Introduction

It is widely accepted that bone responds positively to the mechanical loading and muscle forces associated with physical activity [1]. Athletes have higher bone mineral density than sedentary controls, depending on the loading associated with their particular sport [2, 3], and the influence of site-specific loading on bone mineral density, bone mass and bone size has been further demonstrated in the dominant arm of athletes participating in racquet sports such as tennis [4, 5].

The other components of body composition, namely, fat and lean mass, have been measured in athletes using techniques such as skinfold thickness, near infrared reactance and bioelectrical impedance [6, 7], with very little published body composition data using dual energy X-ray absorptiometry (DXA) [7]. DXA has become more widely accepted as a means of measuring body composition in adults [8], with reference values from NHANES being reported in 2009 [9]. In addition to providing whole body measurements of bone, fat mass and lean mass, DXA can also provide information on the regional distribution of these components of body composition. Regional tissue differences between athletes and controls, as well as between athletes with different playing functions within a team, have been reported by Bell et al. [10] in rugby players, with the aim of contributing to a better understanding of athletic performance and its requirements.

The science and medicine of cricket have been studied and reviewed [11, 12], with a particular focus on the injuries and biomechanics of fast bowlers [13, 14]. However, this does not include any studies on the bone health of these athletes even though stress injuries have commonly been reported in cricketers, most particularly at the lumbar spine [14–16]. Cross-sectional studies have identified that stress fractures of the lumbar spine consistently occur more often in the fast bowling population than in the general population [14–16, 19]. The prevalence for stress fractures is reported to be between 23.5 and 66.7% in fast bowling populations.

Therefore the aim of this study was to compare the bone mineral density and body composition of competitive cricketers to controls.

Materials and methods

Participants

All participants were male, and included 34 high-performance (senior provincial and national level) cricketers and a convenience sample of 23 physically active controls between the ages of 16 and 34 years.

The cross-sectional study was conducted according to the guidelines in the Declaration of Helsinki, and all procedures were approved by the Research Ethics Committee of the Faculty of Health Sciences at the University of Cape Town. Written informed consent was obtained from all the subjects.

Measurements

Whole body (WB), femoral neck (FN), proximal femur (PF) and lumbar spine (LS) BMD, as well as whole body fat mass (WBFM) and lean mass (WBLM) were measured using dual-energy X-ray absorptiometry (DXA) (Hologic Discovery-W, software version 12.1, Hologic Bedford Inc., Bedford, MA, USA). The regional placement of markers to delineate the arms, legs and trunk was determined by the manufacturer algorithm. In vivo precision (%CV) for this machine has been determined for fat-free tissue mass (0.7%), fat mass (1.67%) and whole body bone mineral content (0.9%) by measuring 30 individuals twice on the same day with re-positioning. Low bone mass was defined as a z score below −2.0 (2 SD below the expected value for a healthy young adult) [17]. Appendicular skeletal muscle mass index (ASMI) was calculated as the sum of the lean soft tissue masses for the arms and legs (kg) divided by the squared height (m2).

Statistical analyses

One-way ANCOVA was used to compare BMD between cricketers and controls, after covarying for age and height. The association between lean mass and bone mass was calculated using Pearson's correlation coefficient. The alpha level was set at p < 0.05. One-way ANCOVA, co-varying for age and height, was also used to compare BMD among the different playing positions within the cricketers (i.e. batsmen, spin bowlers and fast bowlers). When a significant p level was achieved, a post hoc Bonferroni test was used to explore these differences further.

Results

Cricketers compared to controls

The descriptive characteristics of the cricketers and controls are presented in Table 1. Although there were no significant differences in weight between the cricketers and controls, the cricketers were significantly younger and taller than the controls. WBFM, in kg and as a % of weight, was not different between the groups; however, the cricketers had significantly greater WBLM and WBBMC compared to the controls, although the correlation between WBLM and WBBMC was significant in both groups (cricketers r = 0.64, p < 0.001; controls r = 0.84, p < 0.001). ASMI was also significantly higher in the cricketers compared to the controls.

Table 1 Descriptive characteristics for cricketers and controls
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Fat, lean and bone mass of the arms and legs are presented in Table 2. Arm and leg absolute lean mass and bone mineral content were higher in the cricketers compared to the controls. There was no difference in absolute or relative fat mass of the arm or legs between the groups, and relative lean mass and bone mineral content, as a % of weight, were not different. Absolute and relative bone mineral content of the trunk was higher in the cricketers compared to the controls (Table 3).

Table 2 Absolute (kg) and relative (%) amounts of fat, lean and bone mass in the arms and legs in cricketers and controls
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Table 3 Absolute (kg) and relative (%) amounts of fat, lean and bone mass in the trunk in cricketers and controls
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LS, PF and FN BMD were higher in the cricketers and controls before and after adjusting for age and height (Table 4). One cricketer (batsman) had a z score below −2 at the lumbar spine, while one control had a z score below −2 at the femoral neck.

Table 4 BMD measurements in cricketers and controls
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Comparison among cricketers

The cricketers were divided into three groups according to their predominant skill: batsmen (B n = 8), spin bowlers (SB n = 17) and fast bowlers (FB n = 9). The fast bowlers were significantly older than the spin bowlers (FB 24.1 ± 2.3 years vs. SB 20.6 ± 3.4 years, p = 0.021), and taller than the other groups (FB 186.3 ± 3.4 cm vs. B 176.3 ± 5.5 cm, SB 178.9 ± 70.7 cm, p = 0.005). There were no differences in weight (FB 81.7 ± 10.1 kg, B 79.1 ± 4.1 kg, SB 75.1 ± 10.7 kg), WBFM (FB 13.3 ± 6.1 kg, B 13.2 ± 3.2 kg, SB 11.5 ± 4.9 kg) or WBLM (FB 64.1 ± 5.2 kg, B 61.7 ± 3.3 kg, SB 59.8 ± 7.0 kg) among the groups.

WBBMD was significantly lower in the spin bowlers compared to the batsmen and fast bowlers, after adjusting for age and height (Fig. 1). There were no differences in LS (FB 1.280 ± 0.181 g/cm2, B 1.225 ± 0.107 g/cm2, SB 1.125 ± 0.133 g/cm2), PF (FB 1.232 ± 0.141 g/cm2, B 1.290 ± 0.129 g/cm2, SB 1.143 ± 0.113 g/cm2) or FN BMD (FB 1.091 ± 0.142 g/cm2, B 1.100 ± 0.129 g/cm2, SB 1.020 ± 0.127 g/cm2) among the groups.

Fig. 1
figure 1

Whole body bone mineral density (WBBMD), adjusted for age and height, in batsmen (B n = 8), spin bowlers (SB n = 17) and fast bowlers (FB n = 9). *p < 0.05, compared to spin bowlers. Adjusted data presented as mean (95% CI)

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Discussion

To our knowledge, this is the first study to examine the bone mineral density of high-performance cricketers, a group of athletes who have been identified to be at high risk of stress fractures [15, 18, 19]. Our results have shown that when compared to males of a relatively similar age and weight, the cricketers had greater bone mineral density at all sites including the lumbar spine, proximal femur and femoral neck. It therefore appears that bone mineral density may not be the only determinant of stress fractures in cricketers and that there may be other factors responsible for the increased incidence of stress fractures reported in these athletes.

Although low bone mineral density has been proposed as a risk factor for stress fractures in athletes [20], other factors unique to the sport itself may also play a role in their aetiology. The biomechanics and excessive loading associated with bowling has been well explored in fast bowlers. Fast bowlers using a ‘mixed’ action technique that involves a counter-rotation of the shoulders during the delivery stride have been found to have an increased risk of developing a stress fracture [14, 18, 21]. A high workload with infrequent rest days is associated with increased risk of injury in both junior and senior bowlers [22, 23]. In addition, adolescent cricketers have been shown to be at a particularly high risk of lower back injury [14, 18, 21, 24], which rather than being a result of low BMD, may actually be due to the incomplete closure of ossification centres in these young athletes.

It is well known, from cross-sectional and longitudinal studies, that athletes have a higher bone mineral density compared to non-athletes [25–27]. We showed a 12.8, 10.4 and 12.5% higher BMD at the lumbar spine, proximal femur and femoral neck, respectively, in the cricketers compared to the controls. When compared to controls, soccer players show similar differences at the lumbar spine (10%) but greater differences at the femoral neck (21%) [28]. The loading characteristics associated with different sports contribute to differences in bone strength, with athletes participating in higher impact sports having a greater bone mineral density compared to athletes participating in lower impact sports or those with no impact at all [29, 30]. It has also been shown that not just high-impact sports, but also sports that involve unusual loading as well as acceleration and deceleration are associated with stronger weight-bearing structures [31]. The characteristics of the game of cricket may fit into the latter category as it is associated with high ground reaction forces of 4.8–6.4 times body weight occurring during the bowling action [18, 32, 33]. In addition, cricketers are required to change direction rapidly, accelerate and decelerate quickly, and fast bowlers in particular place a significant amount of weight on the leading leg when bowling.

The mechanostat theory hypothesizes that the increasing muscle forces that occur during growth influence the size and strength of the bone [1, 34], and that the largest forces on the skeleton are from muscles [35]. The cricketers in our study had higher whole body and appendicular muscle and bone mass compared to the controls. Although we do not know whether this is due to selection bias, the findings of some longitudinal studies show that the bone mass of well-trained athletes still responds to mechanical loading [36]. Therefore, it is likely that the higher bone mass in our cricketers is a consequence of their high lean mass, the result of a regular strength-training regimen. Although the cricketers had higher muscle and bone mass compared to the controls, there was no difference in whole body or appendicular fat mass between the groups. Although we did not collect exercise diaries on the control group, a convenience sample of males who had had DXA measurements on the same machine, most of them were physically active, which may explain a lower fat mass than would be found in a sedentary control group.

The difference in body composition and bone mineral density among athletes playing in different positions within the same sport has been shown in sports such as rugby [10, 37]. In our study the spin bowlers had lower whole body BMD compared to the fast bowlers and batsmen. Although we did adjust for age, which may help to account for the influence of age itself on bone mineral density, it does not take into account the fact that, being younger, the spinners had had fewer hours of competitive cricket, equating to less playing time, less impact on weight-bearing structures and therefore lower bone mineral density. When dividing the spin bowlers into wrist spinners and finger spinners (data not shown due to small sample size), the mean age of the wrist spinners was 18.3 years, approximately 5 years less than the finger spinners who had a mean age of 23.1 years. This age difference, which is a proxy for years of competitive cricket, is reflected in the difference in whole body BMD, which was 8% higher in the finger spinners compared to the wrist spinners. As it is now proposed that peak bone mass is attained by approximately 18 years of age [38, 39], this difference may be due to the hours of competitive cricket, and therefore mechanical loading and impact on the bone.

There were no differences in site-specific BMD between the different playing positions, which may be due to the fact that although cricketers do specialise in a particular playing position, they are required to spend time in all positions during training and competition. This may explain why the differences are not as large as in other sports such as rugby, in which players train very specifically for their playing position.

The small sample size of the cricket and control groups can be considered to be a limitation of this study. The cricketers were recruited from a very select group of provincial and national cricketers, which naturally decreases the sample available for recruitment, and the control group were a convenience sample recruited for another study. In addition, the significant age difference between the cricketers and controls, as well as the spin bowlers compared to the batsmen and fast bowlers, is unfortunate and cannot be adequately adjusted for statistically. The mean age of all the groups was however greater than 20 years, which is above that suggested for the attainment of peak bone mass [39].

In this study, cricketers had a 10–13% greater BMD at all sites compared to controls, suggesting that the mechanical loading associated with cricket is beneficial for bone mineral density. The implications of this in terms of future bone health and fracture risk remain to be determined.