In the current study, a significant positive association was found between GJH and AHSA, with AHSA ranging from 5° to 8° higher than controls, with an increased BT score. Normative data for AHSA ranged from 40° to 52°, depending on age.
Swimmers with GJH ≥7/9 presented with 15 and 20 % higher maximum AHSA mobility than controls at the age of 10 and 14, respectively. These findings are in contrast to a previous study, where no association was found between GJH and shoulder rotation and extension in young swimmers, with only 4 swimmers classified as GJH at cut off point ≥5/9 [16]. Likewise, in non sport-specific adults, there was no association between number of positive BT and passive glenohumeral ROM values [17, 18]. The current data, however, indicates that maximum AHSA is associated with GJH in adolescent swimmers, and that this test may reveal increased shoulder mobility in this group.
Increased ROM of the shoulder is often seen in swimmers, and is generally considered a potential risk factor for shoulder pain and/or injuries. However, such relationship has not yet been confirmed [3, 19–21]. In a cross-sectional study of 32 competitive swimmers aged 15 to 21 years, no association was seen between horizontal abduction and actual shoulder pain, however, classification of GJH was not included in the test battery [14]. Similarly, no association was found between shoulder internal and external ROM and the occurrence of pain in a cross-sectional study including15 participants, with six of these categorised as having shoulder laxity, but none with GJH [22]. The decreased passive stability in GJH along with a higher degree of AHSA requires a large contribution of active stability provided by the rotator cuff muscles to control for glenohumeral translation. Increased demands for active stability may hypothetically result in muscle fatigue, leading to repetitive micro trauma seen as overuse injuries.
At cut off point ≥7/9, maximum AHSA was increased by 8° compared to controls, and a question to be considered is whether GJH and increased shoulder ROM is predictive of shoulder injury and/or pain. The ability to establish a relationship between the BT score including maximum AHSA and shoulder injury in young competitive swimmers is limited by the cross-sectional design of the current study.
The current normative values are in line with a previous study of AHSA in 15–21 year old competitive swimmers [14], however, not tested with BT, but with AHSA presented for the whole group (left shoulder 44° ±14° and right shoulder 44° ±16° versus the current range of 40–51°). The current study included both swimmers with and without GJH in the normative values, representing a typical population of swimmers.
The prevalence of girls at 12 years (mean age) with GJH in the current population of swimmers was about twice as high as found in a normal population (different sports) at similar age; 41 % vs. 20 %, 29 % vs. 15 % and 18 % vs. 5 % for cut off points ≥5/9, ≥6/9 and ≥7/9, respectively [8]. The same pattern was seen for the current 12 year old boys, with a prevalence of GJH also being about twice as high than the normal population of age-matched boys being 19 % vs. 9 %, 10 % vs. 6 % and 5 % vs. 1 % for the three different cut off points, respectively [8]. However, the current high prevalence of GJH is in line with a previous study of adult, competitive swimmers, with prevalence of 20 % for GJH 4≥/9 [7]. Contrary, a previous study of children participating in swimming, found girls aged 9 to have lower BT score than controls (median 2 vs. 3), with no group differences for girls aged 12, but with boys (aged 9 and 12) displaying higher BT score than controls (median 4 vs. 2, and median 2 vs. 1, respectively) [19]. The high prevalence of GJH in the current study of competitive swimmers may be due to the fact that large shoulder mobility with respect to stroke length is recognised as an advantage for competitive swimmer, thereby being seen more prevalent in this sport for those with GJH, as also previously reported [7].
The current study is in line with previous studies showing the prevalence of GJH increasing by age for girls and decreasing by age for boys [8, 23, 24]. Thus, the choice of cut off points for classification of GJH may vary from ≥4/9 to ≥6/9 within this age group [8, 11, 12, 25]. With cut off point ≥6/9 for GJH, almost 33 % of the current girls are considered to have GJH. This is recognised as a relatively high prevalence compared with a normal child population (mean age 13.8 years), where only 7 % of the girls were classified GJH at this cut off point [25]. It seems difficult to determine a single cut off point for children and adolescents, since joint mobility represents a variable fluctuating condition during maturation [24]. It is therefore recommended that future studies present data of GJH prevalence with different cut off points, in addition to age and gender, as presented in the current study.
The limitations of the current study are mainly the unknown reliability and validity for the AHSA test. Previous studies found excellent reliability for inclinometer measurements of shoulder abduction ROM, when tested in a seated position [26]. The purpose of the current test was to measure shoulder mobility, expressed as AHSA, but as AHSA is a coupled motion performed actively, the outcome may rather indicate the swimmers ability to move the arm against gravity, than the actual and specific shoulder ROM. The concurrent validity of the current clinical AHSA test, therefore, remains unknown. However, AHSA was selected due to its similarity with the swimmers freestyle stroke movements, and as such AHSA represents the most swim-related shoulder mobility test.
Another limitation is, that the GJH group accounted for 2/3 girls, which may hamper generalizability. However, since the prevalence of GJH among girls and women is higher in general, the current group may be well representative of the GJH group [27]. The normative values of the current study are distributed by age, but with the small sample sizes in the 10-year-old group (three subjects), the validity of this age group is hampered. A larger sample of this age group is therefore required in future studies for comparison to establish validity.
The strengths of this study are the standardized protocol of the BT and AHSA test, the supervised training phase of the testers and the large sample size. This study was carried out in young, competitive swimmers, making the results largely relevant for this group.