Background Generalised Joint Hypermobility (GJH) is considered an intrinsic risk factor for shoulder injuries and has a high prevalence in swimmers.

Objective The aim of this study was to investigate the influence of GJH on shoulder muscle strength, muscle activity and muscle fatigue during shoulder rotation movement simulating swimming strokes.

Design Cross-sectional study.

Setting Youth elite sports.

Patients (or Participants) 19 elite swimmers with GJH (Beighton's score ≥5 and hypermobile shoulder) were compared to 19 sex- and age-matched control swimmers (Beighton's score ≤3 and no shoulder hypermobility), all 13–17 years.

Interventions (or Assessment of Risk Factors) Concentric isokinetic performance in medial rotation (35^°) and lateral rotation (60^°) was measured at velocities of 60^°/s (5 repetitions) and 180^°/s (10 repetitions). Muscle activity during isokinetic testing was measured with surface electromyography from upper trapezius, lower trapezius, serratus anterior, infraspinatus and pectoralis major.

Main Outcome Measurements Peak torque (Nm/BW), total work (J/BW), electromyographic muscle activity (%MVC), and isokinetic and electromyographic fatigue development.

Results In medial rotation at 60^°/s, there was a significantly lower (14%) total work compared with controls (2.80 vs. 3.26 J/BW; p=0.049), and a tendency to significantly lower peak torque for the GJH group (0.48 vs. 0.55 Nm/BW; p=0.057). There were no group differences in medial rotation at 180^°/s, or in lateral rotation at both velocities. Muscle activity was non-significantly lower in serratus anterior (range: 27–33%) and pectoralis major (range: 19–26%) in the GJH group in medial rotation. Fatigue development did not differ between groups.

Conclusions Young elite swimmers with GJH displayed decreased medial rotation endurance and strength, which may be potential intrinsic risk factor for development of non-traumatic shoulder injury. These swimmers may benefit from medial rotation endurance and strengthening programs; however, further research is warranted to understand the consequences of such deficits and the corresponding preventive concepts.