Body composition analysis is frequently carried out in professional sport to monitor acute changes in physiological status, using a range of techniques including skinfold analysis, body density and volume measurements, bioelectrical impedance methods, and dual-energy X-ray absorptiometry (DXA) (6), to assess individual levels of body mass (BM), fat mass (FM), lean tissue mass (LM), and bone mineral content (BMC).
Professional rugby league players require high muscular power and strength (22) and consequently the LM to FM ratio that will provide optimal performance is often targeted and measured at regular intervals throughout the training and competition period. Because of the highly intermittent nature of rugby league, it is difficult to describe an ‘optimal’ body composition that encompasses all elite players, because of the inherent individual differences that exist between players and teams, and somatotopic requirements for the different playing positions (11). To provide more valuable information, the body composition of rugby league players can be monitored throughout a competition cycle to create an anthropometric profile of each individual player that can be monitored throughout the season. This can also help avoid any changes in body composition that may be detrimental to performance, and provide reference body composition values that can be targeted after a period of detraining or injury, and act as an indicator of physiological status and training adaptations.
The seasonal changes in anthropometric profile have previously been considered in both amateur (10) and junior (26) rugby league players using skinfold analysis. Using DXA, Egan et al. (5) assessed the seasonal variations in body composition of elite soccer players using 5 testing phases over 2 seasons but were not able to collect data at the preseason, midseason, and end-of-season phases because of the international playing commitments of players postseason. This study, in accordance with Gabbett (10) and Holymard and Hazeldine (15), used testing phases that encompassed an entire competitive season, to build a detailed picture of the anthropometric changes that occur as a result of playing and training status. Measurements were taken at the preseason, midseason, and postseason phases.
The physiological and anthropometric characteristics of rugby league players have previously been reported with junior (9), amateur (7), subelite (12), semiprofessional (8), and professional (22) rugby league players, but seasonal changes in anthropometric status have not previously been reported for any professional male players from the English Super League. Furthermore, studies have been limited in that the body fat percentage (BF%) of players has previously been measured using a 4-site skinfold (biceps, triceps, subscapular, and suprailiac) and the Durnin and Womersley (4) equation. Skinfold analysis, which is an example of a 2-compartment (2-C) model of body composition, has previously been shown to significantly overestimate BF% in elite rugby league players when using the Durnin and Womersley (4) 4-site and Jackson and Pollock (16) 7-site equation in comparison to DXA, a 3-compartment model (3-C) (14). Other 2-C models, such as the Jackson and Pollock (16) 3-site skinfold equation, when using either the Siri (29) or Brozek (2) equations to convert body density to BF%, have been shown to significantly underestimate BF% in this population (14). The proportions of FM and LM of the human body can be measured more accurately by increasing the level of compartments considered as part of the whole body from a 2-C to a 3-C model, using DXA.
Dual-energy X-ray absorptiometry uses a 3-compartment model (3-C) to assess body composition (BMC, LM, and FM). Dual-energy X-ray absorptiometry can provide highly precise measurements of soft-tissue composition (18) and can provide details of the body composition of various body segments. DXA has been reported to provide accurate body composition estimates when compared with the 4-compartment (4-C) model in young adults who vary in gender, race, athletic status, body size, musculoskeletal development, and body fatness (25). Conversely, the 4-C method can be very time consuming and expensive, and subsequently, DXA has rapidly gained acceptance as a reference method for body composition analysis (30). In addition to the accurate assessment of FM and LM, the use of DXA in this study enabled the quantification of BMC in elite rugby league players, and the changes that occur in BMC across a competitive season, which has not previously been reported in the literature.
It may be suggested that physiological performance capabilities should peak by the end of the preseason phase and be maintained throughout the entire competition period. However, previous studies have shown that fitness levels in rugby league deteriorate as a season progresses, perhaps because of increased playing commitments (10,26). Traditional periodization of strength training programs is difficult in many team sports because players are often required to play between 1 and 3 competitive matches per week. Therefore, maintenance of body composition throughout the competition phase can be a good indicator of physiological status. The aim of this study was to assess the extent to which body composition values are maintained throughout a competitive season, by the measurement of changes in BM, FM, LM, BF%, and BMC between the preseason, midseason, and postseason phases, in elite male rugby league players.
Experimental Approach to the Problem
A repeated-measures approach was used to assess the anthropometric profiles of the rugby league players in this study, with all athletes being tested on 3 separate occasions during the 2008 Super League season (Figure 1). The first testing phase (T1) occurred at the end of the preseason period (February 2008). The second testing phase (T2) occurred at the midpoint of the competitive season (June 2008). The third and final testing phase (T3) occurred the week after the conclusion of the competitive season, before many of the players had International commitments (September 2008). It was hypothesized that optimal anthropometric profiles would be observed in the early stages of the season, before which the increase in playing intensity, match loads, and injury would result in negative anthropometric adaptations, possibly including an increase in FM and decrease in lean mass.
The Super League is the highest tier of professional competitive rugby league football in England. The league (Super League XIII) consisted of 12 professional teams, which each took part in 27 rounds of domestic competition. Between the beginning and midpoint of the domestic season (0-13.5 games), the subject team in this study played 14.5 games; between the midpoint and end of the season, the subject team played 20.5 games (including cup and play-off games).
Twenty (20) male participants from an English Super League rugby league team agreed to take part in the study (mean ± SD: age, 25.48 ± 3.36 years; stature, 182.41 ± 6.53 cm; and BM, 95.28 ± 11.33 kg). The ethnicity of the study group consisted of 16 Caucasian, 2 black British, and 2 Pacific Island participants. Written informed consent to participate was received from all players before testing, and the project was approved by a radiation protection advisor, the Institutional Review Board and the University's Faculty Research-Ethics Sub Committee.
Participants underwent a total-body dual-energy X-ray absorptiometry (DXA) scan (iDXA, GE Medical Systems, Lunar, United Kingdom), from which estimates of FM, LM, BF%, and BMC were derived based on an extrapolation of fatness from the ratio of soft-tissue attenuation of 2 X-ray energies in pixels, not containing bone (20). Participants lay horizontally on the scanning table with all metal artifacts removed (so as not to interfere with the results). The scan was conducted as the scanning arm travels over the participant's body from head to toe, and the standard mode scans took 6.5 minutes. Scans for heavier participants (those weighing over 100 kg) were conducted using the thick mode, as recommended by Lunar (iDXA, GE Healthcare, United Kingdom). Scans in thick mode account for the greater area of body tissue and take around 15 minutes. All scanning and subsequent analysis were conducted by the same trained operator, on the same day of the week at each testing phase, and before daily training. In our laboratory, the between-trial coefficient of variation for measurement of different variables in 10 adult subjects measured twice, with repositioning, on the same day is (FM, 0.82%; LM, 0.52%; BF%, 0.82%; and BMC, 0.53%). Machine calibration checks were carried out on a daily basis, and showed no significant machine drift during the study. Data were excluded from the study should a player receive an injury that resulted in exclusion from training and match play for a period of longer than 2 weeks.
Statistical analyses were conducted using SPSS Version 16.0 (SPSS Inc, Chicago, IL). Changes in LM, FM, BF%, BM, and BMC between testing phases were made using t-tests for matched samples. Comparisons of anthropometric profiles between playing position were made using t-tests for independent samples. Effect size (±90% confidence interval [CI]) was calculated for each variable to assess the magnitude of the observed differences, in accordance with the method described by Cohen (3). Where presented in the text, data are given as mean ± SD (significance value [p]; effect size [d] ± 90%CI). Statistical significance was set at (p ≤ 0.05).
Group mean ± SD body composition values at the 3 testing phases are shown in Table 1. Mean group BM was not significantly different between testing phases (p > 0.05), with a similar range of BM values being observed across phases, indicated by a consistent SD of the data (Table 1). There was an increase in mean FM of 0.57 ± 1.10 kg (p = 0.002; d = 0.52 ± 0.39) between phases T2 and T3, a mean change of 4.09%. Over the course of the competitive season, between T1 and T3, FM increased by 0.90 ± 1.14 kg (p= 0.031; d = 0.79 ± 0.38), a mean increase of 6.62%. Fat mass measured between T1 and T2 showed a mean group increase of 2.42%; however, this result did not show statistical significance (p = 0.273; d = 0.25 ± 0.38). In line with the significant increase in group FM between the midpoint and end of the competitive season, mean group LM decreased significantly between T2 and T3 by 1.19 ± 1.43 kg (p = 0.001; d = 0.83 ± 0.37), a decrease of 1.54%. LM also decreased over the entire season by 1.17 ± 1.33 kg (p = 0.001; d = 0.88 ± 0.39), a seasonal decrease of 1.51%. Lean tissue mass measured between T1 and T2 was not significantly different (p = 0.95; d = 0.01 ± 0.27). In absolute terms, mean BF% increased by 0.78 ± 1.09% (p = 0.005; d = 0.71 ± 0.39) between T2 and T3, representing a significant change of 4.98%. Body fat percentage also increased significantly by 1.03 ± 1.14% (p = 0.001; d = 0.90 ± 0.4) over the course of the entire season, a relative increase of 6.77%. Mean BF% measured between T1 and T2, although increased, was not significantly different (p = 0.359; d = 0.21 ± 0.39) (Figure 2).
Bone mineral content showed a significant increase between the start and midpoint of the season (30.70 ± 38.00 g (p = 0.002; d = 0.81 ± 0.39), representing an increase of 0.71%. Over the seasonal period, BMC increased by 31.55 ± 33.64 g (p < 0.001; d = 0.94 ± 0.33). Bone mineral content measured between T2 and T3 was not significantly different (p > 0.05) (Figure 2).
Significant differences in baseline anthropometric properties were found between playing position for BM (Forwards 102.72 ± 8.22 kg; Backs 87.84 ± 8.96 kg; p = 0.01), FM (Forwards 15.28 ± 3.29 kg; Backs 11.90 ± 3.33 kg; p = 0.035), LM (Forwards 82.81 ± 7.36 kg; Backs 71.94 ± 8.05 kg; p = 0.005), and BMC (Forwards 4,601.40 ± 547.05 g; Backs 3,988.10 ± 489.58 g; p = 0.17). Body fat percentage was not significantly different between playing positions (Forwards 16.25 ± 3.28%; Backs 14.18 ± 3.68%; p > 0.05). However, no significant between groups differences were observed in changes in BM, FM, LM, BF%, or BMC over the course of the season between forwards and backs, with both groups of players displaying similar trends at each testing phase.
Mean group BM showed no significant change over the course of the competitive season. Although acute changes in BM can be a useful indicator of a change in physiological status of an individual (e.g., increased FM), the absence of a significant change in BM does not indicate that the internal composition of FM and LM has not altered.
In this study, LM showed no significant change between the beginning and midpoint of the season. The significant reduction in LM (−1.54%) across the group, which was found between midpoint and the end of season, may have been because of various factors. The latter stages of the season are often associated with a shift of the focus of training from the development of fitness levels to the maintenance of fitness levels (10). In the present study, the increase in relative match exposure between the first half (14.5 games; 0.86 games/week) and second half (20.5 games; 1.05 games/week) of the season included a period of 4 games in 16 days, and resulted in reduced recovery times between matches in the latter stages of the season. During periods of frequent match exposure, the emphasis of training focused on recovery, rest periods, and match preparation, reducing the training volume and in particular the frequency of resistance exercises. As 1-2 days per week of resistance training has been reported to be an effective strength maintenance frequency (13,23), a reduced training load below the maintenance threshold, because of increased match exposure, may explain the atrophic response observed in the present study. In addition, the prevalence of injuries typically observed toward the end of a season (10) mean that certain players may not able to maintain the weekly training load that has brought about the positive physiological changes in body composition, which can result in the effects of detraining including muscular atrophy.
It may be suggested that in many sports, especially aerobically based team sports, carrying excess body fat may be detrimental to performance, because it is extra weight that has to be lifted against gravity, which will only act to slow a performer (21). Some studies have suggested that rugby league players may benefit from higher subcutaneous fat levels as a means of protection from injury (1,21). However, the majority of evidence suggests that carrying excess body fat has a negative effect on performance (e.g., power/BM ratio, thermoregulation, and aerobic capacity) (11). Therefore, decreasing FM is usually advantageous to athletic performance. The increase in FM (4.09%) in the latter stages of the season coincided with the reduction in LM at the same stage of competition, and it is possible that these outcomes were related. However, although a decrease in LM is most likely caused by a change in training status, the cause of a significant increase in FM, although also possibly because of a change in training practices, could also be caused by dietary and nutritional factors. In particular, maintaining the same weekly energy intake over the course of a season, while decreasing weekly energy expenditure (decrease in training load), would be likely to result in a state of positive energy balance, leading to an increase in the stores of both visceral and subcutaneous body fat levels (19).
The observed decrease in LM and increase in FM over the season resulted in a net increase in BF% of 6.7% over the competition period. This may have significant implications in terms of the speed, agility, power, and functional strength of individual players, as decreased LM, and therefore increased BF%, may contribute to inferior speed and muscular power because of the reduced power to BM ratio and performance in match specific tasks (11). It may be suggested that the aim for strength and conditioning professionals should be to maintain a favorable body composition throughout the season in all players, especially in a sport where excess FM and insufficient LM could have a negative implication for performance. It is also important for players to be in suitable physical condition toward the end of the season where outcomes may decide final league positions (5).
In addition to changes in FM and LM, an important benefit of physical training is its ability to increase bone mass. An exposure to a variety of strain types, magnitudes, rates, and frequencies offers ideal conditions for bone mineralization (17). In particular, the demands of rugby league require frequent and significant impacts, quick accelerations and decelerations, jumping, kicking, and scrimmaging, all of which will offer the stimuli for bone growth according to the Mechanostat theory (28,32). The use of DXA in this study allowed for the measurement of BMC, which has not previously been reported in Super League rugby league players, and also for the assessment of changes in BMC over the course of the competitive season. Maximizing peak bone mass, maintaining bone mass thereafter, and minimizing bone loss over a competition period, are 3 factors that could reduce the risk of osteoporosis and subsequent fractures (24). The ability of bone to adapt to mechanical loading is increased during the growth period (puberty) (27), which is the time when most players in this study began to play rugby league, undertaking training sessions to develop muscular strength and power, and subsequent skeletal loading.
Each bone remodeling cycle typically takes 3 months (31); the duration of this study was 7 months. The significant increase in BMC (0.71%) from the start to the midpoint of the season in this study, over 4 months, may reflect change in training methods between phases. The hypertrophy phase of training toward the beginning of the season may stimulate the greatest rate of bone accrual as a result of the amount of axial loading that occurs through this type of training. The lack of change in BMC between the midpoint and end of the season (0.02%) may reflect the focus of training which changed from muscular development to the maintenance of general fitness levels. This results in less skeletal load being applied during training, therefore decreasing the rate of bone accrual. Future studies to investigate this further would benefit from the investigation of bone turnover through biochemical markers measured at each phase of training, rather than DXA, as bone turnover markers can identify acute changes more effectively.
The findings of this study over the 7-month competition period have important implications for sports science, coaching, and conditioning staff. Fat mass and LM should be measured independently of total BM measures, to maintain a favorable physiological status over the entire competition period. Information on acute changes in body composition may be used to direct training and weight-control practices, to enable performance to be maximized until the conclusion of the competitive season.
Previous studies that have considered the anthropometric profiles of rugby league players have generally used only 1 testing phase with populations of junior (9), amateur (7), semiprofessional (8), and professional (22) players. The seasonal changes in anthropometric characteristics have previously been reported with junior (26) and amateur (10) players using skinfold analysis; however, this is the first study to assess the seasonal changes in body composition using DXA, across 3 testing phases with a population of elite rugby league players from the English Super League. The findings of the present study suggest that negative changes in body composition are observed in the latter stages of the season, resulting in a decrease in muscle mass and increase in FM compared with early season measurements. Carrying less weight as muscle and excess weight as fat can have a negative effect on muscular power and aerobic capacity, which consequently may influence performance capabilities at an important stage of the season, when match load is generally increased because of the demands of competition. The work of coaching, strength and conditioning and medical staff should, where possible, be coordinated to enable the physical condition of the athletes to be maintained throughout the entire competition period.
The authors would like to thank Aleks Gross for his help in organizing and enabling this study, and also Emma Whatley and Marissa Martyn-st James for their assistance with data collection. This study was funded by the Carnegie Research Institute, Leeds Metropolitan University, United Kingdom.
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Keywords:Copyright © 2011 by the National Strength & Conditioning Association.
elite sport; DXA; bone mineral content; team sports