Early Research on Physics of Heading (Part 1 of 3)

Early researcher Charles Babbs in 2001, observed that head-ball contact may be possibly leading to a concussion. Back then, heading was a controversial topic and the risk of serious head injury was a serious health concern. To find out the impact of forces transmitted during heading, Babbs carried out an experiment. Part 1 of 3 of this article discusses his considerations on horizontal head acceleration and trajectories for lofted balls.

A significant research initiative was carried out by Babbs as early as 2001 to understand the safety and risk of heading a soccer ball. In soccer (or football), deliberate and purposeful heading of the ball with the head is termed as “heading”. Babbs’ research was motivated by the incidence of deficits in concentration, memory, alertness, and planning overtime in soccer players based on the number of balls headed. Mild concussions that resulted from high kicks, collisions with other players, and falling to the ground were recorded as 1 per 20 years of active playing, which was a very low rate to support long-term brian injury in soccer players. This data suggests that head-ball contact may be a possible cause of head injury. Babbs also found several studies that indicated deficits in memory and planning in professional soccer players when compared to runners and swimmers. Heading is encouraged in most groups and regarded as an essential aspect of competition, which makes the research topic controversial. However, other studies explain that lifetime exposure puts one at the risk of cumulative injury related to brain acceleration. Furthermore, it was important to note from the estimated population size then (200 million soccer players worldwide), a small percentage risk of permanent brain injury implicated serious public health consequences.

To confirm the available conclusions, Babbs, in an attempt to evaluate forces of heading and mechanisms of heading on impact forces, carried out a study to understand whether the head impact was safe or harmful. Babbs used mathematical models to understand the physics of heading. His experiment was replicable and based on Newton’s second law of motion. The things needed for the project included a meter stick, a stopwatch, and balance. The variables analyzed in the model were:

• The player (a slab-like mass moving forward with a known velocity at the moment of contact)
• The ball (a spherical mass coming from the opposite direction — the ball is cushioned by pressure inside the ball and damped by viscoelastic indentation of its cover; ranges for diameter, mass, and inflation pressure of the ball were obtained from manufacturer guidelines
• Flight of the ball prior to impact
• Effects of the variables on intensity and duration of head acceleration

The forces considered were:

Horizontal Head Acceleration

• Horizontal motion along the X-axis was considered and horizontal aspect of acceleration was the most important from the point-of-view of safety; standing and jumping headers involved upright players, and the force of the ball accelerated the head backward and horizontal; diving headers consisted of a prone body and the force of the ball accelerated the head and the body horizontally; mass, horizontal speed, size, inflation pressure of the ball, and effective mass and speed of the player influenced horizontal acceleration
• Head acceleration is calculated using Newton’s second law of motion (force = mass X acceleration). The acceleration is calculated for a number of game scenarios (realistic ball trajectories: corner kicks, goal kicks, crosses, shots, clearances; large and small players; poor or good technique obtained by specifying small or large effective mass)
• Values for head accelerations obtained from the model were compared to usual activities (jumping, head nodding, dancing) and unusual activities (motor vehicle injury compared against head acceleration criterion defined in motor vehicle crash literature). Research suggests that heading was occasionally dangerous based on the characteristics of the ball and the player.

Trajectories for Lofted Balls

• Observations of actual games were used to determine horizontal ball speeds; effects of air resistance and drag of motion of the ball were incorporated; spring constant of the ball model is a function of inflation pressure; damping coefficient of the ball is calculated from the height to which ball bounces initially from a hard surface when dropped from a height
• Trajectories for lofted balls in games carried out by players between ages 10 and 14, and adult women and men, for different skill levels (amateur and professional levels) were calculated. Equations of motion for spherical projectiles that are slowed by air resistance were calculated. The given variables were time of flight and distance traveled by the balls, obtained from data from observations from actual games. Data was plotted at equal time points, which were separated by 0.1 sec. Distance between data points on trajectory curves was linked to the speed of the ball. Strength and size of players resulted in an increase in distance covered and initial velocities. An increase in air resistance also occurs, which is related to the square of velocity. Air resistance has a longer time to act as balls travel further. The horizontal component of velocity, related to head safety, becomes similar for lofted balls in the descending limb of flight, regardless of strength and age of players (shown on the graph as equally spaced points for all trajectories). Graphical data showed that rising balls kicked by adult players at close range were exceedingly fast (initial horizontal velocities between 25 to 75 meter per second initial velocity for adult players).
• Air resistance had an equalizing effect, and similar distributions were found for head height and horizontal velocities even when there was a wide variation in strength and size of players. One way analysis of variance (ANOVA) was carried out, which did not show differences in descending horizontal head-height velocities, except in the youth (mean value 7.1 meter per second) versus adult (5.1 meter per second) categories. Horizontal velocities of headable descending balls were greater in youth soccer games compared to adult soccer games, but initial velocities of balls leaving the foot were less in youth games compared to adult games. Babbs indicated that differences were counterintuitive, which can be explained by underlying physics, and this has substantial implications for heading safety by youth players.

References

Babbs, C. F. (2001). Biomechanics of Heading a Soccer Ball: Implications for Player Safety. The Scientific World JOURNAL, 1, 281–322. https://doi.org/10.1100/tsw.2001.56