Soccer is regarded as a high intensity intermittent contact sport exposing players to continual physical, technical, tactical, psychological, and physiological demands (5,11,40). At the elite level, the regular demands of match play and training performed during the season’s entirety makes players susceptible to injury. Intuitively, losing players to injury will be to the detriment of team success (2), particularly for teams unable to replace players of similar abilities due to limited resources. Therefore, injury prevention programs have gained greater impetus as part of the player’s daily training schedule.
At the elite male professional level within soccer, the incidence of injuries during competitive match play is suggested to be approximately 24.6–34.8 per 1000 match hours (2,38,47) with injuries encountered during training sessions showing to range between 5.8 and 7.6 per 1000 training hours (2,47). Among the highest number of injuries per season are those players competing in the English League (1.3 injuries per player) (15,22). Almost one-third of all soccer-related injuries are muscle related, with the majority (92%) affecting the following major muscle groups of the lower extremity: hamstrings (37%), adductors (23%), quadriceps (19%), and calf muscles (13%) (17,24). The Football Association Audit of Injuries identified the hamstrings to be the most commonly injured muscle, constituting 12% of all strains. Indeed, players are 2.5 times more likely to sustain a hamstring than a quadriceps strain during a game (24,55).
Although the cause of injury is not always known, there are a number possible factors that may increase its incidence, these may include insufficient warm-up (56), poor flexibility (25,58), muscle imbalances (10,42), muscle weakness (9,31), neural tension (49), fatigue (57), and previous injury (17,42). Thus, devising and implementing training programs that attempt to address some of these issues would obviously be looked upon favorably with strength and conditioning coaches and management team. The justification, for example, of strength training drills within a program is based on the notion that a strength imbalance will increase the likelihood of injury in the weaker leg. It has been reported that players are 2.6 times more likely to suffer an injury in the weaker leg if this imbalance is ∼15% (10,44). Given the contribution of the quadriceps and hamstring during a number of soccer actions (i.e. kicking, jumping, running), it is not surprising that muscular strength is deemed an important facet. The hamstring in particular is a key contributor during deceleration as it works eccentrically to slow the body down and is one reason why eccentric hamstring strength training (Nordic hamstring lowers) is a popular choice from both an injury prevention and performance enhancement viewpoint (2,3). Likewise, the importance of balance training is becoming more apparent in many sporting disciplines, not least soccer (12,28). Through greater functional postural activation, balance training is considered to be an effective (36) strategy shown to reduce the incidence of ankle sprains (13,48,51), hamstring and gastrocnemius strains, patellar tendinopathy, and other lower extremity pathologies (34,37) amongst team sport players. Similarly, the role of core stability in injury prevention has gained greater recognition over recent years as an injury prevention–training method. Core stability is defined as the ability to control the position and motion of the trunk over the pelvis to allow optimum production, transfer, and control of force and motion to the terminal segment in integrated athletic activities (4,50). Made up of the paraspinals, quadratus lumborum muscle, abdominal muscles, hip girdle musculature, diaphragm, and the pelvic floor muscles (1,53), the core is considered important for efficient biomechanical function necessary to maximize force generation and minimize joint loads in various activities (26), and thus possibly decreases the incidence of injury (54).
In a professional club setting, such physical characteristics are rarely trained in isolation and are likely to be delivered as part of a multicomponent training program. However, the effectiveness of multicomponent training programs in soccer has scarcely been reported in the literature. One example that included exercises for stability, flexibility, coordination, power, and reaction time showed a 21% reduction in the incidence of injury per 1000 hours of training and playing soccer, from 6.7 in the intervention group and 8.5 in the control group, albeit in amateur soccer players (30). In contrast, a recent study showed the injury incidence to be almost equal between an intervention group (9.6 per 1000 hours) and control group (9.7 per 1000 hours), although the intervention group sustained significantly fewer knee injuries (50). The tendency within research studies, however, has been to generally investigate their effectiveness (multi component training programs) over relatively short periods. There seems to be few studies reporting the effectiveness of a multicomponent training program in elite-level soccer players for the entirety of the season. Therefore, the aim of the current investigation was to (a) examine the effectiveness of a structured injury prevention program on the number of muscle injuries; and (b) investigate the effect of the program on the total number of injuries across the 2 seasons. It is hypothesized that the injury prevention technique used during the intervention season would significantly reduce the number of muscle injuries when compared with the control season. It is also hypothesized that there be a significant reduction in the total number of injuries when comparing the intervention season with the control.
Experimental Approach to the Problem
The study was conducted over 2 consecutive seasons (2008–2009, 2009–2010). The first season (2008–2009) was the intervention season, and the second was the control season. During the intervention season, the players performed a multicomponent prevention intervention twice weekly (unless 2 × competitive matches per week) and before the technical and tactical training (58 prevention sessions were performed). During the injury intervention program, players were randomly divided into 4 groups, each session players would move through each of the 4 stations (balance; functional strength; core stability; mobility). The control season players performed no set structured prevention intervention pretraining after the preseason phase, that is, 9 prevention sessions were performed. A qualified physiotherapist was present to ensure correct execution of exercises during all sessions. Injury was classified according to training or match incidence, the muscles concerned, the cause, and the nature of the injury.
A total of 26 soccer players participated in the study. The study involved a squad of 26 (first season: 2008–2009) players during the intervention season and a squad of 23 players during the control season whose age, height, body mass, maximal aerobic capacity, lower body strength (1RM squat), 10 and 20 m sprint time, and sum of 8 skinfold sites (taken at the biceps, triceps, subscapular, iliac crest, supraspinale, abdominal, mid-thigh, and calf) were taken at the beginning of each season across the investigation (Table 1). The study was conducted by the Sport Science Department at Rangers Football Club after approval by the Ethics Committee of the Sport Science and Research Department, Claude Bernard Lyon 1 University. Each subject provided a written informed consent in accordance with the Helsinki Declaration with subjects having the option of withdrawing from the study at any time without penalty. The participants were free of all injury and able to participate within the intervention at the initial period of each season. To ensure team and player confidentiality, all injury data were anonymized.
Only training or match injuries were included within this investigation. Recordable injuries were defined as an injury received during training or competition, which prevented participation in normal training or competition for more than 48 hours, not including the day of the injury (24). The classifications and definitions of injuries used within this investigation follow closely the recommended guidelines proposed by International Soccer Injury Consensus Groups (21) and are of similar definitions to those used in other injury-related articles in professional soccer (Table 2) (6,52).
Injury Prevention Program
- The first exercise performed as part of the balance training area was a single leg balance performed on the floor (stable base) for 30 seconds. In pairs, 1 partner serves soccer ball with raised foot to partner opposite to volley back 1 touch. Player then aims to catch ball from return volley and switches leg after 30 seconds.
- The progression to the second exercise performed as part of this area was a single leg balance on an Airex pad (TN, USA) (unstable base) for 30 seconds. In pairs, 1 partner aims to volley soccer ball back with raised foot to server without foot touching the floor. Player switches leg after 30-second duration.
- The final progressive exercise performed within this area involved players performing a single leg balance on a trampet (Reebok, MA, USA) for 30 seconds. Again, in pairs, 1 partner aims to volley soccer ball back to server with raised foot although continually bouncing on the trampet (Reebok, MA). Player switches leg after 30 seconds duration.
All exercises were to be performed single leg (no knee flexion) (Figure 1). The players were instructed to perform 3 sets of 30 seconds on each leg before switching roles (i.e. pass to volley).
The exercises used within this area of the intervention are described below (Figure 2 and Table 3):
- The Nordic hamstring exercise is performed from a kneeling starting position on a soft foundation. The players were instructed to slowly lower their body toward the ground using the hamstrings to control the movement, whereas the feet are held by a partner.
- Players were instructed to hold the resistance band (Theraband, United Kingdom) in position and bend slightly at the knee. Players were then instructed to step laterally for the required number of repetitions to both their left and right side.
- During this exercise players were to start in a single leg stance, holding a kettlebell weight (Adidas, OR, USA) in the opposite hand to the standing leg. Players were then instructed to lower down and touch the weight down to ground although maintaining a straight line through their back before returning to starting position.
- This exercise required players placing a resistance band (Theraband, UK) material around knees while in a side-plank position. Players were then instructed to raise their knee although ensuring their heels remained touching without moving.
- Players for this exercise were instructed to start in a split squat stance although holding a kettlebell weight (Adidas, OR, USA) in each hand. The technique then included dropping the back leg to ground so that their knee touched the ground before returning to the starting position.
- Players for this exercise were required to start on hands and knees and maintain with a straight back throughout the movement. Players then activate the gluteals through alternate reserve leg lifts although ensuring no rotation through the hips occurs.
Exercises used for the core stability development area are described below (Figure 3 and Table 4):
- Players were instructed to lie face down although balancing on forearms and toes ensuring a straight posture is maintained throughout the duration of the static hold.
- Players were instructed to lie on their side although balancing on forearm and side of foot, although trying to maintain a straight posture throughout the static hold.
- Players stand side on to the wall in a fixed open stance. They were then instructed to throw the medball (Reebok, MA, USA) against the wall rotating from 1 side to another via underarm throw in order to rebound ball off the wall.
- Players face wall in a fixed open stance although holding a medball (Reebok, MA, USA) above head. Upon instruction, players throw ball to rebound off wall keeping a tight-controlled posture.
- Players lying on their back with medball (Reebok, MA) placed between knees, which are in a flexed position. Players were then instructed to roll the ball to the left and right with the outside of their knee touching the ground.
- Lying on back with knees bent, players push through heels and raise lower back upwards although contracting gluteals. Players are also instructed to activate core region throughout movement.
Players were instructed to perform various self-selected mobility exercises and movements (Table 5 and Figure 4):
- Forward leg swings (players in pairs perform leg swings forward and backwards to increase range of movement through hamstrings and hip region).
- Sideways leg swings (players in pairs perform lateral leg swings with the aim of increasing mobility around the hip and groin region).
- Cat Stretch (on knees and hands, players lift head up and down although being instructed to arch back to increase movement through lumbar region).
- Forward lunge walks (players walk forward and drop into a lunge position, hold for 1–2 seconds and then alternate legs).
- Zig-zag runs (running forward in a zig-zag formation, cutting from left foot to right foot).
- Hip mobility hurdle stepovers (alternating legs forward and backwards over a 3-feet hurdle) (Barratt, United Kingdom).
Data are expressed as mean and ± SD values. Independent sample t-test was employed to examine the difference between intervention and control seasons (2008–2009 vs. 2009–2010). Pearson product moment correlation coefficient was used to examine the relationship between variables. The magnitude of the correlations was determined using the modified scale (28): trivial: r < 0.1; low: 0.1–0.3; moderate: 0.3–0.5; high: 0.5–0.7; very high: 0.7–0.9; nearly perfect > 0.9; and perfect: 1. Linear regression was used to estimate the number of muscle strains/tears by other measured variables. Significant level was defined as p ≤ 0.05.
Effect size was also used in the present study to provide information on the magnitude of treatment effect. There has recently been a proposed a scale for determining the magnitude of effect sizes in strength training research (46). In this classification, the researcher took the training status of the participants into consideration by separating them into 3 groups as follows: untrained (consistent training less than 1 year), recreationally trained (consistent training from 1 to 5 years), and highly trained (consistent training of more than 5 years) (46). Because the players in this study were professional players, the scale for “highly trained” was selected for interpretation as follows: trivial (effect size < 0.25), small (0.25–0.50), moderate (0.50–1.0), and large (>1.0).
In total, 103 matches were played across the 2 seasons (first season: 48 vs. second season: 55). Players were involved in 154.5 hrs of match play across the 2 seasons (firstt season: 72 hours vs. second season: 82.5 hours) with 4.36 ± 1.57 vs. 5.0 ± 2.19 matches played per month throughout the investigation. Results from the study highlighted a higher total number of injuries sustained within the intervention season (n = 88) when compared with the control season (n = 72); however, no levels of significance were found between them (p = 0.21). Further examination indicated that on average 8 ± 3.16 injuries per month occurred during the intervention season vs. control season 6.54 ± 3.69 per month, respectively. In addition to the previous findings, the study revealed (Figure 5) how number of injuries sustained during competitive match play was higher in the control season when compared with the intervention season (p < 0.001), but less training injuries were observed during the intervention season (25 vs. 26). During both seasons, muscle strains/tears were the most common injury sustained; however, significantly increased numbers were observed during the control season (Figure 6). During the intervention season, the number of muscle strain/tears was less (25% of total injuries) than the control season (52% of total injuries) (moderate effect), and this occurred concomitant with bigger squad size (large effect, p < 0.001).
There were trivial to small effects between the 2 seasons in the number of competitive games played, number of training session performed, and number of match injuries (Figure 5). Linear regression shown that number of training sessions performed, number of prevention sessions performed, and number of competitive games played were effective predictors of number of muscle strain/tear (R = 0.66, R2 = 43%, SEE = 1.82, p < 0.05).
The aim of the current study was to examine the effectiveness of a structured injury prevention program on the number of muscle injuries and the total number of injuries within elite professional soccer. The primary findings from the investigation concur with our hypothesis as the structured injury prevention intervention significantly reduced the number of muscle injuries with a reduction of 43% (large effect) when compared with the control season, even though there were slightly more competitive matches played during this intervention season. However, our second hypothesis, that total number of injuries would also be reduced was not shown.
The majority of injury prevention–training studies have generally examined the effects of individual components on injury incidence. However, this is not representative of a soccer-specific environment where the time constraints dealt necessitates the development of a mixed conditioning approach that allows for the simultaneous development of several fitness qualities. From a practical perspective, injury prevention programs are implemented with the expectation that they will elicit improvements in performance and reduce the incidence of injury; however, this is not always representative of research findings. Although direct comparisons should be avoided, the results of the present study do partially agree with those of Junge et al. (30), where the training intervention program elicited the greatest effect on mild injuries. However, the results of the present study demonstrate that the training program was ineffective at reducing the total number of injuries. In fact, somewhat surprisingly, there was a 9% increase in the total number of injuries when compared with the control season (88 vs. 72 injuries) and a greater number of ligament strains during the intervention season (∼17) than the control season (∼7). Although not directly comparable, the findings from van Beijsterveldt et al. (50) and those of the present study do possibly highlight the need to further understand the etiology of injuries and design injury prevention programs accordingly. The greater number of injuries within the intervention season, however, was largely due to the number (52.27%) of unavoidable contusion injuries (intervention season n = 44 vs. control season n = 23) when compared with the control season. These findings are in agreement with previous research (8,32) who reported how contusion injuries are extremely common and unavoidable in soccer due to the nature of the game. Nevertheless, such findings do raise issues regarding the implementation of new and/or different training methods, particularly for those players at the elite level, and should be a consideration for future training interventions and further investigation. It is not unreasonable to suggest that the greater training history associated with elite-level players may in itself prove problematic when implementing new and/or different training exercises. This seems relevant given that a training prevention program is more likely to elicit greater effects in low-skill than in high-skill teams (30) and the incidence of injuries, albeit youth soccer players, seems to be the lowest in those with the least amount of soccer exposure time (47).
Although often the objective, it is questionable whether some types of exercise-based injury prevention programs actually facilitate true learning of new biomechanical and neuromuscular characteristics (41,43). Whereas, the effects of short duration exercise-based injury prevention programs may induce transient changes in the performance of functional tasks that regress after cessation of the program. To experience biomechanical and/or neuromuscular changes, it is likely that extended duration training periods are necessary to facilitate long-term retention of movement control (41). Yet, performing injury prevention drills over a prolonged period, as done in the present study, may still be insufficient to induce large reductions in the incidence of specific injuries. Essentially, players may respond differently to the training intervention program and should be a consideration for further detailed investigation. Alternatively, despite the program being performed during the entirety of the season, it is possible that the session or individual exercises themselves may have been of insufficient duration required to elicit large training adaptations. This is pertinent to elite-level athletes who generally possess relatively high levels of general fitness and would unlikely experience gains similar to lesser skilled individuals despite being at a greater risk of injury (16).
In summary, the present study shows how performing a multicomponent injury prevention intervention twice weekly throughout the course of a training season can have a significantly positive effect on reducing muscle injuries (strains/tears) within the elite level of professional soccer. However, further research within this area is needed to confirm this. Interestingly, the total number of injuries was greater during the intervention season. Hence, the results from the present study highlight the possible benefits of injury prevention–training programs but also identify that such strategies may not be appropriate to combat different types of injuries. Doing so may require specific individualized training exercises that are relevant to player’s weaknesses or inabilities, particularly for those at the elite level.
Injury prevention–training strategies are a common feature in a practitioners training schedule. Despite a plethora of training exercises being available to use within a multicomponent training program, there seems little evidence-based guidance as to what exercises may be considered effective in actually reducing the incidence of injury. The present study reveals how a multicomponent injury prevention program within the elite level of professional soccer can have significant effect on the incidence of muscle strains and tears, which are regarded as the most commonly encountered injury in soccer across many levels. However, the injury prevention–training program may not be adequate in reducing the total number of injuries. Therefore, when implementing a multicomponent injury prevention–training program, practitioners should clearly justify what they intend to address during the chosen exercises and overall training session. This is an important issue as simply including certain exercises may unlikely reduce the different types of injury equally. Therefore, giving priority to a specific element (exercise, duration, intensity, load, etc.) within a multicomponent training program may be a more appropriate way to address players weakness, previous injury, or susceptibility to injury. Although undeniably a difficult task, by establishing what physical component may be more effective in reducing specific injuries, the practitioner may have greater scope to develop more time efficient and appropriate individualized training drills.
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