Principle and design of a mobile arm support for people with muscular weakness

Headnote

Abstract-This article describes the development of a mobile arm support for people with muscular diseases. The arm support is spring-balanced, with special attention on reduction of operating effort (high balancing quality and low friction), functionality (large range of motion), and aesthetics (inconspicuous design). The spring settings can be adjusted for wearing heavier clothing or picking up an object, a function that can also be used for moving up or down. The device levels itself automatically to compensate for uneven floors, a function that can be overruled to assist forward/backward motion of the arm. Thus, the balancer can compensate for the weight of the arm and be adjusted to generate force to a limited (safe) extent. The principle and design of the mechanism are presented and preliminary field trial results are given. Two users report on 6 months of continuous use of the arm support in their home and social environments.

Key words: adjustable spring mechanism, assistive device, biomechanics, gravity equilibrator, mobile arm support, neuro-muscular diseases, passive orthosis, rehabilitation, static bal-ancing, upper limb, user opinions.

Abbreviations: ADL = activities of daily living, CCM = com-bined center of mass, DOF = degrees of freedom, MAS = mobile arm support, MULOS = Motorized Upper Limb Orthotic System, MGP = Microgravity Products, ROM = range of motion, SMA = spinal muscular atrophy.

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INTRODUCTION

People suffering from neuromuscular diseases have trouble lifting their arms against gravity, although a large number of them maintain sensitivity and residual strength in their hands. Therefore, a device is desired that enables them to use their hands in a larger range of motion (ROM) than they can reach themselves. This is particularly true for patients with spinal muscular atrophy (SMA), a disease having an incidence in the range of 4 per 100,000 [1]. In this disease, the proximal joints (shoulders, hips) are affected first. Over time, performing basic activities of daily living (ADL) unassisted becomes increasingly difficult. For patients, this leads to a feeling of reduced independence.

Available assistive devices can be subdivided in three main groups that are mentioned next with some illustrations [2]. First, a number of rehabilitation robotic manipulators have been developed. Some have been successfully commercialized [3], including the Massachusetts Institute of...