Effects of Contrast Strength vs. Plyometric Training on Lower-Limb Explosive Performance, Ability to Change Direction and Neuromuscular Adaptation in Soccer Players

Abstract

Hammami, M, Gaamouri, N, Shephard, RJ, and Chelly, MS. Effects of contrast strength vs. plyometric training on lower-limb explosive performance, ability to change direction and neuromuscular adaptation in soccer players. J Strength Cond Res 33(8): 2094–2103, 2019—The aim was to compare the effects of 2 differing 8-week in-season strength training programs (contrast strength training [CST] vs. plyometric training [PT]) on selected performance tests (5 and 40 m sprints, S 4 × 5 m change of direction test, squat jump [SJ] and countermovement jump [CMJ], leg peak power on a cycle-ergometer force-velocity test, 1 repetition maximal half-squat, and electromyographic [EMG] activity of the vastus lateralis, vastus medialis, and rectus femoris muscles during vertical jump tests). Forty male soccer players (age = 15.8 ± 0.4 years; body mass = 58.8 ± 6.3 kg; body height = 1.74 ± 0.06 m; body fat = 10.5 ± 1.9%) were divided between a contrast strength group (CSG, n = 14), plyometric group (PG, n = 14), and control group (CG, n = 12). Both training programs enhanced sprint performance (p < 0.001 in 5 m; p ≤ 0.05 in 40 m) and change of direction test scores (p < 0.001) relative to controls. The plyometric group and CSG increased SJ height relative to the CG, with a slightly greater response in CSG compared with PG (p ≤ 0.05). Most CMJ scores increased significantly in both CSG and PG relative to the CG, with no intergroup differences in training response. Most force-velocity scores increased significantly in the CSG relative to PG and CG. The EMG parameters also increased in the CSG relative to both PG and CG. In summary, most measures of athletic performance in male soccer players were enhanced after CST and PT. However, the improvement of physical performance was better with 8 weeks of CST than with PT. Thus, coaches should be encouraged to include CST as an element of in-season conditioning.

Introduction

Soccer is one of the most popular sports among people worldwide (^14,34). It has evolved substantially since its origin in the Middle Ages (³⁶) and is now a complex skilled activity. Optimal performance requires extensive preparation in technical, mental, and especially tactical skills (^3,4), but such preparation must be supplemented by adequate physical preparation. The emergence of new methods of training requires familiarity with the demands of competition, the athlete's abilities, and the qualities required for peak performance (^9,39). Technological developments such as global positioning systems, high-frequency cameras, and computer software now allow for a detailed analysis of movement patterns during a soccer match (^6,12,21,32). The physiological need is, essentially, for a combination of high aerobic and anaerobic abilities. Harley et al. (²¹) recorded a young soccer player (U-16) running a total distance of 7,672 m during a 75-minute game. Similarly, Buchheit et al. (⁶) found youth players (U-17) sprinting an average distance of 8,448 m, 501 m at speeds of 16–19 km·h⁻¹ and 428 m at speeds >19.1 km·h⁻¹ during a 2 × 40 minutes game played on a 100 × 70 m surface. In addition to such high-intensity sprinting, soccer players engage in jumps, tackles, and multiple changes of direction (COD). To win a running or jumping contest or to get the ball before the opponent, youth soccer players also need a combination of strength and power in their lower-limb muscles (^3,6).

Contrast strength training (CST) and plyometric training (PT) constitute 2 different potential tactics to enhance the maximal strength and power of the muscles used in youth soccer. Contrast strength training is characterized by the use of high and low loads in the same training session (³⁸). The details of such regimens have differed. In 1 previous investigation, 6 repetition sets with 70 and 90% of 1 repetition maximum (1RM) loads were alternated with 6 repetition sets of loads between 30 and 50% of 1RM, executed at maximum speed (¹¹). Six or more weeks of CST has previously been shown as effective in improving both muscle strength and power, with adaptations in both neuromuscular function and muscle morphology (^17,20).

Plyometric exercises involve repeated stretch-shortening cycles (SSCs), a rapid muscle stretch (eccentric phase) being followed immediately by a rapid shortening of the muscle (concentric phase) (²⁸). The rapid eccentric muscle contraction facilitates an increased force and power output during the succeeding concentric contraction, provided that the movements are performed rapidly (^19,28,42).

The impact of strength training on measures of athletic performance such as sprinting, agility, and vertical jumping remains controversial (²⁰). Garcia-Pinillos et al. (¹⁷) found that contrast training without external loads (isometric + plyometric) was effective in improving soccer-specific skills such as vertical jumping, sprinting, agility, and kicking speed in young soccer players. Hammami et al. (²⁰) also found increases in sprinting, agility, and the ability to make repeated COD after CST. Maio Alves et al. (²⁵) reported improvements in 5- and 15-m sprinting speeds and squat jump (SJ) performance, but observed no gains in the countermovement jump (CMJ) or agility tests. Chelly et al. (⁹) further noted increases in vertical jump and sprinting in soccer players in response to PT, but Herrero et al. (²³) observed no significant gains in 20-m sprint times unless 4 weeks of PT was supplemented by electromyostimulation. In view of these discordant results, it was believed desirable to assess further the acute responses to strength training, comparing the relative merits of CST and PT.

To the best of our knowledge, this is the first investigation to have compared the effects of CST vs. PT on some crucial physical abilities in junior soccer players. Therefore, the purpose of this study was to compare the effects of CST vs. PT on sprint speeds, ability to change direction, vertical jump performance, leg peak power (LPP), and neuromuscular adaptations as indicated by recordings of electromyographic (EMG) activity. Our null hypothesis was that both training methods would enhance these aspects of physical performance equally.

Methods

Experimental Approach to the Problem

The current study aimed to compare the effects of CST and PT on sprinting, jumping, agility, peak power, and neuromuscular adaptations in junior male soccer players. A team of experienced players was divided randomly between 3 groups: a contrast strength group (CSG; n = 14), a plyometric group (PG; n = 14), and a control group (standard in-season regimen) (CG; n = 12). All participants completed 2 familiarization trials in the 2 weeks before experimental measurements, with the exception of the cycle-ergometer force-velocity test (where participants completed only 1 familiarization trial) and the anthropometric assessments and EMG recordings (which were performed without any familiarization). Experimental measurements began 4 months into the playing season. Baseline data were collected before the start of CST and PT, during the winter rest period (the first 2 weeks of January was a rest period for all players, with no official competitions) and tests were repeated after completion of the 8-week training period. On both occasions, the protocol comprised assessments of sprint performance with measurement of the time at 40 m; change of direction tests (sprint 4 × 5 m [S 4 × 5 m]); a 1RM half-squat; a force-velocity test; and a vertical jump with EMG recordings from the vastus lateralis (VL), vastus medialis (VM), and rectus femoris (RF) muscles. Testing sessions were conducted at a consistent time of the day, and under the same experimental conditions, at least 3 days after the most recent competition. Players maintained their normal intake of food and fluids during the experimental assessments. However, they drank no caffeine-containing beverages in the 4 hours preceding testing and ate no food for 2 hours before testing. Verbal encouragement ensured maximal effort throughout. The posttraining tests were conducted 5–9 days after the last training session, to allow adequate recovery from the acute effects of resistance training.

Subjects

All procedures were approved by ISSEP – Ksar Said University, according to current national laws and regulations. Participants and their parents or guardians signed their informed consent after receiving both a verbal and a written explanation of the experimental design and its potential risks. The participants (40 junior male soccer players, elite-level championship [15-17 years old]) were told that they could withdraw from the trial without penalty at any time. All were examined by the team physician, with a particular focus on orthopedic and other conditions that might preclude resistance training and all were found to be in good health. The 3 groups (CSG, PG, and CG) were well matched in terms of their initial physical characteristics (Table 1).

Table 1.: Physical characteristics of study participants.*

Procedures

The study was performed during an 8-week period from January to March. All participants engaged in the same training sessions, supervised by the coaches of the 3 teams, from the beginning of the competitive season (September) until the end of the trial (March). They engaged in soccer training 4–5 times per week and played 1 official game per week. Standard training sessions lasted 90 minutes; usually, these emphasized skill activities at various intensities, offensive and defensive strategies, and 25–30 minutes of continuous play, with only brief interruptions by the coach. The CG maintained this pattern of training; but for 8 weeks, CSG and PG replaced a part of their standard regimen (the technical and tactical skill activities) by the specific training program under evaluation.

Details of Contrast and Plyometric Training Programs

Details of the CST regimen are given in Table 2. Each Tuesday and Thursday for 2 months, the initial part of the standard regimen was replaced by CST (Table 2). Half-squats were used as a training exercise. Each Tuesday, the CSG performed rising sets (70–90% 1RM) followed by descending sets (90–70% 1RM), and each Thursday they performed rising sets (70–90% 1RM). This resistance training program was supplemented by 3 consecutive CMJs with the arms held on the hip joint after every set in the first 4 weeks, and in the second 4 weeks by 1 CMJ with the arms again held on the hip joint, followed by a 15-m sprint. The loads were calculated using the individual's previously measured 1RM. This value was reassessed at the fourth week and the loadings were correspondingly updated. Strength training sessions lasted for some 45 minutes. Their aim was to obtain an optimal increase in muscle strength, followed by a delayed increase in muscle power.

Table 2.: Details of supplementary in-season program adopted by contrast strength training group (CSG) over 8 consecutive weeks.*

Details of the PT regiment are given in Table 3. Sessions began from rest with a 15-minute warm-up and lasted for some 20 minutes. Jumps were performed vertically on a tartan track. Participants were instructed to perform both hurdle jumps and drop jumps to the maximal possible height, with minimal ground-contact time. Both hurdle and drop jumps were performed with minimal knee flexion; the ground was touched with the balls of the feet only, thereby specifically stressing the calf muscles (²⁷). Hurdling comprised 7–10 continuous jumps over hurdles spaced at intervals of 1 m. Each set of drop jumps comprised 7–10 maximal rebounds after dropping from a 0.6 to 0.7-m box, with a pause of 5 seconds between each rebound (²⁷).

Table 3.: Supplementary in-season training program adopted by the plyometric group.*

Testing Schedule

Testing was integrated into the weekly training schedule, with all field tests performed on a tartan surface. A standardized battery of warm-up exercises was performed before maximal effort. Experimental measurements were performed in a fixed order over 3 days. On the first test day, participants sprinted over a 40-m distance, with timers set at 5 and 40 m; they then conducted the change of direction test (S 4 × 5 m). The second day was devoted to anthropometric measurements, followed by the 1RM half-squat test. On the third day, a combined vertical jump (SJ and CMJ) was performed, with EMG recording, and a cycle-ergometer force-velocity test was completed.

Day 1: 40-m Sprint Performance

The 40-m sprint began with a standardized warm-up (20 minutes). Participants then ran 40 m, with paired photocell timers (Microgate, Bolzano, Italy) placed at 5 and 40 m. Tests started from a standing position, with the front foot placed 0.2 m behind the first photocell beam. Three trials were separated by 6–8 minutes of active recovery, with the best result being recorded.

4 × 5 m Sprint (S 4 × 5 m)

The S 4 × 5 m sprint test required frequent directional changes. Five cones were set 5 m apart and paired photocell timers (Microgate) were placed at the starting and finishing lines. Participants began from a standing position, with a cone between their legs, and the front foot 0.2 m behind the first photocell beam. At an acoustic signal, they ran 5 m to point A; there, they made a 90° turn to the right and ran 5 meters to point B. After a second 90° turn to the left, they ran to point C, where they made a 180° turn to the right and ran to the finishing line (³⁹).

Day 2: Anthropometry

The overall percentage of body fat was estimated from the biceps, triceps, subscapular, and suprailiac skinfolds, using the equations of Durnin and Womersley for adolescent males aged 16–19.9 years (¹³):where density = 1.162–0.063 (LOG sum of 4 skinfolds).

Leg Muscle Volume

Measurements of the circumferences at the maximal level of the calf just above the ankle and skinfolds on the back and each side of the calf plus leg length (from the trochanter major to the lateral malleolus) were added to data for the thigh to calculate the leg muscle volume (²⁴).

Mean Cross-Sectional Area of the Thigh

The mean thigh cross-sectional area (CSA) was calculated from maximal and midthigh circumferences.

Considering the circumference of the thigh as a circle, its radius R was calculated as:

The radius of the muscular component of the midthigh (r) was estimated by allowing for the double thickness of anterior and posterior skinfolds:

The thigh CSA equaled π·r² (cm²).

1 Repetition Maximal Half-Squat at 90° Knee Flexion

Each participant kept an upright position, looking forward and firmly grasping with both hands a bar that was also supported on the shoulders. The knees were bent until they reached an angle of 90°. The participant then raised himself to the upright position, with the lower limbs fully extended. Because the technique was not familiar to the participants, an instructor demonstrated it, and all participants performed 8 technical training sessions during the month preceding definitive 1RM measurements. During familiarization sessions, a pretest RM determined an approximate 1RM value. To measure the experimental 1RM, a barbell was loaded with free weights across the upper back, using 90% of the pretest 1RM as an initial loading. Two consecutive loaded flexion-extensions were performed at 90° knee flexion. Each time the 2 repetitions were mastered, a load of 5 kg was added, after allowing for a recovery interval of 5 minutes. When the participant had performed 2 successful repetitions at the pretest RM value, a load of 1 kg was added after the recovery period. If the individual was unable to complete the second repetition with the new loading, this load was considered as the individual's 1RM. The number of lifting actions required before reaching an individual's 1RM ranged from 3 to 6.

Day 3: Squat Jump and Countermovement Jump

Characteristics of the SJ and the CMJ (jump height, maximal force before take-off, maximal velocity before take-off, and the average power of the jump) were determined using a force platform (Quattro Jump, version 1.04; Kistler Instrument AG, Winterthur, Switzerland). Jump height was determined as the center of mass displacement, calculated from the recorded force and body mass. Participants began the SJ at a knee angle of 90°, avoiding any downward movement, and they performed a vertical jump by pushing upward, keeping their legs straight throughout. The CMJ began from an upright position, making a rapid downward movement to a knee angle of 90° and simultaneously beginning to push-off. One minute of rest was allowed between 3 trials of each test, the highest jump being used in subsequent analyses.

Electromyographic Recording

We applied differential bipolar surface electrodes (Bagnoli Desktop EMG systems; Delsys, Inc., Boston, USA) longitudinally over the muscle belly and parallel to the muscle fibers of the VL, VM, and RF muscles of the right leg, in accordance with European recommendations for surface electromyography (²²). The skin was first shaved and cleaned with an alcohol-ether-acetone solution. Electrodes were then placed on the VL at ≈ 2/3 distance between the anterior superior iliac spine and the lateral aspect of the patella, with a reference electrode attached to a bony prominence on the patella of the same leg. An elastic bandage prevented cable movement during jumping. To ensure consistent electrode replacement from pretest to posttest, pictures of the electrode placement were taken for every athlete.

The signal was amplified, filtered, recorded (Bagnoli-4 EMG System; DelSys, Inc., Boston, USA), and stored for subsequent analysis, using the EMGworks 4.0.4.3 software (Calculation Toolkit 1.5.1.0; Delsys EMGworks, Natick, MA, USA). Electromyographic data were quantified using the root-mean-square (RMS) voltage during both the SJ (upward pushing phase until the beginning of the take-off phase) and the CMJ (during the rapid downward movement to a knee angle of ∼90° until the beginning of the take-off phase). The frequency spectrum of each epoch of EMG data was analyzed using a fast Fourier transformation. Analysis was restricted to frequencies in the range 5–500 Hz, as the EMG signal content outside this range consists mostly of noise.

The Force-Velocity Test

The force-velocity test was performed on a mechanically braked cycle-ergometer (Monark 894 E Ergometer; Vansbro, Sweden). Individuals completed 5 short maximal sprints against braking forces corresponding to 2.5, 5, 7.5, 9, and 11.5% of the individual's body mass, with rest intervals of at least 5 minutes between trials. Software allowed estimation of velocity, braking force, and power output during each trial. Leg peak power was judged to have been reached when additional loading induced a decrease in power output. Relationships between braking force and pedaling velocity were plotted for each individual. Maximal pedaling velocity (V₀) and maximal force (F₀) were calculated using an accepted regression equation (⁴⁰).

Statistical Analyses

Statistical analyses were performed using the SPSS 20 program for Windows (SPSS, Inc, Chicago, IL, USA). Normality of all variables was verified using the Kolmogorov-Smirnov procedure. Levene's test was used to determine homogeneity of variance. Mean values and SDs were calculated, using standard statistical methods. Training-related effects were assessed by 2-way analyses of variance with repeated measures (group × time). If a significant F value was observed, Tukey's post hoc procedure was applied to locate pairwise differences. A p ≤ 0.05 was accepted as a criterion of statistical significance, whether a positive or a negative difference was seen (i.e., a 2-tailed test was adopted). Effect sizes were reported for a main effect of group, a main effect of time, and a main effect of group × time interaction; findings were classified as small (0.00 ≤ d ≤ 0.49), medium (0.50 ≤ d ≤ 0.79), and large (d ≥ 0.80) as suggested by Cohen (¹⁰).

Results

Before the 8-week intervention, data showed no significant differences between CSG, PG, and CG. No significant changes were observed in leg muscle volume, thigh muscle volume, or CSA after completion of either the PT or the CST programs (Table 4). However, after the intervention, both experimental groups showed significant decreases of times for all sprint and change of direction tests relative to controls, with no significant intergroup differences in response between trained groups (Table 5). In most comparisons of SJ and CMJ scores, increases were greater for CSG than for PG (Table 6). The RMS values of the CSG also increased significantly relative to PG and CG for all muscle groups except VL (Table 7). Moreover, the CSG showed substantial gains of 1RM half-squat and force-velocity scores relative to the other 2 groups (Table 8).

Table 4.: Comparison of lower-limb muscle volumes and cross-sectional areas between experimental and control groups before (pre) and after (post) the 8-week trial.*

Table 5.: Comparison of sprint times and ability to change direction between experimental and control groups before and after the 8-week trial.*

Table 6.: Comparison of vertical jump and maximal strength (back half-squat) performance between experimental and control groups before (pre) and after (post) the 8-week trial.*†‡§

Table 7.: Comparison of root-mean-squares of EMG voltages for squat jump and countermovement jump between experimental and control groups before (pre) and after (post) the 8-week trial.*†‡

Table 8.: Force-velocity test scores for lower limbs in experimental and control groups before and after the 8-week trial.*†‡§

Discussion

In the current study, the CST regimen alternated biomechanically comparable strength exercises and sport-specific drills in the same workout (with CMJs and sprinting replacing low loads). The aim was to compare the effectiveness of this type of CST and of PT with respect to gains in linear sprinting, ability to change direction, vertical jumping, power, strength, and neuromuscular adaptations in U-17 male soccer players. The results show that CSG yielded greater improvements in SJ, CMJ, 1RM half-squat, and neuromuscular adaptations, but that the PG showed significantly greater improvements in the SJ test compared with the CG, and that there were no differences between the 2 training regimens with respect to 5 m, 40 m, and S 4 × 5 tests. An original feature of this study was the inclusion of jumping and sprint-training exercises during CST; indeed, to the best of our knowledge, this is the first study that has compared responses to CST and PT in soccer players.

The results demonstrated an improvement of 1RM half-squat in all groups. However, the CSG showed the greatest improvement, with no significant difference of gains between PG and CG. Many previous investigations have found an enhancement of 1RM half-squat performance after strength training (^8,18,37). Gabriel et al. (¹⁶) suggested that strength/power adaptations were largely associated with increases in the CSA of the muscle, but no significant increase of thigh CSA or limb muscle volumes was found in the current study (Table 4). Nevertheless, leg power per unit of the thigh or lower-limb muscle volume did show increases in CSG relative to both PG and CG (Table 8). In this regard, Hammami et al. (¹⁸) also found increases of LPP as evaluated by a force-velocity test after half-squat strength training, despite the absence of changes in thigh CSA and muscle volumes. The effects of CST thus seem due primarily to neuromuscular adaptations, such as more effective motor unit recruitment, rate coding (frequency or rate of action potentials), synchronization, and intermuscular coordination (^1,16).

Furthermore, this study revealed no significant change in 1RM half-squat performance after PT. Other studies have yielded contradictory results. Vissing et al. (⁴¹) suggested that PT induced improvement in all 3 tests of maximal strength (leg extension; knee extension; and hamstring curl), and explained these improvements by shorter training periods or a higher initial training status of the participants.

Many studies have found a high correlation between leg extensor muscle strength and sprint performance (^29,31). An increase in muscle strength of the lower limbs could improve the ability to perform short duration sprints (²⁹). The review of Silva et al. (³⁷) suggested that a 23.5% increase in 1RM half-squat was needed to achieve a 2% improvement in sprint performance over 10- and 40-m distances. The results of the current study are consistent with these findings, with faster 5 and 40 m times in both CSG (5 m: 10.9%; 40 m: 8.3%) and PG (5 m: 7.3%; 40 m: 3.4%). Other studies have yielded contradictory results when evaluating the effects of strength training combined with sports skills. Garcia-Pinillos et al. (¹⁷) observed a significant improvement in 5, 10, 20, and 30 m times (p ≤ 0.05) in soccer players aged 15.9 years after 12 weeks of a contrast training program that included 3 exercises (1 isometric session and 2 plyometric sessions without external loads). Franco-Marquez et al. (¹⁵) reported that a combination of 6 weeks of resistance training with standard soccer training produced greater gains in sprint performance than typical soccer training alone. By contrast, Herrero et al. (²³) found no significant improvements in 20-m sprint times of male physical education students after 4 weeks of electromyostimulation. Likewise, Markovic et al. (²⁷) found no speeding of 20-m sprint times of male physical education students after 10 weeks of PT. These discrepancies could be due to the type of training (intensity and duration, isometrics, resistance, plyometric, or isokinetic programs), and to difference in the sampled population.

In addition to linear sprinting, soccer players make many COD during a soccer game. In the current study, the S 4 × 5 m change of direction score did not change significantly after training. However, Hammami et al. (²⁰) applied a similar program, and they found significant improvements in both strength group and CSG relative to controls, with no differences in gains of agility between strength group and CSG except in terms of the S 4 × 5 m test score. Garcia-Pinillos et al. (¹⁷) found that 12 weeks of a contrast training program (isometric + plyometric) with no external loads improved the performance of the Balsam agility test (p < 0.001) in 15.9-year-old soccer players. By contrast, Cavaco et al. (⁷) found no significant improvements in the 15-m sprint times of youth soccer players after 6 weeks of complex training. Furthermore, Herrero et al. (²³) observed no significant speeding of 20-m sprint times unless electromyostimulation was combined with 4 weeks of PT. The spectrum of possible factors associated with these discrepant results includes the players' age, background, and initial training status, differing training periods, the structure of the training intervention, game exposure, and distinct force/power qualities and technical factors that influence event- or sport-specific COD; in particular, greater change of direction enhancement is seen in adolescents than in adults (³⁷).

Both of the present experimental groups (CSG and PG) significantly improved their SJ height relative to the CG (Table 6), with only slight differences between CSG and PG. For PG, the increase can be explained by the training specificity principle, with a similarity in kinetics and kinematics between jump and half-squat exercises. By contrast, the CSG increased their SJ height relative to the PG and CG, with no significant difference between the latter 2 groups. In young untrained men, Vissing et al. (⁴¹) observed a significant improvement in CMJ in both of their experimental groups (plyometric and conventional resistance training). On average, 24% 1RM improvements during squats resulted in CMJ and SJ increases of approximately 6.8% (³⁷). Hammami et al. (¹⁸) also observed that 8 weeks of strength training with external loads (70–90% 1RM) had a positive effect on SJ and CMJ performance in male soccer players. By contrast, Mujika et al. (³⁰) saw no significant increases in the CMJ performance of junior soccer players after 7 weeks of contrast training. Differences in the strength program (intensity, duration, frequency, and type of exercise) and in methodology (youth vs. young soccer players; elite vs. regional soccer players; duration of intervention) could contribute to these discrepancies. Potential adaptations include an increased neural drive to the agonist muscles, improved intermuscular coordination, and changes in the mechanical characteristics of the muscle-tendon complex. The gains in CMJ performance and in mechanical parameters such as velocity and relative power during CMJ in the CSG support the hypothesis that neurophysiological changes of this type improve the ability to store and release elastic energy during an SSC (^5,16).

Factors other than neuromuscular adaptations may also be involved. For example, individuals may adopt a better technique, their body mass may have decreased, or they may experience favorable psychological or hormonal influences on the day of testing. An increase in neural drive has often been adduced to explain strength gains in the absence of muscle hypertrophy (^5,16). A numbers of articles have reported increases in EMG activity induced by strength training (^5,18,35). To verify this suggestion, the EMG was recorded from VL, VM, and RF during performance of the SJ and CMJ tests. The results demonstrated that 8 weeks of CST significantly improved all EMG parameters relative to the PG and CG, with the exception of RMS values for SJ.VL and CMJ.VL. Hammami et al. (¹⁸) also found that the RMS values of male soccer players during the SJ and CMJ were increased after strength training. Moreover, Arabatzi et al. (²) showed a reduction of medial gastrocnemius EMG activity coupled with smaller kinematic changes after combined weight lifting + PT, suggesting that such training affects mainly knee muscle strength capacity without causing clear changes in the CMJ technique. Prieske et al. (³³) further observed significant gains in maximal isometric trunk extension force (5%, p ≤ 0.05, d = 0.86) after 9 weeks of resistance training. However, Manolopoulos et al. (²⁶) found that 10 weeks of soccer-specific, strength, and technique training induced no EMG changes other than an increase in the averaged EMG for the VM. These discrepancies could reflect differences in the type of training (plyometric, strength, intensity, and duration), or inconsistencies in the placement of EMG electrodes between testing and retesting. The increases of EMG activity seen after CST in the current study seem linked to changes in motor unit firing frequency, rate coding, and impulse synchronization (⁵).

Limitations of this work include the lack of quantification of the Tanner developmental stage for the players, and potential differences in the placement of the EMG electrodes (because the physique developed slightly over the 2 months between initial and final tests). Moreover, we did not evaluate the dominant leg, making it necessary to choose a test with the same number and the same angles of change of direction for left and right sides, to neutralize dominant leg effects.

Practical Applications

Improving muscle function and resulting sport performance are primary tasks for strength and conditioning professionals. From a practical point of view, this study demonstrates that 8 weeks of a CST regimen (exercise intensity: 70–90% of 1RM; sets: 3–5; repetitions: 3–8, with a CMJ + 10 m sprint as a contrast exercise after each repetition) enhances the physical capacities of soccer players more than allocation of a similar time to PT. Inclusion of a twice weekly short-term high-intensity CST program within regular in-season soccer practice enhances many factors relevant to athletic performance in U-17 male soccer players (sprinting, ability to change direction, vertical jumping, strength, power, and neuromuscular adaptations), particularly when this tactic is compared with soccer training plus PT or standard soccer training alone. Coaches may thus opt to use programs that combine strength training with the practice of soccer skills as a helpful tactic to improve the strength and power of their athletes during the competitive season.