Effects of robotic exoskeleton control options on lower limb muscle synergies during overground walking: An exploratory study among able-bodied adults

Summary

Background

The effects of lower limb (L/L) control options, developed for overground walking with a wearable robotic exoskeleton (WRE), on the neuromotor control of L/L muscles [i.e., muscle synergies (MSs)] during walking remain uncertain.

Objective

To gain initial insights regarding the effects of different control options on the number of MSs at the L/L and on their muscle weighting within each MS when walking with a WRE.

Methods

Twenty able-bodied adults walked overground without and with the WRE set at two control options with a predetermined foot pathway imposed by the WRE, and at three other control options with free L/L kinematics in the sagittal plane. Surface electromyography of eight right L/L muscles was recorded. MSs were extracted using a non-negative matrix factorisation algorithm. Cosine similarity and correlation coefficients characterised similarities between the MSs characteristics.

Results

Freely moving the L/L in the sagittal plane (i.e., non-trajectory controlled options) during WRE walking best duplicated typical MSs extracted when walking without WRE. Conversely, WRE walking while fully controlling the L/L trajectory presented the lowest correlations to all MSs extracted when walking without WRE, especially during early swing and L/L deceleration.

Conclusion

Neuromotor control of L/L muscles is affected by the selected control option during WRE walking, particularly when a predetermined foot pathway is imposed.

Significance

This exploratory study represents the first step in informing the decision-making process regarding the use of different L/L control options when using WRE and calls for further research among adults with sensorimotor impairments.

Introduction

Wearable robotic exoskeletons (WRE) allow people with sensorimotor impairments affecting their lower limb (L/L) to stand and walk and rehabilitation professionals to further adhere to the basic locomotor training principles to promote neuroplasticity. Overground walking with a WRE represents an activity-based rehabilitation intervention that may promote neurological and functional recovery after a central nervous system (CNS) lesion [13], [14]. Most of the first generation of WREs provided total and continuous motorised assistance at the hip and knee joints for the foot to follow a predefined planned trajectory during the swing phase. However, to meet the needs and expectations of neurorehabilitation professionals and end users, a wider range of control options are becoming available on the new generation of WREs. Among those, some recently developed WREs now allow self-selected L/L movement trajectory during the swing phase of the gait cycle (i.e., non-trajectory controlled) that promote active participation of the user and allows stepping variability. This control option can also be combined with assistance or resistance being provided to the L/L. However, whether any of these new control options promote typical muscle activation patterns, or which of those control options could best do it, remains unclear. The limited knowledge about what these new options could bring to the rehabilitation field impedes the development of evidence-based WRE locomotor training strategies during neurorehabilitation.

One way of investigating the effects of different WRE control options on the CNS is through muscle synergy (MSs) analyses. The MSs are a series of motor modules, each containing specific muscular activation patterns to simplify the neuromotor control of locomotion [9], [18], [32]. The CNS needs to control only a small number of these MSs, each containing a specific group of muscles associated with specific biomechanical function during the gait cycle [5], [26]. Simulation studies have shown, for example, that altering body weight and external conditions might lead to changes in MSs characteristics, providing evidence of the link between neuromotor control and specific neurobiomechanical adaptations to a task [22]. Hence, given the configuration of some WREs (i.e., backpack command centre), it is plausible that the motor control when walking without and with a WRE differs.

Although previous studies have explored kinematic outputs and individual muscle activity during walking with robotic devices [15], [33], [35], few studies have explored MSs related to robot-aided walking, and these MSs studies have reported contradictory results. For example, the number of MSs and muscle weighting within each MS were similar when able-bodied individuals walked at different speeds on a treadmill with a WRE, with different amounts of weight support or various levels of robotic guidance [11], [24]. Another study showed that muscle weightings within MSs were modified when using passive guidance during overground walking with a WRE compared to without a WRE [20]. These studies are limited by the most commonly available WRE control options used during walking (i.e., total and continuous motorised assistance according to a predefined planned trajectory), and none of them include in their comparison the most recently developed non-controlled trajectory control options. The recently developed control options offering self-selected L/L trajectories that can be assisted or resisted to different extents by the WRE represent promising interventions for rehabilitation purposes, especially for those individuals that preserve the ability to walk after a CNS lesion. However, it is unknown which of these features are valuable and which ones might result in abnormal patterns of muscle coordination or compensatory strategies required to adapt to these control options. These aspects first need to be investigated in able-bodied individuals to explore how an intact CNS might adapt to these conditions imposed by the WRE, before further exploring muscle patterns adaptations to these control options in individuals with sensory motor impairments.

The present exploratory study aims to gain initial insights regarding the effects of different control options on the number and profile of MSs at the L/L and on their muscle weighting within each MS, during overground walking without and with a WRE set at six different L/L control options (i.e., a subset of trajectory-controlled and non-trajectory controlled options used for neurorehabilitation). It is hypothesised that:

• all WRE control options will preserve MS characteristics extracted during overground walking without a WRE, as the intended L/L kinematics remain similar;

• walking with the WRE set to non-trajectory controlled options will lead to variable changes in MS characteristics depending on the control option used, since these options allow increased voluntary participation and step variability by imposing different constraints on the L/L motion.

Stronger evidence is needed to provide new insights into the effects of various control options on biomechanical and neural locomotor control [10], and to inform how different control options may be applied during locomotor training of individuals with sensorimotor impairments and limited walking ability following a neurological event.