During a 90-minute soccer match, professional soccer players make numerous explosive bursts, such as kicking, tackling, jumping, turning, sprinting, and changing pace (5). Speed strength, also known as power, is crucial for performance in sports in which changes in direction, acceleration, and jumps are important (27). Hence, strength and power in leg muscles are important for professional soccer players.
Maximal strength is an important quality for power performance, because power is the product of force (strength) and velocity (speed). Thus, an increase in 1 repetition maximum (1RM) is usually related to improved power abilities (34). In professional soccer players, heavy strength training twice a week on leg extensor muscles has been shown to improve 1RM, vertical jump height, and 10-m and 20-m sprint time (18). However, the study was conducted during the preparation phase, and because there was no control group, it is not possible to conclude whether the improvements were caused by seasonal changes in the concurrent soccer training (i.e., technical, tactical, and endurance) or by the strength training per se. In general, heavy strength training on leg extensor muscles is reported to improve power, jumping height, and sprint performance (11,24,26). Consequently, a wide variety of strength training modes and training protocols have been used to develop lower extremity strength and power.
Plyometric training involves exercises in which the active muscles are stretched prior to its shortening. Plyometric exercises can be done with or without external load, and both modalities have been shown to increase power, jumping height, and sprint performance (26,30,39). However, contradictory results exist regarding the effects of plyometric training on speed and power measurements (23,39) and the effects of heavy strength training (17,20,39). Heavy strength training and plyometric training may affect different aspects of power-related skills. Consequently, it has been suggested that combining heavy strength training and plyometric training improves power and power-related skills to a greater extent than any of the 2 training modalities alone (3). In most studies in this field, data support the hypothesis that combining heavy strength training and plyometric exercises is superior to only training one of the training modalities (3,11,17,20,36). The reason for this could be that these 2 training modes enhance 2 important qualities for high power production: maximal force and rapid force development.
An 8-week combined strength, ballistic, and on-court (including plyometric actions) training program in elite volleyball players resulted in superior jump performance compared to strength or on-court training only (28). The authors are not aware of any controlled studies in professional soccer players that focused on the effects of heavy strength training with or without plyometric exercises on important skills in soccer, such as maximal power, jump, and sprint performances. Therefore, the purpose of this study was to compare the effects of combined strength and plyometric training with strength training alone on power-related skills in professional soccer players. The intervention (7 weeks) took place in the preseason preparation phase, including 6 to 8 soccer sessions a week. It was hypothesized that the combined training would be superior to strength training alone.
Experimental Approach to the Problem
This study was designed to address 2 questions: Does the addition of a 7-week heavy strength training period improve strength, jumping abilities, sprint performance, and peak power production in professional soccer players during the preparation phase with concurrent soccer sessions? Does a combination of heavy strength training and plyometric training result in superior improvements compared to heavy strength training alone on the aforementioned test parameters? To investigate these questions, 2 groups of professional soccer players conducted the same heavy strength training regimen during their preparation phase, and the only difference was that 1 group performed plyometric training, while the other group conducted low-intensity core exercises. The control group consisted of professional soccer players who, instead of heavy strength training, performed core exercises during their preparation period. Changes in the selected variables were tested in the intervention group and control group before and after the 7-week intervention.
Twenty-one Norwegian professional male soccer players (Premier League) (Table 1) volunteered to participate in this study. They had performed on average 5 to 7 training sessions a week during the last 3 years. The study was approved by the Regional Ethics Committee of Norway.
Subjects in the intervention team were randomly divided into 2 groups. Group ST (n = 6) performed the heavy strength training regimen and no extra plyometric training in addition to 6 to 8 soccer sessions a week. Group ST+P (n = 8) performed a plyometric training program in addition to the same training as the ST group. The control group (n = 7) performed 6 to 8 soccer sessions a week in addition to a core training program lasting approximately the same time as the aforementioned strength training program. There were no differences between groups in anthropometric parameters or the test variables before the intervention period (Tables 1 and 2).
Although all subjects had experience with strength training and the half squat exercise, the intervention groups participated in 2 weekly strength training sessions (12-15 RM) the last 3 weeks before initiation of the study to make sure they used a proper lifting technique. This was a part of the transition phase leading to the preseason phase. All subjects performed both their pretests and posttests in 1 day with the same test order. The pretesting and posttesting was done on the same equipment with identical subject-equipment positioning overseen by the same trained investigator. The posttests were accomplished at approximately the same time of day as the pretests and 3 to 5 days after the last strength training session.
All subjects performed a standardized warm-up prior to the sprint test. Subjects jogged for a 15-minute period at a moderate pace. The warm-up was concluded by 4 or 5 40-m submaximal runs. Subjects performed 3 or 4 maximal effort sprints over a distance of 40 m. The subjects in the intervention group performed their presprints and postsprints on a hard even surface in an indoor facility, while control subjects performed all their sprints on an indoor sprint track. All subjects used adapted indoor shoes. Due to the different running surfaces, it is important to carefully interpret the absolute values in the control group and intervention group. The sprints were separated by approximately 3 minutes to ensure full recovery between sprints. Subjects commenced each sprint from a standing (static) position in which they positioned their front foot 50 cm behind the start line. Subjects decided themselves when to start each run with the time being recorded when the subject intercepted the photocell beam. Subjects were instructed to sprint as fast as possible through the distance. Times were recorded by photocells (JBL Systems, Oslo, Norway) placed at the start line and after 10 m, 30 m, and 40 m. Sprint acceleration was measured as the time to complete the first 10 m, and peak sprint velocity was measured as the mean velocity between 30 m and 40 m. The trial with the best 40-m sprint time was chosen for analysis of the sprint times after 10 m, 30 m, and 40 m (CV < 3%).
After 5 minutes of rest, subjects performed a specific warm-up consisting of 3 to 5 submaximal series of horizontal jumps while alternating between the right and left legs. Horizontal jumping performance was evaluated by a 4-bounce test (4BT), in which the horizontal distance covered after a series of 4 forward jumps with alternate left and right foot contacts was measured with a tape. The 4BT started from a standing position, and the subjects were encouraged to cover the longest distance. The best result of 4 to 6 trials was used in the statistical analysis (CV < 1.5%). The maximal vertical jump ability was tested on a force plate (SG-9; Advanced Mechanical Technologies, Newton, Mass.). Subjects performed the CMJ and SJ with their hands kept on their hips throughout the jumps. During the SJ, with knees at 90° of flexion, the subjects were instructed to execute a maximal vertical jump and were not allowed to use any downward movement prior to the maximal vertical jump. The force curves were inspected to verify no downward movements prior to the vertical jump. During the CMJ, the angular displacement of the knees was standardized so that the subjects were required to bend their knees to approximately 90° and then rebound upward in a maximal vertical jump. Force data were sampled at 1,000 Hz for 5 seconds by using an external A/D converter (USB-1408FS; Measurement Computing Corporation, Norton, Mass.), and all data were calculated by using Matlab. Jumping height was determined as the center of mass displacement calculated from force development and measured body mass. Each subject had 4 attempts interspersed with approximately a 1.5-minute rest between each jumps in both the SJ and CMJ, and the best jump from each subject was used in the data analysis (CV < 3%).
Peak Power Measurement
Peak power was assessed during loaded barbell SJ. The test procedure was identical to the SJ procedure except for the increased loading of a barbell and the hands were positioned on the barbell. Subjects performed the SJ against absolute loads of 20 kg, 35 kg, and 50 kg (PP20, PP35, and PP50, respectively) on a force plate. Subjects performed 2 SJs at each load with 2 minutes of rest between each attempt. The highest score of each load was recorded. Mechanical power was continuously calculated as the instantaneous product of vertical force and velocity. Peak power corresponded to the highest instantaneous power output before takeoff at each load (CV < 3%).
One Repetition Maximum Measurement
Maximal strength in leg extensors was measured as 1RM in squat. Before the 1RM squat test, subjects performed a standardized specific warm-up consisting of 3 sets with gradually increasing load (i.e., 40%, 75%, and 85% of expected 1RM) and decreasing number of reps (i.e., 12, 7, and 3). The depth of squat in the 1RM was set to a knee angle of 90°. To ensure a similar knee angle in the pretest and posttest for all the subjects, the subjects' squat depth was individually marked at the pretest depth of the buttock on a list. Thus, the subject had to reach his individually depth (i.e., touch his list with the buttocks) in the posttest to get the lift accepted. The first attempt in the test was performed with a load approximately 5% below the expected 1RM load. After each successful attempt, the load was increased by 2% to 5% until failure in lifting the same load in 2 or 3 following attempts. The rest period between each attempt was 3 minutes. The coefficient of variation for test-retest reliability for this test has been found to be less than 2% (29).
The 7-week intervention period consisted of 2 strength workouts per week on nonconsecutive days. Each workout consisted of a half squat and a hip flexion exercise. The hip flexion exercise was included because it has been indicated that this exercise is important for improvement in sprinting performance (8). After a 15-minute warm-up with light jogging or cycling, subjects performed 2 or 3 warm-up sets of squat with gradually increased weight. All subjects were supervised by one of the investigators at all strength training sessions during the entire training period. The training intensity was 4 to 6 RM and similar for the 2 groups. Subjects were encouraged to continuously increase their RM loads during the intervention. Subjects were allowed assistance on the last rep. Based on the assumption that it is the intended rather than actual velocity that determines the velocity-specific training response (6), strength training was conducted with emphasizing maximal mobilization in concentric phase, while the eccentric phase had a slower speed (i.e., approximately 2 or 3 seconds). Training volume (i.e., number of sets) was altered similarly for the 2 groups. During the first 2 weeks, both groups trained with 3 sets. During the third, fourth, and fifth training weeks, they increased the volume to 4 sets, and during the final 2 weeks, they trained with 5 sets (Table 3).
The plyometric program for the ST+P group consisted of double-arm single-leg forward jumps, single-arm alternate-leg forward bounce, and double-leg hurdle jumps. Progressive overload principles were incorporated into the program by increasing the number of foot contacts. The number of sets and foot contacts in each drill was between 2 and 4 and 5 and 10, respectively, and the rest between sets was approximately 1 minute (Table 3). Subjects were encouraged to perform each drill with maximal intensity, emphasizing a fast switch from eccentric to concentric contraction for optimal quickness off the ground. The control group performed a core training program lasting approximately the same time as the aforementioned strength training program. The core training program focused on the deepest abdominal muscle (i.e., transversus abdominis) and included various balance, abdominal, and back exercises with and without a fitness ball.
A regular training week for both the control and the intervention groups consisted of 6 to 8 soccer sessions lasting between 90 and 120 minutes and focusing on physical conditioning, technical, and tactical aspects of the game (Figure 1).
All values given in this article are mean ± SEM. A 1-way analysis of variance was used to determine significant differences among the ST+P, ST, and control groups in percentage change. After the 2 intervention groups had been pooled, unpaired t-tests were used to compare relative changes from before to after training between the intervention group and control group. Paired t-tests were used to test for significant changes within groups from before to after training. Pearson correlation coefficients were determined for selected variables. The level of significance was set at p ≤ 0.05.
The 1-way analysis of variance detected no significant differences between the ST+P group and ST group in percentage change in any of the test variables from before to after training. However, both groups increased significantly from before to after training in 1RM half squat, 4BT, PP20, and PP50 (p < 0.05) (Table 2). Neither the ST+P group nor the ST group significantly improved in CMJ performance.
Because there were no significant differences between the ST+P group and ST group, the 2 groups were pooled into 1 intervention group. The increase in 1RM squat from before to after training was significantly higher in the intervention group than in the control group (25% vs. 2.5%; p < 0.001) (Figure 2), while no significant change took place in the control group.
There were no differences between groups in total training volume or training intensity in soccer sessions during the intervention period (Figure 1). Of course, there was a difference in the amount of heavy strength training between the intervention group and control group.
The horizontal jumping performance in the 4BT was significantly improved in the intervention group (p < 0.001). Furthermore, the relative improvement in the intervention group was superior to that in the control group (4% vs. 0%, respectively; p = 0.01) (Figure 3).
Regarding vertical jump performance, there were no significant changes in either group in the CMJ test (Table 2), while the intervention group increased significantly (8.5%) in the SJ (p = 0.02). This increase was significantly different from that in the control group (p = 0.03), which had no improvements in the SJ (Figure 4).
The intervention group increased significantly (p < 0.01) in all measurements of peak power (i.e., PP20, PP35, and PP50). In the control group, there was a significant increase in PP20 (p = 0.02); otherwise, there were no changes in peak power (Table 2). There were no significant differences between groups in any peak power measurements, although there was a tendency in PP50 (p = 0.058).
The intervention group significantly increased running performance in the acceleration phase (i.e., the first 10 m of the 40-m sprint) (p < 0.02), peak running velocity (i.e., time between 30 m and 40 m in the 40-m sprint) (p < 0.02), and total time during the 40-m sprint test (p < 0.02), while there were no significant changes in the control group or between groups (Table 2).
There were significant correlations between relative strength in squat (1RM/body weight) and jump performance (r = 0.5; p < 0.05) and sprint performance (i.e., 10 m and 40 m time) (r = 0.4; p < 0.05) at baseline. The only significant correlations between changes in 1RM squat and changes in the power-related measurements were with change in 4BT (r = 0.61; p < 0.01).
Both the ST+P group and the ST group significantly improved in strength, jump, and sprint performance, but there were no significant differences between the 2 groups. Therefore, the groups were pooled into 1 intervention group to increase statistical power compared with the control group. The intervention group significantly improved in all measurements except CMJ, while the control group showed significant improvements only in PP20. After the intervention period, there were significant differences between the intervention group and control group in relative changes in 1RM half squat, 4BT, SJ, and a trend toward difference in PP50.
In contrast to these findings, superior adaptations in power-related measurements are reported when traditional strength training is combined with explosive plyometric exercises in both untrained subjects (11,36) and trained subjects (3,7,17). A point of interest is that none of the combined studies on trained subjects performed additional training, such that the weekly amount of training sessions (i.e., 2-4) was quite normal (3,7,17). Interestingly, in the study by Clutch et al. (7), subjects who combined strength and plyometric training were a weight training class, and the other group was a volleyball team with additional volleyball training 5 days a week during the intervention. There was only a significant difference between the combined training and strength training in the group with no additional training. Whether this lack of difference is due to the jumping performed during the volleyball practice, a state of overtraining, or other possible causes remains unclear. Similar findings have been reported in French handball players (37). The subjects performed standard handball training 3 times a week during the 6-week intervention period in addition to 4 sessions a week of combined strength and plyometric training or only strength training. No significant differences between the 2 intervention groups were observed in jump performance. The similarity of the studies with additional training sessions, including plyometric and explosive movements (e.g., handball, volleyball, and soccer), makes it tempting to suggest that when the total amount of plyometric stimulus is large during the regular team sessions, further advantages of combining a heavy strength training regimen with plyometric exercises are not achieved compared to strength training alone. The latter is further supported by the finding that strength training enhances speed of an unloaded movement only when combined with specific training of that movement (38).
The increase in 1RM half squat is fairly in line with another study on male soccer players from the Norwegian Premier League with a similar heavy strength training regimen (18). It is interesting to note that this relative large increase in maximal strength (i.e., approximately 2% per training session) occurs despite concurrent endurance, technical, and tactical soccer training. There are studies indicating that concurrent strength and endurance training impairs the strength training adaptations in some, but not all tested exercises, when the total training volume of both factors is large (19,21). Interestingly, of the aforementioned studies, Kraemer et al. (21) measured the response of concurrent training on peak power, and Hennessy and Watson (19) measured 20-m sprint and vertical jump performance and found that concurrent training impaired these adaptations.
The lack of significant difference between the training group and control group in some of the parameters may be related to possible effects of overtraining. In a 6-week study on adolescent handball players, the subjects who performed heavy strength training in addition to handball training had a compromised testosterone-to-cortisol ratio during the last 4 weeks (14). It has also been suggested that the lack of direct correspondence between increased strength and other types of performances is partially due to lag time (2,10,35). Lag time is the period of time in which an athlete learns to use his or her increased strength in various sport skills. This is in line with the finding of no improvement immediately after a plyometric training period, but after a recovery period, significant improvement in CMJ was found (22). Unfortunately, no tests could be performed after a recovery period, so whether this is the case in the current study is impossible to determine.
As in the current study, strength trainees may experience no significant increase in their CMJ performance after a short-term strength training period and improvement in maximal strength (32,37). In agreement with the latter, improving strength does not automatically result in more powerful movements and improved performance (20,25,33). Improvements in exercises that include stretch-shortening cycles are often explained through changes in the stretch reflex or increased capacity to store and reuse elastic energy (37). Consequently, it is possible that the intervention program failed to stimulate these factors significantly. However, there was a significant increase in the 4BT, which consists of several stretch-shortening cycles. Therefore, the reason for no improvements in CMJ in the current study remains unclear. Whether the findings can be explained by the aforementioned lag time, lack of recovery period, overtraining, too short an intervention period, or low statistical power remains unknown.
The results of this study indicate that substantial increase in 1RM can be made with little or no increase in body weight. The increase in 1RM values observed may be due to alterations in neural factors caused by the intensity of training (15). There is also a possibility that hypertrophy in leg muscles may account for the increase in 1RM because changes in body composition (i.e., reduced fat mass) is likely in this phase of the training cycle (i.e., preseason). It has also been suggested that a possible response to strength training may be consolidation of the tissue as the muscle fibers increase in girth at the expense of extracellular spaces (13). However, there are no data to discriminate between changes in muscle cross-sectional area or neural activity as the mechanism behind increased leg strength.
Furthermore, it has been stated that maximal strength is the overall most important factor in power output when movement duration is longer than 250 milliseconds (34). There are several possible reasons that the observed increase in maximal strength may increase peak power and performance in power-related measurements. An absolute weight would represent a smaller percentage of maximal strength; thus, this weight would be easier to accelerate. Increased strength is often associated with preferred hypertrophy of type II fibers (4,31), thereby increasing the type II-to-type I cross-sectional area ratio. Type II fibers are the primary motor units that contribute to high power output (16). Enhanced neural drive may increase the muscle activation (1). It is also likely that power production for moving light loads depends on other force-related components, such as initial rate of force development and peak rate of force development (34). Heavy strength training regimens normally result in large improvements of rate of force development (1).
Fry et al. (12) and Wilson et al. (39) reported no significant increases in sprint acceleration or velocity after training programs involving essentially vertical plyometric exercises and weight training in trained subjects. This may be related to the lack of specific sprint training during the intervention period. Furthermore, it has been reported that strength training does not always improve 20- to 100-m sprint time (9,17,20,26,39). Interestingly, the group in the study by Kotzamanidis et al. (20), who performed sprint training in addition to strength training, improved their 30-m sprint performance, while the group who had no specific sprint training did not improve. Similarly, in the study by Delecluse et al. (9), there was a significant increase in sprint acceleration when strength and sprint training was combined. Thus, it seems that the concurrent sprint training, performed during the regular soccer practices in the current study, may be important for sprint adaptations. This is in line with another study on professional soccer players, which found that concurrent heavy strength training and soccer sessions increased the sprint performance (18). Another possible explanation for the improved sprint performance in the current study may be the inclusion of a specific hip flexor exercise in the strength training program, shown to relate to improvements in sprint performance (9).
Previous studies have indicated that heavy strength training and increased maximal strength generally result in greater improvements at the high force end of the force-velocity curve and that high velocity and high power training leads to superior improvements toward the high velocity end (9,17,26). The current study indicates that heavy strength training significantly increases performance in professional soccer players at both the high force end (i.e., 1RM and sprint acceleration) and the high velocity end (i.e., peak sprint velocity and 4BT) as long as the subjects are performing concurrent plyometric and explosive exercises during their soccer sessions. No significant difference was found between the group that combined heavy strength training and plyometric exercises in addition to their regular soccer sessions and the group that conducted only the heavy strength training in addition to the regular soccer sessions.
The current data indicate that professional soccer players can achieve improvements in strength and power-related measurements as a result of a 7-week heavy strength training period. When professional soccer players conduct a 7-week heavy strength training regimen during their preseason preparation phase with an addition of weekly 6 to 8 soccer sessions, there seems to be no further improvements by including a specific plyometric training program. However, the specific mechanisms responsible for the observed findings cannot be determined from the current study. Furthermore, no conclusions can be made on any long-term consequences of the 2 training modalities.
The authors thank P.T. Dan Morten Kleppe for his assistance with training procedures during the study.
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