Power plays
The science behind rugby tackles
It’s a colossal mismatch. All Blacks and Hurricanes midfielder Ngani Laumape, a finely tuned 97kg ball of muscle, vs a 79kg sports reporter.
There are few more destructive players than the 28-year-old powerhouse, a wrecking ball capable of sending would-be tacklers flying and shunting ballcarriers backwards.
So, how much force would Laumape rattle my frame with if I came up against him?
THE SCIENCE
Before we get to the fun stuff – Laumape vs myself – it’s important to understand the basic science behind rugby tackling.
First up: Momentum – a product of mass (size) and velocity (speed). Say there are two objects, or two rugby players. Their total momentum before a collision equals their total momentum after the collision. This is important.
Now, let’s also look at g-force, a measure of acceleration. One g is the amount of acceleration that Earth’s gravitational field exerts on us all, keeping our feet firmly planted on the ground.
Formula One drivers typically experience 5gs while braking, 2gs while accelerating, and 4-6gs while taking corners.
Humans can tolerate 5gs for a long period of time, but sustained g-forces of even 6gs could be fatal.
An F-16 jet pilot can handle up to about 9gs, but only for a matter of seconds. That’s because 9gs would make your body feel nine times heavier than usual, forcing blood to rush to your toes, compromising the brain due to the heart’s inability to pump blood to it.
THE SCENARIO
OK. We got the science nailed so let’s look at the scenario: Laumape has a head of steam up and is racing along at 27kmh and crashes into me hard enough to bring himself to a halt.
The dynamics of the tackle matter. There are many variables which determine how much force goes into a tackle, including speed and mass, which affect momentum. But there’s other stuff, too.
Take two billiard balls, for example. They’re hard and won’t deform when they collide, instead bouncing off each other.
They aren’t in contact for long, meaning the change in velocity occurs over a short time. Essentially, the collision force and subsequent acceleration are high.
On the other hand, the force of two spongy balls colliding would be about half that of the hard balls, given they would somewhat stick, rather than ping off.
It’s the same principle for collisions between rugby players. The less time the players are in contact, the higher the impact is.
THE IMPACT
Before the collision, a rampaging Laumape (97kg) moves at 7.5 metres per second (m/s) – or 27kmh – making his momentum 727.5kg m/s. For this calculation, let’s say I’m stationary (zero momentum) and have been tossed a hospital pass.
After contact, total momentum equals 727.5kg m/s, but Laumape is stationary (zero momentum). The total momentum has transferred to me.
We’d be in contact for only a quarter of a second, and my 79kg frame would ping off at 33.2kmh (9.2m/s).
Going from zero to 9.2m/s in a quarter of a second would see me hit with 3.75gs (that’s the gravitational force, remember?). For perspective, 50gs is the equivalent of a car hitting a brick wall at 80kmh per hour.
In the extreme case I bounced off Laumape within a 10th of a second, I’d be subjected to about 10gs, says Geoff Willmott, an associate professor in the departments of physics and chemistry at the University of Auckland.
But that’s acceleration, on average, across the whole body. The reality is some parts of the body could reach significantly higher gs.
In fact, as concussion expert Doug King discovered while conducting research for his second doctorate, the force to specific parts of the body can be ‘‘frightening’’.
Using accelerometers (which registered anything over 10gs of force) in mouthguards and in patches behind the ears of Hutt
Old Boys Marist premier grade players, King saw readings as high as 160gs.
One player – blasted by three players in quick succession – hit the deck and was knocked out. The first hit registered 98gs, the second 160gs, and the third 70gs.
Whiplash was a huge factor. He was first hit on the side of the body. As he looked to his left, another defender whacked him from the right, smashing his head into the shoulder of a third defender.
Hence, the dynamics of the tackle matter when dissecting the impact of collisions.
If I were to actually attempt to tackle a rampaging Laumape, it would be ludicrous to attempt to blast him. There would be bracing, and an attempt to wrap him up and drag him to the ground.
Like squishy balls, all going well, the impact would be drawnout. The majority of tackles, particularly in amateur rugby, are of the softer variety – drawn-out and not hugely forceful.
BRAIN INJURIES
Global research has concluded most concussions occur at roughly 90-100gs.
The death of
Wainuiomata league player Leonardo Va’a from a head injury in 1998 was the catalyst for King’s concussion research.
Va’a, who had been advised not to play again after a serious concussion in 1993, died from a cranium bleed. He was tackled around the waist.
Between 30-40 per cent of players concussed during King’s research were struck on the body, highlighting the damage whiplash can do.
More information will soon be available, too, given the recently announced study in Otago, where 700 male and female players at adult, under-18, under-15 and under-13 level will participate in head impact research.
ACC receives about 2000 claims annually for concussion, an area increasingly in the spotlight.
Utilising RugbySmart, an injury prevention programme launched in 2001, ACC is focusing on the prevention aspects of head injuries, most notably tackle technique.
That should help lower the number of tackles with the same, or more, force as Laumape’s tackle on me, ones with the potential to record devastating damage to some parts of the body.