Sports Science Part II: Anatomy of a Hit in Football


What's up fellow classmates? I'm back with the second of my six-part sports science blog post series. My first sports science blog post talked about the science behind shooting the perfect jump shot in basketball.

In this blog post, I'll explain to you the anatomy of a hit in football by giving you an in depth look at the physics behind a tackle made by former Seattle Seahawks cornerback Marcus Trufant. The tackle was made on former Philadelphia Eagles wide receiver Greg Lewis in a nationally televised Monday Night Football Game on December 2, 2005 (Seahawks blew out the Eagles 42-0 in Philadelphia). As a diehard Eagles fan, it's like I'm reliving this loss all over again as I write this post. Have some sympathy for me.

Researchers have been studying the science behind one of football's most basic fundamentals: the tackle. The tackle made by Marcus Trufant caused Greg Lewis to drop the football, resulting in an incomplete pass. Unfortunately for players, incomplete passes aren't the only consequences from hard hits such as Trufant's tackle. There are also fumbles, and a widespread of injuries from a torn ACL to the growing concern of head injuries and concussions. But given the speed and size of today's NFL athletes, players around the league are required to wear high-tech football equipment to protect themselves from the physics of the sport's gruesome collisions.

According to Timothy Gay, a physics professor at the University of Nebraska and author of The Physics of Football, "A defensive back's mass combined with his speed on average, 4.56 seconds for the 40-yard dash, can produce up to 1600 pounds of tackling force." Marcus Trufant is average-size for a NFL defensive back, 5 ft. 11 in. and 199 lbs., to be exact. There are close to 500-plus NFL players who weigh somewhere between 200 to 300-plus pounds. But Trufant still has the ability to make a forceful impact when he tackles players who are much bigger than him in height and weight.

A tackle isn't complete unless the tackler is able to fully bring the ball carrier to the ground. Therefore, the field (turf or grass) plays a part in tackling. Researchers use a metric called G-Max, to rate a field's shock absorbency. The field's shock absorbency is measured by dropping an object that 'approximates a human head and neck (about 20sq. in. and 20 pounds) from a height of 2 ft. Low G-Max concludes that the field can absorb more energy than the player. On the specific play that we're analyzing, Trufant and Lewis both end up landing on grass, which covers Lincoln Financial Field (Philadelphia Eagles Stadium). The grass has a G-Max of over 60, while synthetic surfaces have a 120 G-Max rating.

Many people often associate G-force (measurement of acceleration felt as weight; force per unit mass and can measured by accelerometer), with people who are astronauts or fighter pilots. What those people may not know, is that there are some 'earthbound' events that can boost G-force. There aren't a ton of things that can match the G-force of a hard-nosed football tackle, which can reach 150 on the G-force scale.

So far I've discussed Trufant's tackling side of the play, but let's not forget about Greg Lewis. According to John Melvin, an injury biomechanics researcher for General Motors and NASCAR, the human body can handle a tackle with a half a ton of force, and then some, as long as the impact is well-distributed. In fact, the body can handle twice that amount. Being able to well-distribute the forceful impact of a tackle is handled by the equipment a football player wears. The equipment spreads out the incoming energy, which lessens the severity of the tackle. Good thing for Greg Lewis, who is a pretty skinny guy.

Speaking of equipment, the design of today's shoulder pads aid players who are dishing out, and taking viscous hits. According to Tony Egues, head equipment manager for the Miami Dolphins, "Shoulder-pad plastic hasn't changed much in 25 years, but it is now molded into designs with more right angles to deflect impacts." Also, football helmet's solid shell and face mask helps redistribute the energy of a full force tackle.

The foam padding underneath the plastic components of equipment absorbs and compresses energy, while reducing the speed of impact. Visco elastic foam retains its shape better than conventional foam, that's why it's used in the NFL. The shape of the foam returns to form rapidly after a hit. It was invented by NASA in order to protect astronauts from G-forces during liftoff.

The tackle made by Marcus Trufant caused Lewis' head to accelerate in his helmet at 30 to 60 g's, according to a study done by Virginia Tech. Researchers at VT collect data with the Head Impact Telemetry System, which plants sensors and wireless transmitters in helmets. Stefan Duma, director of the university's Center for Injury Biomechanics, states, "We see 100-g impacts all the time, and several over 150 g's."

Greg Lewis is a wide receivers coach at San Jose State University and Trufant is currently a NFL free agent. While both of them sustained careers that were relatively healthy, they have been subjected to serious knee injuries playing football. According to the Pittsburgh Tribune-Review, more than 1200 knee injuries were reported by the league between 2000 and 2003, accounting for one out of every six injuries -- by far the highest percentage in the NFL. The knee's anterior crucial ligament can stand close to 500 pounds of pressure, but it can easily tear because of hits from the side and evasive maneuvers.

Whew. That was a lot to cover wasn't it? But then again, I bet that you probably learned a few new things about how science relates to football that you hadn't known before. Football is a great sport, no doubt about that. It's probably America's favorite sport at that. But after writing this blog post, I have a better understanding of the physics behind those viscous tackles that we love to witness every Sunday. For an example, speed and force go hand in hand. A guy who runs 4.5 second 40-yard dash can hit with the force a 1600 pounds. That's insane! Who knew that a knee could handle withstand 500 pounds of pressure, but remain highly vulnerable to tear if hit in a specific way? I learned a lot writing this blog post, and I hope that you guys did too.

Oh yeah. Somebody say a prayer for the Eagles to go undefeated the rest of the season and win the Super Bowl. It's a pipe dream, yeah I know. 


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