The main findings of this study display major differences between males and females in shoulder performance fatiguability with arm position, dominance and muscle group playing an important role. It has previously been established that the arm spatial orientation affects muscle activation and strength of the shoulder muscles [17, 25]. In overhead athletes it has been found that these individuals are at a higher risk of shoulder related injuries, with the ligaments around the shoulder weakened due to the overload and repetitive stress [26, 27] produced by reduction in the subacromial, with the higher angles of abduction , especially when the rotator cuff muscles are in a fatigued state . Therefore, it is important to use a test which demonstrates high levels of reproducibility, in order to be able to detect any worthwhile changes when individuals are fatigued (a major factor which affects shoulder performance and functional dynamic stability). Horobeanu et al.  previously assessed the reliability of two isokinetic set-ups: lying supine with arm abducted at 45⁰ and at 90⁰ in frontal plane, for assessing shoulder rotators fatigability properties and found both set-ups demonstrating very high reliability. Further, it has also been established that lying in a supine position is not only comfortable for the individual, but it provides the best support base. Assessing the degree of variation for isokinetic strength Forthomme et al.  reported a “good” coefficient of variation below 12 %. The largest advantage of this position is that the gravitational forces are evenly distributed either side of zero (vertical) when testing the shoulder IR and ER muscles. Therefore, the results are not influenced by unquantified forces (such as gravity) especially during fatigue resistance assessment of a population which are not involved in “upper limb” sport participation. In addition, the specificity of the testing position is suitable for testing sport activities in overhead athletes.
The most notable finding of our study was that all measures related to performance were statistically and clinically different between IR and ER shoulder muscles in both males and females. It was found that the IR muscles were more fatigue resistant (IF: 8.19%), able to perform higher amounts of work over 30 repetitions (C.Perf: 45%) and able to develop more work during a single repetition (BR: 39.23%) compared to the ER muscles. Many studies have established differences in several performance variables between the IR and ER shoulder muscles. The IR muscles have always been found to be significantly stronger than the ER muscles [16, 29]. These observed differences are due to the muscle-size differences between both muscle groups, where the IR muscles can produce a larger amount of force due to the larger cross-sectional area. In addition, the IR muscles have a larger lever arm than the ER muscles, meaning more force can be produced . In addition, there are differences from a biomechanical perspective between IR and ER shoulder muscles in relation to size and volume which further accentuates the significant differences found in favour of the IR muscles. The results of our study also found similar results to previous reports related to ER:IR ratios which seems to be between ~ 1:2 and 9:10 [16, 30, 31], for pain-free sedentary individuals.
Another finding of the current study was that no differences in performance (IF, C.Perf, BR) were present between supine position with the arm abducted to 90⁰ angle and a supine position with the arm abducted to a 45⁰ angle when considering only the arm position without differentiation of other possible interactions. In addition, arm dominance also showed no variation between the dominant and non-dominant arm. Therefore, considering previous literature findings related to changes in IR and ER muscle performance between different abduction angles, it was important to further investigate possible interaction of gender and dominance. Looking at whether possible interactions between arm position, dominance, muscle group and gender on muscle performance during 30 reciprocal maximal contractions at 180°/s will further explain findings between these. Our data suggests than increasing the abduction angle to 90° as opposed to 45° positively impacts only the work of ER regarding BR in both males and females and in both the dominant and non-dominant arm while there is no influence on IR. These results contradict the findings of Golebiewska et al.  who observed a decrease in muscular strength from 45° to 90° of abduction in frontal plane for both muscle groups. It must be noted that their performance measure was peak torque, while we considered the induced fatigue, work done over all repetitions and the best repetition. Peak torque looks at only one point of the movement, the highest one on the angular curve, while the work represents the whole area under the curve , which explains potential differences between these studies. Peak torque is not necessarily the etalon of all other torques developed through the entire range of movement, having a consistent occurrence between certain degrees of movement. However, the work accounts for the overall modification of the curve, not only its peak . In addition, considering that their participants performed the test in a seated position, could further explain the disagreement. Regarding fatigue resistance, IR were deemed to be less fatigued than ERs after 30 repetitions in both the dominant arm (-38.71% vs. -46.69%) and non-dominant arm (-39.09% vs. -46.94%) for males at the 90° position while the females were less fatigued on non-dominant side and only at 45° (-40.11%vs. -46.71%). Similar profiles have previously been found in the literature with IR displaying more fatigue resistance than ER in a study performed by Ellenbecker & Roetert,  looking at young tennis players. Differences in fatigue rates between IR and ER have a clinical relevance due to ER functioning as a humeral head stabilizer  especially for the athletic population where it has been shown to alter performance  and be a potential shoulder injury risk factor. Differences in shoulder position affect the muscle activation around the shoulder and its rotational strength [26, 34]. Studies on a larger population and from variate age groups are necessary to explore their influences and possible implications for different rehabilitation programs of non-athletic populations.
Finally, it is well known that males are stronger than females when comparing different muscles groups [35–38]. Gender differences have previously been found for knee antagonist muscles, quadriceps and hamstrings, with males performing between 25–40% more work compared to their female counterparts [36, 37] during maximal concentric reciprocal contractions. Our study observed shoulder rotator muscles in males to be significantly stronger than females (P < 0.05) by 50%, for both arms and at both angles of shoulder abduction. The amount of difference between genders should be viewed from the perspective of raw data, without normalisation to body weight. The choice of comparing raw values has been agreed in order to make possible comparisons with other findings in the literature. However, there was no gender influence on fatigue resistance of the shoulder rotator muscles, showing both genders having a similar reduction in performance, ranging from 38–47%, after 30 reciprocal concentric contractions, respectively. Present results are in line with findings established by Senefeld et al. , who reported no significant gender difference but in contradiction with previous findings by Avin et al. , who showed females were more fatigue resistant than men. However, in their study the subjects were tested for their isometric sustained capabilities at elbow level, as opposed to the shoulder. Therefore, we support the affirmation of Hunter  that the gender differences in regard to fatigue resistance is task specific because different neuromuscular sites will be stressed when the requirements of the task are altered, and the stress on these sites can differ for men and women. Task variables that can alter the gender difference in fatigue resistance include but are not limited to the type, intensity and speed of contraction, the muscle group assessed, and the environmental conditions. In addition, it is also important to understand the impact of menstrual cycle phase on the effect of study design and potential outcomes, with different phases showing different fluctuations in performance levels [39, 40]. In order to gain a better understanding, monitoring females during different stages of the menstrual cycle will provide a clearer picture. Nevertheless, differences between males and females are apparent, but explanations for these differences remain unanswered and more research is required in order to supplement the literature.