Training Loads and Incidence of Injury During the Preseason in Professional Rugby League Players

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TRAINING LOADS AND INCIDENCE OF INJURY DURING THE PRESEASON IN PROFESSIONAL RUGBY LEAGUE PLAYERS NATASHA M. KILLEN,1 TIM J. GABBETT,1,2

AND

DAVID G. JENKINS1

1

School of Human Movement Studies, The University of Queensland, Brisbane, Australia; and 2Brisbane Broncos Rugby League Club, Brisbane, Australia

ABSTRACT Killen, NM, Gabbett, TJ, and Jenkins, DG. Training loads and incidence of injury during the preseason in professional rugby league players. J Strength Cond Res 24(8): 2079–2084, 2010—Research into rugby league has found a significant, positive relationship between training load and injury rates. However, there has been limited research investigating this relationship in the preseason period, and the relationship between training load, and injury among professional rugby league players is yet to be examined. The primary aim of this study was to examine the relationships between training load, various psychological data, and the incidence of injury during preseason training at a professional rugby league club. Thirtysix male professional rugby league players undertook a 14-week training program. Each player’s training time, intensity rating, and injury status were recorded after each training session. In addition, players rated their sleep, food, energy, mood, and stress on a scale of 1–10 (with 1 being extremely poor and 10 being excellent) biweekly. Over the entire preseason period, a total of 2,877.9 training hours were recorded for the players, with an overall incidence of injury of 6.9 per 1,000 training hours. Higher training loads during the first half of the preseason corresponded to a higher injury rate in comparison to the second half of the preseason. No significant relationship was found between the preseason weekly injury rate and the weekly load, nor was there a relationship between injury and psychological data. These findings suggest no relationship between training load, psychological data, and injury incidence during the preseason training period in professional rugby league players. However, results suggest that players may have an increased risk of injury during the early preseason period. The findings of this study may be particularly useful in professional rugby league teams to determine when a player

Address correspondence to Natasha Killen, [email protected]. 24(8)/2079–2084 Journal of Strength and Conditioning Research Ó 2010 National Strength and Conditioning Association

is at increased risk of injury, using their training loads and psychological data.

KEY WORDS team sport, football, collision sport, training, performance

INTRODUCTION

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ugby league football is a physically demanding, full-contact, team sport in which players are required to compete in a challenging contest involving frequent bouts of high-intensity activities such as running, passing, tackling, and kicking (20). The game’s highly demanding nature and frequent physical collisions, combined with the minimal protective equipment, mean injuries are inevitable. Prevention of injuries in professional rugby league is important; injuries negatively impact not only the individual but also the entire team and its success (22). Research into rugby league has found a significant, positive relationship between training and match load and injury rates (8–10). Gabbett (9) studied the incidence, site, severity, and cause of training and match injuries in semiprofessional rugby league players over a playing season and demonstrated a significant relationship (p , 0.05) between training injuries and the training load (r = 0.86) along with match injury incidence and match load (r = 0.86) (9). Research involving rugby league has widely acknowledged that as the intensity, duration and load of a training session or match increases, so too does the incidence of injury (8). Reynolds et al. (21) examined the incidence of injury in women undergoing a rigorous 24-week training program and reported a relationship between the injury incidence and training load. They also reported that a well-designed, periodized training program can elicit significant improvements in performance with a low incidence of injury. However, there has been limited research investigating this relationship in the preseason period and research is yet to examine the relationship between training load and injury rate among professional rugby league players. Preseason training for rugby league players in Australia runs from early December until the beginning of March. Training intensities and durations during this period are high as the VOLUME 24 | NUMBER 8 | AUGUST 2010 |

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Training Load and Injury players undergo rigorous conditioning to raise their fitness, skill, and strength in advance of the upcoming season (10). Injury rates in semiprofessional players have been shown to increase gradually throughout the preseason from December to March, followed by a decline through to the end of the season. Indeed, the preseason incidence of injury at the end of February (205.6 per 1,000 training hours) was much higher than at the beginning of the preseason (105.2 per 1,000 training hours) (9). In addition to the effects of training duration, intensity, and load on injury occurrence, the contribution of psychological factors on injury has been examined in several studies (6,19,23). Lavalle´e and Flint (19) investigated the contribution of anxiety, mood state, and social support to injury in athletic populations and found that high tension and anxiety levels were significantly related to the injury incidence. High life stress has also shown be an independent and significant (odds ratio = 1.84; 95% CI 1.10–3.11) predictor of sporting injuries (23). These findings were supported by Dvorak et al. (4) who examined this relationship in 264 football players over the course of a year. Psychological factors, training intensity, duration, and load are well-established, significant predictors of injury. The particularly high preseason injury rates warrant research. If those factors that influence the incidence of injury during the preseason training period in professional rugby league players can be identified, coaches may be able to modify them. The primary aim of the present study was to examine the relationship between training load and incidence of injury during a preseason training period at a professional rugby league club. A secondary aim was to investigate the relationship between the players’ physical and psychological status and their preseason injury rates.

METHODS Experimental Approach to the Problem

The present study used a prospective experimental design to identify the relationship between training load, physical and psychological status, and injury incidence in professional rugby league players. The club physiotherapist assessed all injuries, and these were expressed relative to exposure hours. The injury definition (8) and experimental design (13) employed in this study were identical to other rugby league studies. It was hypothesized that a significant relationship would be detected between training load, physical and psychological status, and injury incidence. Subjects

A squad of 36 National Rugby League (NRL) players were involved in this study. Participants were professional rugby league players (i.e., participants generated their entire income from their involvement in rugby league) (7), and their ages ranged from 17 to 32 years. Participation in the study formed part of the players’ routine training commitments. At the time of the study, players had completed a 6-week active

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off-season and returned to preseason training with an average maximal oxygen consumption of 53.8 6 0.6 ml
kg21
min21. The average NRL playing experience of the participants was 55.7 6 11.3 games. All participants received a clear explanation of the study, and written consent was obtained. The Institutional Review Board for Human Investigation approved all experimental procedures. Preseason training for the season was conducted over a period of 14 weeks, beginning in early December and finishing in mid-March. During this time, players participated in 6–9 training sessions every week, except for 11 days over the Christmas and New Year period. The duration of training sessions ranged from 25 to 105 minutes, and the number of players at each session varied from 11 to the full squad of 36. Sessions involved general conditioning, specific speed, agility, and skills training along with upper and lower body strength sessions in the gymnasium. Not all players were able to attend all sessions for various reasons such as illness, injury, or other personal or professional commitments. Procedures

Injury was defined as ‘‘any pain or disability that occurred during participation in a rugby league training activity that was sustained by a player, irrespective of the need for training time loss (18).’’ A modified rating of perceived exertion (RPE) scale was used to estimate exercise intensity (1). The scale was explained to the players on multiple occasions, and players were asked for their RPE within 10 minutes of completing each training session. The training load from each session was then calculated by multiplying the RPE training intensity and the duration of the session. Rating of perceived exertion has previously been shown to be an acceptable tool for estimating the intensity of training sessions, is effective for extended aerobic exercise sessions (6), and can be reliably used for resistance training sessions (2). When compared to heart rate and blood lactate concentration, the RPE scale has been shown to provide a valid estimate of exercise intensity (3,5,12,16). In addition, before commencing the study, we investigated the relationship between heart rate and RPE, and blood lactate concentration and RPE on a subset of subjects during typical rugby league training activities. The correlations between training heart rate and training RPE, and training blood lactate concentration and training RPE were 0.89 and 0.86, respectively. A subset of players (n = 11) also completed 2 identical preseason training sessions, performed 1 week apart, before the commencement of the study, to determine test–retest reliability. The intraclass correlation coefficient for test–retest reliability and typical error of measurement for the RPE scale were 0.99 and 4.0%, respectively. Collectively, these results demonstrate that the RPE scale offers an acceptable method of quantifying training intensity for collision sport athletes. Psychological data were collected on the players’ perceptions relating to sleep, food, energy, mood, and stress. Physical data included players’ perceptions of how their body was

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Journal of Strength and Conditioning Research feeling physically. Each player rated how they felt in each category on a scale of 1–10 (with 1 being extremely poor and 10 being excellent), and these ratings were recorded immediately before 2 training sessions each week. An average weekly figure for the entire team was calculated for each category throughout the preseason. The intraclass correlation coefficient for test–retest reliability and typical error of measurement for the psychological data were 0.95 and 1.2%, respectively. Statistical Analyses

Data were analyzed using SPSS 15. Normality of distribution for each measure was tested using the Kolmogorov–Smirnov test and analysis included standard descriptive statistics, paired t-tests, Spearman and Pearson correlations, and 1-way analysis of variance. Injury rates per 1,000 training hours were calculated by dividing the total number of injuries by the exposure hours and multiplying this by 1,000. The chi-squared (x2) test was used to determine whether the observed injury frequency was significantly different from the expected injury frequency. Training monotony was calculated by multiplying the weekly training load and the SD of the weekly training load; training strain was calculated as the product of total weekly training load and training monotony. Based on an alpha level of 0.05 and a sample size of 36, our beta level (power) was $0.80 for detecting correlations of 0.85 or greater among injury, training load, and psychological data. All results are reported as means and SDs or medians with interquartile ranges.

RESULTS Incidence of Injury

A total of 2,877.9 training hours were recorded for the players over the entire preseason training period, with an average of 221.4 6 44.3 exposure hours completed by the training group each week. A total of 20 injuries were recorded during the preseason training period with an overall incidence of injury of 6.9 (95% CI: 3.7–10.1) per 1,000 training hours.

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TABLE 1. Site of injury during the preseason period in professional rugby league players.*† Site of injury Thigh and calf Ankle/foot Knee Thorax/abdomen Hip Shoulder Elbow Shin Head/neck Face

Injury number

Injury rate (95% CI)

%

7 3 3 2 2 1 1 1

2.4 (0.6–4.2) 1.0 (0.0–2.2) 1.0 (0.0–2.2) 0.7 (0.0–1.7) 0.7 (0.0–1.7) 0.3 (0.0–1.0) 0.3 (0.0–1.0) 0.3 (0.0–1.0)

35 15 15 10 10 5 5 5

*CI = confidence intervals. †Injury rate expressed per 1,000 training hours.

TABLE 2. Type of injury during the preseason period in professional rugby league players.*† Injury type

Injury number

Injury rate

%

Inflammation Sprains/strains Degenerative Overuse Hematoma Contusions Other

5 5 4 1 1 1 3

1.7 (0.21–3.3) 1.7 (0.21–3.3) 1.4 (0.03–2.8) 0.3 (0.0–1.0) 0.3 (0.0–1.0) 0.3 (0.0–1.0) 1.0 (0.0–2.2)

25 25 20 5 5 5 15

*CI, confidence intervals. †Injury rate expressed per 1,000 training hours.

Site of Injury

Lower body training injuries were most common (5.6 per 1,000, 80%), compared to injuries to the trunk (0.7 per 1,000, 10%) and upper limbs (0.7 per 1,000, 10%). The most common training injury was sustained to the thigh and calf (2.4 per 1,000, 35%). Injuries to the ankle (1.0 per 1,000, 15%), knee (1.0 per 1,000, 15%), back (0.7 per 1,000, 10%), hip (0.7 per 1,000, 10%), shoulder (0.3 per 1,000, 5%), elbow (0.3 per 1,000, 5%), and shin (0.3 per 1,000, 5%) were less common (Table 1). Type of Injury

Inflammation injuries and sprains and strains were the most common types of training injury (1.7 per 1,000, 25%). The incidences of degenerative injuries (1.4 per 1,000, 20%), overuse injuries, hematomas, and contusions (0.3 per 1,000, 5%) were low (Table 2).

TABLE 3. Average training load, monotony, and strain, and psychological data during the preseason period for professional rugby league players.* Training variable Training load Training monotony Training strain Exposure hours Total psychological data†

2,788.6 6 913.6 3.1 6 0.6 8,729.7 6 3,710.1 221.4 6 44.3 38.7 (10)

*Data are mean 6 SD. †Nonparametric data, expressed as median and interquartile range.

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Training Load and Injury preseason (2,169.4 6 597.7 arbitrary units) (F = 15.212, p = 0.002). The higher training loads during the first half of the preseason corresponded to a higher (x 2 = 2.3, df = 1, p . 0.05) injury rate (8.7 6 3.4 per 1,000 training hours) in comparison to the second half of the preseason (5.3 6 6.3 per 1,000 training hours).

DISCUSSION This study is the first to examine the relationship between training load and injury incidence over the preseason period in professional rugby league playFigure 1. Preseason weekly training loads and injury rates for professional rugby league players. ers. This relationship is of great interest to many involved in rugby league, particularly coachSeverity of Injury ing and conditioning staff, whose aim is to find a training The majority (6.6 per 1,000, 60%) of training injuries were program that will elicit an improvement in playing perfortransient, resulting in no loss in training and requiring no mance and physical fitness without increasing the incidence modification to the training program. Only 5% of the total of injury (9). injuries resulted in the player needing more than 2 weeks to In contrast to much of the related research (8,9), the present recover and resume normal training. study found no significant relationship between training load and the incidence of training injuries during the 14-week Relationship between Training Load, Physical and preseason period. In addition, the majority of injuries Psychological Status, and Injury Incidence sustained were only minor, resulting in no loss in training There was no significant relationship between players’ time and no necessary modifications to the training program. training loads and psychological data (r = 0.248, p = 0.414) Analysis did identify a trend toward higher injury rates with or the total physical and psychological status and training greater psychological scores (rho = 0.501, p = 0.081). This load (r = 0.216, p = 0.478). Additionally, there was no may suggest that when players feel healthier, they can train at significant relationship between the preseason weekly injury higher intensities, which may increase the incidence of injury. rate and the weekly training load (r = 0.023, p = 0.941), Weekly training loads were higher during the 14-week training monotony (r = 0.323, p = 0.281), training strain (r = preseason training period compared to the competition phase 0.088, p = 0.776), and total psychological data (rho = 0.501, of the season. These findings are consistent with the work of p = 0.081). The weekly preseason team data are shown Gabbett (9,10) who also reported higher training intensities in Table 3. and durations during the preseason period. The relationship between training load and injury rate is The weekly training loads of 2,809 arbitrary units are shown in Figure 1. The average weekly preseason training relatively low for professional athletes. In addition, an injury load of positional playing groups was compared to ensure rate of only 6.9 per 1,000 training hours was recorded, with that there was no difference among the subgroups. The the majority of injuries being transient in nature. These ÔadjustablesÕ playing group included the halfback, hooker, findings may indicate that training loads were adequate and fullback positions, and the Ôoutside backsÕ included the to improve fitness without unduly increasing the incidence of wing and center positions. The training load for the forwards injury. was 2,665.4 6 926.5 arbitrary units, compared with the The majority of injuries sustained in this study were to the adjustables 2,890.2 6 954.3 arbitrary units and the outside lower body, which is consistent with results from previous backs 2,809.4 6 942.3 arbitrary units. There was no studies (11,15,17). Previous investigations have suggested that significant difference during the preseason in the training most injuries are received by the ball carrier while being loads among the positional playing groups (F = 0.190, p = tackled (14). Most coaches will instruct players to aim tackles 0.827). The average weekly training loads for the playing around the hips or thighs of the attacking players (24), and group were significantly greater during the early preseason this makes this area more prone to injury. Additionally, when (3,510.9 6 641.9 arbitrary units), compared to the late tackles are aimed at the upper body, the arms and shoulders

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Journal of Strength and Conditioning Research can be used to defend and protect, whereas the legs are more exposed to heavy contact. Several factors distinguish this study from the majority of others that have investigated the relationship between training load and injury rates. None of the previous studies have examined the relationship between training load and injury rate in the preseason period among professional athletes; all previous studies have examined the training–injury relationship in amateur or semiprofessional rugby league players. With vastly different training loads, injury rates, support staff, and training programs, it would be difficult to compare ÔamateurÕ players (i.e., those who do not receive match payments) and ÔsemiprofessionalÕ players (those who receive moderate remuneration to play) to professional rugby league players, who generate their entire income from their involvement in rugby league (7). Firstly, professional players would be expected to have a higher base level of fitness entering the preseason, compared to amateur and semiprofessional players. Higher fitness levels would enable them to exercise at higher intensities and for longer periods before fatigue. It has also been shown that well-developed maximal aerobic power offers a protective effect against injuries in rugby league players (11). One would also expect professional rugby league players to be more skilfull, which would potentially help them avoid certain situations or positions that could cause injuries. Professional sporting clubs also have a far more thorough injury prevention program than amateur and semiprofessional teams, with a greater emphasis on preventive strategies including flexibility and stretching, appropriate warm-ups and cool-downs. Dietitians are often available to assist in recovery from exercise, and physiotherapists address musculoskeletal problems. In summary, the present study found no relationship between training load and injury rates in a cohort of professional rugby league players during a 14 week preseason training period. However a trend toward greater injury rates with higher psychological scores was identified.

PRACTICAL APPLICATIONS Monitoring training loads is critical to ensure that players receive a progressively overloaded periodized training program and are given adequate recovery between highvolume and high-intensity sessions. It is important for sport scientists and strength and conditioning coaches to determine the appropriate training loads and recovery periods to maximize improvements without unduly increasing injury incidence. The increased injury rate, coupled with the higher training loads in the early preseason period, suggests that professional, male rugby league players returning from the off-season period may be at greater risk of training load-related injuries. Gradual increases in training loads during this period, and ensuring players return to training with a minimum standard of physical fitness, may reduce the incidence of injury in this training period. Of interest was the ÔspikeÕ in injury rates

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toward the end of the preseason period, when training loads were lowest. Although these results are difficult to reconcile, it is possible that increased fitness may increase training intensity and subsequently increase injury rates (9). Although the present study has found no significant relationship between training loads and injury rates, there was a trend toward a higher injury incidence with higher psychological data scores. Psychological data may therefore be useful in determining when a player is at increased risk of injury.

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