Sample Entropy Improves Assessment of Postural Control in Early-Stage Multiple Sclerosis.

Postural impairment in people with multiple sclerosis (pwMS) is an early indicator of disease progression. Common measures of disease assessment are not sensitive to early-stage MS. Sample entropy (SE) may better identify early impairments. We compared the sensitivity and specificity of SE with linear measurements, differentiating pwMS (EDSS 0-4) from healthy controls (HC). 58 pwMS (EDSS ≤ 4) and 23 HC performed quiet standing tasks, combining a hard or foam surface with eyes open or eyes closed as a condition. Sway was recorded at the sternum and lumbar spine. Linear measures, mediolateral acceleration range with eyes open, mediolateral jerk with eyes closed, and SE in the anteroposterior and mediolateral directions were calculated. A multivariate ANOVA and AUC-ROC were used to determine between-groups differences and discriminative ability, respectively. Mild MS (EDSS ≤ 2.0) discriminability was secondarily assessed. Significantly lower SE was observed under most conditions in pwMS compared to HC, except for lumbar and sternum SE when on a hard surface with eyes closed and in the anteroposterior direction, which also offered the strongest discriminability (AUC = 0.747), even for mild MS. Overall, between-groups differences were task-dependent, and SE (anteroposterior, hard surface, eyes closed) was the best pwMS classifier. SE may prove a useful tool to detect subtle MS progression and intervention effectiveness.

Centre of pressure or centre of mass feedback in mediolateral balance assessment.

The mediolateral balance assessment method (MELBA) consists of tracking a sinusoidal or multisine target with the center of pressure as feedback (CoPfb). The aim of the CoP trajectory is to elicit weight-shifting, i.e. movement of the center of mass (CoM). However, it is not known whether CoPfb elicits consistent mediolateral displacements of the CoM, whether CoM feedback (CoMfb) is required to achieve this and whether CoPfb or CoMfb elicit different kinematic strategies. The aims of this study were to determine (1) the extent to which CoP imposes CoM displacements (CoMd) during CoPfb, (2) whether larger CoMd are elicited by CoMfb and (3) whether different kinematic strategies arise when using CoPfb or CoMfb. Nineteen young adults performed MELBA with CoPfb and CoMfb from which coherence, gain and phase-shift between CoP-CoM and leg-trunk kinematics were calculated. CoMd and CoPd and leg and trunk excursions were also calculated. Results show that for CoPfb tasks, CoP-CoM coherence was high, while the gain dropped with increasing frequency. The drop in gain was highly consistent between subjects. Reasonable trunk-leg coherence (≈.6) was found over all frequencies and tasks. The leg-trunk angle gain increased with frequency in all tasks and was significantly higher in the CoMfb compared to the CoPfb. Significant interaction indicated that this difference increased with frequency. CoPfb in MELBA elicits consistent CoMd. However, different kinematics are employed in CoMfb with more trunk movement and an ankle-to-hip shift as frequency increases. Hence CoMfb may be preferable over CoPfb despite the larger measurement effort involved.

Frequency domain mediolateral balance assessment using a center of pressure tracking task.

Since impaired mediolateral balance can increase fall risk, especially in the elderly, its quantification and training might be a powerful preventive tool. We propose a visual tracking task (VTT) with increasing frequencies (.3-2.0Hz) and with center of pressure as visual feedback as an assessment method. This mediolateral balance assessment (MELBA) consists of two tasks, tracking a predictable target signal to determine physical capacity and tracking an unpredictable target signal to determine sensorimotor integration limitations. Within and between sessions learning effects and reliability in balance performance descriptors in both tasks were studied in 20 young adults. Balance performance was expressed as the phase-shift (PS) and gain (G) between the target and CoP in the frequency domain and cut-off frequencies at which the performance dropped. Results showed significant differences between the MELBA tasks in PS and G indicating a lower delay and higher accuracy in tracking the predictable target. Significant within and between sessions learning effects for the same measures were found only for the unpredictable task. Reliability of the cut-off frequencies at which PS and G performance declined and the average values within cut-off frequencies was fair to good (ICC .46-.66) for the unpredictable task and fair to excellent for the predictable task (ICC .68-.87). In conclusion, MELBA can reliably quantify balance performance using a predictable VTT. Additionally, the unpredictable tasks can give insight into the visuomotor integration mechanisms controlling balance and highlights MELBA's potential as a training tool.