Background:A quantitative and accurate measurement of the range of hip joint flexion (RHF) is necessarily required in the evaluation of disordered or artificial hip joint function.This study aimed to assess a novel method to measure RHF more accurately and objectively.Methods:Lateral radiographs were taken of 31 supine men with hip joints extended or flexed.Relevant angles were measured directly from the radiographs.The change in the sacrofemoral angle (SFA) (the angle formed between the axis of the femur and the line tangent to the upper endplate of S1) from hip joint extension to hip joint flexion,was proposed as the RHF.The validity of this method was assessed via concomitant measurements of changes in the femur-horizontal angle (between the axis of the femur and the horizontal line) and the sacrum-horizontal angle (SHA) (between the line tangent to the upper endplate of S l and the horizontal line),the difference of which should equal the change in the SFA.Results:The mean change in the SFA was 112.5 ± 7.4°,and was independent of participant age,height,weight,or body mass index.The mean changes in the femur-horizontal and SHAs were 123.0 ± 6.4° and 11.4 ±-3.0°,respectively.This confirmed that the change of SFA between hip joint extension and hip joint flexion was equal to the difference between the changes in the femur-horizontal and SHAs.Conclusions:Using the SFA,to evaluate RHF could prevent compromised measurements due to the movements of pelvis and lumbar spine during hip flexion,and is,therefore,a more accurate and objective method with reasonable reliability and validity.