Participants
This cross-sectional descriptive study targeted individuals, male or female, aged 18–45 years with a body mass index (BMI) ranging from 18.5–24.9 kg per square metre (kg/m2) who were able to perform and follow research instructions. Participants were excluded if they were pregnant and/or had lower-extremity pain or injury within the six months prior to data collection, current lower extremity pain, a history of surgery and/or fracture of the spine or lower extremity, diagnosed neurological deficits such as stroke and spinal cord disorder, spinal scoliosis, rheumatoid, gout and/or systemic lupus erythematosus [9, 14, 15]. The study protocol was approved by the local centre for ethics in human research (Registration number: HE602301). Individuals who live in local province were invited to participate in this study using an advertisement (i.e., a poster) and face-to-face meetings. Prior to data collection, all potential participants gave their written informed consent.
Sample size calculation
The optimal number of participants was calculated based on an agreement analysis of two different measures with Cohen’s kappa and aimed to detect a significant substantial agreement (kappa = 0.80) between them. To obtain the optimal sample size, the present study consulted the table for sample size estimation for kappa analysis [16]. The present study defined the kappa to detect as approximately 0.80, with the null hypothesis value of kappa equal to 0.40 (two-tailed) and the power of the test set to 80%. Hence, the optimal sample was 102 individuals.
Clinical measures of foot type classification
The level of agreement was determined using the right foot of all participants [14, 17]. The measurement procedures were conducted as follows.
The NNHt (Fig. 1) was conducted as the uni-planar measure to classify normal, pronated and supinated feet [9, 18]. The NNHt referred to the ratio of the navicular height (mm) to the truncated length of the foot (mm). While barefoot, participants were asked to stand still with their arms by their sides in the double-stance relaxed position on top of a 90x80x25-cm stool. The perpendicular distance between the most prominent part of the navicular tuberosity and the supporting floor (H) was measured using a card size of 7 × 12 cm2, and a dial calliper (precision = 0.05 mm; Oxford Precision, Oxford, GB) was used to measure the H. Then the truncated length of the foot (the perpendicular distance between the first metatarsophalangeal joint and the most posterior end of the heel) was measure using a steel ruler (L). Three repeat measures were conducted, and the average value was used to classify foot posture. Participants with NNHt scores < 0.17 had a highly pronated foot, whereas those with scores from 0.17–0.21 had a pronated foot, 0.22–0.31 a normal foot and 0.32–0.35 a supinated foot. Scores > 0.35 represented a highly supinated foot [9].
The FPI-6 was conducted as the tri-planar measure that combined the semi-quantitative examination of foot posture from the forefoot, midfoot and rearfoot segments, and it can identify normal, pronated and supinated feet [11]. While barefoot, participants were asked to stand still for two minutes with their arms by their sides in the double-stance relaxed position. The rater observed and palpated the participant’s foot and scored each criterion of the FPI-6. The FPI-6 consists of six criteria: a) talar head palpation, b) lateral malleoli curvature, c) calcaneal inversion/eversion, d) talonavicular bulging, e) the height and congruence of the medial longitudinal arch, and f) the forefoot on rearfoot abduction/adduction (Fig. 2). Each criterion was rated on a 5-point score ranging from − 2 to + 2. A negative score represented a supinated foot, a zero score represented a normal foot, and a positive score represented a pronated foot [12]. The total scores ranged from − 12 to + 12 and were used to classify foot posture. Participants with total scores ≥10 had a highly pronated foot, whereas those with scores from 6 to 9 had a pronated foot, 0 to 5 a normal foot and − 1 to − 4 a supinated foot. Those with scores ≤ − 5 had a highly supinated foot [19].
Procedure
A rater (a physiotherapist) with one year of extensive training in management for musculoskeletal disorders, including the use of the NNHt and FPI-6 measures [12, 20], conducted both methods to all participants. Prior to data collection, the inter- and intra-rater reliability of the NNHt and FPI-6 were determined in 30 asymptomatic adults (these participants were not recruited in the main part of the present study). For inter-rater reliability, two investigators participated in this step: the rater and an experienced physiotherapist who expertise in foot/ankle management with intensively conducted the NNHt and FPI-6 to classify foot posture. For the intra-rater reliability, the rater conducted both methods on the same day, with a 10-minute break between the methods. The results demonstrated excellent inter- and intra-rater reliability (ICCs: 0.98–0.99) [21].
For data collection, the eligible participants were asked to provide their personal demographic data by filling out a form (e.g. age, gender, weight and height). The rater conducted the static measurements for foot classification, with a 10-minute rest between the methods [14]. The measurement sequence was simply randomised for each participant. After completing all measures, the NNHt and FPI-6 scores were calculated to classify the foot type for each method.
Statistical analysis
All data analyses were performed using Microsoft Excel 2016 (MicrosoftCorp, Washington, USA), SPSS version 26 for Windows (IBM SPSS Statistics, New York, USA) and Stata version 10 for Windows (StataCrop, Texas, USA). The baseline characteristics (e.g., age, weight and height) were represented as follows: mean ± standard deviation (SD). The nominal data (e.g., gender) were represented as numbers, percentages or proportions when appropriate. Prior to the statistical analyses, the normal distribution was assessed using the Kolmogorov-Smirnov test. The present data were normally distributed. The Weighted Kappa (Kw) was used to determine the agreement between the methods. The weights were calculated and expressed in the equation below [14], with foot classifications being coded as highly pronated, 0.2; pronated, 0.4; normal, 0.6; supinated, 0.8; highly supinated foot, 1.0.
$$\textrm{Weight}={\left({\textrm{i}}_{\textrm{n}}-{\textrm{j}}_{\textrm{n}}\right)}^2$$
Where; i = row.
j = column in test-retest matrix.
Weighted Kappa (Kw) was interpreted as ≤0.4 is fair, 0.41–0.6 is moderate, 0.61–0.8 is substantial, and > 0.8 is excellent agreement [22].
