Discussion
We evaluated a series of novel isometric long lever shoulder tests performed on a force platform and found excellent reliability (ICCs 0.94–0.98) in the three ‘IYT’ positions. There are no upper limb isometric force platform tests in the literature for comparison, but this compares favourably to the reliability reported for upper limb HHD tests such as internal and external rotation strength measured in a range of positions: standing, ICC: 0.92–0.9631; seated, ICC: 0.68–0.9932 and in multiple positions, ICC: 0.93–0.99.17 Athletic Shoulder (ASH) test reliability was also higher than peak force reliability in a lower limb isometric test (ICC 0.86–0.95),6 which also used a 3 s maximum effort against a force platform. Based on the criteria (CV<10%) used by McCall et al,6 and others, all test positions (CV 5.0%–9.2%) can be considered reliable, except for the non-dominant arm I-test (CV 11.3%).30 Therefore, practitioners can be confident that the force platform IYT tests are reliable in detecting between subject and within subject differences.
In practical terms, high reliability does not necessarily mean that a test has the sensitivity to detect a real change,6 10 17 and MDC is useful in a monitoring context to establish how far ‘the needle needs to move’ before a change can be considered meaningful. IYT values for MDC90 were 13.3–25.9 N (MDC% ranged from 10.7 to 20.1), meaning that a change of >13.3 N in a dominant arm T-test would be considered meaningful with a confidence level of 90%. For comparison, MDC% 7.5%–19.3% was reported in HHD IR and ER strength tests in sitting, supine and prone positions,17 and MDC%≤17.8% in prone shoulder extension.33 Holt et al
34 (2016) compared average internal and external rotation strength in neutral using HHD with and without external fixation (EFD) and found the range of intrarater %MDC’s were lower for HHD (17.3%–19.7%) than EFD (19.3%–29.0%). McCall et al6 reported MDC90 of between 26.2 and 36.9 n across limbs and the two positions of a posterior chain isometric force platform test. Thus, the power to interpret change in the ASH test is comparable with relevant isometric tests in the literature.
In addition to reliability and interpretability, a new test should also demonstrate validity and clear purpose.10 The force platform is a gold standard force measurement tool and valid for use in assessing isometric strength.6 22 The principle purpose of the IYT tests was to quantify force production and potential interlimb or postinjury deficits in rugby specific shoulder positions. The T-position is similar to the ‘arm tackle’ position, while the I-position closely represents the ‘try-scorer’ injury mechanism.35 Both tests are performed using a straight arm with force delivery at the end of a long lever, corresponding to the higher torques and force transfer requirements experienced in sporting actions than the majority of short lever (bent arm) tests in the literature. Therefore, the ASH test may have value in tackling sports and in other sports, occupations and tasks that expose the shoulder to longer lever stress or require the ability to transfer forces similarly across the shoulder girdle. It is our preliminary observation from pilot testing that the long lever test demonstrated unilateral deficits that were not evident in short lever testing using HHD which warrants further investigation. To our knowledge, the only straight-arm test not using an isokinetic dynamometer but instead using HHD was developed for swimmers, with a similar subject set up to the I-test position of the current study.24 The authors reported lower interday reliability values in the dominant arm (ICC 0.81–0.95; SEM% 4.0–8.8) than we observed in the I-position (ICC 0.95–0.97; SEM% 5.6–7.5). Furthermore, one strength of the present study in terms of applicability to elite athletes was the assessment of reliability in an elite population, which few shoulder strength studies have done. The higher forces produced by more elite athletic populations are more likely to reduce HHD test reliability,34 suggesting that a stable force platform would be more appropriate for these populations, eliminating the negative influence of tester-athlete strength imbalances on reliability, particularly in stronger athletes.17 34 36
Limitations
There were small differences in reliability between the three test positions, which could be related to different requirements of the subject to stabilise their trunk against rotational forces. Without randomisation of test order for each subject however, comparing reliability between test positions is not possible,19 and as such our study design does not allow this comparison. It could be argued that a standing test would be a more sports-specific position to evaluate the ability to produce force in a standing tackle. However, a standing test has greater potential for test-retest variability due to the additional degrees of freedom, while the prone position on the floor has the advantage of minimising the involvement of other body segments, which could explain the high reliability observed. The excellent reliability reported in the current study could also be related to the removal of inconsistencies outlined by other authors.19 In addition, trials were discarded if there were visible compensatory strategies. The duration of the test and the rest period between trials can influence results.26 In pilot testing, we determined that 20 s was the minimum optimal rest period needed for subjects to consistently repeat a maximum effort without deterioration in scores across the three trials. In other studies, test duration ranged from 3 to 5 s, with rest periods ranging from 10 s17 to 5 min,33 factors that may have influenced intratest reliability. The authors recommend that the full ASH test protocol be completed in preseason to establish individual baselines for comparison (duration 6 min excluding standardised warmup). In-season monitoring can focus on one or two positions (duration 2 to 4 min) to reduce time requirement. Although the method required that players were familiarised prior to testing to minimise any learning effect and 72 hours rest prior to the study protocol, it is possible that small increases in mean NPF between day 1 and day 2 were influenced either by a learning effect or additional recovery time. This emphasises the importance of familiarisation prior to interpretation of test results. It is important to recognise that results were obtained from elite male rugby players and therefore clinimetrics need to be verified in other populations.
Future studies
The portability, simplicity and rapid implementation of the test in a gym/training environment means that it has the potential to be used; with individual athletes during rehabilitation to assess progression in force production following injury; to inform return to competition decisions and to regularly monitor recovery of shoulder performance following competition induced fatigue in groups of athletes. Further work is needed to evaluate the sensitivity of the ASH test to competition induced fatigue and the profile of strength decrement and recovery as has been demonstrated in isometric lower extremity testing.6 Preliminary data in the present group of athletes suggest that the test may be a useful tool in this capacity as we noted ASH test strength deficits at 48 hours after a competitve rugby game with recovery at 72 hours. We have also observed changes beyond the 25 N MDC threshold in a variety of glenohumeral as well as cervical spine rehabilitation cases. However, further studies that show the sensitivity of the ASH test to alterations in training load, relationship to injury and the response to rehabilitation are the necessary next steps in determining the usefulness of this test across a wide range of environments. NPF was highest in the I-test followed by Y-test and T-test, respectively, and it may be of interest to determine normative ratios across the IYT positions in relation to sport and injury risk in larger samples.