Intra-subject sample size effects in plantar pressure analyses

Abstract

Background Recent work using large datasets (>500 records per subject) has demonstrated seemingly high levels of step-to-step variation in peak plantar pressure within human individuals during walking. One intuitive consequence of this variation is that smaller sample sizes (e.g., 10 steps per subject) may be quantitatively and qualitatively inaccurate and fail to capture the variance in plantar pressure of individuals seen in larger data sets. However, this remains quantitatively unexplored reflecting a lack of detailed investigation of intra-subject sample size effects in plantar pressure analysis. Methods Here we explore the sensitivity of various plantar pressure metrics to intra-subject sample size (number of steps per subject) using a random subsampling analysis. We randomly and incrementally subsample large data sets (>500 steps per subject) to compare variability in three metric types at sample sizes of 5–400 records: (1) overall whole-record mean and maximum pressure; (2) single-pixel values from five locations across the foot; and (3) the sum of pixel-level variability (measured by mean square error, MSE) from the whole plantar surface. Results Our results indicate that the central tendency of whole-record mean and maximum pressure within and across subjects show only minor sensitivity to sample size >200 steps. However, <200 steps, and particularly <50 steps, the range of overall mean and maximum pressure values yielded by our subsampling analysis increased considerably resulting in potential qualitative error in analyses of pressure changes with speed within-subjects and in comparisons of relative pressure magnitudes across subjects at a given speed. Our analysis revealed considerable variability in the absolute and relative response of the single pixel centroids of five regions to random subsampling. As the number of steps analysed decreased, the absolute value ranges were highest in the areas of highest pressure (medial forefoot and hallux), while the largest relative changes were seen in areas of lower pressure (the midfoot). Our pixel-level measure of variability by MSE across the whole-foot was highly sensitive to our manipulation of sample size, such that the range in MSE was exponentially larger in smaller subsamples. Random subsampling showed that the range in pixel-level MSE only came within 5% of the overall sample size in subsamples of >400 steps. The range in pixel-level MSE at low subsamples (<50) was 25–75% higher than that of the full datasets of >500 pressure records per subject. Overall, therefore, we demonstrate a high probability that the very small sample sizes (n < 20 records), which are routinely used in human and animal studies, capture a relatively low proportion of variance evident in larger plantar pressure data set, and thus may not accurately reflect the true population mean.

Human foot form and function: variable and versatile, yet sufficiently related to predict function from form

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Plantar pressure and falling risk in older individuals: a cross‐sectional study

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Regional differences in the biological variability of plantar pressure as a basis for refining diagnostic gait analysis

“…During familiarization with the testing procedure participants were instructed to look straight ahead to avoid targeted movement that might alter the gait. Following current recommendations 30 regarding the number of repetitions needed, we recorded 200 steps of each participant—100 each for the left and right foot, taken alternately.…”

“…The high informativeness of such data has allowed researchers to establish that there is a high level of idiosyncrasy in anatomical and functional features of the foot, manifested in the processes of both standing and of body weight shifting 28 , 29 . Step-by-step analysis of the variation within plantar pressures found that the minimum number of measurements required for reliable analysis results is 200 30 . There is a generally positive correlation between dispersion and mean plantar pressure from successive measurements, the ratio of which can be expressed using a coefficient of variation (CV), which is higher in areas with lower mean plantar pressure values, such as around the periphery of the foot and at the midfoot, but lower in the heel and forefoot 31 .…”