Short sprint performance is of great importance in many sports, with elite soccer players spending approximately 11% of the game sprinting, which equates to a 10- to 15-m sprint every 90 seconds (2,33). The outcome of the game tends to be determined during these periods of sprinting, whereby athletes are making a break, intercepting, tackling, or shooting.
Sprinting requires high levels of acceleration and as such high levels of strength to overcome the inertia of body mass. A number of studies have investigated the relationship between strength and sprint performances, demonstrating that, in general, stronger athletes perform better during sprint performances (1,8,11,25,32). Moreover, Comfort et al. (9) recently found that increases in back squat strength mirrored improvements in sprint performance, with similar findings previously reported in junior soccer players after 8 weeks of resistance training (5,6). This may be explained by the fact that peak ground reaction forces and impulse are strong determinants of sprint performance (19,29–31,34).
Studies have used various methods to assess strength, including isokinetics (1,3), machine squats (16,17), and free weight squats (1,8,11,25,32), when investigating the relationship between strength and sprint performance. Others have also looked at multiple tests, usually including assessment of power, to develop a model to predict sprint performance (3,26). The strongest relationships have been observed between free weight squats and short sprint performance (25,32). Wisloff et al. (32) found a strong correlation (r = −0.94) between 1 repetition maximum (1RM) squat performance and 10-m sprint time; however, in contrast, Harris et al. (17) found a weak and nonsignificant correlation between Smith machine squats and 10- and 40-m sprint times in athletes. More recently, McBride et al. (25) demonstrated a stronger relationship (r = −0.605, p = 0.01) between relative strength and sprint performance (40 yard) in individuals with a high relative strength (≥2.1 kg·kg−1) compared with individuals with a lower relative strength (<1.9 kg·kg−1). However, in contrast, 5-yard sprint times and relative strength showed a nonsignificant correlation (r = 0.45, p = 0.0698). Each of these studies has used well-trained adult athletes.
Youth athletes demonstrate a high trainability, with Keiner et al. (21) concluding that well-trained athletes aged 16–19 years should be able to back squat a minimum of 200% body mass. In addition to the trainability of youth athletes, research has demonstrated that resistance training is safe, improves performance, and reduces injury risk (12,13,15). No investigation has identified if the relationships between squat strength and sprint performances occur in younger athletes, although Chelly et al. (6) previously reported increases in sprint and jump performances after 8 weeks of resistance training and demonstrated a relationship between squat strength and peak velocity during a 5-m sprints (5). Similarly, Christou et al. (7) demonstrated that combining strength training with soccer-specific training resulted in greater improvements in lower-body strength, sprint, and jump performance than soccer training alone.
The aim of this study, therefore, was to determine the relationships between back squat strength, 5- and 20-m sprint performances, and jump performance in well-trained youth soccer players. It was hypothesized that the relationships between maximal squat strength and sprint and jump performance would be similar to those previously identified by Wisloff et al. (32) and McBride et al. (25) because of the level of training of the youth athletes. It was further hypothesized that relative strength would demonstrate a stronger relationship with sprint and jump performance than absolute strength because body mass has to be accelerated during both sprinting and jumping.
Experimental Approach to the Problem
This study was designed to investigate the relationships between maximal back squat strength, sprint performance (times over 5 and 20 m), and jump performance (squat jump [SJ] and countermovement jump [CMJ]) in well-trained (≥3 times per week for ≥2 years) youth soccer players. Sprint performances over 5 and 20 m were selected because these are representative of sprint distances covered during competitive soccer matches (2,33), whereas the jump and squat protocols were selected because they are commonly used to assess such athletes. After data collection was complete, associations between dependent variables were determined via Pearson’s correlations.
Young male soccer players (n = 34; 17.2 ± 0.6 years; body mass, 72.62 ± 7.42 kg; height, 179.27 ± 6.58 cm) participated in this study. All players were fully informed of the requirements of the investigation and provided appropriate consent to participate, with consent from the parent or guardian of all players under the age of 18. The investigation was also approved by the institutional review board.
Testing was conducted in the mid-season during which time, all subjects were training full time, with sessions comprising all the elements of performance including 4–5 soccer-specific training sessions, plus 2 resistance training sessions each week. At the time of testing, subjects were at the end of a power mesocycle. All athletes rested the day before testing and were asked to attend testing in a fed and hydrated state, similar to their normal practices before training. In addition, participants were asked to avoid caffeine consumption for the 24 hours before testing.
On arrival, all participants had their height (Stadiometer; Seca, Birmingham, United Kingdom) and weight assessed (Seca Digital Scales, Model 707) while in bare feet and wearing shorts.
Standardized progressive warm-ups were applied before all tests to control potential variables and improve the reliability of all test. Warm-ups included 10 minutes of nonfatiguing activation and mobilization exercises, including various bodyweight lunges and squats, interspersed with footwork and sprint mechanics drills, followed by some low-level plyometric drills, replicating the athlete's standardized warm-ups before training. The 20-m sprint test was administered as a test of acceleration and sprint ability. All subjects performed 3 trials, with a 2-minute rest between trials, on a third-generation artificial rubber crumb surface using “Brower photocell timing Gates” (model number BRO001; Brower, Draper, UT, USA) setup at 0, 5, and 20 m. Players started 1 m behind the first gate, to prevent any early triggering of the initial start gate, from a 2-point staggered start after. Testing was conducted after a standardized warm-up protocol. The best performance from each of the 3 trials was used for correlation analysis.
Squat jump and CMJ were tested using a portable jump mat (Just Jump; Probotics, Huntsville, AL, USA). After the standardized warm-up, players completed 3 trials of each jump, with hands on hips throughout the jump, with a 1-minute rest between trials. The SJ was performed with a 3-second pause at approximately 90° of knee flexion. If players failed to adhere to the strict protocol and either performed a countermovement or moved their hands off their hips, the trial was repeated after an additional 1-minute rest. After a further 5-minute rest, subjects performed 3 CMJs, with a 1-minute rest between repetitions, with the player squatting to approximately 90° angle of knee flexion before immediately extending into a maximal vertical jump.
A progressive standardized warm-up was completed by each subject as described, followed by one set to positive failure with a weight estimated to be their 5RM, established through training loads used in strength training sessions. The Brzycki (4) regression equation was then used to predict 1RM back squat based on the subjects performance in the repetitions to failure protocol (22,24). Although there may be potential problems in using a regression equation to establish 1RM for the back squat, it has a lower risk of musculoskeletal injury and is more appropriate for athletes that are unaccustomed to training with maximal loads (4,22). The purpose of using the back squat was to establish lower-body strength; however, at high loads, the mechanical stress on the back may be the limiting factor in the ability to complete lifts with a high percentage of 1RM (20,27,28).
All assessments took place on the same day, in the sequence described above, with approximately 10-minute rest between assessments. All subjects were tested in the morning on the same day.
Intraclass correlation coefficients were used to assess the repeatability of performances between trials for sprint and jump performances, using the criteria of Cortina (10), where r ≥ 0.80 is excellent. Relationships between variables (sprint performances, jump performances, and absolute and relative strength) were determined using Pearson’s correlations using SPSS software (version 17.0; SPSS, Inc., Chicago, IL, USA). Apriori power calculations performed using G*Power (3.1; University of Dusseldorf, Dusseldorf, Germany) (14) determined that a minimum of 17 subjects were required for a statistical power ≥0.90, for an α level of p ≤ 0.05.
Intraclass correlation coefficients showed a high level of reliability (r ≥ 0.872, p < 0.001) between trials for sprint and jump performances (Table 1).
Pearson’s correlations demonstrated moderate-to-strong inverse relationships (r = −0.519 to −0.672) between strength and sprint times. In addition, strength showed strong relationships (r = 0.619–0.762) with jump performances (Table 2).
Absolute strength showed the strongest correlations with 5-m sprint times (r = −0.596, p < 0.001, power = 0.99) (Figure 1), SJ height (r = 0.762, p < 0.001, power = 1.00), and CMJ height (r = 0.760, p < 0.001, power = 1.00), whereas relative strength demonstrated the strongest correlation with 20-m sprint times (r = −0.672, p < 0.001, power = 0.99).
In line with our hypothesis, this study found good relationships between maximal squat strength and sprint and jump performances, which were similar to those previously reported in adult male athletes (8,25,32). Relative strength showed a slightly stronger relationship with 20-m sprint performance (r = −0.672, p < 0.001) compared with absolute strength (r = −0.645, p < 0.001), similar to previous findings regarding relationships between squat strength and 10 and 40-yard sprints (25), and the relationships between 20-m sprint performance and hang power clean and front squat performances (18).
In contrast to the hypothesis, however, absolute strength demonstrated the strongest relationships with 5-m sprint times, SJs, and CMJs (r = −0.596, p < 0.001; r = 0.762, p < 0.001; and r = 0.760, p < 0.001, respectively). These results are interesting as it was expected that a relative strength measure would better predict performances during sprinting and jumping because body mass has to be accelerated and therefore would be expected to be inversely related to performance. Similarly, Wisloff et al. (32) also previously reported strong correlations between absolute back squat strength and both 10- and 30-m sprint performances in well-trained adult male soccer players. More recently, however, and in line with the findings of this study, Comfort et al. (8) reported a good correlation (r = −0.613) between back squat strength and 5-m sprint times in adult athletes. In youth soccer players, only a relationship between squat strength and peak velocity during a 5-m sprint has previously been reported (5). In contrast to our findings, McBride et al. (25) failed to find a significant relationship between strength and 5-yard sprint performances in well-trained adult athletes, which is surprising in light of the fact that peak ground reaction forces and impulse are strong determinants of sprint performance (19,29–31,34).
With regard to the jump performances, Wisloff et al. (32) previously reported similar correlations (r 0.78) between squat strength and CMJ performance in adult male soccer players. In line with previous research (1,8,9,11,18,25,32), the results of this study illustrate the importance of developing high levels of strength to enhance sprint and jump performance in youth soccer players, with stronger athletes tending to demonstrate the best sprint and jump performances and the weaker athletes demonstrating the worst sprint and jump performances. Importantly, strength training has also been shown to decrease the incidence of injuries in college soccer players (23).
The authors acknowledge that a strong correlation does not imply cause and effect; however, Comfort et al. (9) previously noted that increases in back squat strength were accompanied by improvements in short sprint performance, with similar findings also reported in young soccer players (6,7,15). It is likely, therefore, that increases in back squat strength may result in an improvement in sprint performance, although it is still essential to ensure that athletes have optimal technique in terms of both sprint mechanics and exercise technique, especially in youth athletes. In addition, the authors acknowledge that one limitation of this study is that physical maturation was not assessed and therefore differences based on the maturation status may have been overlooked; however, this study provides a description of the relationships between lower-body strength and performance in an ecologically valid manner.
The results of this study illustrate the importance of developing high levels of lower-body strength to enhance sprint and jump performance in youth soccer players, with stronger athletes demonstrating superior sprint and jump performances. Coaches and strength and conditioning coaches should ensure that youth soccer players develop squat strength, once athletes' are technically proficient, which should result in improvements in both sprint and jump performances, in line with previous research (6,7). Squat strength should be improved as part of a periodized training program, ensuring that technical proficiency in all performances (e.g., back squat and other lifts, jumps, sprinting) is not neglected.
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