This study was approved by the institutional review board of our university, and informed consent was obtained from all participants. In total, 15 female participants with hip OA and 15 female healthy controls were recruited. The inclusion criteria of the hip OA group were women who were scheduled to undergo unilateral total hip arthroplasty (THA) for treatment of moderate to severe OA and aged < 85 years. The severity of OA was determined on radiography according to the Kellgren and Lawrence (KL) grade  in the participants with hip OA. The exclusion criteria of the hip OA group included a history of (1) immunosuppression or autoimmune deficiency, (2) inflammatory arthritis, (3) local or systemic infections, (4) knee arthritis and/or total knee arthroplasty, or (5) symptomatic spinal cord disease. The Harris hip score, which includes sections on pain, function, absence or presence of deformity, and passive range of motion and is scored from 0 (worst) to 100 (best), was used to evaluate participants with hip OA. None of the healthy controls had a history of bone fracture or surgery in the lower limbs; history of neurological, respiratory, or cardiovascular diseases; musculoskeletal disorders within the past 6 months; or previous history of trauma.
The participants wore specifically designed shorts with sensors while using the LBPP treadmill. The height of the chamber was fixed to accommodate the participant, and sensors from the shorts were attached to the LBPP treadmill. Then, it was set equal to the height of the greater trochanter of the participant’s femur (Fig. 1). To determine the correlation of gravity and the internal pressure of the chamber, calibration was performed for each participant as previously described . The participants walked at a self-selected speed on the LBPP treadmill (Anti-Gravity Treadmill M320, AlterG, Inc., Fremont, California, USA) at 100%, 75%, and 50% body weight (BW) conditions. The walking speed was consistent across the loading conditions. The participants walked for 30 s under three conditions selected randomly (100%, 75%, and 50% BW) for the testing procedure. Before recording the walking trials, they were asked to familiarize themselves with walking on the LBPP treadmill for 3 min and given 90 s to adapt to each BW condition (Fig. 1). The participants in the hip OA group were asked to assess their hip pain using a numeric rating scale (NRS) in which the scores 0 and 10 represented no pain and the worst pain, respectively , during walking under 100%, 75%, and 50% BW conditions.
Data collection using the motion analysis system
All data collections were performed on the OA side in the OA group and on the dominant leg in the control group. The dominant side in the control group was defined according to which leg the participants used for kicking. Data were collected using a motion analysis system (H-Gait system, Laboratory of Biomechanical Design, Hokkaido University, Sapporo, Japan) where wearable sensors analyzed the 3-D gait kinematics [8, 12]. Briefly, seven wearable sensor units (TSDN121, ATR-Promotions, Inc., Kyoto, Japan), which consisted of triaxial acceleration and gyro sensors, were placed on seven lower-limb body segments (pelvis, right and left thighs, right and left shanks, and right and left feet), as shown in Fig. 2. Acceleration and angular velocity data were collected simultaneously during gait via wireless connection (Bluetooth) in real time. Sensor specifications were the same as those mentioned in the previous studies [8, 12].
According to a previous study , a calibration test for each participant was performed to measure the acceleration data of the sensors in the upright and inclined positions to calculate the initial inclination of each sensor with respect to the gravity. Before each trial, an initial static phase was acquired in the upright position. When the participants started walking, subsequent 3-D orientations from the initial one were estimated by integrating the angular velocity with the drift removal using the MATLAB software (MathWorks, Natick, MA, USA) . The 3-D angular displacement from the initial upright position was calculated in a quaternion according to a previous study . From these data, the spatiotemporal gait parameters; hip joint angles in the sagittal, coronal, and transverse planes; and knee and ankle joint angles in the sagittal plane during walking under each BW condition were evaluated in each participant. This H-Gait system divided 30 s of walking into gait cycles and calculated the angles of each joint for every gait cycle. The median gait cycle represented by this system during 30 s of walking under each BW condition was used for analyses. For the gait cycle, one gait cycle from the heel contact to the next heel contact was normalized to 100%. The swing and stance phases were defined using the heel contact and toe off timings of both legs. The heel contact and toe off timings were detected using the peak angular velocity data of the shank as previously reported [12, 14]. With regard to the validity and reliability of the gait analysis system, Tadano et al. analyzed the kinematics of the lower limbs in walking using the H-Gait system and compared them with that using a camera-based motion analysis system . The correlation coefficients of the hip and knee flexion and ankle dorsiflexion angles were 0.98, 0.97, and 0.78, respectively. The minimal detectable changes were as follows: 5.1° for the peak hip flexion angle; 3.4°, peak hip extension angle; 3.9°, peak hip abduction angle; 2.5°, peak hip adduction angle; 3.3°, peak hip external rotation angle; 3.6°, peak hip internal rotation angle; 7.0°, peak knee flexion angle; 3.5°, peak knee extension angle; 5.9°, peak ankle dorsiflexion angle; and 5.2°, peak ankle plantar flexion angle.
The demographic characteristics and walking speed between the groups were compared using independent Student’s t-tests. One-way ANOVAs with post hoc Bonferroni tests were used to investigate the differences in the NRS pain scores during walking under 100%, 75%, and 50% BW conditions for the hip OA group. Two-way repeated ANOVAs (3 BW conditions × 2 groups) were performed to assess the main effects of the BW conditions (100%, 75%, and 50% BW) and groups (control, OA) on the spatiotemporal gait parameters and peak angles of each joint. When the interactions were nonsignificant, the main effects were assessed. If the main effect of the BW condition was statistically significant, post hoc Bonferroni tests were performed to evaluate the significant differences in the spatiotemporal gait parameters and peak angles of each joint among the BW conditions. In addition, the effect sizes for the main effect and interaction between unweighting and group were calculated to determine the magnitude of the differences using eta squared (η2). The significance level was set at 0.05. Statistical analyses were performed using the IBM SPSS version 17 software (SPSS Inc., Chicago, IL, USA).