Abstract
ccer is a complex sports discipline that requires players to engage in diverse high- intensity and multidirectional activities. The optimization of strength and conditioning programs requires a comprehensive understanding of the physical attributes influencing player performance. While previous research has demonstrated the influence of knee and hip extensor muscles on the performance in sprints and other explosive movements, this study aimed to establish the relationship between plantar flexor muscle strength and high-intensity actions. Back squat (BS) and calf raise (CR) one-repetition maxima as well as linear sprint (5-, 10-, 30 m) and drop jump performance from different heights (15, 30, 45 and 60 cm) were measured in 45 elite youth players (age: 16.62±1.1 years). Results showed significant negative correlations between BS strength and sprint times (r =−0.60 to −0.61), confirming the importance of lower limb extensor muscle strength in short-distance sprints. While no significant correlations were found with sprint performances, CR strength was significantly
performance from different heights (15, 30, 45 and 60 cm) were measured in 45 elite youth players (age: 16.62±1.1 years). Results showed significant negative correlations between BS strength and sprint times (r =−0.60 to −0.61), confirming the importance of lower limb extensor muscle strength in short-distance sprints. While no significant correlations were found with sprint performances, CR strength was significantly associated with drop jump test results from 45 and 60 cm drop height (r = 0.36 to 0.46). These findings demonstrate that isolated CR strength positively influences the performance in actions involving rapid stretch-shortening cycles, which suggests that current strength and conditioning programs for youth soccer players should be extended to also include exercises specifically targeting the plantar flexor muscles. While this cross-sectional study provides novel insights into the complex interplay between muscle strength and soccer-specific performance, its findings need to be corroborated in longitudinal studies directly testing the impact of plantar flexor strength training. Keywords:plyometric; test; speed 1. Introduction Soccer is a sports discipline characterized by a highly intermittent load structure. While a plethora of parameters, including technical and tactical factors, determine team performance, the physical demands of elite soccer players are high and have significantly increased in recent years [1]. In particular, high-intensity and multidirectional tasks, such as accelerations, decelerations, sprints, jumps or changes in direction are essential prerequi- sites for success. Match play analyses suggest that, in dependency of the playing position, soccer players perform between 20 and 39 sprints at velocities > 21 km/h [2] and may cover more than 1 km running at high speed or sprinting [3]. While the sprints account for only ~2–3% of the total distance covered in a match, it is the high-intensity efforts that mainly determine the outcome of matches. Indeed, more than 80% of all goals scored in Sports2024,12, 87.
Sports2024,12, 87 2 of 10 the 2007/08 season of the German national league were preceded by a powerful action, such as straight sprints (45%), jumps (16%), rotations and change-in-direction sprints (each 6%) [4]. The importance of well-developed jumping abilities is further underlined by the observation that 44–56% of all goals scored in the Spanish and Italian soccer leagues are immediately preceded by headers [5]. Hence, muscle power generation in explosively performed actions is a critical performance-limiting factor, the development of which must be emphasized in youth players [6]. In an attempt to examine the determinants of performance in explosive actions, several studies have tested the correlations between parameters of fitness and the ability to sprint, jump and change direction. One of the earlier studies performed in young soccer players by Comfort et al. showed strong correlations between squat, sprint and jump performance [7]. These results have recently been confirmed by Keiner, Brauner, Kadlubowski, et al. [8] in elite youth players of various ages. Further studies investigated the associations between leg extension [9] or Nordic hamstring strength [10] and sprinting speed, as well as between the performance in the endurance plank and step 505 agility test [11]. Surprisingly, few studies performed to date have established the correlation between soccer players’ plantar flexor strength and performance in activities involving explosive muscle action. The strength of the plantar flexor muscles is critical for the performance in activities involving ankle joint stretch-shortening cycles (SSCs), such as hopping, skipping, jumping and sprinting. This is because the contractions of the plantar flexor muscles are expected to tauten the respective muscle–tendon units, thus benefitting the storage of elastic energy, and enhance the potential for contractile potentiation during the eccentric phase of movement; during the subsequent concentric phase, they would directly increase the propulsive impulse [12,13]. Despite this generally accepted understanding, there is limited research directly testing the influence of plantar flexor muscle strength on performance in SSC activities. One of the few correlational studies, which was performed in a sample of physical education students, reported moderate to strong correlations between calf raise strength and sprint performance [14].
they would directly increase the propulsive impulse [12,13]. Despite this generally accepted understanding, there is limited research directly testing the influence of plantar flexor muscle strength on performance in SSC activities. One of the few correlational studies, which was performed in a sample of physical education students, reported moderate to strong correlations between calf raise strength and sprint performance [14]. The notion that the potential importance of the plantar flexor muscle strength may be a largely overlooked factor is also highlighted by the fact that training interventions aimed at improving the performance of young soccer players in high-intensity tasks typically rely on exercises aimed at strengthening the knee and hip extensor muscles [6,15–20], plyometric jumps or sprint training [21–26] or a combination of both [18,19,27–31]. By contrast, strength and conditioning programs for soccer players rarely include exercises to increase the maximum strength of the plantar flexors, such as calf raises [32,33], although maximum (isometric) strength and rapid force production are known to be correlated [34]. If a study were to reveal robust correlations between the maximal strength of the plantar flexor muscles and performance in high-intensity tasks, it would underscore the importance of prioritizing the enhancement of calf muscle strength within the training regimen of soccer players. Considering the scarcity of data reflecting the association between maximum strength and performance in high-intensity, explosive tasks in youth soccer players, the present study aimed to investigate the relationship between the back squat and calf raise one- repetition maxima and the performance in linear sprints as well as countermovement, squat and drop jumps. It was hypothesized that both back squat and calf raise strength would be correlated with better sprint and jump performance. 2. Materials and Methods 2.1. Experimental Approach to the Problem To test the above hypothesis, a cross-sectional study design was used. A sample of elite youth soccer players were tested for their one-repetition maxima (1-RM) in the back squat (BS) and standing calf raise (CR) exercise, linear sprint times over 5, 10 and 30 m as well as countermovement (CMJ), squat (SJ) and drop jump (DJ) performance from heights of
To test the above hypothesis, a cross-sectional study design was used. A sample of elite youth soccer players were tested for their one-repetition maxima (1-RM) in the back squat (BS) and standing calf raise (CR) exercise, linear sprint times over 5, 10 and 30 m as well as countermovement (CMJ), squat (SJ) and drop jump (DJ) performance from heights of 15, 30, 45 and 60 cm (DJ15, DJ30, DJ45, DJ60, respectively). The tests were performed on
Sports2024,12, 87 3 of 10 2 test days separated by 2 days of recovery. One week prior to test day 1, the soccer players completed two preparatory sessions performed on separate days to familiarize with all the tests. On test day 1, the DJ, CMJ and SQ performance as well as the 1-RM BS were tested. On test day 2, the tests to assess linear sprint times and the 1-RM CR were performed. 2.2. Subjects A total of 45 male youth soccer players (age: 16.62±1.1 years old; height: 1.78±0.06 m; weight: 67.7±8.5 kg) were recruited from the under 17-years-old (U17, n= 44) and under 19-years-old (U19,n= 43) teams of one elite youth training center. Both youth soccer teams were engaged in the second highest league (U17/U19 Westfalenliga). The included soccer players participated in 4 training sessions per week and competed on weekends. All subjects had played soccer since early childhood and were not familiar with strength training. The training volume did not deviate between the teams. The subjects did not engage in fatiguing training sessions for a minimum of 3 days before testing. None of the subjects reported any injuries at the time of testing. All participants (and their parents, in the case of subjects who were younger than 18 years old) were informed of the experimental risks involved with the research before providing written informed consent to participate in this study. Approval for this study was obtained from the institutional review board at the German University of Health and Sport (DHGS-EK-2022-002). This study was performed in accordance with the Helsinki Declaration. 2.3. Measures and Procedures Testing included the determination of the 1-RM BS (high bar). The barbell was posi- tioned on the descending part of the trapezius muscle below the seventh cervical vertebra. The participants stood in an upright position with a self-selected distance between their feet, flexed their knees and hips to reach a deep squat position with proper form (top of thigh breaking parallel) and returned to the starting position. Attempts were considered invalid if the experienced examiner visually identified improper form, such as a rounding of
the seventh cervical vertebra. The participants stood in an upright position with a self-selected distance between their feet, flexed their knees and hips to reach a deep squat position with proper form (top of thigh breaking parallel) and returned to the starting position. Attempts were considered invalid if the experienced examiner visually identified improper form, such as a rounding of the back or inadequate squat depth. A warm-up (2 sets of 6–8 repetitions) was performed with a submaximal, non-fatiguing load. The 1-RM was subsequently established through a series of maximally 5 trials, which were interspersed by at least 5 min of passive rest. The 1-RM BS is a highly reliable measure, with test-retest reliability intraclass correlation coefficients reportedly ranging between 0.91 and 0.99 [35]. Absolute maximum strength values were further normalized to body mass to determine relative maximum strength (Relative 1-RM BS). The 1-RM CR test was performed on a standing calf raise machine (Flame Sport, Siauliai, Lithuania), which allows for a highly reliable test execution (test-retest reliability intraclass correlation coefficient of 0.91 (0.98–0.99)). Subjects were instructed to elevate their heels by plantar flexing the ankle joint; in agreement with previous studies [36,37], the range of motion was set to 4 cm, which was visually controlled through the use of a camera. Trials were considered invalid if participants failed to keep an upright posture, flexed their knees or were unable to plantar flex their ankles to raise their heels to the requested height. Proper execution was simultaneously controlled by 2 examiners. Just as for the assessment of the 1-RM BS, the actual test was preceded by a specific warm-up consisting of 2 sets of 6–8 repetitions performed with a submaximal, non-fatiguing load. Results were also normalized to body mass to yield relative maximum strength (Relative 1-RM CR). The warm-up for the jump and sprint tests consisted of nonspecific running at low-to- medium intensity for approximately 5 min. Then, running coordination drills, including high knee skips and butt kicks (i.e., heel lift running), as well as side steps were performed for approximately 5 min. Subsequently, 3 acceleration runs over approximately 30
yield relative maximum strength (Relative 1-RM CR). The warm-up for the jump and sprint tests consisted of nonspecific running at low-to- medium intensity for approximately 5 min. Then, running coordination drills, including high knee skips and butt kicks (i.e., heel lift running), as well as side steps were performed for approximately 5 min. Subsequently, 3 acceleration runs over approximately 30 m were performed with short intervening walking breaks. Overall, the total warm-up time on each test day was 15 min. To assess sprint performance, each athlete was granted 3 attempts interspersed by 5 min breaks, and the 5, 10 and 30 m linear sprint time was measured via timing gates (Brower TC Timing System, Biederitz, Germany).
Sports2024,12, 87 4 of 10 Jump performance was measured using a commercially available contact mat (Re- fitronic, Schmitten, Germany), which allows for jump height to be calculated from flight time (gt 2 /8; g = the gravitational acceleration [9.81 m·s −2 ] and t = flight time). The test- retest reliability for this system has been reported to be high, with intraclass correlation coefficients ranging between 0.85 and 0.93 [36,38]. The subjects performed 5 trials for each jump, and their best result was used for analysis. The athletes rested for 15 min between jumps of different kinds and for 1 min between individual attempts. The SJ was initiated at a knee angle of 90 ◦ without counter-movement and arm swings. The DJ was performed from different heights (15, 30, 45 and 60 cm). With their hands positioned above their hips, the participants were instructed to take a horizontal step from a box and jump as explo- sively and high as possible immediately after ground contact. To warrant minimal ground contact times (<250 ms), subjects were instructed to prevent their heels from touching the floor. A reactive strength index (RSI) was calculated as the ratio between jump height and ground contact time (RSI = jump height in cm/contact time in s) and interpreted as gross parameter indicative of DJ performance. Figure sequence of measurements.Sports 2024, 12, x FOR PEER REVIEW 4 of 11 performed for approximately 5 min. Subsequently, 3 acceleration runs over approxi- mately 30 m were performed with short intervening walking breaks. Overall, the total warm-up time on each test day was 15 min. To assess sprint performance, each athlete was granted 3 attempts interspersed by 5 min breaks, and the 5, 10 and 30 m linear sprint time was measured via timing gates (Brower TC Timing System, Biederitz, Germany). Jump performance was measured using a commercially available contact mat (Re- fitronic, Schmitten, Germany), which allows for jump height to be calculated from flight time (gt 2 /8; g = the gravitational acceleration [9.81 m·s −2 ] and t = flight time). The test-retest reliability for this system has
was measured via timing gates (Brower TC Timing System, Biederitz, Germany). Jump performance was measured using a commercially available contact mat (Re- fitronic, Schmitten, Germany), which allows for jump height to be calculated from flight time (gt 2 /8; g = the gravitational acceleration [9.81 m·s −2 ] and t = flight time). The test-retest reliability for this system has been reported to be high, with intraclass correlation coeffi- cients ranging between 0.85 and 0.93 [36,38]. The subjects performed 5 trials for each jump, and their best result was used for analysis. The athletes rested for 15 min between jumps of different kinds and for 1 min between individual attempts. The SJ was initiated at a knee angle of 90° without counter-movement and arm swings. The DJ was performed from different heights (15, 30, 45 and 60 cm). With their hands positioned above their hips, the participants were instructed to take a horizontal step from a box and jump as explo- sively and high as possible immediately after ground contact. To warrant minimal ground contact times (<250 ms), subjects were instructed to prevent their heels from touching the floor. A reactive strength index (RSI) was calculated as the ratio between jump height and ground contact time (RSI = jump height in cm/contact time in s) and interpreted as gross parameter indicative of DJ performance. Figure 1 shows a schematic diagram reflecting the sequence of measurements. Figure 1.Sequence of tests and measurements performed before (T1) and after (T2) the training period. CMJ = Counter-Movement-Jump height; DJ = reactive strength index from 15/30/45/60 cm drop height, respectively; BS 1-RM = one-repetition maximum for the back squat; CR 1-RM = one-repetition maximum for the standing calf raise. 2.4. Analysis The significance level for all statistical tests was set atp< 0.05. Normality of data was tested using the Kolmogorov–Smirnov test, and data were expressed as mean±SD. Intra- class correlation coefficients (ICC) were calculated as measure of test-retest reliability and 95% confidence intervals (95% CI) were calculated for all variables. The ICC magnitudes
all statistical tests was set atp< 0.05. Normality of data was tested using the Kolmogorov–Smirnov test, and data were expressed as mean±SD. Intra- class correlation coefficients (ICC) were calculated as measure of test-retest reliability and 95% confidence intervals (95% CI) were calculated for all variables. The ICC magnitudes
Sports2024,12, 87 5 of 10 were interpreted as follows: ICC < 0.5 = poor agreement,0.5≤ICC≤0.75 = moderate agreement, 0.75≤ICC≤0.9 = good agreement, ICC≥0.9 = excellent agreement. Pear- son’s coefficient was calculated to establish correlations between the outcomes of the maximum strength tests (1-RM BS, 1-RM CS) and sprint and jump performance measures. The respective effect sizes were interpreted as per the following scale: 0 < r < 0.1 = very weak correlation, 0.1≤r < 0.3 = weak correlation, 0.3≤r < 0.5 = moderate correlation, 0.5≤r < 0.7 = strong correlation, 0.7≤r < 0.9 = very strong correlation,0.9≤r < 1.0 = nearly perfect correlation and perfect correlation [39]. A post hoc test of the correlation coefficients was made to determine the power of the results. 3. Results The outcomes of speed and strength measurements as well as the respective test-retest reliability statistics are shown in Table. All parameters were normally distributed. In the drop jumps, the players consistently achieved ground contact times of <250 ms, irrespective of the drop height. Table 1.Mean, standard deviation, 95% confidence intervals and reliability statistics of all outcome variables.Fitness Test Mean ±SD 95% CI ICC (95% CI) 5 m linear sprint (s) 0.98 (0.05) 0.97–1.00 0.897 (0.83–0.94) 10 m linear sprint (s) 1.70 (0.06) 1.68–1.72 0.935 (0.89–0.96) 30 m linear sprint (s) 4.14 (0.14) 4.10–4.18 0.972 (0.95–0.98) CMJ (cm) 41.21 (4.53) 39.85–42.57 0.968 (0.95–0.98) RSI 15 (cm/s) 156.82 (33.69) 150.54–170.05 0.943 (0.91–0.97) RSI 30 (cm/s) 163.82 (39.99) 151.81–175.84 0.951 (0.92–0.97) RSI 45 (cm/s) 163.24 (26.83) 155.18–171.30 0.847 (0.76–0.91) RSI 60 (cm/s) 152.59 (36.54) 141.48–163.70 0.888 (0.80–0.94) 1-RM BS (kg) 94.09 (19.92) 88.03–100.15 0.989 (0.98–0.99) 1-RM CR (kg) 99.09 (21.10) 92.87–104.47 0.991 (0.98–0.99) Relative 1-RM BS (kg/kg) 1.36 (0.22) 1.28–1.42 / Relative 1-RM CR (kg/kg) 1.43 (0.32) 1.33–1.53 / SD = Standard deviations; ICC = Intraclass correlation coefficient; 95% CI = Confidence intervals; CMJ = Counter- Movement-Jump height; RSI 15/30/45/60 = Reactive strength index from 15/30/45/60 cm drop height, respec- tively; 1-RM BS = Back squat one-repetition maximum; 1-RM CR = Standing calf raise one-repetition maximum; Relative 1-RM CR/Relative 1-RM BS = Back squat/Standing calf raise one-repetition
Description
Investigates the correlation between strength and performance in sprinting and jumping.