Competitive soccer can be characterized as an intermittent high-intensity strenuous sport during which short bouts of very intense activity are interspersed with lower intensity movements (26). Time-motion analysis studies have demonstrated that soccer players spend 8–12% of total match distance undertaking high-intensity running or sprinting (18). Although soccer players only sprint between 100 and 1,320 m per match (26), these actions are considered critical, and straight sprinting is the most frequent action in goal situations (11). Moreover, sprint ability is able to discriminate players from different standards of play (23). Therefore, it can be argued that sprint ability is crucial in competitive soccer, and accordingly, the development of different training strategies to improve these actions is recommended.
It is well known that sprint ability can be improved by means of strength training (19). Several strength-training methods have been used to improve sprint by investigating its relationship with athletic performance in soccer players, such as resistance training (27), plyometric training (17), contrast training (14), or resisted sprint (RS) training (28). Resisted sprint training, which involves the player sprinting with added load, or using other forms of resistance such as hills and stairs, is a form of sprint training designed to increase the neural activation and strength of the hip extensors, thus sprinting velocity, without substantial changes in running form (2). This method includes limb loading, uphill running, parachutes, elastic cord, partner-resisted drills, resisted towing, and weighted vests (WVs) (8).
Despite the popularity and the theoretical benefits associated with RS training, research validating this training method for improving physical performance in soccer is limited. To date, only a few studies have examined the chronic effects of RS on physical performance in soccer players using resisted towing (9,25,28). Generally, published research in soccer has produced unanimous results regarding the effectiveness of RS training reporting improvements of resisted towing on strength (25), jump performance (9), and maximum velocity (28). According to the training specificity principle, the player's movement patterns during the training exercise should closely resemble those used when performing the sport (1). In this respect, WVs, due to the fact that external loading is transported around the torso, may result in fewer joint kinematic alterations than other RS means in soccer players (8). Additionally, unlike other RS means, the WVs offer the possibility to make ballast motor actions such as jumps, change of direction, or even soccer-specific actions such as duels or hitting. However, the benefits of WVs are relatively unexplored, and no previous studies have examined the effects of vest sprinting on physical performance in soccer players.
During the most intense periods of a soccer game, the occurrence of short sprint sequences with brief recovery periods suggests that repeated sprint ability (RSA) may be considered as a soccer-specific capacity (15). Therefore, training interventions aimed at improving RSA may be a priority for soccer coaches. Although the previous studies on RS have focused on short performance outcomes (i.e., jump or sprint), no studies have assessed whether or not changes in RSA can be achieved with RS training. Information in this regard could result of practical interest for coaches and strength and conditioning professionals in soccer and other team sports.
Therefore, in the light of the aforementioned considerations and limitations, the aim of this study was to determine the effects of a 6-week, 12-session training program using WVs as compared with traditional unresisted sprint (US) training on jump, sprint, and RSA performance in male adult soccer players. We hypothesized that the use of WVs would provide greater training overload and improve performance to a greater extent than the US training. This study was designed to provide coaches and players with relevant information regarding the availability and efficacy of training methods for improving jump, acceleration, maximum velocity, and RSA performance.
Experimental Approach to the Problem
This study used a 2-group, randomized controlled trial design to compare the training effects of sprint training with (WV) and without (US) WVs (Figure 1). To determine training effects, the following tests were selected: (a) countermovement jump (CMJ), (b) 10-m sprint, (c) 30-m sprint, and (d) RSA test. After pretesting, the subjects were randomized by a coauthor not directly involved in testing or the training intervention into one of 2 groups (WV = 10 or US = 9). The WV and US groups completed 12 sessions spaced over 6 weeks. The intervention program of each group was added to the usual training routines. In all other respects, all subjects completed identical training activities. To reduce the influence of uncontrolled variables, all soccer players were instructed to maintain their habitual lifestyle and normal dietary intake before and during the study.
Nineteen male amateur soccer players (ranged from 21 to 27 years old) participated in this study (subject characteristics are presented in Table 1). All participants were classified as experienced soccer players with 14.7 ± 4.1 years of systematic soccer training. The players regularly performed 4 weekly soccer sessions with their team and on average exercised 8.3 ± 0.9 h·wk−1 in their normal training cycle. The team also regularly completed one official match per week. The study protocol took place during the second half of the competitive period of the season (i.e., February to March). At the time of the investigation, players were performing 4 weekly training sessions containing low-intensity aerobic training, plyometric, agility, and soccer-specific drills. The players had never participated in a regular/systematic RS training program. Only players who participated in full training were considered for inclusion. Exclusion criteria were injuries resulting in loss of one or more soccer matches/training sessions in the preceding 3 months before the initiation of the study. The study was approved by the University of Vigo Institutional Review Board for the use of Human Subjects. In accordance, before participation, the investigator informed all subjects as to the benefits and possible risk associated with the participation in the study and all subjects signed a written informed consent document indicating their voluntary participation.
Vertical jump, sprint velocity, and RSA are considered as determining factors of soccer players (26). These indicators were used in this study as simple and reliable measures of anaerobic performance. The testing procedures were performed at baseline (pretest) and at the end of the 6-week training program (posttest) (Figure 1). During testing sessions, the participants were required to wear the same athletic equipment and measurements were conducted at the same time of the day to minimize the effect of diurnal variations on the selected parameters during the 2 experimental sessions. All data collection and test sessions were performed in an indoor court where ambient temperature ranged from 18 to 21 degrees Celsius. All tests were performed after 72 hours of rest and at the same venue under identical conditions and supervised by the same test leaders. Before each testing session, players complied with the following pretest guidelines: (a) not to consume any energy/performance-enhancing drinks or supplements 48 hours before testing; (b) not to take beverages containing caffeine or alcohol at least 3 hours before testing; and (c) not to consume food at least 2 hours before testing. Before testing, all participants performed 10 minutes of standardized warm-up comprising 2 minutes of light active static stretching (10 repetitions for hamstrings, quadriceps, and calf muscles) and 5 minutes jogging, followed by short distance accelerations (3 submaximal sprints, progressing to 90% of their maximal velocity for the shuttle distance [30 + 30 m]). This routine was supervised by the team's physical trainer before the tests.
Countermovement jump was performed on a force plate (Ergo Jump Bosco System; Globus, Treviso, Italy) according to the procedures proposed by Bosco et al. (4). Players were allowed 2 trials, with a 1-minute recovery period between each trial. The best trial was used for subsequent analysis. Countermovement jump was performed with a squat starting position, that is, knees flexed to 90° and hands on hips. From this position, the soccer players were required to bend their knees to a freely chosen angle and perform a maximal vertical thrust (24). The hands are held on the hips during the jump to avoid any effect of arm-swing. Participants were instructed to keep their body vertical throughout the jump, avoiding undue lateral and frontal movements, and to land with knees fully extended. Any jump that was perceived to deviate from the required instructions was repeated. The intraclass correlation coefficient (ICC) for test-retest trials was 0.98 (95% confidence interval [CI] 0.95–0.99).
Ten- and 30-m Sprint Tests
Sprint time was measured by means of a dual infrared reflex photoelectric cell system (DSD Laser System; León, Spain). The photoelectric cells were attached to tripods, raised to a height of 0.9 m and placed in pairs 1 m apart. All players began from a standing start, with the front foot positioned 0.5 m from the first timing gate, and were instructed to perform all the sprints with a maximal effort. Players were allowed 2 trials, with a 2-minute recovery period between each trial. The best trial was used for subsequent analysis. During the 2 experimental sessions, the participants were required to wear the same shoes to avoid the effects of different athletic equipment. The ICCs for test-retest trials were 0.95 (95% CI: 0.91–0.97) and 0.97 (95% CI: 0.93–0.98) for the 10 and 30 m, respectively.
Repeated Sprint Ability Test
Photoelectric cells (DSD Laser System; León, Spain) were used to measure the soccer players' performance and to increase test reliability. The RSA protocol was consisted of 6 maximal 25-m sprints. After each sprint, there was a period of active recovery (25 seconds), while the athlete positioned themselves for a new start. Recovery was measured (stopwatch) to ensure that subjects returned to initial point of course between the 23rd and 24th second. Verbal feedback was given at 5, 10, 15, and 20 seconds of the recovery. The average time (AT), fastest time (FT), and total time (TT) were recorded during the RSA test according to previous studies (16). The percentage of decrement score (%Dec) was then calculated using the following formula proposed by Fitzsimons et al. (12), which has been demonstrated as the most valid and reliable method of quantifying fatigue in multiple sprints test (30):where ideal sprint time = 6 × FT.
After pretesting, the subjects began one of the 6-week training protocols presented in Table 2 in addition to the usual soccer training. The intervention program had to be performed 2 times a week (total of 12 sessions), on nonconsecutive days (48 hours rest). Training took place on the same surface as the testing sessions. Both groups completed the same amount of sprints and distance (Table 2). The only difference between the 2 interventions was that the WV group performed all the sprints with an additional weight of 18.9% ± 2.1% of body mass (a moderate load according to previous studies (22)) by mean of a WV (Xvest model X4040; Houston, TX, USA). Before each session, participants completed a standardized warm-up (same as pretesting and posttesting), as prescribed by a certified strength and conditioning specialist. The players were instructed to give maximal effort in each training session. A certified strength and conditioning specialist supervised all training sessions to ensure that all warm-up activities and sprints were completed with correct technique and with maximum effort. To quantify the intensity of the training sessions rating of perceived exertion (RPE) using Foster's 0–10 scale were recorded respectively (13).
All variables were normally distributed (Shapiro-Wilks test). Data are presented as means with SD. A 2 (group: WV and US) × 2 (time: pre, post) repeated-measures analysis of variance (ANOVA) was calculated for each parameter. Partial eta-squared (ηp2) effect sizes for the time × group interaction effects were calculated. An effect of ηp2 ≥ 0.01 indicates a small, ≥0.059 a medium, and ≥0.138 a large effect, respectively (7). Additionally, Cohen's d effect sizes for identified statistical differences were determined. Effect sizes with values of 0.2, 0.5, and 0.8 were considered to represent small, medium, and large differences, respectively (7). In addition to this testing, for each variable percentage difference in the change scores between WV and US from pretest to posttest was calculated. The chances that the differences in performance were better/greater (i.e., greater than the smallest worthwhile change [0.2 multiplied by the between-subjects SD, based on the Cohen d principle]), similar, or worse/smaller were calculated. Quantitative chances of beneficial/better or detrimental/poorer effects were assessed qualitatively as follows: <1%, almost certainly not; 1–5%, very unlikely; 5–25%, unlikely; 25–75%, possibly; 75–95%, likely; 95–99%, very likely; and >99%, almost certainly (20). A substantial effect was set at >75%. If the chances of having beneficial/better and detrimental/poorer performances were both >5%, the true difference was assessed as unclear. Correlations between change in sprint performance and change in RSA performance were assessed by Pearson product-moment correlation coefficient. Magnitude of effect for the correlations was based on the following scale; <0.10, trivial; 0.10 to 0.29, small; 0.30 to 0.49, moderate; 0.50 to 0.69, large; 0.70 to 0.89, very large; and >0.90, nearly perfect (20). Reliability for test-retest trials was assessed using ICC, with a value of 0.7–0.8 being questionable and >0.9 being high (29). Significance was set at an α level of 0.05. All statistical analyses were conducted using the statistical package SPSS for Macintosh (version 21.0; Chicago, IL, USA).
Rating of perceived exertion was reported throughout the intervention period with a median between 1 and 3 (Figure 2). There were no significant differences in RPE between groups during the training period. Absolute values for each parameter at pretest and posttest, together with the ANOVA results are displayed in Table 3. In the within-group analysis, significant improvements in 10-m (F1,17 = 71.99, p < 0.001) and 30-m sprint performances (F1,17 = 99.89, p < 0.001) were found in WV (d = 1.77 and 3.30, respectively) and US (d = 2.50 and 1.86, respectively) from pretest to posttest. Players in both WV and US also showed significant enhancements in RSA AT (F1,17 = 99.21, p < 0.001; d = 2.45 and d = 2.31, for WV and US, respectively), TT (F1,17 = 99.21, p < 0.001; d = 2.45 and d = 2.31, for WV and US, respectively), and FT (F1,17 = 108.27, p < 0.001; d = 2.15 and d = 3.12, for WV and US, respectively) from pretest to posttest. Results from the between-group analyses are illustrated in Figure 3. Contrary to our hypothesis, there were no differences between the training groups (WV and US) in any variable (p > 0.05) (Figure 3). Percentage changes in 30-m sprint performance, for both groups combined, had a very large correlation with percentage changes in AT in RSA (r = 0.878; Figure 4).
This is the first study designed to examine the chronic effects of sprint training with and without a WV on physical performance in soccer players. The main findings of this study indicate that (a) WV and US training seem to be effective in increasing sprint performance; and (b) both training regimens were equally efficient at enhancing AT, FT, and TT of RSA.
Resisted sprint training is expected to improve the player's ability to generate horizontal or vertical forces depending on the direction of the applied load during training (21). In this sense, sled towing has been proposed to offer a greater horizontal vector-training stimulus, whereas WV is more vertically oriented (8). However, in this study, WV training failed to promote significant improvements in a vertically oriented test such as CMJ performance, which might suggest that more specific training strategies (e.g., plyometric training, resistance training) than those used in this study may be required to improve jump capacity in soccer players.
Sprinting speed is an essential fitness component for soccer playing (11). Resisted sprint training in the form of WV is a training protocol often prescribed for soccer players in an effort to improve sprinting performance. However, no previous studies investigated this issue. The results of this study showed a large training effect on 10-m and 30-m sprints for WV and US training. However, there were no significant differences between both training groups, indicating that both WV training and US training were equally effective in developing sprint performance. Therefore, the results did not support the experimental hypothesis that players training with WV would demonstrate significantly greater improvements in sprint performance than subjects completing US training. To the best of our knowledge, only one previous study has investigated the use of WV on physical performance in team sports athletes (6). Clark et al. (6) examined the effects of 7-week weighted towing and WV sprint training on maximum velocity sprint performance and kinematics in male collegiate lacrosse players. The results indicate that weighted towing and WV training had no more beneficial effect compared with US training. Thus, the results from Clark et al. (6) generally agree with this study. Previous evidence with resisted towing, however, is in conflict with the findings of this study (9,25,28) that generally found positive effects of RS training compared with US training in reactive strength, jump, and sprint performance and velocity. The differences between our findings and those of the previous studies (9,25,28) may be related to a number of factors, including sex, competitive level, or training status of the group, or more importantly, they may be related to the differences in the form of overload the athlete (i.e., horizontal or vertical oriented) or training load used. In this respect, the results of this study are in accordance with the ideas set forth by Petrakos et al. (22) in a recent literature revision, according to which moderate loads (10.0–19.9% BM) as the one used in this study (18.9% BM) seem to be of no more benefit than US training.
Research has documented that WV may increase the eccentric strength of the extensor muscles during the braking phase of ground contact and to increase muscle and leg spring stiffness, potentially increasing the muscles' capacity to store elastic energy and improve power output (6,8,31). However, despite this theoretical basis, the present data showed that both training programs were equally efficient. These generic improvements may suggest that the sprint, independent of the mode of training, had a positive effect on maximum velocity sprinting performance (6). Both training programs may have induced increases in key determinants of acceleration and speed, such as peak horizontal and vertical impulses, peak force, and rate of force development (22). This positive adaptation to training is not entirely unexpected because previous research has demonstrated that both resisted and US training protocols may elicit significant sprint performance improvements in team sports athletes (6,25). It is important to note that the nature of the improvement in sprint performance differed between both training modalities (i.e., WV and US). The US group achieved greatest improvements in the 10-m sprint test, whereas the WV group had the greatest increase in the 30-m sprint test (Figure 3). That is, the training modalities impact velocity slightly differentially as the result of their effect on the rate of change in velocity (30 m) or acceleration (10 m). This fact has important practical implications as 10-m sprints are likely to occur more often than 30-m sprints in soccer players (10) and thus US training may provide more specific adaptations.
As a result of significant amounts of intermittent sprinting and multidirectional changing movements, RSA has been recognized as one of the most relevant physical components in soccer (15). Repeated sprint ability is a complex quality related to both neuromuscular (i.e., acceleration and maximal sprint speed, e.g., neural drive or motor unit activation) and metabolic factors (5,16). Thus, training strategies targeting the development of sprint performance may account for an improvement in RSA (5,16). Different training approaches have shown positive effects on soccer players' RSA. However, the influence of periodized RS training on RSA has not been studied yet. The results of this study showed a large training effect on TT, AT, and FT for US and WV training. The lack of significant improvements in CMJ and the similar large improvements in 10-m and 30-m sprints in both training groups suggest that the observed improvements in RSA in this study were likely related to changes in specific coordination and agility rather than due to enhancements in explosive force or sprinting mechanisms (5,31). This conclusion is supported by the association observed between the changes in 30-m sprint and the percentage change in AT (Figure 4). Thus, given the importance of RSA in soccer, coaches should consider adding focused WV or US to improve this quality in addition to normal training. Additionally, the %Dec remained unchanged in both groups after the training period. This suggests an increase in the overall sprint performance, but not in the ability to recover between sprints (3).
The interpretation and broader implications of the current data must be undertaken within the limits of the specific data collection undertaken. Although the study had many unique aspects, there are some limitations to note. First, although participant numbers in this study were similar to other studies that have assessed RS methods in team sports, our sample size was relatively small. A larger sample size may have provided more conclusive results. To circumvent this issue and prevent potential misinterpretations, different statistical approaches were used, including the magnitude-based inference, which allow detecting any possible changes in the performance that might be relevant in a sports setting. The second limitation is that sprint kinematics adaptations were not included in the study as an attempt to keep it simple, noninvasive, and practical.
This study was the first to investigate the effects of WV training on specific physical attributes of soccer players. Based on the present results, 6 weeks of WV and US training seem to represent a time-efficient stimulus for a simultaneous improvement of sprint fitness as well as for RSA in soccer players, given the lower training volume required. Weighted vest and US can be easily integrated one time per week as a part of the normal in-season training. Thus, from a practical point of view, desired adaptations can be obtained with a substantial reduction in exercise training time, allowing the players to spend more time on court and optimizing technical and tactical skills. Because small differences may be important for competitive soccer, the nonsignificant superiority of US over WV in improving acceleration (10 m) may suggest that US is still a preferred training method for this variable. The improvements in RSA in response to the 2 training regimens tested in this study illustrate the concept of training specificity and suggest that both training contents could be part of the training program in soccer players. These training-specific adaptations offer coaches and practitioners the possibility to individualize training content specific to the athletic qualities in soccer.
The results of this study do not constitute endorsement of the products by the author or the National Strength and Conditioning Association.
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