Mimetic Jar Device Capable of Measuring Dynamic Opening Forces: Development and Validation

Abstract

INTRODUCTION On a daily basis, individuals open household jars without considering the biomechanical requirements involved, but for some people this task can be difficult to complete. Those with osteoarthritis, fibromyalgia or other neuromuscular conditions need to make accommodations to their hand kinetics to successfully open a jar with minimal pain. Unfortunately, there exists little data to help clinicians suggest proper hand kinetics for these individuals. Many jar instruments have been developed to attempt to address these large populations; however, none of these jar instruments include a combination of : a six-axis load cell capable of measuring the total compensatory forces applied to the jar lid, multiple lid grip sensors, and a torque limiter to repeatedly simulate the experience of opening a sealed jar . Similarly, those who have quantified the forces and moments acting upon a jar have created instruments that are not true to form and presumably alter the natural requirements of the task. Thus, we developed a life-like device to measure the full kinetics of the hand during jar opening, including both grip forces and resultant forces applied to the jar in six degrees of freedom. This instrument was validated by bench top and healthy human subject testing. Subsequent testing with this jar device may help clinicians advise suitable hand kinetics for individuals with neuromuscular conditions to maximize their effective forces on the jar and minimize pain during jar opening. METHODS A jar device was designed and constructed so as to measure the 6-axes of forces and moments applied to the jar between the two hands as well as the grip forces on the lid (Fig. 1). This jar device utilized a torque limiter set to 2.4 Nm and the jar lid would open freely for 45o to provide a similar experience to opening a real jar. The lid was equipped with 5 mm force sensing resistors (FSR) to measure the force magnitude and direction of force application while gripping the lid (Fig. 1B). The instrumentation of the jar was designed to work with two interchangeable lids: 83 mm (Fig. 1A) and 55 mm (Fig. 1C). Each of these jars utilizes the same load cell and torque limiter and a size appropriate instrumented lid with identical force sensing transducers. The large jar is outfitted with 6 sensors equally spaced radially around the lid, while the small jar has 4.