Field hockey is a high paced and intense team sport1 with an international popularity second only to soccer.2 Playing field hockey is physically demanding and the demands on the shoulder joint and girdle during play cannot be underestimated.3 Although field hockey is not considered an overhead sport, the shoulder is highly loaded during practice and competition.

Rotator cuff pathology is considered to be a principal cause of shoulder pain,4 particularly in athletes with repetitive overhead activities, such as throwing athletes,5 of whom shoulder loading during sport participation is known to be extremely high. Acromiohumeral distance (AHD) is a common investigated variable in these shoulder-loading athletes. It is the quantification of the size of the subacromial space6 and although under debate, is assumed to be clinically relevant since a reduction of this space is linked to possible mechanisms in the etiology of rotator cuff tendinopathy.6–8

Generally, it has been established that AHD varies with shoulder position and is greatest at rest, in a neutral shoulder position. Norm values range from 10 to 15mm in a healthy shoulder,9–11 with no systematic difference between the dominant and non-dominant shoulder in a non-athletic population.12 The source of this wide range may be due to interindividual variability or the variety of measurement protocols used.6 AHD reduces with shoulder elevation or abduction9,10 and is smaller in patients with rotator cuff tendinopathy compared to asymptomatic individuals.11,13–16 An AHD equal to or smaller than 6mm is considered the cut-off for an abnormal AHD, suggesting a clinically significant rotator cuff pathology.12,17 Maintenance of the subacromial space in the shoulder girdle is essential for normal shoulder function.

In shoulder-loading athletes, no uniform conclusions can be made concerning sport-specific adaptations of the AHD. While some studies found a significantly larger AHD on the dominant side compared to the non-dominant side in these athletes,10,18,19 the opposite result9,20,21 as well as no difference between sides22 has been concluded in other studies. Moreover, comparing shoulder-loading athletes to a non-athletic population, a smaller AHD in tennis players compared to controls has been recorded23 while a larger AHD was found in swimmers.24 Comparison of elite shoulder-loading athletes with recreational athletes, proved a larger AHD in the first group compared to the latter.10 Whether sport-specific adaptations in the shoulder joint also occur in field hockey players is yet to be studied. Since field hockey players do not load their shoulders in the exact same way as athletes practicing more conventional overhead sports, such as handball and tennis, the comparison with study results in these overhead athlete populations is not entirely suitable. However, field hockey players are allowed to make plays above shoulder height and thus use their shoulders in a wider range of abduction than conventionally suspected.

In contrast to the extensive shoulder-related research in throwing sports and in spite of the popularity of field hockey, limited research has been done within the population of field hockey players. The scarcity and heterogeneity of the executed research concerning field hockey injuries does not allow for comprehensive statements on this matter.25 According to the limited available research, lower limbs are the most affected in these athletes, with contusions, abrasions and sprains being the most common types of injury.25,26 However, the most severe injuries involve direct trauma to the upper limb and head or face. These upper limb injuries comprise 4.9–39.4% of all hockey injuries.26 Within the upper limb, the shoulder has been suggested to be a vulnerable joint, with a significant risk for traumatic shoulder injury during training and competition.3,27,28

Since susceptibility to shoulder injuries for field hockey players is not far-fetched and the demands of the shoulder are repetitive of nature and extremely high, especially in elite players, a better understanding of the field hockey shoulder is a logical goal to aspire. Therefore, the purpose of this study was to bilaterally examine AHD at different angles of shoulder abduction within a group of elite field hockey players and compare these measurements with an age-and gender matched control group of healthy individuals. It was hypothesized that the AHD would differ between athletes and controls, however, it was uncertain whether to expect this difference in favor of the athletes or the controls: comparing results of similar studies in other shoulder loading, athletic populations does not lead to conclusive conclusions regarding this matter.

The purpose of this study was to investigate the AHD in elite field hockey shoulders and to determine whether sport-specific adaptations concerning this variable occur. To this end, the complete national male field hockey team was tested bilaterally and compared to an age- and gender matched control group of non-athletic healthy participants. In total, 50 participants (25 athletes and 25 controls) were included. This is the first study addressing AHD in field hockey players.

A first finding was a significant difference between both groups for AHD at 45° and 60° of shoulder abduction in the frontal plane. At both these degrees of abduction, the AHD was larger in field hockey players compared to their matched controls, in the dominant as well as in the non-dominant shoulder. At 0° of abduction, no group difference was found. A second finding implies a side difference established in both groups: at 60° of shoulder abduction, a significant larger AHD was noted in the dominant shoulder compared to the non-dominant shoulder. At 0° and 45° of abduction, no significant differences between sides were found in either group.

A significant AHD group difference was established at 45° and 60° abduction, while no significant difference was found at 0°. This may be explained through the difference in muscle activity in both situations. The AHD measurements at 45° and 60° required active shoulder abduction of the participant, whereas the measurement at 0° of abduction was a passive measurement, with the participant's upper arm hanging alongside the body and the hand resting on the thigh.

It is well established that, when actively moving the shoulder, both glenohumeral and scapular muscle balances are of upmost importance to retain normal shoulder function. A good balance between the trapezius muscles and serratus anterior is necessary to assure optimal scapular orientation, thus optimizing the glenoid cavity positioning. Good co-activation of, as well as proper muscle balance within, the deltoid and rotator cuff couple are necessary to maintain a sufficient AHD, thereby avoiding compression in the subacromial space. The rotator cuff is responsible for maintaining normal humeral head position in the glenoid during shoulder elevation movements. The compressive forces of the cuff stabilize the humerus against the glenoid, thereby providing dynamic stabilization of the glenohumeral joint and counteracting the upwardly directed force of the deltoid.30 When these dynamic stabilizing forces are insufficient, an increase in superior and anterior migration of the humeral head occurs, narrowing the subacromial space, which may lead to impingement or could make the shoulder more susceptible to pathology.31,32 Changed scapular biomechanics (e.g. diminished upward rotation and reduction of posterior tilt) are associated with mechanical impingement.33 A more efficient working of, and cooperation between, rotator cuff and scapular muscles, results in better biomechanics and larger AHD and prevents tendon overload in the subacromial space. From a biomechanical point of view, stronger rotator cuff and scapular muscles could be associated with a larger AHD. Leong et al.14 confirmed this relationship between the subacromial space and scapular muscle strength by showing the correlation between decreased strength of the middle and lower trapezius and a reduction in subacromial space.14 As confirmed in a recent study, elite field hockey players have stronger rotator cuff muscles than non-athletes.34 This could explain the larger AHD in field hockey players at 45° and 60° of (active) shoulder abduction, with no group difference for the AHD at 0° (measured in a resting shoulder position). Since an insufficient AHD can be associated with shoulder pathology (e.g. subacromial pain syndrome, rotator cuff tendinopathy),11,13–16 an increased AHD with active shoulder movements may contribute to a better resistance against shoulder injuries. The larger AHD in field hockey players compared to controls may be a sport specific adaptation. This adaptation may also, contribute to the protection of these athletes against rotator cuff related injury. However, this type of shoulder injury does not seem to be common among these athletes.

The established group differences are quite small, ranging from 1.07mm to 1.81mm. Nevertheless, they are larger than the calculated minimal detectable change (MDC) for AHD, with exception of the group difference concerning AHD measured at 45° of abduction in the non-dominant arm. Three out of 4 group differences are thus larger than the MDC.

The finding of a larger AHD in elite field hockey players compared to controls is contrary to the findings of Silva et al.,23 who demonstrated a significant smaller AHD in junior elite tennis players, compared to control participants. Hibberd et al. compared the AHD of competitive adolescent swimmers with a control group of non-overhead athletes and found a bilaterally larger AHD in the swimmers group. These findings are in line with the results of this study, implying a sport-specific adaptation in these athletes, resulting in a larger AHD.23

One significant side difference was found in both groups at 60° of shoulder abduction, with a larger AHD at the dominant side compared to the non-dominant side. This difference between both shoulders was almost exactly the same for both groups: 0.78mm in the athlete group, 0.79mm in the control group. These side differences are smaller than the MDC and therefore of less clinical relevance. At 0° and 45°, no significant AHD side differences were found. Previous studies have stated different, compatible as well as incompatible, conclusions concerning whether or not side differences are present for AHD. In some studies involving shoulder-loading athletes a side difference is noted. However, direction of this difference differs across studies with some revealing a larger AHD on the dominant side, while others note a smaller AHD on this side compared to the non-dominant one.9,20,21 In the study of Thomas et al.22 in college baseball players, AHD was measured at 0° and 90° of abduction and in this study no side difference was found.22 No side differences were also concluded by Petersson et al.,12 who studied the AHD in a general, non-athletic population.12 Putting together these results with the result of this study, no uniform conclusion can be made concerning the presence of side differences in AHD.

The relatively limited study sample could be considered a limitation of this study. The sample size would preferably be larger but testing of the national team in isolation preserves the inclusion of elite players only. Expanding the sample size to include sub-elite players diminishes one of the strengths of this study, namely the investigating the shoulder of one of the best field hockey teams in the world.

An obvious strength of this study is the fact that this is the first study addressing AHD in field hockey players. In this aspect, this study can contribute to a better understanding of the shoulder of these not yet widely studied athletes. Secure and consistent reporting was of upmost importance in describing the measurement procedure, since key components of the protocol must be clearly stated to support consistent standardization of the measurement method (e.g. probe positioning, AHD definition). Additionally, excellent ICC values assure a good test–retest reliability of the measurements. Reliability of the AHD measurement through ultrasound has been previously investigated and found to be good to excellent.6

A limitation of the study concerns the current debate regarding the clinical relevance of AHD measurements. Inconsistent AHD changes in rotator cuff tendinopathy patients compared with pain-free shoulders have for example been reported in a recent review by McCreesh et al.6 However, the latest studies by Navarro et al. did show an association between AHD and shoulder pain and function in chronic shoulder pain patients.7,8

The clinical implications of this study may be found in the results themselves. With active shoulder abduction, the AHD in the shoulders of elite field hockey players was found to be larger compared to non-athletic participants. This may be a protective sport-specific adaptation, to better guard the highly loaded joint from injury. In light of injury prevention, AHD measurement may help physical therapists and coaches decide which athletes could benefit from specific, additional exercises aimed at enlarging the subacromial space. Adding appropriate exercises to the training program of athletes with smaller AHD values compared to their colleagues could contribute to the tenability of the shoulder. Since optimal functioning athletes are a necessity in any team sport, injury prevention should be a distinct priority. Prospective research should however further clarify the relationship between AHD and shoulder injury risk.

Expanding the study sample would be an appropriate step in further research. Recruiting more elite field hockey players add to the current study sample would be interesting, as well as repeating the same study with a sample of sub-elite and/or recreational field hockey players to investigate if the same observations can be made in lower level field hockey players. Second, instead of the cross-sectional design operated in this study, a prospective study design would be suitable to examine whether AHD could be predictive for shoulder complaints in these athletes. The set-up of a prospective study would therefore be greatly encouraged to contribute to the understanding of the influence of this variable. Specific research focusing on pinpointing the explanation for this difference in AHD between field hockey players and non-athletes would also favorably add to this understanding. Lastly, uniformity in AHD measurement protocols should be pursued, with clear and unambiguous reporting of the measurement procedure.

In both shoulders, elite field hockey players have a statistically significant and clinically relevant larger AHD than their matched controls, when actively abducting their shoulder to 45° and 60° in the frontal plane. This may be a protective sport-specific adaptation to shield these athletes from certain shoulder problems and may be linked to superior shoulder girdle strength in the field hockey players. At 60° of abduction, a statistically significant larger AHD in the dominant side compared to the non-dominant side was noted in both groups. However, this difference was not clinically relevant. Further research should focus on expanding the study sample and investigating AHD in a prospective manner.

The authors declare no conflicts of interest.