Abstract
Throwing athletes involved in overhead sports exhibit a higher occurrence of overuse shoulder
injuries when compared to both non-throwing athletes and the general population. Overuse
shoulder injuries affect both junior and senior athletes and are mainly attributed to overuse,
training errors, biomechanical errors, and possible muscle imbalances.
This study aimed to assess the prevalence of potential shoulder muscle imbalances among
junior track- and field throwing athletes participating in discus, javelin, shot put and hammer
throw. By making use of a descriptive-, explorative-, quantitative cohort study, isokinetic
testing was used to determine possible shoulder muscle imbalances amongst these junior trackand
field throwing athletes. The reason for focusing on junior athletes is due to their significant
muscle growth and physical development during adolescence. A sample of 60 under 17 (u/17)
to u/19 male (n=30) and female (n=30) track- and field throwing athletes from the Central and
Northern Gauteng region were recruited. Isokinetic testing consisted of shoulder internalexternal
rotation on a Humac Norm isokinetic dynamometer. Isokinetic testing of shoulder
external- and internal rotation was conducted with participants seated, positioning the shoulder
in a horizontal, scapular plane, and maintaining the elbow at 90° of flexion. Concentric
isokinetic testing, corrected for gravity, was executed at a velocity of 60°/s. The main reason
for testing athletes concentrically as well as at 60°/s is due to the main muscles, which is
responsible for throwing, acts predominantly concentrically, while the stabilizer muscles act
eccentrically. Testing at 60°/s was more reproducible than the faster speeds. Statistical
significance was set at p≤0.05.
In total, 35 junior participants (n=35); 21 males (average age=17.76) and 14 females (average
age=18.07) completed the study. Significant differences were found between the dominant
shoulder (DS) and non-dominant shoulder (NDS) during concentric external (p=0.000) and
internal (p=0.000) shoulder rotation peak torque at 60°/s for the males (DS > NDS), and
between the DS and NDS during concentric internal shoulder rotation peak torque (p=0.000)
for the females. There was no significant difference between the DS and the NDS for concentric
external shoulder rotation peak torque for females (p=0.090). When comparing the bilateral
agonist/antagonist ratio for males, there was a significant bilateral difference of 11.7%
(p=0.008) between the DS and the NDS external/internal shoulder rotation ratios. However,
there was a non-significant difference of 6.05% for females between the DS and the NDS
XIV
external/internal shoulder rotation ratios (p=0.310). The findings of this study emphasize the
presence of significant differences between the DS and NDS in concentric peak torque of the
shoulder internal and external rotator muscles, but also between the DS and NDS
external/external rotation ratios in male junior track- and field throwing athletes. In the female
athletes the only significant difference observed was between the DS and the NDS for
concentric internal shoulder rotation peak torque. In summary, both male and female throwers
had bilateral asymmetry in their shoulder rotator muscles and they had lower agonist/antagonist
ratios compared to previous research (external/internal shoulder rotation ratios less than 60%),
possibly predisposing the athletes to increased risk for shoulder rotator cuff injury.
Future researchers may combine other testing items like scapular stability, posture, and
flexibility measures with isokinetic testing to produce a more comprehensive testing protocol.
This study contributes to the establishment of reference values that can be used for concentric
isokinetic peak torque (at 60°/s) for the shoulder rotator muscles among male and female junior
track and field throwing athletes. The findings may prove valuable for throwing athletes,
coaches, and biokineticists in evaluating shoulder muscle strength, tracking training-induced
strength changes, and in the prevention or rehabilitation of shoulder injuries.