Nylon Questions: How can we quantify the effect of injuries on expected future performance?

Sports analytics is a constantly evolving field and keeping up can be a challenge, especially with so much work being divided between the public and private spheres. As we head into the 2018-19 season, Nylon Calculus wanted to take stock of where we are and of what’s coming next.

This project is a throwback to work Keith Woolner did for Baseball Prospectus nearly two decades ago, and an update to Kevin Pelton’s basketball-specific version from five years ago. Our staff compiled 10 questions whose answering will likely guide the next few years of public analytic work. Not knowing what has been accomplished in private by NBA teams and consulting firms, we focused on questions that could be worked on in the public sphere, wouldn’t have to be answered with existing datasets (we don’t want to imagine the data we have now is all we’ll ever have) and things that would theoretically have an effect on how teams operate on and off the court.

Hopefully, these questions will help spark, refocus, and recalibrate conversations and lead to collaborative progress here at Nylon and everywhere else sports analytic work is being done.

7. How can we quantify the effect of injuries on expected future performance?

Injuries are an unfortunate part of sports and all athletes have at least some risk of injury every time they step onto the court. Collision sports such as football, rugby, and hockey are known for the risks associated with concussions, broken bones, and even missing teeth. Soccer has issues with concussions too, but also ankle and knee risks. Even non-contact sports such as tennis have injury risk.

Basketball has common injuries associated with the game as well. Ankles, knees, and backs are at risk of injury due to basketball being played on a hard surface with a lot of quick lateral movement and jumping is required. The constant impact of running and jumping on a hard surface puts a strain on the joints of the lower body and back. Basketball, being a contact sport, also has risks associated with concussions but at a lower probability compared to collision sports.

The NBA is aware of the risks associated with their sport and the increase in injury risk associated with fatigue. Individual teams have been known to take the idea of rest and injury prevention seriously. At the tail end of the Duncan era in San Antonio, the Spurs were known to rest their aging stars on a regular basis and were even fined for resting their players. Being rested plays an important part in injury prevention as a tired body is a weak body. Weaknesses and imbalances within the body typically lead to increased injury risk. By attempting to limit back-to-back games and increasing rest between games, the NBA has embraced the idea of rest and injury prevention. Like most major sports, the NBA has also started taking concussions more seriously with rule changes and concussion protocols. The NBA has even recognized the importance of mental health and take steps to help players cope with anxiety and depression.

Ideally, the NBA and the individual teams care about the well-being of their players. I like to think this is the main driver behind the new rules and programs designed to keep players healthy. However, the NBA is also a business that pays millions of dollars to their players. In order to stay financially viable, it is necessary for the team and the league to get the most out of their players.  The teams expect their investment in a player to lead to more wins and better entertainment for fans. More fans lead to more money for the league and the owners, and therefore it is to their benefit to make sure the players are happy, healthy, and have all they need to handle the stresses of a long NBA season. Keeping players injury free keeps them on the court and is important for both the players and the league.

Like the NFL, the NBA had adopted the multiple day combine event in an effort to measure and evaluate incoming players. While measurements such as vertical leap, shuttle runs and other physical attributes are impressive, the NBA can further innovate to focus even more in injury prevention starting at the combine. One popular test is the Functional Movement Screen, that measures an athlete’s muscle imbalances and range of motion of their joints. Getting a baseline score for each player could help prevent injuries and determine appropriate timelines to return to play after injuries.

Also helping these players learn to move correctly is important for basketball players. These are some of the longest humans alive. Typical professional basketball players have long limbs, broad shoulders, and long torsos. According to basic physics, forces applied at longer distances (limbs) equals more torque at the anchor (core and low back). Teaching these players to move correctly using their core and improve their hip mobility could reduce the impact of back problems.

Unfortunately, injuries will happen no matter how hard we try to prevent them. From the minor joint sprains and muscle strains to the major ligament and tendon tears and repeated concussions, injuries are a part of the game. Being able to quantify the impact of an injury will play an important role in team decisions going forward.

Research does provide some good news in terms of minor injuries and even many types orthopedic surgery. It seems that for the most part only ACL reconstruction and Achilles tendon surgeries have large negative effects on future performance. Jones fractures do not seem to negatively affect future performance. While microfracture surgery does not seem to shorten career length it does seem to lead to slightly lower games played per year and performance.

ACL injuries are common among basketball players, especially women basketball players. And the bad news is that it that in many of the studies that I found, performance does seem to be affected by ACL reconstruction. One study found that players play about 22 fewer games per year and have a career post injury PER that is 2.45 lower than the control group. This study includes 80 players from 1984-2014. With advances in ACL surgery and rehab, it could be that some of the older examples of the 80’s and 90’s did alter a player’s career more than more recent ACL injuries.

Newer studies of WNBA players and NBA players paint a slightly better picture of post-injury performance. Both studies seem to suggest that there is no statistically significant decrease in performance beyond two seasons after the injury. The season immediately following the injury does tend to show a decrease in performance. The second season post-injury seemed to show players performing at or near pre-injury levels. Which begs the question are these players returning too soon? Something to think about with a player like Kristaps Porzingis coming back from an ACL injury this season. The one drawback of each of these studies is that they have very small sample sizes of 18 and 12 respectively.

The ugliest injury is the dreaded Achilles rupture. From the research I found, an Achilles rupture is the biggest long-term detriment to player performance.  Possibly the most relevant study showed that Achilles injuries lead to less playing time and significant drops in PER. PER was on average 4.57 lower in the first year and 4.38 lower in the second year. Almost 40 percent of the players did not even return to play after the injury.

Is there any good news for Achilles injuries? There are three reasons to be somewhat optimistic about Achilles injuries. In many of the studies, age and body mass index (BMI) were predictors of return to play. Younger and thinner players have a better chance of returning to play and better performance. Another possible reason for an optimistic outlook is that biomechanics and body alignment can help improve recovery. Simple tests like single leg hop, the range of motion, functional movement screens, and atrophy of the injured leg can tell a lot about the recovery. A final reason for hope for Achilles injuries is evident in ACL injuries. Until the last decade, ACL injuries were assumed to be career-ending injuries much like the Achilles injuries are now. However much of the research I found shows that recent ACL injuries typically take two full seasons but athletes can get back to pre-injury performance. With advances in medical procedures and rehab, it might be possible one day that players can return from a torn Achilles much like they can from ACL injuries today.

So what does this all mean for the question of how we can quantify the effect of injuries on future performance? First of all, surgeries of the knee and ankle are generally bad for basketball players. But there are some predictors of the amount of decrease in performance. Age and BMI play important roles as well as the range of motion of the joint.

We can use this information to look at two quality players and their recent injuries. DeMarcus Cousins ruptured his Achilles tendon last season. At the age of 28 and with a BMI of 27.4 kg/m2, he is on the border of being in danger of a significant drop. Age 30 or greater leads to worse outcomes while a BMI of 27 or more also tends to lead to worse outcomes for Achilles injuries. The biggest thing to look out for with Cousins is how he plays when he does return this year. Since Achilles injuries do not tend to improve after the second year,  whatever level of play he performs at this year will most likely be consistent with the rest of his career. A second player to keep an eye on is Zach LaVine. He has received criticism recently for not being worth his big contract. However, last season was his first full season back from an ACL injury. It typically takes two full seasons for players to get back to playing near their pre-ACL injury levels and LaVine might be able to silence some of his critics due to finally being fully healthy. We will find out a lot more about these players and the future performance after this season.