Supine lying center of pressure movement characteristics as a predictor of normal developmental stages in early infancy

1. Introduction

Qualitative evaluation of spontaneous movements is often utilized to predict neurodevelopmental impairments in young infants. One such tool is Prechtl’s General Movement Assessment (GMA), which has been established as a predictor of cerebral palsy (CP) diagnoses [1, 2, 3]. The GMA classification is age-dependent since the characteristics of spontaneous movements evolve with age [4]. The classification of general movements via GMA is based on the quality of the infant movements focused on complexity, fluency, and variation during supine lying positioning [5]. The GMA classifies general movements as “normal”, “absent”, or “abnormal” based on the impression of the clinicians. Thus, the current GMA method is purely observational and qualitative.

Fidgety movement is a type of general movement characterized as small movements of the neck, trunk, and limbs at moderate speed with variable acceleration in all directions [1, 6, 7]. Fidgety movements may be seen as early as 6 weeks post-term but are usually predominantly observed from 9 to 20 weeks post-term. If the fidgety movements are never observed from 9–20 weeks post-term, infants are classified as absent fidgety on the GMA. Additionally, the fidgety movements which are moderately or greatly exaggerated in amplitude, speed and jerkiness are considered as abnormal fidgety movements. Absent or abnormal fidgety movements have been associated with subsequent diagnoses of developmental disorders such as CP [1, 2, 3], and the GMA is the current clinical tool for identifying these infants early in life. Early intervention can optimize motor, cognition, and communication outcomes with promotion of learning and neuroplasticity as well as prevent secondary impairments and minimize influence of complications [8], yet the GMA is solely based on clinicians’ observation, with poor inter-rater reliability [9]. Although the GMA shows high sensitivity and specificity (95% and 96%) of fidgety movement assessment [1], it still provides limited discrete values rather than continuous values. Thus, a robust and objective quantification of general movements in the fidgety age of infancy is necessary to provide a refined assessment tool for better identification and classification of the risk for neurodevelopmental disorders and would benefit from early intervention.

Entropy measures of infants’ center of pressure (CoP) have previously been utilized to identify abnormal movement patterns in early infancy [10, 11, 12, 13]. Sample entropy (SampEn) quantifies the structure of variability and randomness in time series data [14]. Supine lying entropy measures have previously been used to determine differences in CoP movement patterns between full term vs. pre-term babies [10, 11]. SampEn measures of CoP have also been used to distinguish sitting postural control between infants with typical vs. delayed development [12]. A recent study reported that infants at high risk of developmental delay showed smaller supine lying SampEn values of spontaneous leg movement, as measured by wearable accelerometers [13]. Supine lying SampEn of infants, therefore, may be a feasible tool to assess abnormal infant movements related to potential developmental disorders. The purpose of this study was to determine whether quantitative CoP measurements during supine lying were associated with age in healthy full-term infants. We hypothesized that the complexity of CoP (SampEn) during supine lying would be significantly correlated with age in infants with normal fidgety movement.

2. Methods

After recruiting caregivers of potential participants via flyers, phone screen interviews were performed to see if their infant would qualify for this Institutional Review Board-approved study. Infants were excluded if they had any orthopaedic or neurological conditions or had received vaccinations within two weeks prior to data collection. Infants included in the study were born $>$ 37 weeks gestation and between 5–95th percentile in height and weight [15]. Infants’ ages were adjusted for pre- or post-maturity from a term pregnancy (40 weeks) using the chronological and gestational ages, per GMA guidelines [16]. As the movement pattern changes over time and is determined based on post-menstrual age, it is important to correct for all infant’s age born at less than 40 weeks gestational age, for accurate assessment for their movement patterns [6]. For example, a baby born at a gestational age of 38 weeks is 2 weeks premature. If the baby’s actual age is 20 weeks, then the corrected age would be 18 weeks old. Twenty-four healthy full-term infants (age: 16.3 ± 5.6 weeks; 10M, 14F; length: 65.5 ± 4.6 cm; weight: 6.4 ± 1.2 kg) were enrolled after caregivers consented and signed a permission form as approved by the University of Arkansas for Medical Sciences Institutional Review Board. Infants were placed supine on an AMTI force plate (Watertown, MA; 1000 Hz) in orthogonal view of a GoPro camera (San Mateo, CA). Data was collected for 2 minutes (Fig. 1). A force platform is the most commonly used type of force transducer in human movement science, considered as a gold standard for measuring 3-D ground reaction forces (GRF). The force platform incorporates stain gages to determine a single 3-D GRF vector. The location of the force vector usually called center of pressure (COP) as the force vector results from a distribution of forces to a contact area on the surface of the force platform. Thus, COP is located within (x, y) coordinate pair of the force platform [17]. The camera was used to record infants’ movements while lying on their back on the floor (force platform). Three trained raters (TJ, NK, and BM) assessed the video by observing spontaneous movements of the infants and using GMA. One- or two-minutes representative data are sufficient to observe normal fidgety movement [18]. Infants with any discrepancies on the GMA scoring between the three raters were considered as abnormal fidgety. Nineteen infants (adjusted age: 14.7 ± 4.5 weeks; 9M, 10F; length: 59.9 ± 2.7 cm; weight: 6.2 ± 0.8 kg) were deemed fidgety per the GMA and were included in further analyses.

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3. Results

Significant correlations were observed for SampEn of RCoP ( r = 0.501, p = 0.027) and range of RCoP ( r = 0.609, p = 0.006) with adjusted age (Table 1). However, other variables such as SampEn of RCoP velocity, SampEn of RCoP acceleration, total path length, mean absolute velocity, and mean absolute acceleration were not significantly associated to the adjusted age (Table 1). In the linear regression model, SampEn of RCoP and range of RCoP were significant predictors, explaining 25% and 37% of the variance in adjusted age, respectively (Fig. 2).

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The purpose of this study was to determine whether quantitative CoP measurements during supine lying were associated with age in healthy full-term infants. Complexity of RCoP parameters (RCoP, RCoP velocity, RCoP acceleration) and traditional CoP parameters (range, total path length, mean absolute velocity, and mean absolute acceleration of RCoP) were calculated and then correlations between these CoP parameters and infant age were assessed. Infants in the fidgety period exhibited lower complexity with increasing age. Interestingly, SampEn of RCoP was significantly associated with infant age. A non-linear data analysis approach has been previously used to identify infants at high risk for developmental delay. Dusing et al. reported that full-term infants at 1 to 3 weeks of age had greater entropy values of CoP (higher complexity) compared to pre-term infants of the same age during supine lying, which indicates the pre-term infants (who have a higher risk of developmental delay compared to full-term infants) exhibited lower complexity of their CoP movement [10]. Deffeyes et al. also used entropy measures of CoP to detect differences between infants and toddlers with typical vs. delayed development and reported greater entropy values for infants with typical development during sitting postural control [12]. Furthermore, Smith et al. found that infants with typical development showed increased SampEn of spontaneous leg movements compared to infants at risk of developmental disorders [13]. These previous findings support the idea that the entropy measure of CoP can be a useful tool to detect infants at high risk of developmental disorders. Our results also indicate that this approach might be useful to develop a screening tool for neurodevelopmental disorders in early infancy (fidgety period: ∼ 6–22 weeks) [26] aimed at early diagnosis and interventions, but this needs to be further explored.

In addition to the nonlinear SampEn parameters, range of RCoP was a significant predictor of infant age while other traditional CoP parameters appeared not to be associated with infant age. Range of RCoP had the highest correlation ( r = 0.609; Table 1) with infant age among the dependent variables and infants’ CoP movements tend to be greater with increasing age. In order to determine if range of RCoP was simply related to increased body size which occurs as infants age, we performed a post-hoc calculation of the correlation between infant height and range of RCoP. An insignificant correlation was found ( r = 0.320, p = 0.182), indicating that infants’ length did not affect the CoP results as a potential confounder. In general, as babies grow by 26 weeks (6 months) of age, reflexes tend to disappear and be replaced with cortex-directed voluntary movements including head control, posture, and various type of locomotion [27]. Greater range of RCoP with increasing age may reflect this concept of the development stage of infants, indicating that infants develop their movements to be more gross and regular (evidenced by lower SampEn) movements such as changing posture, kicking and initiation of rolling from supine to prone. Therefore, range of CoP is a highly correlated parameter with age and should be included in developing a diagnosis method when considering using a multivariate approach.