Shared and Task-Specific Muscle Synergies during Normal Walking and Slipping

Abstract

Falling accidents are costly due to their prevalence in the workplace. Slipping has been known to be the main cause of falling. Understanding the motor response used to regain balance after slipping is crucial to developing intervention strategies for effective recovery. Interestingly, studies on spinalized animals and studies on animals subjected to electrical microstimulation have provided major evidence that the Central Nervous System (CNS) uses motor primitives, such as muscle synergies, to control motor tasks. Muscle synergies are thought to be a critical mechanism used by the CNS to control complex motor tasks by reducing the dimensional complexity of the system. Even though synergies have demonstrated potential for indicating how the body responds to balance perturbations by accounting for majority of the data set's variability, this concept has not been applied to slipping. To address this gap, data from 11 healthy young adults were collected and analyzed during both unperturbed walking and slipping. Applying an iterative non-negative matrix decomposition technique, four muscle synergies and the corresponding time-series activation coefficients were extracted. The synergies and the activation coefficients were then compared between baseline walking and slipping to determine shared vs. task-specific synergies. Correlation analyses found that among four synergies, two synergies were shared between normal walking and slipping. However, the other two synergies were task-specific. Both limbs were contributing to each of the four synergies, suggesting substantial inter-limb coordination during gait and slip. These findings stay consistent with previous unilateral studies that reported similar synergies between unperturbed and perturbed walking. Activation coefficients corresponding to the two shared synergies were similar between normal walking and slipping for the first 200 ms after heel contact and differed later in stance, suggesting the activation of muscle synergies in response to a slip. A muscle synergy approach would reveal the used sub-tasks during slipping, facilitating identification of impaired sub-tasks, and potentially leading to a purposeful rehabilitation based on damaged sub-functions.

Muscle Activity Analysis Using Higher-Order Tensor Decomposition: Application to Muscle Synergy Extraction

“…We showed that higher-order tensor decomposition models can achieve direct identification of shared synergies without relying on any similarity metric such as correlation coefficient. This is in contrast with the current approaches for shared synergy estimation [23], [24], [33], [34], [57] which apply NMF repetitively on multi-channel EMG recordings of different repetitions and then rely on maximising the correlation coefficients between the estimated synergies with regard to a reference one. Then, shared and task-specific synergies are identified through the correlation coefficient threshold.…”

“…This implies that shared synergies can be found in diverse motor tasks sharing some mechanical or physical characteristics. Support for this idea comes from animal studies (frogs [30], [31] and cats [32]) as well as human studies where shared and task-specific synergies distinctive for one motor task or movement have been investigated across activities such as walking and cycling [33], postural balance positions [34], [35], and normal walking and slipping [23], [24].…”

“…NMF [25] is used in this study as a comparative benchmark for shared synergy identification [23], [30], [33], [57]. NMF processes the multi-channel EMG recording as a matrix X with dimensions channel × time.…”

“…Finally, the arranged synergies are averaged to compute the first and second mean synergies for the task. Then, to identify the shared synergy of one DoF, the mentioned method is applied on the two tasks forming the DoF in question, the correlation coefficients between the resulting four mean synergies (two for each task) are calculated so that the highly correlated synergies between the two tasks are identified as shared, while the other two are considered as task-specific [23], [35], [57].…”

Angular momentum regulation may dictate the slip severity in young adults

“…Additionally, prior studies have shown that one’s risk of fall is affected by both pre-slip control (gait control) and post-slip response (slip control) (14–17). In other words, mild slippers possess different control techniques for both walking and slipping compared to severe slippers.…”