Dalvin Cook

        Minnesota Vikings running back Dalvin Cook is one of the best backs in the league. He has consistently put up great rushing numbers and is his team’s best offensive asset. He was drafted with the 41st pick in the 2017 NFL draft. Best known for his elusiveness and toughness, Cook seamlessly made the transition from college to the NFL. But in week 4 of the 2017-2018 NFL season, Cook’s rookie season against the Detroit Lions, Cook took a handoff from Case Keenum up the middle of the field for a 10-yard gain. As soon as he cut to his right to avoid being tackled by Lions safety Tavon Wilson, Cook reaches down to grab his left knee, fumbling the ball. Vikings team doctors and coach Mike Zimmer described Cook’s injury as “a normal, typical ACL tear”. 

The absence of contact from another player means we can classify Cook’s injury as non-contact. The direction at which force is applied to the knee can cause the ACL to tear, especially in game situations which typically require more load and speed to produce faster movements. Between 2013 and 2016 about 10% of ACL injuries were running backs, and about 71% of these were noncontact. Running backs, especially those with playstyles similar to Dalvin Cook’s, rely on sudden change of direction to be effective. While this movement does improve their on-field performance, it statistically puts them at a higher risk for injury. Cook was injured because he changed direction while cutting. To fully understand the nature of Dalvin Cook’s injury, it would be beneficial to look at certain biomechanical risk factors that apply.

An effective running back is able to change directions quickly to mislead defenders. However, changing direction with poor technique and body posture can overload the ACL without being hit. When changing direction, an athlete will first plant their foot and decelerate their body, then push off in a different direction. If the leg is not aligned properly, the athlete risks falling into valgus collapse: a combination of knee movements that has been shown to play a significant role in overloading the ACL. Valgus collapse is when the knee moves toward the midline of the body, with the kneecap pointing inwards. Oftentimes the thigh is the only part of the leg to rotate in the direction of the knee. The lower leg, on the other hand, rotates in the opposing direction. This causes the ACL to stretch and twist, significantly increasing the chance it tears. Parts of the body beyond the leg contribute to valgus collapse, the largest of which is the trunk. If the trunk lags behind the plant leg as the athlete changes direction, the likelihood that the leg falls into valgus collapse increases. All of the risk factors and mechanisms we just discussed can be seen in the video of Dalvin Cook’s injury.

Dalvin Cook cuts to avoid safety Tavon Wilson and gets injured

Cook cut to his right to evade a defender when he was injured. To perform the juke, he shifted his hips to his left then quickly flipped them to his right. Cook loads his left leg and prepares to push off to continue his run. This change of direction forces Cook to slow down and load his knee. The hip fake alters his torso position by pulling it over his plant leg in the direction of the fake, leading to more prominent valgus collapse in the knee during the juke. His hip and knee both come away from the midline of his body, and his shin starts to face inwards. The speed at which these movements were performed eventually overloaded the ACL, resulting in the ligament tearing.

ACL injuries are very common in the NFL. Studies estimate that a player is more at risk for an ACL injury up to ten times more than other sports. From the 2013-2015 season, 50% of running backs who suffered an ACL injury returned to play. The study showed that there was no significant difference in RTP for contact and non-contact injuries, and younger players tended to return faster and at the form they were in before injury. Cook was 22 at the time of injury, so he was a reasonable candidate to return to play at pre-injury level, and eventually returned for the start of the 2018 season and has shown no signs of his injury hampering him. While he has dealt with a variety of injuries including shoulder and ankle problems, Cook has not experienced further setbacks in his knees. When describing his recovery, Cook said he felt great about coming back the next season and “I’m ready to deliver”. Cook put up solid numbers in his 2018 season, then exploded for back-to-back 1,000-yard seasons in 2019 and 2020, earning him Pro Bowl selections in both seasons. Although Dalvin Cook suffered a possibly devastating ACL injury early in his rookie season, he has been able to rebound spectacularly and become one of the best running backs in the NFL today.

Bibliography

Dalvin Cook finally gets drafted, heads to Minnesota: NFL draft. PFF. https://www.pff.com/news/draft-dalvin-cook-finally-gets-drafted-heads-to-minnesota. Published April 28, 2017. 

Cronin C. Dalvin Cook has torn ACL, will miss remainder of rookie season. ESPN. https://www.espn.com/nfl/story/_/id/20895191/dalvin-cook-minnesota-vikings-torn-acl-miss-remainder-rookie-season. Published October 2, 2017. 

Johnston JT, Brophy RH. Video analysis of Anterior Cruciate Ligament Tears in professional American Football Athletes: Response. The American journal of sports medicine. https://pubmed.ncbi.nlm.nih.gov/30499707/. Published December 2018. 

Dos Santos T. The effect of training interventions on change of direction biomechanics associated with increased anterior cruciate ligament loading: A scoping review. Sports medicine (Auckland, N.Z.). https://pubmed.ncbi.nlm.nih.gov/31493206/. Published December 2019. 

Alentorn-Geli E. Prevention of non-contact anterior cruciate ligament injuries in soccer players. part 1: Mechanisms of injury and underlying risk factors. Knee surgery, sports traumatology, arthroscopy: official journal of the ESSKA. https://pubmed.ncbi.nlm.nih.gov/19452139/. Published July 2009. 

Boden BP. Noncontact anterior cruciate ligament injuries: Mechanisms and risk factors. The Journal of the American Academy of Orthopaedic Surgeons. https://pubmed.ncbi.nlm.nih.gov/20810933/. Published September 2010. 

Minnesota Vikings Star Rookie RB Dalvin Cook Out with "Nearly Complete" ACL Tear. The Fumble; 2017. https://www.youtube.com/watch?v=NrbFNVW8Eeo. 

Eisenstein ED. Variables affecting return to play after anterior cruciate ligament injury in the National Football League. Orthopedic journal of sports medicine. https://pubmed.ncbi.nlm.nih.gov/27826598/. Published October 2016. 

Dalvin Cook stats. Pro Football Reference. https://www.pro-football-reference.com/players/C/CookDa01.htm. 

Krammer A. Vikings running back Dalvin Cook talks about recovery from ACL surgery. Star Tribune. https://www.startribune.com/vikings-running-back-dalvin-cook-talks-about-recovery-from-acl-surgery/489473781/. Published July 30, 2018. 

Joe Burrow

        During week 11 of the 2021 NFL season against the Washington Football Team, Cincinnati Bengals rookie quarterback Joe Burrow dropped back to throw. During the play, defensive tackle Jonathan Allen beat his man and fell into Burrow’s knee as he released the ball. After being carted off, it was clear he had suffered a serious injury. A few hours later, Adam Schefter reported that Burrow had suffered a contact ACL injury. He tore his anterior cruciate ligament (ACL), medial collateral ligament (MCL), and partially tore his posterior cruciate ligament (PCL) and meniscus. Contact ACL injuries occur when something comes into contact with the lower leg resulting in an ACL injury. These injuries can be devastating to NFL players, forcing them to go through a long recovery, and possibly hamper them when they return to play. Between 21-37% of players who suffer ACL injuries never appear in another NFL game. We can learn more about Burrow’s injury by looking at the anatomy of the knee.

To fully understand what happened to Burrow, it is important to know more about the ACL. Stretching from the back of the femur (thigh) to the front of the tibia (lower leg), it resists hyperextension and rotation of the knee. The MCL is another ligament in the knee. It connects from the bottom of the thigh bone to the top of the shin bone while resisting the knee collapsing too far inwards. 

Outside

Inside

The body puts more load on different structures based on how it is positioned. Certain postures can cause more load to be applied to the ACL, influencing injury risk. One such position is when the leg is fully extended and the knee is locked. Any contact with the knee will increase the load on it, possibly resulting in an injury. Forces on the knee are strongest when the knee is fully extended, and there is greater knee abduction movement. This happens when the knee bends too far inwards. Valgus collapse is a combination of the movements above and is one of the predominant recognized mechanisms when diagnosing ACL injuries. Video analysis supports valgus collapse with external tibial rotations, the direction the shin is facing. Tibial rotations also correspond with foot, knee, and hip position. When tibial rotation is greater, the foot is more abducted and the hip is extended more.  It is important to note that ACL tears only happen when the stress on the ligaments exceeds what they can bear, not necessarily a certain set of positions. Contact ACL injuries are often associated with MCL and meniscus injuries. As the knee collapses from contact, the pressure on the ACL is shared by the MCL and menisci. This was the case in Joe Burrow’s injury, as he also suffered a torn MCL and additional structural damage.

Burrow steps into a throw under pressure and gets injured

When Joe Burrow got injured, he was stepping up in the pocket to make a throw when a defender fell into his knee with the additional weight of Burrow’s teammate. Upon contact, Burrow’s knee bends backwards and he goes down in serious pain. He had just released the ball, so he was in a standard throwing position. His foot was firmly planted in the ground, all his weight was on his one locked leg, and his knee was fully extended. When he is hit, his knee is in the worst position possible, being almost fully extended. As he steps into his throw, we can see he has external tibial rotation with his foot being abducted and him having high flexion in his hip. As soon as two 250+ lb players crash directly into his knee, Burrow’s knee buckles and his ACL and MCL tear. 

This ACL injury can have long term effects for Joe Burrow. Looking at the risk of reinjuring his knee, athletes who suffer ACL tears are 15-25 times more likely to suffer another ACL injury to the same or different knee. Athletes also tend to favor their uninjured knee up to two years post operatively. However, quarterbacks are not at as much of a risk. A study analyzed 69 out of the 156 ACL injuries in the NFL during 2013-2016 and found that about 7% of tears belonged to quarterbacks, with 80% being non contact injuries. Athletes between 23 and 28 with ACL/MCL tears have a 70.9% return to play rate. As a 24 year old male, Burrow falls in this demographic and reasonably aims to return next season. He has also stated that he is aiming for a comeback before this upcoming season. The normal timeline for recovering from an ACL injury for NFL players is from 250-350 days after injury. But Burrow also tore his MCL and PCL. The average return to play for players with ACL and PCL injuries is from 200-700 days. While this is a very broad range, Burrow didn’t suffer a complete PCL tear, so his recovery won’t take more than a year. 

Although he will wear a brace for at least the near future, his play on the field shouldn’t be impacted by his injury. Burrow is more of a pocket passer, and doesn’t have to put too much stress on his knee by running. Burrow’s doctor compared his injury to that of former Bengals quarterback Carson Palmer. Palmer suffered a contact ACL injury during the 2005-2006 playoffs, tearing his ACL, MCL, and meniscus. The following season, his completion percentage dropped from 67.8 to 62.3, and his touchdowns fell by four and interceptions rose by one. While he experienced a down year after his injury, Palmer was a reliable quarterback for the rest of his career. He retired with over 46,000 passing yards and almost 300 touchdowns. When Joe Burrow was hit, he wasn’t aware of the defender falling into his knee due to him looking downfield. He was very unlucky and found himself in the worst possible spot he could be in. He couldn’t minimize the impact by going down immediately after contact. In the future, it isn’t only up to Burrow to prevent another injury. The Bengals have prioritized investing in their offensive line this offseason. They signed tackle Riley Reiff in free agency and drafted Clemson guard Jackson Carman in the second round in April. They also drafted Ja’Marr Chase, Burrow’s top wideout in college at LSU. Joe Burrow has a bright future ahead of him in the NFL, but his contact ACL injury will always be a part of his career.

Bibliography

Acevedo, Rafael J. “Anterior Cruciate Ligament INJURY: Identification of Risk Factors and Prevention Strategies.” Current Sports Medicine Reports, U.S. National Library of Medicine, 2014, pubmed.ncbi.nlm.nih.gov/24819011/.

Baby, Ben. “Recovery of Bengals' Joe Burrow: What to Expect, How It Compares to Others.” ESPN, ESPN Internet Ventures, 16 Dec. 2020, www.espn.com/blog/cincinnati-bengals/post/_/id/32845/recovery-of-bengals-joe-burrow-what-to-expect-how-it-compares-to-others.

Bakshi, Neil K. “Return to Play after Multiligament Knee Injuries in National Football League Athletes.” Sports Health, U.S. National Library of Medicine, 2018, pubmed.ncbi.nlm.nih.gov/29638200.

Dodson, Christopher C. “Anterior Cruciate Ligament Injuries in National Football League Athletes from 2010 to 2013: A Descriptive Epidemiology Study.” Orthopaedic Journal of Sports Medicine, U.S. National Library of Medicine, 3 Mar. 2016, pubmed.ncbi.nlm.nih.gov/26998501.

Hewett, Timothy E. “Mechanisms, Prediction, and Prevention of ACL Injuries: CUT Risk with THREE Sharpened and Validated Tools.” Journal of Orthopaedic Research : Official Publication of the Orthopaedic Research Society, U.S. National Library of Medicine, Nov. 2016, pubmed.ncbi.nlm.nih.gov/27612195/.

Johnston, Jeffrey T. “Video Analysis of Anterior Cruciate Ligament Tears in Professional American Football Athletes: Response.” The American Journal of Sports Medicine, U.S. National Library of Medicine, Dec. 2018, pubmed.ncbi.nlm.nih.gov/30499707/. 

“Carson Palmer Stats.” Pro Football Reference, www.pro-football-reference.com/players/P/PalmCa00.htm.