Heading in Football. Part 3: Effect of Ball Properties on Head Response N Shewchenko, C Withnall, M Keown, R Gittens, J Dvorak
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i33 Br J Sports Med: first published as 10.1136/bjsm.2005.019059 on 25 July 2005. Downloaded from SUPPLEMENT Heading in football. Part 3: Effect of ball properties on head response N Shewchenko, C Withnall, M Keown, R Gittens, J Dvorak ............................................................................................................................... Br J Sports Med 2005;39(Suppl I):i33–i39. doi: 10.1136/bjsm.2005.019059 Objectives: Head impacts from footballs are an essential part of the game but have been implicated in mild and acute neuropsychological impairment. Ball characteristics have been noted in literature to affect the impact response of the head; however, the biomechanics are not well understood. The present study determined whether ball mass, pressure, and construction characteristics help reduce head and neck can impact response. Methods: Head responses under ball impact (6–7 m/s) were measured with a biofidelic numerical human model and controlled human subject trials (n = 3). Three ball masses and four ball pressures were investigated for frontal heading. Further, the effect of ball construction in wet/dry conditions was studied with the numerical model. The dynamic ball characteristics were determined experimentally. Head linear See end of article for authors’ affiliations and angular accelerations were measured and compared with injury assessment functions comprising ....................... peak values and head impact power. Neck responses were assessed with the numerical model. Results: Ball mass reductions up to 35% resulted in decreased head responses up to 23–35% for the Correspondence to: N Shewchenko, Biokinetics numerical and subject trials. Similar decreases in neck axial and shear responses were observed. Ball and Associates Ltd, pressure reductions of 50% resulted in head and neck response reductions up to 10–31% for the subject Ontario, Canada; trials and numerical model. Head response reductions up to 15% were observed between different ball shewchenko@biokinetics. constructions. The wet condition generally resulted in greater head and neck responses of up to 20%. com Conclusion: Ball mass, pressure, and construction can reduce the impact severity to the head and neck. It is ....................... foreseeable that the benefits can be extended to players of all ages and skill levels. arly footballs (soccer balls), developed in 1855, consisted heading older generation balls, especially when wet, is of a rubber bladder and leather covering. The ball generally acknowledged, but little research has been con- Econstruction was a great improvement over previous ducted to quantify the effects of ball impact on head response generations that had used an inflated pig’s bladder. Further and the associated risks to the player. Isolated ball impacts improvements were made over the following years including onto steel plates and numerical studies have provided some better consistency in shape and durability but it was not until insight but do not elaborate on the aetiology of injury or http://bjsm.bmj.com/ 1872 that the spherical shape was made mandatory by the biomechanics of the impact.2–5 English Football Association. Specifications for both the To date, there is much controversy on the effects of circumference (686–711 mm) and mass (368–425 g) were repeated head impacts where it concerns both the risk of included. The official mass was later increased in 1937 to injury after having sustained a concussion and the effects of 397–453 g but the remaining requirements were unchanged. repeated low level impacts to the head such as in heading the These requirements formed the basis for the current ball. This controversy remains partially due to the difficulty in specifications in the Fe´de´ration Internationale de Football assessing the severity of concussion and to the unknown Association (FIFA) Laws of the Game specifying a circumfer- effects of repeated head to ball impacts. For many years, on September 25, 2021 by guest. Protected copyright. ence of 680–720 mm and mass of 410–450 g. Inflation recommended practices and guidelines for proper heading pressures of 600–1100 g/cm2 (0.6–1.1 bar) are also specified techniques have been based on the experience of players and along with leather or other suitable cover material. trainers alike but these lacked quantification of the physical In more recent years the materials used in ball construction event and potential sequelae. Recently biomechanical studies have changed considerably, improving upon the leather in heading have started to be conducted but must be matured exterior with manmade materials. This has helped reduce to develop a better understanding of the cause and effect for water absorption and degradation of the balls, which was a single and repeated impacts. Once an understanding has problem with the older generation balls. Recent efforts to been reached, preventive measures could be taken to reduce standardise the balls further were made upon the release of the mechanical insult to the head and associated risks, if any. the FIFA Quality Concept (FQC) for Footballs1 which Studies related to ball–head response under different stipulates the circumference (680–700 mm), mass (410– impact conditions have been conducted in parallel with 450 g), shape, size, water absorption (,20%), pressure loss, biomechanical studies. These have provided data concerning rebound, durability, and balance. All balls used for FIFA the peak force exerted to the head.6–9 Estimates of the severity sanctioned match games must comply with the FQC of the impact were made where peak forces were noted to be requirements. as high as 2000 N for ball speeds of 36 m/s. Continuing improvements to the balls have resulted in Impact studies relevant to work herein have involved more consistent game play but little attention has been given assessment of the severity of the impact of the ball in relation to the severity of ball impact with the head during heading. to the pressure and mass when wet.2 The tests were carried With the recent media attention on neurophysiological and out by having the ball strike an instrumented flat steel neuropsychological dysfunction related to heading, a better surface. Twenty Size 5 balls inflated to 0.4, 0.6, and 0.8 bar understanding of this issue is required. The discomfort of were impacted at a nominal velocity of 9.7 m/s2. Peak www.bjsportmed.com i34 Shewchenko, Withnall, Keown, et al Br J Sports Med: first published as 10.1136/bjsm.2005.019059 on 25 July 2005. Downloaded from transmitted force, rise time, and impulse were computed. sensitivity of ball characteristics with the aid of a numerical Increases in ball pressure were commensurate with an model in an initial effort to determine whether impact increase in peak force and impulse. Decreases in the rise reductions are possible, and if so, which parameters are the time for the force and reduced contact durations were also most significant. The paper is the third of a three part series noted. Balls conditioned in the wet experienced increases in published in this supplement. The first part, ‘‘Development of peak force likely due to the increased mass. Ball construction Biomechanical Methods to Investigate Head Response’’, methods were also found to be of interest where sewn described analysis techniques and test methodology capable assemblies were noted to absorb a greater percentage of of measuring both kinematic and kinetic responses of the water than moulded balls. Sewn balls were also observed to player.10 The second part, ‘‘Biomechanics of Ball Heading have higher peak forces, rise times, impulse, and contact and Head Response’’, focused on the implementation of a duration. Although statistically significant differences were numerical model and guidelines for reducing head response.11 found between groups, absolute differences of 4–9% in peak force were obtained. It was suggested that the playing METHODS conditions within the game such as ball speed, impact We used a validated numerical model and human subjects to direction, and actual inflation pressure may have an equal or quantify the biomechanical response of ball impact under a greater influence. However, control of the ball pressure and limited set of heading scenarios. Initial tests were conducted mass was recommended to reduce the severity of head with human subjects (n = 7) to obtain a preliminary under- impact. standing of heading techniques on biomechanical response A similar test series was conducted on an instrumented and to validate the movements of a 50th percentile male plate with 20 balls inflated to 0.6 bar and impacted with a numerical model. Linear and angular head accelerations and 2 3 speed of 17–18 m/s . Significant differences were found neck loads in the midsagittal plane were provided by the between sewn and moulded balls. Higher values of peak numerical model.11 Subsequently, we used the numerical force, rise time, and impulse were noted for the sewn balls model to investigate the sensitivity of ball mass, pressure, although the average ball mass for the sewn construction was construction, and condition on head response. higher than for the moulded balls. Concern was expressed Study of the ball impact characteristics and interaction about the peak forces as these were in the range of published with the head first required detailed measurement of the skull and facial bone fracture levels. However, it should be ball’s dynamic stiffness. We accomplished this with physical noted that the study failed to address the issue of load impact tests at 7 m/s onto an instrumented hemispherical distribution, which would increase the effective load toler- anvil having a radius of 98 mm, approximating a 50th ances. Impulse values were also noted to be below published percentile male forehead. Measured anvil forces and ball concussion levels but movement of the head towards the ball deformations obtained by high speed video analysis provided under game play conditions would increase the impulse data for assessment of the stiffness.