How and Why Minimal or Non-Contact Injuries Occur in NFL Workouts

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How and Why Minimal or Non-Contact Injuries Occur in NFL Workouts
Tony Gutierrez/Associated Press

Last year, an outbreak of preseason ACL tears throughout the NFL drew attention to the injury like never before. Other serious injuries—Achilles tendon ruptures and triceps tears, for example—also dotted last year's summer football landscape, and many teams felt the effects throughout the entire 2013 regular and postseason.

Unlike regular-season injuries, early offseason injuries usually do not come as a result of high-flying collisions. Rather, a significant percentage do not involve a direct hit to the injured area. Instead, they often occur due to seemingly minor secondary contact elsewhere on the body—and even sometimes without contact at all—in combination with physical subtleties and momentum shifts.

Each year, the injuries first start during OTAs.

As noted by theScore's Michael Amato and David P. Woods, OTAs ideally constitute non-contact drills and practices. In fact, NFL rules prohibit contact drills at this stage in the offseason. That said—as noted by Stephen Holder of The Indianapolis Star—the line between non-contact and contact practices is probably quite blurry at times.

Either way, serious injuries are, regrettably, already starting in 2014.

Lynne Sladky/Associated Press
A third ACL tear likely ended Domenik Hixon's 2014 season—and possibly his career.

For instance, late last month, Dallas Cowboys linebacker Sean Lee suffered a torn left ACL during an OTA drill. Though he could conceivably return in 2014, the injury almost certainly ends his season. Chicago Bears wide receiver Domenik Hixon also tore his ACL—his third such injury, ending his season and possibly career.

Elsewhere, hamstring strains and ankle sprains continue to pop up from time to time.

How do these types of injuries occur, and why do they occur? To come up with an answer, let's use some anatomy and a bit of physics to analyze two of the most serious kinds of non-contact injuries: The ACL tear and the Achilles tendon rupture.

Non-contact ACL Injuries: Positioning, Momentum and Direction Change

Soft-tissue injuries occur when the ligament or tendon in question stretches beyond a critical point, beyond which damage, tearing or rupture occurs. A number of factors determine that critical point, such as flexibility and tensile strength.

Movements that cause injury are tissue-specific. For example, ACL injuries occur when something forces the knee to twist down and inward—one of the types of motion it tries to prevent.

Wikimedia Commons with edits by the author.
The ACL is one of the four main knee ligaments. It connects the tibia to the femur and prevents the knee from twisting inward and the tibia from moving forward.

To create the sensation of the ACL limiting such internal rotation, stand with both feet planted on the ground with both sets of toes pointed forward. Then, bring one knee inward and slightly down—toward the body's midline. It can only move so far, largely due to the ACL.

In the case of direct contact ACL injuries, a blow to the outside of the knee is frequently to blame. On the other hand, non-contact ACL injuries often come as a result of a perfect storm of positioning, momentum and direction change. Often, subtle contact elsewhere on the body also plays a significant role.

That said, the relative roles of anatomical and biomechanical factors leading to non-contact ACL tears remains a subject of research. However, some basic anatomical knowledge provides insight into the underlying mechanisms of two recent examples of minimal-to-no contact ACL tears in the NFL.

To start, here is a video of Sean Lee's injury—courtesy of the club's official website. Lee wears No. 50. A closer look at the play shows the series of events leading to injury.

As Lee sidesteps, he first plants his left foot out to the side of his body as he leans to the right and back. Importantly, he also points his toes out and away from the play. Then, his midsection twists back and to the left as a result of relatively minimal contact to his upper body, but his planted left foot remains in place.

Because his toes pointed out to the left, his knee could not bend in the normal plane as he went to the ground—as it could if his toes pointed inward, for example. As a result, his knee twisted inward relative to his toes and hips—toward his body's midline—most likely causing the tear.

Official video (linked above) with edits by the author.
This series of screenshots from the video linked above shows the likely point of Sean Lee's injury frame by frame. The yellow arrow represents the relative inward motion of his knee, probably causing the ultimate injury.

Interestingly, in 2010, Dr. Barry P. Boden and colleagues published a study in the Journal of the American Academy of Orthopedic Surgery that may relate to Lee's injury. In the study, the authors suggest landing on either a flat foot or the back of the foot may predispose to ACL injury—as may a flexed hip at the time of impact—along with several other factors. Indeed, Lee landed on his heel with his thigh bent upward.

Admittedly, the contact to Lee's upper body probably played a role in the injury, as it contributed to the change in direction of Lee's torso as well as his hip flexion and resulting momentum shift. However, the placement of Lee's foot started the sequence of events, and his knee received no contact whatsoever.

A somewhat similar situation occurred last season to Sam Bradford—an injury this author reviewed last October. The GIF below—courtesy of Fox Sports via AOL's Sporting News—makes it clear Bradford's knee did not receive significant contact.

Fox Sports (via AOL's Sporting News).

As shown, a slight nudge to Bradford's side—while he simultaneously turned to his left at least partly on his own accord—created a mismatch between his momentum and the direction his knee faced relative to where his toes pointed. As a result, his knee collapsed inward without direct contact—a moment that becomes painfully clear with the following screenshot.

Screenshot via Fox Sports (via AOL's Sporting News) with edits by the author.
The yellow circle highlights the exact moment Sam Bradford's injury most likely occurred.

Immediately after the above moment, Bradford dropped the football seemingly suddenly—possibly a reaction to the onset of extreme knee pain.

Non-contact ACL Injuries: Planting, Cutting and Hyperextension

In addition to preventing internal over-rotation of the knee, the ACL also keeps the lower leg from moving forward with respect to the thigh.

Knee hyperextensions—or over-straightening, leading to a bending of the leg in the opposite direction—can cause the lower leg to shift forward with respect to the thigh. If it does so suddenly and sharply enough, the ACL may not be able to keep up.

A number of scenarios—such as landing from a leap onto a forcefully straightened leg—can result in a knee hypertension and ACL injury. Hyperextensions can also damage other knee ligaments.

Elsewhere, sudden stops, plants and cuts may injure the ACL if the momentum and direction imbalance mentioned in the previous section results. For example, running forward and planting the right foot—with the toes pointed to the right—before attempting to cut to the left can cause the injury.

Non-contact Achilles Injuries: Forced Planting of the Foot

Located at the back of the ankle, the Achilles tendon attaches the calf muscles to the back of the heel. When the calf contracts, it pulls on the tendon—which in turn pulls on the heel. As an end result, the toes point down.

Wikimedia Commons.
The Achilles tendon—seen here as a thick white band—connects the muscle bodies in the calf to the back of the heel.

In football, the Achilles tendon plays a large role in jumping, as it allows an athlete to use the toes to push off the ground. It also helps in planting and cutting.

For example, when a wide receiver wants to quickly change direction, he will most likely stop suddenly and push off with his toes. Doing so brings the toes upward—the opposite motion the Achilles helps bring about. If the receiver does so too suddenly and too forcefully, the Achilles can stretch too far, rupture and pull off the heel.

Last year, San Francisco 49ers wide receiver Michael Crabtree fell victim to the above mechanism of injury when he suffered a torn Achilles tendon in May. According to the AP's Janie McCauley, Crabtree went down at the start of a route.

Marcio Jose Sanchez/Associated Press
Michael Crabtree returned for the end of the 2013 season following his Achilles tear.

Most likely, Crabtree ran in one direction and planted his right foot in order to turn to his left and start his route. By using his toes to push off the ground, he contracted his calf—and thus tightened his Achilles. However, driving his heel into the ground stretched the contracted tendon, and in this case, the force proved too great.

For a clear video representation of a non-contact Achilles injury, look no further than New England Patriots lineman Vince Wilfork's injury last year. This video from shows the play that ended Wilfork's 2013 season.

Wikimedia Commons with edits by the author.
The above screenshot shows the exact moment Vince Wilfork likely over-stretched and tore his Achilles.

As seen, Wilfork plants his right foot behind his body in an attempt to move forward. However, his weight overwhelmed his tightened Achilles tendon.

Bottom Line

Despite the relative safety of non-contact OTAs, injuries remain a threat. Relative deconditioning may play a role in some, while a combination of unfortunate positioning and bad luck reigns supreme in others. Regardless, whenever a motion, plant or cut stretches tissue beyond its capacity, a sprain or strain becomes a possibility.

ACL sprains and Achilles tendon ruptures represent two of the most serious non-contact injuries, as either can end a player's season before it starts.

Hopefully, this year's OTAs and training camps do not produce as many devastating injuries as in 2013. Nevertheless, the smart money says that at least a few more will come down the pipeline before the 2014 preseason gets underway.

Dr. Dave Siebert is a resident physician at the University of Washington who plans to pursue fellowship training in Primary Care (non-operative) Sports Medicine. The information above is for informational purposes only.

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