long jump and agility. Furthermore, similar improvements were found in sporting performance, including softball, basketball and golf (39). The traditional warm-up incorporates a brief period of submaximal aerobic activity followed by a series of static and/ or dynamic stretching routines and sport-specific exercise (32,35). In more recent years, the addition of a post-activation performance enhancement (PAPE) complex within a warm-up has grown in popularity amongst strength and conditioning (S&C) coaches (11,26). This method involves performing a conditioning exercise (CE) prior to an explosive movement with similar biomechanical characteristics (15,26,44).
To date, a number of studies have examined the acute effects of a heavy resistance CE within a warm-up on sprint performance (42,56). This commonly involves the execution of a multi-joint free weight exercise at loads exceeding 80 to 85% of an individual's one repetition maximum (1-RM) (44,56). The most frequently used heavy resistance CE is the back squat with findings demonstrating significant improvements in sprint performance over distances of 10 to 40-metres following 1 to 5 repetitions at >85% of 1-RM (p = <0.05) (5,9,29,34,57,58).
The ability to attain the accompanying PAPE benefits are, however, thought to be associated with the physical conditioning of the individual (4,37,57) and their level of muscular development (40). These characteristics can be directly influenced by the age of the participant as children possess less voluntary muscle speed, strength and power, even when corrected for age or biological maturation (1,38). Likewise, children and adolescents possess a significantly lower percentage of type IIx muscle fibres when compared with adults (27,28,46) and have a reduced ability to utilise higher-threshold motor units (13,16,19,23,40,43) which are more responsive to heavy resistance exercise (24,54). The combination of these factors may influence the ability of adolescent participants to derive benefits from heavy resistance CE. Therefore, alternative methods of warming-up that may be better tolerated are appealing.
Recently, Grimes et al. (22) investigated 20-metre sprint performance 5-, 6-and 7-minutes after a sled push at 50 and 100% of body mass amongst male soccer players. Despite no significance in either condition, a moderate effect size at 6-and 7-minutes (d = -0.71 ± 0.57 and -0.62 ± 0.65, respectively) in the 50% of body mass condition was observed alongside no decrements in sprint performance following both conditions and all recovery durations. It is likely that the lack of significance may have been as a result of the sled pushing altering participants natural sprint mechanics through removal of the arm action, excessive forward trunk lean and an increase in ground contact times (30). Likewise, the load imposed during the 100% of body mass condition may have been too great, lacking transfer to that of maximal sprint speed and the ability of youth participants to derive benefits from heavy resistance CE (47). The ability to access sleds and sufficient load may also be restrictive to youth-based team sports, hence, alternate means of adding load to dynamic movements may be more applicable.
One such method is the use of a weighted vest (WV) CE included within a warm-up (7,50). For example, Faigenbaum et al. (18) found that when adolescent athletes (15.3 ± 1.2 years) continually wore a WV of 2% body mass during a dynamic warm-up, jump performance was significantly improved compared with a dynamic warm-up alone (42.1 ± 5.2 cm vs 37.1 ± 5.1 cm, respectively; p = 0.04). A weighted vest of 6% body mass, however, did not improve performance, suggesting that the additional load may have been too great. Similarly, Reiman et al. (41) also found no advantageous effects of a dynamic warm-up with the addition of a WV at 5% of body mass on subsequent lower body power output. When a weighted vest of 10% body mass was worn only for the last 4 of 12 dynamic exercises, jump performance was significantly improved compared to dynamic movements alone (49). These findings suggest that improvements can be established following heavier WV activity, however, the amount of time participants wear the WV during the warm-up must be carefully considered to ensure fatigue is managed and potentiation is realised (50). In further support, Turner et al. (55) observed significant improvements in 20-and 10-metre sprint performance after 3 sets of alternate leg bounds with the addition of a WV at 10% body mass, albeit in adult participants. It is evident from these findings that to benefit from the acute effects of WV drills contained within a warm-up, factors such as the volume and intensity of added mass along with the duration of recovery prior to performance must be well-thought-out.
When considering the recovery period following heavy resistance CE, performance is initially impaired, potentiation is then realised, peaks and decreases in an inverted U-shaped fashion (6,52,56). However, when considering a lighter CE, that of a WV, the same level of agreement is yet to be reached with the reporting of equivocal results. Nonetheless, previous investigations have found potentiating effects in numerous physical performance tasks (i.e., vertical jump, long jump, medicine ball toss and 10-yard sprint) 2-minutes post warm-up containing dynamic exercise with a WV at 2% of body mass (18). Similar results have been demonstrated after <60 seconds (33,53) and 3-minutes (12,48,49). It would therefore appear that the recovery duration necessary to observe a potentiated response may be lower than would be required following heavy resistance CE given the significant reduction in system mass loading (21). Despite the aforementioned investigations suggesting that a lighter CE requires less recovery prior to subsequent physical performance, very few have attempted to examine the effects on more functional indices of sporting performance in youth populations, such as sprinting. Hence, further research is warranted to determine an optimal load and recovery period following a warm-up containing WV activity prior to sprint performance in youth soccer players.
The potential that a dynamic warm-up protocol with the inclusion of a WV could result in enhanced subsequent sprint performance could have significant implications for S&C coaches who work with young soccer players. Accordingly, this study aimed to examine the acute potentiating effects of WV sprints at 10, 20 and 30% of body mass on subsequent 20-and 10-metre sprint performance after 4-, 8-, and 12-minutes of rest, relative to C. It was hypothesised that all WV conditions would significantly enhance 20-and 10-metre sprint performance in comparison with C.

Participants
Participants (n = 12) were a mixture of attackers, midfielders and defenders from the male under 16 age group of a sub-elite soccer club (mean ± standard deviation (SD) age 16 ± 0.60 years, height 175.17 ± 5.92 cm and body mass 61.85 ± 5.88 kg). All participants took part in an average of 6 ± 1 hours of combined soccer specific training and competitive play per week and had 1.5 ± 1 years of plyometric and sprint training experience. Following a full explanation of the procedures and potential risk, parental consent and participant assent were gained. Procedures were pre-approved by the Plymouth Marjon University ethics committee. Participants were healthy, non-smokers and had all been injury free for at least 8 months prior to taking part. In the 12-hours leading up to data collection, participants were encouraged to replicate their normal diet, avoiding alcohol and caffeine. Recommendations were made to refrain from strenuous physical activity 48-hours prior to testing, in line with similar previous research (55). .

Study Design
A within participants repeated measures study design was used to compare the effects of WV sprint running within a warm-up on 20-and 10-metre sprint time, relative to C. Participants completed four experimental trials involving a standardised warm-up followed by an unloaded 20-metre sprint (C) or a CE consisting of 20-metre WV sprint running with an additional mass of 10, 20 or 30% of body mass (WV10, WV20 and WV30, respectively). After performing one of the four conditions, 20-metre sprints were re-tested without additional mass at 4-, 8-and 12-minutes to profile transient fatigue and potentiating effects. The order of CE for each participant was determined using an online block randomisation tool (Research Randomizer, Version 4) and participants were informed of the condition they would be exposed to at the start of each visit.

Familiarisation
Prior to the main experimental trials, participants attended a familiarisation session whereby anthropometric measurements of stature and body mass were taken (Seca, mBCA 514, Birmingham, UK) and sprint performance testing was practiced. Each participant also familiarised themselves with sprinting whilst wearing a WV to ensure they were sufficiently prepared for the main experimental trials. The first of the main experimental trials was performed 48-hours after familiarisation and no less than 48-hours separated all subsequent main trials. Test-retest reliability for our participants during 20and 10-metre sprints met recently recommended criteria (intraclass correlation = 0.957-0.978; coefficient of variation percentage = 2.47-2.46) (2).

Sprint Time
Sprint performance was assessed over 20-metres using a timing gate system (Brower Timing Systems, Utah, USA) positioned at 0-, 10-and 20-metres at a height of approximately 60-cm. These methods were similar to those of Turner et al. (55) to ensure light beams were not broken by the lower arm or lower leg. Participants began each sprint following a 3-second countdown from a 2-point staggered stance at a distance of 0.3-metres behind the first set of timing gates to ensure light beams were not broken before the start of each sprint. The countdown was used to make sure that participants were focused and ready to give maximal effort. A 10-metre deceleration zone was marked to encourage participants to accelerate beyond the 20-metre finish line. Timing started and finished when the beams of the first and last gates were broken respectively.

Conditioning Exercise
During the main experimental trials, participants reported to the Sport and Exercise Science Laboratory between 18.00 and 20.00. After being informed of their condition, participants underwent Participants consumed water ad libitum during the trials and a single member of the research team administered all tests such that the potential variation in test instruction was minimised. All tests were conducted in a laboratory that was maintained at an air temperature of 20 ± 0.6 °C.

Statistical Analysis
All statistical analyses were performed using SPSS software (Version 28) and data are presented as mean ± SD.  (25).

RESULTS
Mean ± SD for 20-and 10-metre sprint times following each of the CE and rest periods are shown in Figures 1 and 2, respectively. Percentage change values and effect sizes are provided in Table 1.

20-Metre Sprint Time
A two-way repeated measures ANOVA found no effect of group (F ( were moderate at 4-and 12-minutes post WV10, 12-minutes post WV20 and 12-minutes post WV30.

DISCUSSION
In the current investigation, adolescent soccer players performed WV sprints at 10, 20 and 30% of body mass to determine whether this intervention would acutely enhance 20-and 10-metre sprint performance relative to C after 4-, 8-and 12-minutes of rest. The main finding was that all of the WV conditions improved 20-and 10-metre sprint performance in comparison with the C. Although no statistical significance was found, the greatest reductions in 20-metre sprint times were observed after WV10 at 12-minutes. Similarly, WV10 was also superior in improving 10-metre sprint performance after 4-minutes. Nevertheless, the results fail to support our hypothesis.
The augmentation of sprint performance through the use of different warm-up protocols has been well documented when using a heavy resistance CE such as a barbell back squat or deadlift (5,14,44). However, the validity of employing a heavy resistance CE prior to training or competitive performance has often been questioned (31,55). This type of protocol requires heavy loads, approaching 1.5 to 2x an athlete's body mass, with the addition of equipment such as a squat rack, barbell and plates (14). The ability of younger athletes to realise positive effects following a heavy resistance CE are also likely to be diminished given their relatively little quantity of type IIx muscle fibres and reduced ability to recruit higher-threshold motor units compared with adults (3,13,16,23,40). The incorporation of WV sprint exercise into an athlete's warm-up is therefore a far more practical approach and one which the current investigation has demonstrated as being efficacious in improving 20-and 10-metre sprint times. That the load of the WV does not need to be very heavy is also in support of similar observations (18,41,50,55).
Despite the lack of statistical significance, the results of this study demonstrate the WV10 condition to induce the greatest improvement in 20-and 10-metre sprint performance. Though improvements were noted, it is likely that WV20 and WV30 resulted in comparatively higher amounts of fatigue and thus, interfered with the potentiation response. To date, however, there is a limited amount of research in support of these findings, specifically regarding adolescent soccer players. Turner et al. (55) reported that following WV alternate leg bounds at 10% body mass, subsequent 20-and 10-metre  sprint performance was impaired at 15-seconds by 1.4 ± 2.5% and thereafter, improved at 4-minutes (20-metre = -2.3 ± 2.6%; 10-metre = -2.2 ± 3.1%) and 8-minutes (20-metre = -2.6 ± 2.8%; 10-metre = -2.9 ± 3.6%), though the participants were adults. When using participants of a similar age to those of the current study, Needham et al. (36) and Faigenbaum et al. (17) observed improved sprint and jump performance following a dynamic warm-up containing numerous plyometric type exercises and front squats with 20% body mass. Interestingly, Faigenbaum et al. (18) also found significant improvements in jump performance after participants were instructed to wear a WV at 2% body mass throughout the entire warm-up. Though, when this was increased to 6%, no further improvements were observed. However, given that the WV was worn continually throughout the warm-up protocols, it is likely that high levels of fatigue were evoked, compromising any possible potentiation effect. Furthermore, only 2-minutes of rest were given post warm-up. Factors such as the type, volume and load of CE and the recovery duration therefore must be considered prior to the application of WV drills. In the current study no significant improvements were found following one set of WV sprints at 10, 20 or 30% of body mass, despite reductions in sprint times. Consequently, perhaps this protocol may benefit from an additional WV set to ensure a larger amount of potentiation is realised.
From studies where a heavy resistance CE has been used during a warm-up, performance is consistently compromised immediately after the CE (6,26,52). However, speculation exists regarding the rest duration required following a lighter mode of CE. In contrast to previous investigations (18,50), the current findings suggest that both 20-and 10-metre sprint performance were enhanced after each of the WV conditions at 4-, 8-and 12-minutes post, relative to C. The greatest improvements in 20-metre sprint performance were observed after 12-minutes rest, across all conditions. For 10-metre sprint performance, a 5.73 ± 4.90% reduction in sprint time was realised after 4-minutes, representing the largest improvement. When considering similar investigations, the majority have observed potentiating effects following the use of a lighter load CE after <3-minutes (10,12,49) and even <60-seconds in some cases (53). However, several methodological differences must be acknowledged. Firstly, the participants in each of these studies were either Division 1 Collegiate or professional athletes with far greater strength and plyometric training experience in comparison with those of the current investigation. Secondly, the CE ranged from depth jumps to a combination of various hopping and bounding drills, which, in comparison with the WV sprints are less intense and fatiguing (17,18). Therefore, it is likely that the conditioning of participants and the nature of their experimental procedures explains the differences in findings.
Although performance in the current study was optimal after 12-minutes, it is also important to note that both 20-and 10-metre sprint performance was enhanced as early as 4-minutes following a CE.
A potential limitation of the current investigation is related to the intensity of the WV warm-up, specifically the number of sets. In an attempt to evoke potentiation, the protocol was designed using a WV CE that involved the completion of one set of a 20-metre sprint at 10, 20 or 30% body mass. Theoretically, a multiple-set WV CE would've produced greater amounts of fatigue which may reduce the ability to express high levels of potentiation (37,57

CONCLUSION
Training for sprint speed is an important consideration for S&C coaches who work with team sport athletes.
The results of the current investigation demonstrate that, although not statistically significant, 20-and 10-metre sprint times in youth soccer players can be improved using WV sprints as part of a dynamic warmup at 10, 20 and 30% of body mass relative to C, after 4-, 8-and 12-minutes of rest. More specifically, the WV10 warm-up protocol demonstrated the greatest improvements in 20-metre sprint performance. It is, however, important to acknowledge that the specific effect of each WV warm-up is largely individualised and as such, coaches should adopt a trial-and-error approach to identify the most suitable protocol for their athletes. Having only implemented one set of each WV condition, future research should look to include a multiple set WV CE whilst replicating the current study design to examine the effectiveness of this type of intervention.

International Journal of Strength and Conditioning. 2022
The Acute Effects of Weighted Vest Protocols on 20-Metre Sprint Performance in Youth Soccer Players

PRACTICAL APPLICATIONS
The application of a WV within a warm-up has more recently been popularised as a result of a growing interest in more practical PAPE protocols. The principle is that through the addition of a WV an athlete can enhance their subsequent performance. In the current investigation, a WV warm-up at 10, 20 and 30% of body mass was superior to C in improving 20-and 10-metre sprint performance amongst trained adolescent soccer players. Despite no statistical significance, the moderate effect sizes observed following some of the conditions and rest periods demonstrates the practical implications of these findings. Table 2 shows an example of how a WV warm-up could be structured based upon the findings of this study. The suggested prescriptions are guided by the results but should be determined appropriately on an individual basis. The recovery duration necessary to observe a potentiated response in 20-metre sprint performance is less than previously reported post heavy resistance CE. Thus, this type of warm-up may be a more time-efficient way of harnessing potentiation.