Subjects
Fourteen healthy males (age, 24.9 ± 3.2 y; height, 1.73 ± 0.05 m; weigh, 67.5 ± 9.5 kg) and 15 healthy females (age, 23.2 ± 1.4 y; height, 1.60 ± 0.04 m; weight, 49.4 ± 4.4 kg) participated in this study; their right foot was measured. Subjects with orthopedics or neurological disorders were excluded. Informed consent was obtained from all participants before their inclusion in the study, and the ethics committee of the Faculty of Medicine, Kagoshima University approved the study protocol (ref no. 359).
Procedure
The relation between arch height during walking as measured by a tablet computer and motion capture system, and static foot alignments tests were estimated in order to test the validity of dynamic assessment using 2D analysis. Arch height of the right foot during walking was simultaneously measured using a tablet computer (iPad Air2, Apple, Inc., CA, USA) and motion analysis system consisting of 6 cameras (VICON MX3, Oxford Metrics, Oxford, UK) and two force plates (BP600400, OR6-7, AMTI Inc., MA, USA). Prior to gait measurement, reflective markers were attached to the lateral and medial epicondyle, the lateral and medial malleolus, the first and fifth heads of the metatarsal bone, the medial and dorsal point of the calcaneus, and the navicular, according to a previous study [21]. The tablet computer was placed vertically on the floor at the left side of the walkway and 1.3 m from the midline, so that it could capture the medial aspect of the foot (Fig. 1). One central stance phase during an 8 m comfortable walking gait was analyzed. The measurement was performed after a warm-up period, and the mean of 10 samples was adopted as the representative value. The sampling frequencies of the motion capture system and the tablet computer were 100 Hz and 120 Hz, respectively.
Static foot posture was assessed using the arch height index (AHI) and foot posture index (FPI) during relaxed standing after several steps. AHI was the ratio calculated by dividing the dorsal arch height at 50% of total foot length, measured by height gage (VHK-15, Niigata Seiki Co, Ltd, Niigata, Japan), by the total foot length [9]. The FPI consisted of 6 components: talar head palpation, supra and infra lateral malleolar curvature, calcaneal frontal plane position, bulging in the region of the talo-navicular joint, height and congruence of the medial longitudinal arch, and abduction/adduction of the forefoot on the rearfoot [7]. Each component was scored on a scale ranging from -2 to +2, and the total score ranged from -12 to +12; a low value indicated pronation, high values indicated supination.
Data analysis
Arch height at the mid-stance (Mst) phase, the moment when the right tibia is positioned vertically, and at the pre-swing (PSw) phase, the moment of heel strike of the opposite side, were obtained from kinematic data measured by the tablet computer and motion analysis system, respectively. In 2D analysis using the tablet computer, arch height was calculated as the distance between the navicular tuberosity and the baseline connecting to the medial aspect of the calcaneus and the first metatarsal head, and calculated as a percentage of the baseline using ImageJ (National Institute of Mental Health, MD, USA) by Windows PC (FMV-BIBLO NF/G50, Fujitsu, Kanagawa, Japan). In three-dimensional (3D) analysis, arch height was calculated as the distance between the navicular tuberosity and a plane consisting of the first metatarsal head, fifth metatarsal head, and the dorsal point of the calcaneus.
Pre-statistical analysis showed no sex difference in foot alignment, therefore we treated males and females as one group. Furthermore, we performed power analysis to estimate the validity of sample size by referring to a previous study, which reports that the correlation coefficient between 2D analysis and 3D analysis, and between static foot alignment and dynamic foot alignment, were 0.76 and 0.56, respectively [25]. Thus, power analysis was performed using the G * Power, r = 0.50, α = 0.05, and power (1 – β) = 0.8, indicated that the required sample size was 26. Thus, we accepted that this study had a suitable sample size.
To examine the validity of foot assessment using 2D analysis, we conducted correlation analysis between the arch height at Mst and PSw as measured by a tablet computer and a motion capture system. Meanwhile, FPI and AHI were used to test the relation to arch height at Mst as measured by a motion capture system because of the similarity of their measurement posture. These relationships were analyzed using Pearson’s correlation coefficient or Spearman's rank correlation coefficient after data were tested for normality using the Shapiro-Wilk test. All statistical analyses were performed using R (2.8.1) statistical software, and significance was set at 5%.