Hamstrung! MLB sets new record for Hamstring DL’s
Stan Conte PT, DPT, ATC
(The term “Hamstrung” actually originates from Biblical writings and referred to ancient warriors cutting the hamstring tendons of their enemy’s horses so they could not be used in battle)
I received an email in early June from Jeff Zimmerman, a baseball injury blogger for several internet outlets including fangraphs.com. He asked me the following: “What is up with all the hamstring injuries? Cooler weather?”. I told him that I did not know but I would take a look. I have been following the hamstrings since.
There are several factors or questions that come into play in trying to answer that question.
What is the “normal” amount of hamstring injuries in a season?
What was the average number of hamstring injuries per month in order to project where they might end up.
What data is available and which data should be utilized.
Are there other studies that can answer these questions?
Where do you get Injury Data? It is important to determine what injury data is being used in the analysis. There really are only two sets of data that are available. The first and most public, is the Major League Disabled List (DL). This can be obtained from several public sources (spotrack.com is one of the better ones). I have collected DL stats going back to 1991 through various sources. DL data is the most historical and the rules for placement on the DL have remained constant over the years except for the 7-Day concussion DL that was instituted in 2010. This allows us to compare years. 1998 is a good year to start a study since the number of teams (30) have been constant since that time. That gives us 18 seasons of reasonable data for comparisons. I and others have used this data for several published studies including a recent study in The American Journal of Orthopedics (see graph below). However, there are several problems with using this data. The first and foremost is that the DL is NOT an injury database. It is a roster management tool to replace injured players on the 25 and 40 man rosters and thus protecting them from being claimed by other teams through a waiver claim process. Therefore, all the data on the DL may not be precise or even correct and it includes only Major League players.
The other potential database is from the Electronic Medical Record (EMR) System that was established by an agreement between MLB and the Players Union in 2010. Athletic Trainers from every team and at nearly every level of play enter injury data in this system daily. De-identified injury data from the EMR, can be extracted through a database called the HITS system. Through the use of this system, injury data can be obtained that is more accurate and granular than that of the DL. In addition, the HITS system includes major and minor league injury data. There are few issues with this system in looking at the historic trends of hamstring injuries. One is that the data only goes back to 2010. In addition, one can only obtain this data if you are connected with a major league organization and 2016 data will not be available until several months after the end of the season.
Because of these reasons, I decided to utilize the Disabled List dating back to 1998 as my reference to see if there has been an increase hamstring injuries. Below is a chart showing the DL Days accumulated for all teams by years 1998-2015.
Other Studies on Baseball Hamstring Injuries One of the more recent and comprehensive studies on Hamstring Injuries in professional baseball was authored by Dr. Christopher Ahmad, the NY Yankee Orthopedic Team Physician. This study, Major and Minor League Baseball Hamstring Injuries: Epidemiologic Findings From the Major League Baseball Injury Surveillance System was published in 2014 in the American Journal of Sports Medicine. The study utilized the HITS system and reported hamstring injuries in the 2011 season. He reported that in the 2011 season there were 50 major league and 218 minor league players who sustained hamstring injuries, missing an average of 24 days and 27 days, respectively. The study also showed that the highest number of strains occurred in the month of May.
Hamstring DL Placements 1998-2015
Let’s look at some historic data on hamstring injuries. The total number of players placed on the DL from 1998-2015 was 621. The number per year averaged 34.5 per season with a low range of 24 in 1998 and 2011. The highest recorded year was 48 placements in 2014. See table below
The graph below indicates a very slight upward trend increase of Hamstring DL placements over the years but this was not statistically significant with a p value = 0.41 and standard deviation of 6.83. This shows wide variations from year to year with almost an undulating pattern.
Hamstring DL’s by Month
Ahmad’s hamstring article showed that the month of May had the highest incident of hamstring injuries and this is further demonstrated utilizing the DL.
Notice that DL placements in March and September are included in this table but both of these months are deceiving. This is because of the limitations of the DL. March typically includes only the last 7-10 days of the month. This is because the start of the Major League season is on or around April 1st. The rules of the DL allow teams to retroactively place injured players on the DL eight days prior to the beginning of the seasons. There certainly are hamstring injuries and injuries of all types, that occur in spring training but are healed and the players have returned to play prior to the start of the season and therefore are not reported on the DL. September is also unique since on September 1st, all teams are allowed to expand their roster with up to 15 additional players. Therefore there is less of a need to place an injured player on the DL. This is one of the principal reason that March and September are very low DL months.
Lost Days due to Hamstring Injuries The total of lost days due to placement on the DL for 1998-2015 was 20,090. This was an average of 34.5 days per injury. This differs from Ahmad, et al, results that showed in the 2011 season only 24 days were lost to hamstring injuries. There can be several reasons for this difference. One is that the DL has a minimum of 15 days on every DL while the HITS data will more accurately show players that lost only 1 to 14 days thus bringing the average down. However, if the hamstring injury is severe enough to be placed on the DL, the average stay on the DL is 34.5 days. The second reason maybe because 2011 was a low DL year for hamstring injuries showing only 24 DL placements. This was one of the two lowest DL placements in the 18 seasons studied. Also, 2011 had one of the lowest average stays on the DL with 22.9 days. This is in comparison to highs at 46 in 2001 and 2005.
How does all of this compare to 2016 numbers? As of 9/17/16 as indicated by the table below, there have been 53 placements of hamstring injuries on the DL. This surpasses the previous high of 48 hamstring DL placements in 2014. It also demonstrates the two highest levels of hamstring DL’s ever in two of the last three seasons. Also note that most of the months in 2016 were higher than the previous averages with June being very significant.
Re-Injury Rates Ahmad, et al, reported a 20% re-injury rate in 2011 based on the HITS data that included major and minor league players. This is a high re-injury rate. As way of comparison, Oblique injuries have a 8-10% re-injury rate. Our study here using only MLB players on the DL for the past 18 seasons revealed that out of 621 Hamstring injuries, 103 had at least one recurrence of a hamstring strain. Further break down of the 1998-2015 DL by players shows 78% (N=357) never had a recurrence vs. those with recurrence regardless of side (n=103, 22%). This 22% re-injury rate is in line with the Ahmad study. For those with a recurrence, the number of players with ipsilateral (n=75, 73%), contralateral (n=56, 54%), and side unknown (n=3. 3%) recurrences are provided. You’ll notice that these three numbers add up to more than 100%, but that is because some players had both ipsilateral and contralateral recurrences. The ipsilateral recurrences are then broken down by the number of recurrences. This demonstrated in the following flow chart below. This analysis was done by Dr. Christopher Camp, an orthopedic surgeon at Hospital of Special Surgery in New York. See Flow Diagram below.
Our 2016 DL data indicates that 11 players were re-injured in the 2016 season or also had a previous injury in 2015 (one player had 2 previous injuries between 2015-16 for a total of 3 hamstring DL’s). This calculates to a re-injury rate of 18% and is very consistent with Ahmads’s paper’s conclusion of a 20% re-injury rate.
So after all this data, what can we do to decrease hamstring injuries in baseball? Is there a successful prevention program? We need to look to the European soccer leagues to find the answer. Hamstring injuries are very prevalent in European with one study indicating that 80% of all soccer injuries occurred in the lower extremity and that 47% of them were hamstring strains. (Askling, et al, Scand J Med Sci Sports. 2003). This study and others have shown that instituting eccentric hamstring strengthening exercises decreased their prevalence. So why don’t we do that with baseball? One team did exactly that. The Los Angeles Angels just recently did a study where they implemented the strengthening program in their organization and 213 of their players. They found a 25% reduction in major league injuries and a 40% reduction in minor league injuries compared to a control group. In addition, those that did get injuries lost less time and recovered faster than the control group. Many teams have implemented this program and hopefully we will see an overall reduction in hamstring injuries.
After looking at all the historic and current DL that resulted from hamstring injuries, we see that 2016 is a MLB record year for players placed on the DL with 53 total. This also shows that that during the past 3 seasons, 2014 and 2016 had the greatest number of hamstring injuries resulting in DL in the past 5 years. Even though the overall trend over the past 18 seasons does not show a statistically significant increase, there is a trend upward. The re-injury rate utilizing the DL is similar to the Ahmad study utilizing the HITS system. It further indicates the best predictor of a future hamstring injury may be a previous hamstring strain. The final question is of course, why? To this question, I do not have a reasonable answer other than as we have seen, all injuries are pointing upward especially in 2015 and 2016 and hamstring strains are part of that group.