Benchmarking of boccia balls: Roll distance, accuracy, stiffness, rolling friction and coefficient of restitution

Boccia is a Paralympic sport for athletes with cerebral palsy and certain other conditions, such as muscular dystrophy, that cause impairments similar to cerebral palsy. Boccia balls are either thrown or rolled down the court using an assistive device, i.e. a ramp. The balls are filled with plastic granules. The aim of this study was to characterise the balls based on their mechanical properties and to rank them according to their performance (distance of roll, consistency of placement, and angular deviation). The distance of roll was determined with a custom-made ramp on a wooden gym floor. Additionally, the balls were compressed with a materials testing machine and their stiffness and energy loss determined. The rolling friction coefficient was determined on a force plate, as was the coefficient of restitution with additional analysis of high-speed video files. Five ball models were tested in this study. Boccia balls deform visco-plastically and lose energy due to internal movement of the plastic granules. Their force-deflection function on compression is exponential. Softer boccia balls are characterised by a better performance (in terms of more accurate and repeatable placement of balls), due to higher coefficient of friction and higher energy loss (coefficient of restitution) from plastic deformation. However, better performance is compromised by a shorter roll distance. Harder balls are less spherical with a smaller contact area and thus prone to react to sideward moments at lower velocity, thereby negatively affecting the accuracy of placement.