Generalizability of motor modules across walking-based and in-place tasks - a distribution-based analysis on total knee replacement patients

Abstract

Introduction: There are evidences that the nervous system produces motor tasks using a low-dimensional modular organization of muscle activations, known as motor modules. Previous studies have identified characteristic motor modules across similar tasks in healthy population. This study explored the generalizability of motor modules across two families of walking-based (level-walking, downhillwalking and stair-decent), in-place ascending (sit-to-stand, squat-to-stand), and in-place descending (stand-to-sit and stand-to-squat) motor tasks in a group of six individuals undergone total knee replacement (TKR) surgery. Methods: Motor modules were extracted from the EMG data of CAMS-Knee dataset using non-negative matrix factorization technique. A distribution-based approach, employing three levels of k-means clustering, was then applied to find the shared and task-specific modules, and assess their representability among the whole task-trial data. Results and Discussion: Results indicated a four- and a seven-subcluster arrangement of the shared and task-specific motor modules, depending upon the membership criteria. The first arrangement revealed motor modules which were shared across all tasks (min coverage index: 76%; modules' distinctness range: 7.08-8.91) and the latter among tasks of the same family mainly, although there remained some interfamily shared modules (min coverage index: 81%; modules' distinctness range: 7.17-9.89). It was concluded that there are shared motor modules across walking-based and in-place tasks in TKR individuals, with their generalizability and representability depending upon the analysis method. This finding highlights the importance of the analysis method in identifying the shared motor modules, as the main building blocks of motor control.

Keywords: clustering; k-means; non-negative matrix factorization; shared modules; synergy analysis.

FIGURE 1

(A) Box plots and ranges of the number of muscle synergies for trials of each task. The numbers inside the boxes indicate the most frequent numbers (MFNs) of motor modules, and the red dots represent the outlier data for the WDH. Also, The * sign indicates a significant difference (p < 0.05). (B) The average VAF results for different numbers of muscle synergies of each task. WLV: level walking; WDH: downhill walking; WSD: stair descent, ASQ: squat to stand; AST: sit to stand; DSQ: stand to squat; DST: stand to sit.

FIGURE 2

Four-cluster arrangement of shared motor modules. (A) Characteristic motor modules of different tasks (first seven rows) and the representative motor modules of each cluster and their coverage rate across all task-trial data (last row). The main contributing muscles into each module (relative weights larger than 0.3) are colored and * sign indicates a significantly different muscle weight from that of the representative module (p < 0.05). (B) Activation profiles of the muscle modules. The lines above each profile indicate activation levels above 50% of the maximum and red lines separate the stance and swing phases of the gait cycle. WLV: level walking; WDH: downhill walking; WSD: stair descent, ASQ: squat to stand; AST: sit to stand; DSQ: stand to squat; DST: stand to sit.

FIGURE 3

Seven-subcluster arrangement of shared motor modules. (A) Characteristic motor modules of different tasks (first seven rows) and the representative motor modules of each subcluster and their coverage rate across all task-trial data (last row). The main contributing muscles into each module (relative weights larger than 0.3) are colored and * sign indicates a significantly different muscle weight from that of the representative module (p < 0.05). (B) Activation profiles of the muscle modules. The lines above each profile indicate activation levels above 50% of the maximum and red lines separate the stance and swing phases of the gait cycle. WLV: level walking; WDH: downhill walking; WSD: stair descent, ASQ: squat to stand; AST: sit to stand; DSQ: stand to squat; DST: stand to sit.

FIGURE 4

t-SNE plots for the (A) four cluster and (B) seven-subcluster arrangements of the shared motor modules. The center of each circle represents the centroid of a cluster/subcluster and hollow and solid circles illustrate preserved and eliminated data, respectively.