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Within a fraction of a second, it became clear that New England Patriots running back Stevan Ridley suffered a concussion during the AFC Championship (h/t Jeff Rowe) following a brutal hit by Baltimore Ravens safety Bernard Pollard.

The events leading up to the tackle, the hit itself and Ridley's reaction following the blow could be placed in a textbook on NFL concussions. And the sight of Ridley leaving the game immediately afterward did not shock anyone.

What made it obvious that Ridley suffered at least the 171st concussion this season, according to PBS' Concussion Watch?

Here is a closer look, beginning with a clip of the injury itself.

The Events Immediately Prior to Contact

On the play in question, Ridley took a handoff from Patriots quarterback Tom Brady and reached full speed over the course of a six-yard gain before lowering his head as the defense closed in. Though Pollard did not reach full speed, he met Ridley while traveling in the exact opposite direction.

That is the beginning of the recipe for a concussion.

Once believed to be a brain bruise, scientists now know that concussions actually occur when the brain moves abnormally within the skull—it does not need to "contact" the skull per se.

For a better picture, think of a bowl of Jello being shaken. The bowl is the skull, and the Jello is the brain.

How much the brain needs to move within the skull before a player suffers a concussion—and which direction it needs to go—remains a subject of intense and fascinating research. However, the sobering reality is that according to a 2007 study out of Wayne State University, it might not be much.

In the study, Warren N. Hardy and colleagues discovered that the brains of cadavers—preserved bodies of those so graciously willing to donate their bodies to science—moved an average of less than seven millimeters in response to blows to the head similar to those that may occur in a football game.

That bears repeating—seven millimeters.

In other words, concussions are not caused by a brain doing back flips within the skull. In fact, miniscule amounts of movement might be to blame.

Of course, there is no way to monitor a player's brain during a game in real time. Yet suffice to say, it is now clear that Ridley and Pollard were definitely moving quickly enough at the time of impact to cause injury.

The Hit Itself

Video replay clearly shows that Ridley lowered his head into Pollard, precipitating helmet-to-helmet contact. Pollard's helmet struck the upper portion of the left side of Ridley's, and the hit sent Ridley spinning to his right and down to the ground. His brain felt at least two different types of forces on the play—linear and rotational.

When Ridley and Pollard first made contact, Ridley's skull immediately stopped moving forward, sending his still-moving brain toward the left side of his skull—a result of linear force. At the same time, Ridley rebounded upward and spun to his right, meaning his brain rotated counterclockwise (to the left)—in relation to his skull—a result of rotational force.

In short, Ridley's brain moved in several different directions as a result of the hit.

For still unclear reasons, abnormal brain movement leads to the following, causing symptoms:

Unregulated activity of neurotransmitters—the molecules that carry signals from neuron to neuron.

An impaired ability of neurons to use glucose—the brain's exclusive fuel source.

Decreased brain blood flow.

One theory holds that abnormal stretching of neurons and blood vessels by brain movement disrupts their normal cellular activity.

Ridley's brain certainly underwent those changes, but it appears that Pollard's did not. Why is that the case? That is the million-dollar question.

One explanation is that Pollard—and thus his brain—was not moving as fast at the time of contact as Ridley, suggesting his brain did not come to a halt or rotate quite as sharply.

It is also possible that Pollard has a higher so-called "concussion threshold" than Ridley, meaning it takes a larger combination of linear and rotational forces to cause his brain to undergo concussion-like changes.

Most likely, it is a combination of the two.

Every athlete touts his or her own unique concussion threshold, and countless factors likely influence an individual's unique level of concussion susceptibility. These unanswered questions are one of the hottest areas of research in sports medicine today.

The Events Immediately Following the Hit

Ridley immediately lost consciousness due to the hit, made obvious by his body going limp, the fumble that resulted and his posture on the ground immediately following the tackle. The large amount of rotational force applied to Ridley was likely to blame.

The brain stem is particularly sensitive to rotational forces, and within the brainstem is the reticular activating system (RAS)—the part of the brain responsible for maintaining consciousness. By causing the cellular changes mentioned above, those rotational forces briefly stunned Ridley's RAS and caused him to lose consciousness.

Similar changes in the brain stem also caused Ridley's "fencing response."

When Ridley hit the ground, he held his left arm—the side of the head that was hit—straight out in front of him and right arm—the side of the head not hit—bent in toward his side for a few seconds.

However, he didn't know he was doing so. Believe it or not, his motions were actually reflexive.

During the first year of life, humans exhibit a number of primitive reflexes. For example, before four or five months of age, gently lifting an infant partially off the bed by the hands and letting go will cause the infant to:

Reach his or her arms outward, forward and then inward

Cry

Called the Moro reflex, this sequence of events represents an infant's natural response to losing support—in this case, being let go—and searching for something to grab onto.

As the brain grows during the first year of life, infants gain the ability to suppress primitive reflexes such as the Moro reflex. However, brain injuries can reactivate them through a complex sequence of events in the brain stem, and the fencing response is thought to occur for the same reason.

Ridley's Walk to the Locker Room

With the severity of his injury already obvious, Ridley quickly regained consciousness and left the game. Yet before he did so, he gave one more glimpse into how he was really feeling, as an astute observer may have noticed Ridley bump into the wall on his way to the locker room.

That was no accident.

Concussion affects all areas of the brain, including cognitive ability and motor control. Deficits in motor control most readily manifest themselves as difficulty with balance, as maintaining proper balance requires the flawless cooperation of several areas of the brain.

Ridley's Next Steps

Ridley will rest both his mind and his body until his symptoms resolve. He will then gradually return to both athletic and cognitive activity.

Since the Patriots' season is now over, doctors will play it very safe with Ridley. Such a severe injury may take time to resolve, and there is no reason for Ridley to rush back to training.

He should make a full recovery.





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Dave Siebert is a medical/injury Featured Columnist for Bleacher Report who will graduate from medical school in June. He plans to specialize in both Family Medicine and Primary Care (non-operative) Sports Medicine and has a special interest in concussions. Injury and anatomical information discussed above is based on his own knowledge and experience evaluating concussions under the direct supervision of Sports Medicine physicians and concussion specialists. For further, more technical reading, please see: