Soccer is a high-intensity intermittent contact sport that exposes elite-level players to continual physical, technical, tactical, psychological, and physiological demands (33,36). The stressors encountered during actual match-play have been suggested to show no detrimental effect of consecutive games' physical performance, but a greater injury risk (8,12). From a training perspective, cardiovascular and neuromuscular adaptations are suggested to be stimulated through a high training load (TL), induced through manipulation of intensity, duration, and frequency of training (10). However, if the intensity or volume is increased by an amount above the level at which various physiological systems can adapt, injury may result (26). Therefore, it is important to understand the optimal TL at which adaptation occurs without raising the risk of injury.
The implications of a high number of training days and matches lost because of injury is suggested to be to the detriment of team success (2), especially for soccer teams unable to replace players of similar abilities because of limited resources. Recently, Eirale et al. (16) even showed a clear relationship between teams' ranking and injury rate. Indeed, in the Qatari Professional league, it resulted that lower injury incidence rate was strongly correlated to team success over an entire season.
In soccer, high-intensity training has increasingly been advocated and used to elicit cardiovascular adaptations (11,30,36). Often, this type of training is reported to require players working at high to very high intensities, indicated through high heart rate responses (>85% maximal heart rate [HRmax]) (36). Within the literature, mean match HRs of ∼85% HRmax have been reported; however, these are averaged values that include numerous, recovering low-intensity bursts that follow extremely high-intensity efforts.
At the elite level, the accumulative high intensity nature of soccer match-play combined with multiple high-intensity sessions may cause excessive bodily strain (3,34). The continual strain over a prolonged period of time may lead to performance decrements and increase the risk of injury (5,35). Injury incidence in soccer is high with approximately 20–35 injuries occurring per 1,000 hours of match exposure (14,17) and injuries during training sessions ranging between 5.8 and 7.6 per 1,000 hours (2,38). Some authors report that a greater number of overuse injuries occur in the preseason training period (28,29), because of a greater intensity and volume of training causing a residual fatigue that may be attributed with an increased injury risk. It may be important to consider whether training intensity relates to injury incidence, to educate coaches or other sport professionals involved within the physical development of players. Program or session design must ensure that TLs = volume × intensity are not significantly exceeded, eliciting accumulative fatigue responses and predisposing players to overuse injuries.
Gabbett and Dumrow (25) found that TL increased the odds of sustaining an injury; however, this particular study involved rugby players, and injury exposure was estimated based on average training duration rather than calculated per player. Furthermore, ratings of perceived exertion (RPE) scores were used as a measure of intensity (TL = RPE × training duration). Although popular owing to its ease of use (1,6), this method depends on the personal perception of physical effort a period after (6,26). Therefore, although it does provide a valid estimate of total session mean intensity, it does not provide data about the periods spent at various training intensities (31). Heart rate training zones have been used as an alternative and objective measure of estimating training intensity (6,15,23,36), with HR being reported as a valid and reliable indicator of exercise intensity within soccer training (30,36). Currently, there are no studies investigating the relationship between injury incidence and high intensity quantified through HR within elite-level professional soccer.
Because of the limited amount of research within this area, the purpose of the investigation was to examine whether a relationship exists between training volume and intensity on injury incidence and also the odds of sustaining an injury in both training sessions, and competitive matches. Many publications have highlighted specific audits of injury types at many levels of the game; however, no literature exists regarding the potential effect of TL on injury rates. It was hypothesized that a greater time spent in the very high-intensity HR zones would increase the odds of injury, and therefore be associated with a higher injury incidence.
Experimental Approach to the Problem
This prospective, cohort surveillance study was carried out over 2 competitive soccer seasons. To examine if a greater time spent at 85–<90% (time spent in high-intensity zone [T-HI]) and ≥90% (time spent in high-intensity zone [T-VHI]) of HRmax is associated with a higher injury incidence, the relationship between training intensity and injury incidence was determined. Secondary outcome measures of the study included injury severity, type, and frequency. Furthermore, odds ratios (ORs) were determined to examine if a higher individual TL would increase the odds of injury. Injury incidence was presented as the number of injuries per 1,000 hours of exposure, with exposure recorded for each player rather than being estimated for the group (27). Because relationships have been previously reported between injury incidence and players' age, body composition, maximal oxygen consumption (V[Combining Dot Above]O2max), and vertical jump height (4,17,18) these variables were measured in this study to examine their impact on injury.
Twenty-three elite, male professional soccer players participated in this investigation. At the initiation of the study, players involved had a mean ± SD age of 26.8 ± 4.6 (range: 18–38) years, stature of 181.8 ± 6.8 (1.70–1.92) cm, and body mass of 79.3 ± 8.1 (62.5–93.6) kg. Percentage body fat for the 23 players was 10.1 ± 2.7 (5.1–16.3) %, and mean V[Combining Dot Above]O2max was 53.7 ± 4.3 (52.1–68.6) ml·kg−1·min−1. All participants had been playing soccer for 8 years or more, and all but 3 of them were competing at the international level. Participants were informed that they were free to withdraw from the study at any time. Procedures followed were in accordance with the Helsinki Declaration of 1975, approved by the Ethical Committee of the collaborating University and followed the standards of the Sport Science and Medical Department of the researching soccer club.
Measure of Injury
The injury definitions and method used for analysis of each injury followed the guidelines recommended by the International Soccer Injury Consensus Group (22,27). In this regard, an injury was defined as any physical complaint sustained by the soccer player either in training or in competition, which prevented the injured player from participating in competition or normal training for at least 1 day, but not including the day of the injury (22). This type of injury has been referred to as a time-loss injury (22). Injury incidence was categorized according to incidence per match (i.e., the number of match injuries in relation to the time spent in matches), and incidence per training (the number of injuries during time spent in training), and total injury incidence (sum of training and match injuries in relation to overall training and match exposure) (22,27). The severity of each injury was defined by the time lost from usual training or competition and was categorized in the following way: slight as 1–3 days, minor as 4–7 days, moderate as 8–21 days, and major as >21 days (28). Injuries were classified according to whether they were overuse or traumatic (22). Other information recorded about the injury included the nature of the injury (sprain, fracture, etc.), the location (body part), the date, and whether or not the injury was a preceded by a previous one (recurrent), the latter being defined as an injury that had occurred previously at the same location and of the same nature (22). All injuries were diagnosed and recorded by the club's medical staff. Injuries were recorded throughout both seasons. Illness was not taken into account within this study.
A laboratory-based, maximal incremental running treadmill test (Run 500 model; Technogym, Cesena, Italy) was conducted once during preseason before the commencement of the training period to determine maximal oxygen uptake (V[Combining Dot Above]O2max) and HRmax to determine individual HR training zones. Players followed the V[Combining Dot Above]O2 max running protocol of Hoff et al. (30), and a precalibrated breath-by-breath metabolic system was used (Medical Graphics Cardiopulmonary Exercise System, CPX/D; Medgraphics Corp., St. Paul, MN, USA). Individual HRmax and V[Combining Dot Above]O2max were derived by using the mean of the 2 highest 15-second averages achieved during the final stage of the V[Combining Dot Above]O2max test. A true HRmax and V[Combining Dot Above]O2max were considered to have been achieved, if both variables failed to increase despite an increase in exercise intensity (37). The protocol is commonly used for testing endurance performance in professional football players (26) and involved participants running on a treadmill at a 3° incline with speed increases of 1 km·h−1 every minute until exhaustion. Before the protocol test, players performed a 3-minute warm-up eliciting an intensity of around 70% HRmax in addition to self-selected stretching exercises.
Heart rate was recorded and assessed at 5-second intervals by portable HR monitors (Polar Team System 2; Polar Electro OY, Kempele, Finland) throughout the training sessions for the duration of the investigation (36). Goalkeepers were not included within the data group. After each testing session, HR data was downloaded to a computer using dedicated software (Polar Precision S-Series Software SW 3.0; Polar Electro OY) and stored for analysis. The mean and %HRmax achieved during each game was calculated for each player, with each player's total time spent in specific HR zones following methods used in previous studies (25,36): ≤50%; 50–<60%; 60–<70%; 70–<85%; 85–<90% HRmax (T-HI) and ≥90%HRmax (T-VHI). However, for the purpose of this particular study, the time spent within the higher intensity zones, T-HI and T-VHI, has been assessed and reported to differentiate between “training intensity” (11). These 2 HR zones were chosen because previous and current research investigating elite-level soccer has reported how HR >85% is key when discussing training adaptations (36).
Before analysis, injury incidence and HR data were explored (and confirmed) for normality and for equality of variances. Data are expressed as mean ± SD, percentages, and 95% confidence intervals (CIs), where relevant. Statistical significance was set at p ≤ 0.05.
Pearson's correlations were used to examine the relationships between TL and injury (incidence, severity, type, and frequency), as well as between injury incidence and physiological/anthropometrical data. The magnitude of the correlations was determined using the modified scale by Hopkins (2000): 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.
A stepwise, multiple linear regression analysis was used to predict injury incidence; variables having a higher correlation coefficient than r = 0.50 (and a significant relationship) were included in the analysis. The adjusted R2 was used to assess the proportion of the variance explained by the independent variables.
Odds ratios were used to examine whether the TL increased or decreased the odds of injury. Odds ratios were derived by tallying the frequency of injury on a monthly basis, because training was organized into mesocycles (4 weeks). Training load per mesocyle was categorized according to whether it was considered to be a “high TL” or a “low TL,” by using a median split of the data. Odds of TL increasing the frequency of match injuries, training injuries, traumatic and overuse injuries, and total injuries, and of increasing the frequencies of injury severity, were examined.
A χ2 test was used to determine whether the observed injury frequency differed from the expected injury frequency. Expected injuries were calculated as the same proportion of the total injuries as the mesocycle TL score was of the total TL score, following the method of Gabbett (23).
The TL within the previous mesocycle before the injury being sustained was determined and assessed to provide an accurate picture of the relationship between TL and injury. This was due to anticipation that TLs would be lower in the month when an injury was sustained because of reduced training availability. An independent samples t-test was then used to examine whether TL, training exposure, and match exposure differed significantly in the mesocycle before injury, compared with the mesocycles when injury did not occur.
Over the 2 seasons, the players were exposed to a total of 1,704.4 hours of match-play, and 5350.0 hours of training, which equated to an average of 4.8 ± 3.8 hours of training time per match hour. The team played 116 matches, 54 in season 1 and 62 in season 2, with the higher number in season 2 because of UEFA Champions League fixtures. There were a total of 130 injuries recorded over the 2 seasons. Type and site of injuries (for both match-play and training) are given in Tables 1 and 2, respectively.
Injuries and illnesses sustained outside of soccer hours were excluded from further analysis regarding injury incidence, injury type, cause, site, and severity, leaving a total of 119 soccer-related injuries. Recurrent injuries were also excluded when describing injury type, cause, and severity, so as not to falsely elevate these values. Of the remaining 119 injuries, 87 were match injuries and 32 were training injuries. Thirty-nine of the 87 match injuries (44.8%) were overuse, and 48 (55.2%) were traumatic. Seventeen of the 32 training injuries (53.1%) were overuse, and 15 (46.8%) were traumatic. Total injury incidence was 18.8 (95% CI, 14.7–22.9) injuries per 1,000 hours of exposure. Training injury incidence was 6.7 injuries per 1,000 hours of training exposure (95% CI, 3.7–9.6), and match injury incidence was of 54.1 (95% CI, 39.7–68.6). Of the match injuries, 9.2% (n = 8) were slight, 35.6% (n = 31) were minor, 33.3% (n = 29) were moderate, and 21.8% (n = 19) were major. Of the training injuries, 28.1% (n = 9) were slight, 21.9% (n = 7) were minor, 28.1% (n = 9) were moderate, and 21.9% (n = 7) were major.
Data for injury frequency and training intensity are given in Figure 1. There was a significant correlation between total injury incidence and training intensity (T-HI: r = 0.57, p = 0.005; T-VHI: r = 0.568, p = 0.005). There was also a significant correlation between training injury incidence and training intensity, but only for T-HI (r = 0.48, p = 0.02). Correlations were low between match injury incidence and training intensity (T-HI: r = 0.09, p = 0.69; T-VHI: r = 0.19, p = 0.38). Correlations were significant for number of days off because of injury (an indication of injury severity) and training intensity (r = 0.51, p = 0.01 for T-HI, and r = 0.47, p = 0.02 for T-VHI). There was a significant correlation between training intensity and total number of traumatic injuries (r = 0.42, p = 0.04 for T-HI, and r = 0.44, p = 0.03 for T-VHI).
A significant negative correlation was observed between injury incidence and percentage body fat (r = −0.43, p = 0.04), but correlations between injury incidence and all other anthropometrical/physiological variables were low and nonsignificant.
A forward, stepwise linear regression, with T-HI and T-VHI in the model, gave an adjusted R2 of 0.28, p = 0.014 for injury incidence; hence, training intensity explained 28% of the variance in injury incidence.
The ORs of sustaining an injury because of training intensity are given in Table 3. Only one of these values was significant, with a greater time spent in T-HI resulting in a greater odds of sustaining a match injury (χ2 = 7.22, p = 0.059).
There was a significant difference between the observed total injury frequency and the expected injury, as determined as a proportion of training intensity (mean T-HI, χ2 = 33.2, p = 0.04; mean T-VHI, χ2 = 33.5, p = 0.04). Differences were also significant when separately analyzing training injuries (for T-HI, χ2 = 38.0, p = 0.01; and T-VHI, χ2 = 36.7, p = 0.02) and match injuries (for T85, χ2 = 48.4, p < 0.001; and T90, χ2 = 48.3, p = 0.001).
Mean differences (including significance) in time spent in each intensity zone in the month preceding an injury and in the mesocycle when an injury did not occur are given in Figure 2.
The main purpose of this study was to examine whether training intensity, as assessed using T-HI and T-VHI, increased the odds of sustaining an injury. Findings from this study have revealed that individual training intensity and load were highly related to total injury incidence (r = 0.57, p = 0.005). Training intensity (T-HI and T-VHI) explained 28% of the variance in injury incidence. Odds ratios for training intensity and injury incidence were negligible with no discernible pattern apparent. For instance, an increased proportion of time spent at 85–<90% HRmax significantly increased the odds of sustaining a match injury but did not increase the odds of sustaining a training injury (Table 2).
Accumulative fatigue derived from high intensity training and competitive match-play may have played a fundamental role because of its residual effect in explaining the higher ORs for match injury incidence. In addition, the significant increase in match contusion injuries (n = 28) vs. training contusion injuries (n = 9) as a result of contact with competitors during match-play should be highlighted as a key factor of increased odds. Using χ2 analysis, the observed total injury frequency and the observed training injury frequency were significantly different (p ≤ 0.05) from the number of injuries that were expected to occur based on training intensity, which suggests that training intensity and injury frequency were not associated. It seems, therefore, that TL approach, assessed using specific HR zones, has only a moderate effect on injury incidence and does not increase the odds of injury. This finding is contrary to what was reported by Gabbett and Dumrow (25), who found that high TL increased the odds of injury in rugby players. The discrepancy in relation to this study may be explained by how Gabbett and Dumrow (25) estimated training intensity through RPE as opposed to HR zones used in this study. It should also be noted that the previous study (25) defined exposure as the number of players multiplied by session duration to give an average exposure, as opposed to individual exposure data used in this study. Similar to this study, Killen et al. (32) found no relationship between TL (as assessed using RPE) and training injury incidence in rugby players. Their suggestion was that the high-caliber nature of the athletes was protective against injury, which may also account for the current findings. However, significant relationships have been found between individual session–RPE and HR-based TL, therefore strengthening the use of HR as a valid method of assessing TL in sports.
It may be suggested that injury results from an accumulation of TL. For these reasons, an attempt was made to analyze whether differences in TL occurred before the injury being sustained, through consideration of the TL in the preceding period. On the occurrence of injuries, duration sustained within T-VHI was significantly greater in the preceding mesocycle when compared with the duration accumulated in T-VHI if an injury had not occurred (Figure 2). Although training intensity was related to injury incidence but not increased odds of training injuries, the accumulation of training at high intensity combined with match-play may affect injury incidence; however, further research is needed to confirm this. When analyzing injuries, both training intensity and volume over time should be considered, and coaches should try to ensure that excessive durations' training at these high levels are avoided through appropriate periodization.
On a month-by-month basis, using χ2-analysis, the frequency of training injuries reflected training exposure (Figure 3); to note, months when training duration was greater, such as in the preseason period, injury frequency was high. The frequency of training injuries did not, however, reflect the training intensity. Therefore, it could be suggested that when training intensity is ≥85% HRmax, injury does not necessarily result, but when exposure to training is prolonged, more injuries maybe more prevalent. In agreement with the present findings, other researchers have found a similar relationship between exposure and injury frequency (14,17,18). In this study, the players did not train for long periods of time in comparison to that reported by others (13). Indeed, training was often of a high intensity but short duration, using a predominance of soccer-specific intermittent training. This approach to training might explain the high relationship found between injury incidence and training intensity but further specific investigations are warranted.
The low injury incidence for training (6.7 injuries per 1,000 hours of training exposure) was comparable with that previously reported (13), although match injury incidence (54.1 injuries per 1,000 hours of match exposure) was higher (13), although this could be a direct link to the increased number of games played. Match injury incidence has been found to be higher in certain circumstances. For instance, Dupont et al. (12) reported a match injury incidence of 97.7 per 1,000 hours of match exposure when players played 2 matches a week. Dvorak et al. (14) reported a match injury incidence of 81 per 1,000 hours of match exposure in the 2002 FIFA World Cup.
The site, type, and severity of injuries reported within this investigation were within the ranges reported previously among other professional soccer players. Previous literature purports how a greater proportion of injuries occurred to the lower extremities (Table 1) (20,27–29,36). Results from this study concur with previous findings as a greater number of injuries, both during training and match-play, consisted of muscle strains, contusions, and ligament sprains (Table 2) (17,36). Therefore, the present sample of injuries in the studied team is representative of usual soccer injuries, and the conclusions of the study are more likely to be interpreted as providing knowledge on actual soccer.
Relationships between injury incidence and anthropometrical and physiological variables were weak in this study, as also reported by others (18,24,35), possibly explained by the homogenous nature of the players. Considering that training intensity did seem to affect injury incidence, interestingly anthropometrical and physiological variables did not relate to injury incidence, other intrinsic risk factors, such as joint instability, functional skill, psychology (34) and other extrinsic factors such as playing surface and foul play (34,35) may have contributed to injury. Based on this information, it can be confirmed that the cause of injury is multifactorial, with this study highlighting training intensity being a significant factor.
Participants within the study were elite-level footballers, and unique with respect to comparisons of previous research in the area of injury incidence and training intensity (19,25). Killen et al. (32) described that it is difficult to compare results obtained from semiprofessional/amateur players with those from professional players, therefore suggested research on professional players was required. Amateur players generally have lower cardiovascular endurance capability, strength, and skill base, which may predispose them to a greater injury risk (25). To date, this study is the only one having investigated the relationship between HR intensity and injury incidence in professional soccer players. As a result, the practical implication that injury incidence is highly related to high-intensity training is important considering the professional nature of the players, and the individual auditing. Continual bodily strain eliciting greater training durations at T-VHI levels during training may increase the odds of sustaining a match injury, but not the odds of sustaining a training injury.
In professional soccer, training is generally comprised of a variation of small-, medium-, and large-sided games alongside high-intensity intermittent bouts used as a time efficient and effective means of enhancing cardiovascular fitness (9,30,36). Such training methods may impose more stress on the body than more traditional training methods, with heart rates of >85% often being elicited (9,36). This is the first study to have examined the relationship between HR-based assessment of training intensity and injury incidence in soccer. Based on the data collected in this study, it is recommended that training intensity be considered as one of the many factors in injury prevention. Reducing the number of competitive match injuries among elite-level professional players may be possible if greater focus is placed on the training intensity and volume over a period of time, ensuring the potential reduction of fatigue or overuse injuries. In addition, it is important to understand the optimal TL at which adaptation occurs without raising the risk of injury. Therefore, monitoring training to ensure that optimal loading is not significantly exceeded, should be considered as vitally important at the elite level of professional soccer with respect to injury incidence.
The authors thank Prof. Anthony Stewart, who advised on statistical approaches, and critically reviewed the article for statistical accuracy; Mr. Jordan White, who assisted with initial data compilation; the soccer players for their participation in the study; and the sport science and medical staff.
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