Participants and experimental procedure
The investigation was implemented as a descriptive, cross-sectional laboratory study analysing the intra- and interday reliability of dynamic navicular drop and drift measurements during walking gait. The study was approved by the ethics committee of the canton of Bern (KEK number Z007/12) and all participants signed informed consent. The examined sample consisted of twenty-one healthy participants, 12 females (foot length 239 (SD 11) mm, age 27.9 (SD 5.0) years, height 166 (SD 6) cm, body mass 62.3 (SD 6.9) kg) and nine males (foot length 266 (SD 12) mm, age 32.8 (SD 10.3) years, height 181 (SD 7) cm, body mass 75 (SD 7.4) kg). Exclusion criteria were back pain or other spinal disorders, any kind of musculoskeletal affections, neurological injuries or diseases, thrombosis or fractures in the past 12 months, implants or surgery on the lower extremity in the past 12 months, bone tumours, angiopathies, alcohol abuse, dementia, acute infections (such as common cold) or high-intensity exercise the day before the measurements.
Each subject underwent a test-retest study work flow, where the navicular height (NH) and width (NW) during the stance phase (SP) were assessed with a set of four markers (Fig. 1) on the left and right foot. A testing session consisted of ten barefoot walking trials and all participants completed two subsequent (no pause between) sessions on a first day (M1a, M1b) to assess intraday reliabiltiy and a third session (M2) one week apart from their first day to assess interday reliability. An optical motion capture system (cameras: 2× Bonita10, 8× Bonita3; volume: 5.5 m × 1.2 m × 2 m; 200 Hz; Nexus 1.8.5 software, Vicon Motion Systems Ltd., Oxford, UK) was used for kinematic measurements. Initial contact and toe off events were determined based on ground reaction force measurements with two force plates (AMTI OR 6, Watertown, USA, 1000 Hz, threshold 25 N).
Markerset and model outputs
Reflective skin surface markers (diameter 16 mm) were placed by a single experienced physical therapist on the left and the right foot to track the following anatomical landmarks (Fig. 1):
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MPM
first Metatarso-Phalangeal joint, metatarsal head, dorso-Medial aspect.
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MPL
fifth Metatarso-Phalangeal joint, metatarsal head, dorso-Lateral aspect.
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CA
CAlcaneal tuberosity, posterior aspect of the calcaneus (Achilles’ tendon insertion).
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NA
NAvicular tuberosity, most medial aspect of the navicular bone.
MPM, MPL and CA markers served to define the XY-plane (Fig. 1) which was calibrated to be parallel to the foot’s plantar surface based on a static trial and CA marked the origin of a Cartesian coordinate system. The foot’s longitudinal axis (Y) was the bisecting line [26] of the MPM, MPL and CA triangle (Fig. 1). According to the right-hand rule convention, the foot’s vertical axis (Z) was perpendicular to the XY-plane pointing towards cranial and the foot’s lateral axis (X) pointed laterally for the right foot and medially for the left foot. The navicular height described the distance of the navicular marker from the XY-plane and the navicular width described the medial distance of the navicular maker from the YZ-plane.
Post-processing and statistics
Post-processing and statistical analysis were performed using Matlab (Version R2016a, The MathWorks Inc., Natick, USA). Navicular height and width were calculated and low-pass filtered (4th order zero-lag Butterworth, 4 Hz). Stance phase intervals were extracted, resampled to 200 samples and subsequently averaged among ten trials. Navicular height and width from the static trials (NHS and NWS) were taken as reference positions to express the navicular height and width from gait trials. The curve features, dynamic navicular drop and drift, were extracted from the navicular height and width time-series, respectively, and calculated as differences between minimum (for navicular height, Eq. 1) or maximum (for navicular width, Eq. 2) excursions during the stance phase and values at foot strike (NHFS, NWFS):
$$\begin{array}{*{20}l} dNDrop & = NH_{\text{FS}} - NH_{\text{Min}} \end{array} $$
(1)
$$\begin{array}{*{20}l} dNDrift & = NW_{\text{FS}} - NW_{\text{Max}} \end{array} $$
(2)
Respective time points of dynamic navicular drop and drift were determined in %SP and denoted tdNDrop and tdNDrift. Reliability was primary analyzed by the Bland-Altman method [27] and completed by the intraclass correlation coefficient ICC21 and the standard error of measurement (SEM, Eq. 3) [28] with SDd being the standard deviation of the differences from the Bland-Altman analysis.
$$ SEM = \frac{SDd}{\sqrt{2}} $$
(3)
Left and right feet measurements were not treated separately but treated as independent samples making up a total of 42 feet that were assessed at the time points M1a, M1b and M2. Reliability of NHFSNHMinNWFSNWMaxtdNDrift and tdNDrop was also considered to explore how each parameter contributed to intra- and interday variations in dynamic navicular drop and drift, respectively. All variables and the Bland-Altman differences were tested for normal distribution by the Kolmogorow-Smirnow test. The significance of the bias from the Bland-Altman analysis was evaluated by paired t-tests. The level of statistical significance was set as p<0.05.
Instrumental error assessment
To estimate the instrumental errors, the marker set was applied to a ski boot, which served as a rigid frame. A person wearing the ski boot completed ten walking trials. Model outputs were calculated as described in the “Post-processing” section based on marker distances measured with a caliper. Under the assumption that the markers built a rigid cluster, all marker distances would have been constant throughout the stance phase and deviations from the rigid cluster model outputs were taken for instrumental error estimation by calculating the root mean square errors among the stance phase.