Investigating the Impact of the User Interface for a Powered Hip Orthosis on Metabolic Cost and User Comfort: A Preliminary Study
Authors: Lee Seung Eun , Kilpatrick Claire , Kang Inseung , Hsu Hsiang , Childers Walter Lee , Young Aaron 
Published: June 2020

ABSTRACT

Introduction
Powered orthoses have the potential to benefit pathologic or geriatric populations by increasing the quality of their mobility, reducing metabolic cost, and helping restore functional status. However, user interface design of powered orthoses is rarely considered, and may limit their clinical impact. The aim of this study was to design and evaluate a novel user interface for a powered hip orthosis. We hypothesized that our interface design would reduce metabolic costs, reduce skin irritation, increase user comfort, and reduce pain during ambulation when compared with an off-the-shelf (OTS) interface.

Methods
A novel, custom-fit torso user interface was designed to have extended trimlines and load the iliac crests. This allows for improved purchase over anatomic structures and potentially improves weight distribution of the powered hip orthosis. The design was compared with an OTS user interface. Subjects ambulated in three conditions: without a powered orthosis, with an OTS interface, and with the novel interface. Five healthy subjects (23.6 ± 2.2 years) with no neuromuscular limitations ambulated on a treadmill for 6 minutes at 0.8 m/s while measuring metabolic cost for three levels of torque assistance. Subjects repeated this procedure for all three conditions. After ambulating in each interface, skin was inspected at t = 0, 10, and 30 minutes. They completed the OPUS (Orthotics and Prosthetics User’s Survey) Satisfaction with Device survey and reported pain on a 1-to-10 scale (0 = no pain).

Results
The novel interface reduced metabolic cost for all conditions when compared with OTS componentry. Maximal difference was at the 13% torque assistance level where the difference was 0.18 W/kg ± 0.11 (SEM) (9.48%). The novel design generally reduced pain scale and skin irritation. The average pain rating decreased from a 3/10 ± 1.17 (SEM) in the OTS condition to 2/10 ± 0.84 (SEM). The novel design generally increased OPUS Satisfaction with Device score when compared with the OTS condition; the OPUS score increased from an average of 32/45 ± 2.56 (SEM) for the OTS condition to 36/45 ± 2.1 (SEM).

Discussion
The novel design tended to reduce metabolic cost for all tested powered orthotic conditions. This may be explained by the novel design's more proximal trimlines and increased loading of anatomic structures. These features may be due to maximized biomechanical leverage and minimized compensatory motions during ambulation.

Conclusions
The user interface may impact metabolic cost of walking and user comfort. Moving forward, it should be considered an essential element of powered orthosis design. It is critical to optimize the user interface in future powered hip orthotic designs due to minimal surface area available for weight-bearing and reduced number of actuated joints. Therefore, those interested in optimizing powered hip orthotic user interface designs should focus on loading anatomic structures, including the shoulders and iliac crests, and on supporting the curvature of the lumbar spine.