Influence of Power Delivery Timing on the Energetics and Biomechanics of Humans Wearing a Hip Exoskeleton

Abstract

A broad goal in the field of powered lower limb exoskeletons is to reduce the metabolic cost of walking. Ankle exoskeletons have successfully achieved this goal by correctly timing a plantarflexor torque during late stance phase. Hip exoskeletons have the potential to assist with both flexion and extension during walking gait, but the optimal timing for maximally reducing metabolic cost is unknown. The focus of our study was to determine the best assistance timing for applying hip assistance through a pneumatic exoskeleton on human subjects. Ten non-impaired subjects walked with a powered hip exoskeleton, and both hip flexion and extension assistance were separately provided at different actuation timings using a simple burst controller. The largest average across-subject reduction in metabolic cost for hip extension was at 90% of the gait cycle (just prior to heel contact) and for hip flexion was at 50% of the gait cycle; this resulted in an 8.4 and 6.1% metabolic reduction, respectively, compared to walking with the unpowered exoskeleton. However, the ideal timing for both flexion and extension assistance varied across subjects. When selecting the assistance timing that maximally reduced metabolic cost for each subject, average metabolic cost for hip extension was 10.3% lower and hip flexion was 9.7% lower than the unpowered condition. When taking into account user preference, we found that subject preference did not correlate with metabolic cost. This indicated that user feedback was a poor method of determining the most metabolically efficient assistance power timing. The findings of this study are relevant to developers of exoskeletons that have a powered hip component to assist during human walking gait.

The exoskeleton expansion: improving walking and running economy

“…Rather than placing actuators on the exoskeleton's end-effector, researchers began placing them off-board and attached them through tethers (e.g., air hoses and Bowden cables) to streamlined exoskeleton end-effectors [45,53,54]. This approach enabled researchers to conduct high throughput, systematic studies during treadmill walking and running to determine optimal exoskeleton assistance parameters (e.g., timing and magnitude of mechanical power delivery [27,55]) for improving walking and running economy. Furthermore, the high-performance motors on recent tethered exoskeleton testbeds have relatively high torque control bandwidth that can be leveraged to render the dynamics of existing or novel design concepts [43,56].…”

Biomechanical Analysis of an Unpowered Hip Flexion Orthosis on Individuals With and Without Multiple Sclerosis

“…This suggests that there is an upper limit on device effectiveness as band stiffness increases, and presents the need for exploration of lowerstiffness bands in future studies. It has been shown that there are diminishing returns with increased assistance magnitude for a powered hip exoskeleton (50), as well as with assistance onset timing for the same exoskeleton (33). A similar phenomenon may be observed in the HFO, because both the magnitude and timing of its energy delivery are a function of the device con guration.…”

“…These devices aim to augment human gait through the controlled actuation of motor-driven cables or pneumatic arti cial muscles that span various joints in the lower limbs (22)(23)(24), or motors situated concentrically with joints (25,26). Studies have demonstrated that such technology can reduce muscle activation during unloaded (22,23,(27)(28)(29)(30) and loaded walking (22,31), and reduce the metabolic cost of walking in both healthy subjects (23,27,(32)(33)(34) and subjects with post-stroke hemiparesis (35,36). While promising, the costs, power demands, environmental adaptability, noisiness, and size of these devices are hurdles that must be overcome for widespread adoption of this technology (37)(38)(39).…”

“…Despite the practical barriers currently preventing most exosuits from reaching consumers, the research related to these devices has been vital to learning about the biomechanical effects of augmenting forces about the lower-limb joints. Exosuit studies have demonstrated the virtues of carefully-timed assistive forces about the hip during walking (28)(29)(30)(31)33,34). Considering the crucial role that hip exors play in the swing phase of gait (40,41), the lack of clinically available hip-centric orthoses for mobility assistance suggests a need for more investigation in this area.…”

A Biomechanical Comparison of Proportional Electromyography Control to Biological Torque Control Using a Powered Hip Exoskeleton

“…Similarly, with hip exoskeletons, there is a need for testbed system results (Lewis and Ferris, 2011; Lenzi et al, 2013; Young et al, 2017) to help inform the control of new autonomous systems (Giovacchini et al, 2014; Buesing et al, 2015; Seo et al, 2016; Karavas et al, 2017; Sugar et al, 2017). Ankle exoskeleton studies on testbed devices have shown that human users lower their muscle force with added ankle exoskeleton assistance to keep the total joint moment around the ankle consistent with unassisted walking (Lewis and Ferris, 2011).…”