Here we show that novel, energy-recycling stairs reduce the amount of work required for humans to both ascend and descend stairs. Our low-power, interactive, and modular steps can be placed on existing staircases, storing energy during stair descent and returning that energy to the user during stair ascent. Energy is recycled through event-triggered latching and unlatching of passive springs without the use of powered actuators. When ascending the energy-recycling stairs, naive users generated 17.4 ± 6.9% less positive work with their leading legs compared to conventional stairs, with the knee joint positive work reduced by 37.7 ± 10.5%. Users also generated 21.9 ± 17.8% less negative work with their trailing legs during stair descent, with ankle joint negative work reduced by 26.0 ± 15.9%. Our low-power energy-recycling stairs have the potential to assist people with mobility impairments during stair negotiation on existing staircases.

Introduction

Stair negotiation is a demanding task that limits the independence of individuals with mobility impairments such as muscle weakness, joint pain, or reduced sensorimotor control. Joint moments in the knee are over 3 times greater during stair negotiation compared to level walking during both stair ascent and descent [1–3]. Stair negotiation is ranked among the top 5 most difficult tasks in community-residing older adults [4, 5]. Patients—such as those with hip osteoarthritis—adopt altered joint movements to reduce pain during stair negotiation [6]. Moreover, even if they are capable of using stairs, people with mobility impairments often avoid stair negotiation [5, 7].

Current solutions providing assistance in stair negotiation are costly, energy-consuming, and do not help to retain the user’s ability to negotiate stairs independently. Elevators or stair-lifts are often impractical to install because they require substantial household remodeling. Further, an elevator can consume over 12,000 kWh annually [8], equivalent to 50% of the average household energy consumption in the United States in 2009 [9] and over 200% of that in the United Kingdom in 2004 [10]. Perhaps more importantly, elevators or stair-lifts replace the need to negotiate stairs altogether, regardless of a user’s level of motor function. Because studies suggest that disuse of a specific motor function can further accelerate its loss [11–13], it is important to provide motor assistance that allows users to retain their ability to use stairs and to prevent further motor decline.

Cheap, low-power, yet effective human movement assistance is possible by applying the principle of energy recycling [14]. Collins et al. showed that a simple exoskeleton with passive springs can store and return energy at each step to assist joint motion during walking [15]. The exoskeleton takes advantage of the alternating braking and propelling action of the legs during gait at the level of the ankle (Fig 1). During braking, the lower-leg exoskeleton stores mechanical energy in a passive spring, reducing the negative work generated by the ankle to brake the leg. The stored energy is later released to propel the body forward, reducing the positive work generated by the ankle for propulsion. Through appropriate detection and response to gait events, the exoskeleton reduces the metabolic cost of walking by 7.2% without consuming energy.

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larger image TIFF original image Download: Fig 1. Positive and negative work generation during walking and during stair negotiation. Blue represents the phases during which negative work is generated by the right leg (braking). Red represents the phases during which positive work is generated by the right leg (propulsion). In walking, the right leg both brakes and propels within one stride. In stair negotiation, however, the right leg generates predominantly negative work throughout stair descent (T UL ), and predominantly positive work throughout stair ascent (T L ). The gait cycle during stair negotiation follows the definition in [1]. https://doi.org/10.1371/journal.pone.0179637.g001

In contrast to level walking, storing and returning energy within each gait cycle cannot be applied to stair negotiation. The legs produce predominantly positive work during stair ascent, and predominantly negative work during stair descent [2]. Thus, a more effective approach for energy-recycling in stair negotiation would be to store a large amount of energy cumulatively during stair descent, and to then release that energy during stair ascent. Specifically, energy recycling could be targeted to two phases where the greatest increase in joint power are observed compared to level walking: the loading phase of stair ascent (T L ) when energy is generated by the leading leg, and the unloading phase of stair descent (T UL ) when energy is absorbed by the trailing leg (Fig 1). In the loading phase (T L ), center-of-mass (CoM) elevation occurs early in the gait cycle, between foot strike of the leading leg and into the mid swing phase of the trailing leg. Sagittal knee joint power throughout T L is higher than in level walking, both in adults over 40 years old [3] and healthy young adults [2], with peak knee power of roughly 2 times greater than in level walking [1, 2]. In the unloading phase (T UL ), the center-of-mass (CoM) is lowered as the trailing leg does negative work (absorbs energy) until toe-off. In healthy young adults, knee joint power throughout T UL is higher than in level walking [2], and peak knee joint power reaches 3.8 times that seen during level walking [1, 2]. Further, the ankle joint generates large negative power in sagittal plane during T UL but negligible negative power during overground walking. Based on previous findings [1–3], we hypothesized that assistive stairs could store energy during T UL and release it during T L , reducing both positive (ascent) and negative (descent) work generation in the legs during stair negotiation.

Therefore, our objective was to design, build, and test energy-recycling assistive stairs (ERAS) that store energy during stair descent and return it to assist the user during stair ascent. We built two prototype modular steps that can be placed on an existing staircase. Energy is stored in passive springs and released based on gait events detected by pressure sensors on each tread. We then measured the amount of work generated or dissipated by the leg joints during assisted and unassisted stair negotiation in naive users. Our results show that ERAS reduces positive work generation in the leading leg during ascent by 17.4 ± 6.9%. In particular, a 37.7 ± 10.5% reduction was observed in the knee, which is one of the main contributors of positive work during ascent. In addition, ERAS reduces negative work generation in the trailing leg during descent by 21.9 ± 17.8%. In particular, a 26.0 ± 15.9% reduction was observed in the ankle-sagittal degrees-of-freedom (DOF), which is one of the main contributors of negative work during descent. Together, our work demonstrates the feasibility of a low power, modular, interactive device to assist those with difficulty in stair negotiation in their homes.