Reliability and validity of a low-cost luggage scale for hip external rotation strength measurements

[Purpose] The purpose of this study was to determine the intra- and inter-rater reliability of strength testing of the hip external rotators using a dynamometer and a luggage scale. Furthermore, the secondary purpose was to examine the validity of the luggage scale when compared to the dynamometer. [Participants and Methods] Twenty-two participants were strength tested bilaterally (n=44). Hip external rotation strength was tested using a Lafayette handheld dynamometer (HHD) and an analog luggage scale (LS) with the participant prone with their knee flexed to 90 degrees and hip maintained in neutral. [Results] The results indicated good to excellent intra-rater reliability for the HHD (ICC=0.921, 95% CI=0.864, 0.99, ICC=0.886, 95% CI=0.772, 0.923) and excellent intra-rater reliability for the LS (ICC=0.971, 95% CI=0.948, 0.984, ICC=0.926, 95% CI=0.871, 0.958) for each examiner. Inter-rater reliability was good for the HHD (ICC=0.891, 95% CI=0.837, 0.927) and excellent for the LS (ICC=0.938, 95% CI=0.907, 0.958). Strong correlations were found between the HHD and LS by both examiners (r=0.869, r=0.750, respectively). [Conclusion] Both an HHD and LS are reliable for measuring hip ER strength. The luggage scale is a valid, cost-effective means for reliable measurement of hip ER strength.