Non-linear analysis of the structure of variability in midfoot kinematics
Introduction
A good understanding of the methods available to evaluate foot function is a requirement for clinicians to provide meaningful interventions to foot-related disorders. One of the methods frequently used to evaluate foot movement is the navicular drop [1]. The navicular drop describes the range of sagittal deformation of the midfoot during the stance phase of gait. Navicular drop has been suggested to be the most appropriate parameter for the assessment of foot pronation [2] as it is a valid indicator of talonavicular motion [3] and rear foot movement [4]. In addition a large navicular drop has been associated with an increased risk of medial tibial stress syndrome (MTSS) [5], [6].
In general gait analysis focuses on the properties of subject's average gait pattern and does not take in consideration its fluctuations [7]. These fluctuations may be observed during pathology or aging as subtle changes in the underlying structure of the signal [8], [9]. The assessment of the variability during gait can be a way to gain knowledge about important aspects of motor control not addressed when using a traditional approach [7]. Variability in a time series can be described using linear analysis such as mean and standard deviation. However, linear analysis fails to detect dynamic changes in the patterns of the system output. Non-linear analysis of variability provides information of the spatio-temporal or structural characteristics of time series [10] such as the dynamics of the navicular bone during several consecutive steps.
Non-linear analysis has previously been applied to various biological signals and has shown a decrease in the complexity of the physiological system output in relation to aging and disease [8]. A general decrease in signal complexity in relation to pathology has led to the “loss of complexity hypothesis” [11] where high variability has been described as healthy and low variability associated with pathology. Even though different measures of non-linear variability have shown it can separate healthy subjects from subjects with pathology [11] differences have also been reported in the healthy population. Some part of this inter-subject variation in variability can be explained by gender [12]. Previous studies have shown lower variability during running at different speeds [13] and during short and sustained contractions [14] among women compared with men.
To our knowledge non-linear analysis has not been applied to foot kinematics in a large randomly selected sample. Interestingly, a link between a pronated foot type and large navicular drop has been suggested as a risk factor in the development of overuse injuries [5]. Then, it is likely that subjects with a pronated foot posture and large navicular drop as well as women, will be characterized by lower complexity values of their foot kinematics.
The purpose of this study is (1) to calculate sample entropy (SaEn) to establish normal values of variability in midfoot kinematics; (2) to examine if factors like foot posture, navicular drop and gender are associated with SaEn in kinematic midfoot data.
Section snippets
Subjects
280 adults were randomly selected from the Danish Central Personal Register (CPR). Demographic and anthropometric characteristics are presented in Table 1. Healthy volunteers aged 18–68 years were included. Inclusion criteria were (a) no lower extremity deformities, (b) no history of trauma in the lower extremity, and (c) no pain in the lower extremity during the last three months.
Measurements and instrumentation
The six-component Foot Posture Index (FPI-6) [15] was used to classify the standing foot posture. The FPI-6 score
Results
The groups resulting from the FPI scores and SaEn are presented in Fig. 2. Mean values indicated that a supinated foot type has the highest SaEn value, whereas the pronated foot type has the lowest SaEn value for both men and women. For women, SaEn were 1.10 ± 0.19 (supinated group), 0.96 ± 0.17 (neutral group) and 0.77 ± 0.16 (pronated group) (F133,2 = 15.744, p < 0.001). For males, SaEn were 1.25 ± 0.24 (supinated group), 1.06 ± 0.23 and 0.86 ± 0.19 (F141,2 = 23.45, p < 0.001). The two-way ANOVA revealed a
Discussion
The purpose of this study was to establish normative values of the variability in midfoot kinematics using a non-linear technique, and to investigate which factors influence SaEn in midfoot kinematics measured in the sagittal plane. We established normative values of SaEn that indicated lower SaEn values in women compared to men. The SaEn values were influenced by dND, foot posture, gender and preferred walking speed.
Acknowledgements
Foot and Ankle Research Northern Denmark.
Obelske Family Foundation.
Conflict of interest statement
There are no commercial relationships of the authors which may lead to a conflict of interests, whether financial or personal, with third parties or persons.
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