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

In part 3 of 3 of this article discussing early research findings on heading by Charles Babbs, key aspects of promoting heading safety with reduced pressure balls, the effect of touch and playability, and more information on head acceleration with regard to ball pressure, effective body mass of young players, and head impact criterion is discussed. Babbs proves the cumulative effect of repeated subconcussive blows, and how dangerous head accelerations are caused. Babbs then demonstrates an ideal heading technique with his biomechanical approach, supporting proofs with Monte Carlo simulations.

Reduced Pressure Balls

Heading safety may be increased for young players by reducing inflation pressure. Smaller, lighter weight individuals can practice safe heading with balls inflated to 0.3 atmosphere / 5 PSI and are softer than standard balls. Reduced inflation have reduced bounce on hard surfaces which may not be necessarily be flat. These balls are similar to older practice balls, which are checked for pressure and loose air overtime. Youth players with 30 kg weight (in the range 7–12 years) can head a soft rubber playground ball safely in practice.

Touch and Playability

Reduced inflation pressure of standard soccer balls can also be demonstrated by calculating head and ball accelerations. Considering that the ball impactor is the instep of the foot (area = 100 cm2), and not the foot, v1 is the foot speed (such as 2 meters per second), v2 is the ball speed (for example, zero). The football model during play measures of playability and touch i.e. maximal contact area and contact time of peak indentation and contact time. Greater contact area and greater contact time mean greater control over the ball, i.e. greater touch. Measures of touch increase when inflation pressure decreases. Softer balls improve shooter’s touch and control, and goalkeeper’s time to react is increased only slightly. In this way, softer balls increase heading safety and goal scoring by increasing control. It is important to understand the role of touch factors in order to prefer softer balls over hard over-inflated balls.

Head acceleration

Babbs applied Newton’s second law of motion to explain safety in heading in soccer. Head acceleration with a high ball during a practice or game are governed by the interaction of masses moving at known masses.

• During soccer, balls move faster than players, and their speed and mass determine forces acting on the head.
• The horizontal component of the ball velocity, as described above is relevant to heading safety. Range of horizontal speeds of balls in a position to be headed is constant in actual games, even where there are variations in strength and size of players.
• Head is accelerated in inverse proportion to effective body mass of player, which is dependent on the technique and body size of the player.
• Balls with lower pressure cause less head acceleration over longer contact times, when compared to high pressure balls
• Flatter acceleration across time causes lower HIC values when compared to shorter and higher intensity accelerations.
• Larger contact area and longer contact time of low-pressure balls played with the foot or the head causes players to have more control over the ball.
• Low-pressure balls improve safety and enjoyment in the game, which requires a change in the negative attitude of soccer players towards softer balls owing to their touch and safety of the ball.
• The current model demonstrates two masses connected by a linear spring and a damper in a single dimension. The model uses HIC that does not include the complexities of the head injuries, and a complex 3D model may produce more realistic insights.
• According to the present research, proper heading in adults generates HIC values less than 0.1% of what is required to produce a brain injury (younger players having a higher risk — less strength, practice, effective body mass, headed balls slowed less by air resistance, slightly greater mean horizontal velocity; HIC 1% of what is required to produce brain injury).
• Repeated concussions produce prolonged and severe symptoms when compared to isolated single blows. Repeated subconcussive blows have significant cumulative effect. A player who repeatedly heads the ball accidentally risks cumulative brain damage.
• When the neck is loose and head wobbles, the effective mass of the player reaches the mass of the head. This causes highest dangerous head accelerations during blind-sided hit (accidental) or half-hearted effort. The situations when this happens is [1] when a goalkeeper is unaware and other players are taking practice shots [2] more than one striker shoots at a live goalkeeper who is hit by the ball [3] a player is heading a resign ball that did not have time to be slowed down by air resistance — short reaction time does not allow the player to have sufficient effective body mass. A rule change making heading a rising ball dangerous and that ducking must be practiced is a recommended strategy.
• A high ball that has encountered enough air resistance may not necessarily be dangerous (high balls have less horizontal velocity). Other safety guidelines may include : [1] Diving headers are safer from standpoint of the brain due to greater effective body mass in the horizontal direction [2] softer, under-inflated, lighter are suitable during intensive practice, especially for effective body mass of young players [3] players should be advised to put maximal body mass behind the ball with a strong and stiff neck. Neck strengthening exercises must be explained during training [5] Players must be advised to avoid blind-sided balls and rising balls.

Biomechanical Approach

• The values gained are better than through the use of accelerometers, human subjects, and high-speed video tape. Monte Carlo simulations are used to explore rare events with unusually high risk. Small errors may be introduced by approximations (of the order 1% to 5% and are rare as there may be wide variation in ball speeds, playing conditions, weight, technique, size, player, and angles). Babbs used the approach to study dangerous head accelerations and underlying variables. The following assumptions are made:
• The head in neither unconnected from the body nor is the entire mass of the body lumped with the head for purpose of calculating effective body mass. In a grounded player, the body may rotate backwards at the point of foot contact with the turf. In a jumping player, the body is free to rotate along the center of gravity.

Effective mass opposing horizontal acceleration is m’=Fx /ax (ratio of horizontal force to horizontal acceleration). Effective body mass depends on inclination angle of the player w.r.t. ground and axial distribution of mass along the length of the body i.e. θ.

• Effective mass of the player inclined at angle θ is

m’ = m / 2sin2θ. (grounded player)

m’ = m / 2sin2θ + cos2θ (jumping player)

Effective mass for uniform density player at right angle to ground i.e. θ = 90 degrees is one half mass for jumping or grounded player

Effective body mass for perfectly horizontal diving header with feet off ground (θ = 0 degrees) is 100% of the body mass.

Most headers are taken at 80 to 100 degrees. The cosines of these angles are similar to zero, and the sines of the angles are similar to one.

Routine heading with ideal technique is to keep neck stiff and no head wobble, using one half body weight.

The ideal technique is modeled by using the equation

m’ = m / 2

In the worst heading (hit unaware or blind sided) only the mass of the head is taken into consideration

When the head wobbles slightly, as in the mediocre technique, the effective body mass is between the mass of the head and half of the total body mass

• The ball is a spherical ball, that is partially flattened on one side due to the impact of the forehead

Monte Carlo Simulation

During lifetime soccer play, combinations of poor technique and high-speed balls tends to produce dangerous HIC values even with occasional impact. A Monte Carlo simulation can validate this assumption. The averages assumed are:

• 10,000 headers in 20 years
• 20 weeks of play per year
• 5 games per week
• 5 headers per game

Ball velocities and techniques are chosen from real or random sampling distributions for youth and adult play.

The mean and standard deviations calculated were:

• Effective body mass 24 + or — 10 kg (never ❤ kg for head mass)
• Horizontal ball velocity 6.0 + or — 2.5 meter/sec
• Ball inflation pressure 1.0 atm

The simulations predicted:

• 64 unlucky headers
• HIC in the range 30 to 100 (3 to 10% of single impact danger level)
• These impacts represent harmful subconcussive blows or low-grade concussions even when a good technique is used
• Reducing inflation pressure to 0.3 atm resulted in a prediction of one header with HIC between 10 and 30

Researchers studied accidental heading, i.e. dangerous head-ball impacts, which could be caused when balls are kicked with full force at close range strike players. Players are often caught off guard and do not react quickly to protect themselves. Accidental heading occurs when players are in a defensive “wall” and struck by a free kick at a range of 10 yards It may also occur when players get accidentally hit fetching balls near the goal during shooting practice. Multiple occurrences of such accidents during a player’s career may cause subtle brain injury. An example of a dangerous concussion is:

• Rising ball kicked
• Average force 10 meters away
• Horizontal speed 50 meters per second
• Ball strikes a player with effective head mass of 5 kg
• Mean head acceleration would be 106 g, HIC would be 844 g2.5/sec

Researchers measured speeds of regulation. Soccer balls kicked by professional football players (24 to 34 meters per second)

• Speeds of 27 to 54 meter per second ball inflated to 1.0 atm, HIC range from 190 to 1020 g2.5/sec
• Balls inflated to 0.3 atm, HIC range from 70 to 380 g2.5/sec

Final thoughts

Babbs developed mathematical formulas for the effective body mass of the player, the ball, the impact, motion of the balls in the air prior to impact, damping factor of head-ball impact, and applied Monte Carlo to the data using reference for safe and dangerous accelerations.

Results indicated that heading was safer in certain circumstances (low risk of dangerous headers):

• Player had greater effective body mass (determined by strength, size, and technique) when heading the ball
• Lower inflation pressure of the ball (reduced risk of dangerous head accelerations). Balls with lower pressure had greater playability and touch measured as contact area between ball and foot during a kick and the contact time.

The at-risk population for dangerous headers included:

• Smaller youth players with lesser body mass even when using youth size balls

Babbs made important recommendations:

• To teach the right technique
• To re-design age-appropriate balls for young players having reduced inflation pressure and reduced weight.
• Young players must be taught to avoid head contact with fast rising balls kicked at close range to reduce the risk of subtle brain injury in players using heading in soccer

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