The aim of this work was to evaluate the reliability of a functional and user-friendly test to measure hip abductor strength in older people. This is particularly important since hip abductor strength is known to be associated with a risk of falls . The results showed that testing hip strength in a standing position is feasible and rapid. All participants successfully completed the test in less than four minutes. Moreover, intra-rater reliability was good for the total sample as well as for both the faller and non-faller subgroups. As testing hip strength in a standing position is feasible, reliable, and better reflects real life situations than strength tests in supine or side-lying positions, we recommend this position for clinical practice.
The test-retest reliability for MVIS found in the present study was similar to that found by Wang et al. in supine position . They evaluated the reliability of strength measurements using a HHD in 8 lower limb muscle groups in community-dwelling, older fallers. However, this study included only fallers without a comparison with non-fallers. In our study, ICC values for MVIS were better in non-fallers than fallers, while the intra-rater reliability for RFG was lower for the non-fallers than for fallers. Nevertheless, ICC values remained above 0.75 which is a good reliability and an appropriate threshold value for clinical use . Our results and those of Wang et al.  showed that hip abductor strength can be measured reliably in older subjects at risk of falls both in supine and standing position.
Widler et al. assessed intra-rater reliability in young adults . They compared intra-rater reliability in standing (ICC = 0.88), side-lying (ICC = 0.90), and supine positions (ICC = 0.82). They found the highest ICC in side-lying, although reliability was also good in the two other positions. The level of test-retest reliability found in the present study was higher than in the study by Widler et al. . This difference might be due to more heterogenic strength values for our study sample, but also due to differences in the test position . In the present study both forearms of the participants were supported on a high treatment table, while in the study by Widler et al. subjects stabilized themselves with only one hand . The importance of stabilization on reliability has been confirmed by a recent study . Awwad et al.  who assessed hip abductor strength in older adults in a standing position showed values close to our results. In contrast to our study their participants stabilized the pelvis against a wall and participants indicated to perceive fatigue in the opposite hip to that being tested . This was probably due to the fact that standing leg was necessary to stabilize the pelvis in this position. Awwad et al.  further showed an increased intra-subject variability in strength values recorded in the standing position compared to the supine position (in which the pelvis was also stabilized against the wall). In our study the pelvis was not stabilized against a wall. Nevertheless, we found similar ICC’s to Awwad et al. and none of the patients complained of fatigue in the supporting leg. These facts support the use of the testing position described in the current study. In addition, our test has the advantage of being closer to different motor functions during which the pelvis is never stabilized.
Another explanation for the discrepancy between the study of Widler et al.  and our results could be the differences in the subjects’ ages. The greater strength of young subjects induced certainly greater difficulties to avoid compensations . As the muscle strength in fallers was lower than in non-fallers, it is likely that fallers compensated less which might have increased reliability. In the supine position with belt-resisted dynamometer fixation during strength testing, Kramer et al. found higher test-retest reliability in older subjects (ICC > 0.98) than in young adults (ICC > 0.92) . The authors attributed this ICC difference to the compensation during test but also to higher between-subject strength variation in older adults. In standing position, the intra-rater reliability of MVIS measured in older adults in our study was higher (ICC > 0.94) than the intra-rater reliability of MVIS in young healthy subjects of the study of Widler et al. (ICC > 0.88) . The results of these studies corroborate the results of Kramer et al. who tested older and young adults in a side-lying position . Among the study population fallers were eight years older than non-fallers. However, the ICC of hip abductor MVIS were similar in fallers and in non-fallers whereas higher ICC’s were found for the RFG measures in fallers (ICC = 0.93) than in non-fallers (ICC = 0.88). It might be that the RFG is more influenced by age than the MVIS and thus induces more heterogeneity and consequently a higher ICC values in RFG parameter. It is well known that movement slows down with advancing age which is thought to be partially strategic in that older adults give emphasis to movement accuracy at the cost of movement speed . In addition, slower information processing due to an increase in neural noise and other synaptic changes may also affect the RFG . Nevertheless, we cannot exclude that several other unknown factors playing a role in these differences.
In both the present study and the study by Widler et al. , the HHD was fixed. Several studies have shown that when a HHD is fixed externally, subjects generate higher magnitudes of peak force, and test-retest reliability is higher than for manual, examiner resisted HHD measurements [15, 31]. In our study, the HHD was fixed to the wall, thus the procedure was standardized, and no bias was created by variations in rater-force. Moreover, the procedure is faster when the HDD is fixed externally.
Comparison of the strength parameters showed that reliability was higher for MVIS than RFG. To our knowledge, no other study has evaluated the test-retest reliability of the RFG of the hip abductors in a standing position, despite the fact that RFG is an important parameter relating to dynamic balance control in older subjects [4, 17, 18]. Although the reliability of RFG was lower than MVIS, it was still good (ICC > 0.88). Thus, according to clinical recommendations by Portney and Watkins , this parameter can be used to evaluate hip abductor strength in clinical practice. We believe it is relevant to assess both MVIS and RFG since they are not correlated , and both are associated with an increased risk of falls in older women . In clinical practice, the dynamometer usually directly shows MVIS, but not RFG. Thus, in the future, it would be useful to develop HHD tools with RFG values directly displayed on the screen facilitating the use by clinicians.
ICCs are highly dependent on the heterogeneity of the study sample while SEM, SDD and LOA are more appropriate to evaluate changes over time . SEM values are also interesting to clinicians in terms of decision making, since they describe errors in the same units of measurement, and can be used to calculate SDD between two measurements . In standing, the SEM for MVIS was very similar between fallers and non-fallers, however, the SDD was higher in fallers (respectively 32.7 and 20.3%). In whole group, the SDD of RFG was higher (44.3%) than the SDD of MVIS (25.2%). Mentiplay et al. showed the same results for young subjects measured in a supine position with a SDD of RFG of 34.65% and a SDD of MVIS of 20.23% . The SDDs found in the present study were lower than the ones in our previous study that measured hip abductor strength in older adults in a side-lying position (RFG: 51%, MVIS: 32%) . Thus, we recommend in clinical practice testing hip abductor strength in older persons at risk of falls in a standing position. Similarly to Widler et al.  and Awwad et al. , who evaluated maximal hip strength value in a standing position, Bland and Altman analysis for intra-rater reliability showed no systematic session effect for the MVIS. This suggests that no learning effect or fatigue occurred in the patients or raters and confirms the quality of our procedure without previous practical trials for this parameter. Nevertheless, Bland and Altman analysis showed somewhat higher mean between sessions differences for RFG, especially for the fallers, indicating a small bias, i.e. a small learning effect between sessions. The greater mean differences between session for RFG than for MVIS have also been mentioned by Mentiplay et al. . The good intra-rater reliability and the small systematic effect which has previously been observed for MVIS in older people , the lower SDD observed in older persons at risk of falls in a standing position compared to the ones of older persons in a side-lying  and the feasibility highlight the interest of the clinical abductor test in standing position.
It should be noted that the present study only evaluated intra-rater reliability. In addition, the test position of participants was only verified visually without the use of precise sensors. However, this reflects clinical practice. In addition, the rater was not blinded to the faller or non-faller status of the participant. In this study, before tests, the subjects haven’t had a separate familiarization session. We decided for this procedure according to our pilot test as well as to avoid additional fatigue for these older subjects. Indeed, we think that fatigue is a more important bias than the lack of a familiarization session. However, we believe that the reliability could be improved for patients with low fatigue and who have had a practice trial, especially for RFG parameter. The MVIS and RFG parameters were assessed in the same trial, in order to avoid too many repetitions for the participants. This implicated a more complex test instruction (push hard and fast) which may have been difficult for some participants to achieve. Testing MVIS and RFG in two separate trials may produce more accurate results for the assessment of strength in an older population.
This study focused on immediate isometric hip abductor strength without considering endurance, which is an important parameter for activities of daily living. Moreover, it is well known that type 1 muscle fibers tend to degenerate in older people [34, 35]. Van Cant et al.  found good test-retest reliability for both isometric and isotonic hip abductor endurance in a side-lying position in young adults. Therefore, future studies should determine the reliability of testing both maximal isometric strength and endurance of the hip abductors in a standing position in older subjects.