# Biomechanical Task-Based Gait Analysis Suggests ReWalk Gait Resembles Crutch Gait
## 1 Introduction  
Bipedal gait is the primary method of ambulation during everyday life. While most people can walk without much effort, the underlying mechanism requires intricate control of the individual muscles, the ensuing joint movements, and, moreover, whole-body balance [1, 2, 3]. What is interesting is that energy consumption is optimized during this complex process because of an inherent motor program shared by the general population [4, 5]. Thus, a common repetition of sequential patterns can be observed during natural gait [1, 6]. These patterns exist, in part, to execute specific biomechanical tasks, namely, weight acceptance, single-limb support, and limb advancement [1, 7]. Constraints that limit the execution of certain motor programs not only affect the pertinent biomechanical task but, because of the reciprocity, also elicit compensation patterns that can be observed throughout the gait cycle. These compensation patterns can be found in abnormal gaits and, moreover, are indicative of innate pathological conditions [1, 8].  
Current gait rehabilitation strategies are based on motor learning principles, which require active patient participation and accurate feedback via intense training of biomechanical task-related motor activities [9, 10, 11, 12, 13, 14]. When circumstances such as injuries or diseases inhibit the patients' ability to independently perform gait, physical therapists are burdened with biomechanical tasks such as supporting bodyweight, moving the extremities, and maintaining balance during rehabilitation sessions [15, 16]. This is not only physically demanding but, due to limited human resources and facilities, also impacts the patients' training frequency and duration negatively.  
Robot-assisted gait training (RAGT) has been proposed to alleviate this predicament and further promote quantitative rehabilitation sessions [17; 18; 19; 20; 21]. Contrary to high expectations, studies report that the full-bodyweight-supporting RAGTs, compared to conventional methods, fall short regarding rehabilitative efficacy [22; 23]. It is widely accepted that this was due to a lack of patient participation, which was evident from diminished electromyography (EMG) signals [3; 24]. More recently, exoskeleton-type robots, initially developed to assist with the ambulation of spinal cord injury (SCI) patients, have been proposed for RAGT. It was postulated that patient participation could be further facilitated because sensors were incorporated to identify gait intent and the patient controlled one's own weight [22; 25]. These studies indeed report a significant increase in EMG activity. However, it is of particular concern that the EMG patterns do not well match those of natural gait [3; 26; 27]. It is vital that the biomechanical task-related activities incorporated during RAGT promote the recruitment of appropriate muscle groups because rigorous training with deviant patterns may elicit non-physiological gait characteristics [10; 11; 13].  
In this paper, we investigate how gait patterns change with ReWalk, an FDA-approved exoskeleton-type robot previously reported to improve quality of life for SCI patients [28], in order to investigate its potential use as a gait rehabilitation robot. Given that it is vital that specific interventions directly train the biomechanical tasks, we examine the gait patterns according to the three major gait-related biomechanical tasks, which are identifiable by three separate stages of the gait cycle [1]: weight acceptance (WA), single limb support (SS), and limb advancement (LA). We hypothesize that the use of elbow crutches and the immobilized ankle conformation of ReWalk, change the user's natural gait patterns. The comparison was made by evaluating the lower limb kinematics as well as the EMG patterns during overground gait with ReWalk, elbow crutches, and AFOs. We find, from six healthy subjects that participated in this experiment, that walking with ReWalk closely resembles that of crutch gait, and, more importantly, deviates considerably from that of natural gait.