## 1 Introduction  
It has been reported by the World Health Organization that adults should participate in 150 min/week of physical activities to maintain good health [1, 2]. However, it is difficult for bedridden patients suffering from muscle atrophy, caused by long-term disuse, to fulfill this requirement [3, 4, 5, 6, 7]. This elicits a vicious cycle where the ramifications include, but are not limited to, an increased risk of cancer, cardiovascular diseases, diabetes, and sarcopenia [8, 9]. Current strategies to avoid the deconditioning cycle consist mainly of gait exercises provided with assistance from therapists and personal trainers. While effective, limited resources often prevent patients from receiving the required exercise. Furthermore, it is imperative that physiological gait patterns are preserved during these exercises as compensation patterns may lead to gait deficits [10]. Recent advancements have led to the development of gait rehabilitation robots [11, 12, 13, 14, 15, 16, 17, 18, 19]. These devices are designed to provide assistance by supporting bodyweight, lower limb trajectory control, and, furthermore, quantitative monitoring of the patient. However, studies have shown that full-bodyweight-supporting robot-assisted gait training (RAGT) exhibits lackluster rehabilitative efficacy [20, 21, 22]. The primary reason behind this is thought to be due to the lack of patient participation [23, 24, 25]. In order to resolve this issue, more recently, various wearable robots have been incorporated into the exercise regimen [26, 27, 28, 29]. While hard exoskeletons were initially developed to assist patients with total loss of lower extremity function, they are now being utilized for more general RAGT purposes [30, 31]. Due to added inertia and the wearability and comfort issues of hard exoskeletons, soft-type exoskeletons comprising textiles and Bowden cables have also been developed to boost users' mobility [32; 33; 34; 35; 36]. However, both types are relatively expensive and require specialist assistance; thus, they have fallen short of being immersed in the rehabilitation scene. While there exists a plethora of assistive devices, simple rollators do not provide any force or trajectory assistance, and it has been reported that RAGT devices restrict free movement and have an inherent difficulty in providing assist-as-needed control [24; 25]. Unpowered exosuits and devices consisting of only passive elements and no actuators have become a popular research topic [37; 38; 39; 40; 41]. The main goal of such devices has been lowering the metabolic costs during gait. The strategy behind this is storing and releasing elastic energy among the joints. [PERSON] et al. proposed an ankle orthosis consisting of a spring, rope, and clutch that harvests energy and assists the calf muscle, resulting in reduced metabolic energy during muscle contraction [37]. More recently, [PERSON] and [PERSON] proposed a bianticular passive exosuit that is capable of reducing metabolic costs by 10% during gait [42]. These promising results suggest that further research is warranted in the field of passive exosuits. In this paper, we present a novel apparatus, that can be independently used by the user without external assistance, that attaches to a conventional rollator consisting of only passive elements that aims to support patients with diminished lower extremity capabilities. Our device, unlike traditional rollators, provides force assistance according to the gait cycle while allowing relatively free lower limb movements compared to RAGT devices. This system (Figure 1a) comprises a spring-bar system with an elastic cord that is attached to a specialized shoe which has two straps, one anchored at the heel and one above the metatarsal head. Our device offers plantar flexion torque during the late stance phase which immediately transitions to dorsiflexion torque during the early swing phase.