This study examined 58 Major League Baseball players who sustained a hamstring injury over four seasons. Number of games played and official at-bats were analyzed for 2 seasons before injury, the season of injury, and 2 seasons following injury. Games played for each season before injury were significantly greater than for the season of injury (p = 0.002 and p = 0.003 respectively). Games played for each season following injury were not significantly different than for the season of injury (p = 0.26 and p = 0.08 respectively). At-bats for each season before injury were significantly greater than the season of injury (p = 0.01 and p = 0.007 respectively). At-bats for each season following injury were not significantly different than the season of injury (p = 0.26 and p = 0.07 respectively). Seventeen players played one or less seasons after injury and were already playing less in 2 seasons before injury. If a hamstring injury occurs in a player with fewer games and at-bats in the 2 seasons before the injury, his career is at risk for ending soon after the injury. However, if the injury occurs in a player with high participation in games and at-bats in the 2 seasons leading up to the injury, he has a good chance of returning to the same level of participation after injury.
Keywords: hamstring; baseball; Major League; athletic injuries
1. Introduction
Hamstring injuries are a common in sports. A recent report of Major League Baseball (MLB) players estimated that among position players hamstring injuries are the most common lower extremity injuries, and second most common overall [1]. These injuries often result in a significant length of stay on the disabled list. Purported risk factors for hamstring injuries across all sports include previous injury [2-6], reduced hip flexor flexibility [4], hip weakness [7, 8], and reduced core stability [9, 10].
Earlier reports have suggested that injuries in MLB are not decreasing [1, 11]. Most of the injuries reported on the disabled list occur early in the season or pre-season [1, 12], which can potentially effect a player’s overall performance for the remainder of the season. In this report, we sought to characterize the epidemiology of hamstring strains occurring among position players in MLB and its association with future return to play.
2. Methods
2.1. Data
The Major League Baseball disabled list (DL), which is publicly available, was reviewed retrospectively from 2002 to 2005 [13]. The DL is an injury database verified by the trainer and team physician from each team. According to MLB rules, to be placed on the disabled list, the player must be certified as unable to play, with a specific diagnosis made by the team physician[14]. Once on the disabled list, the player cannot return to the active roster for a minimum of 15 days. The player can remain on the disabled list for as many days as necessary for him to return to play.
All players on the DL from 2002 to 2005 with a listed injury as “hamstring strain,” “hamstring pull,” “hamstring tear,” or “hamstring” injury were identified as the initial subject population. Seventy-six total players were identified. Since hamstring injuries occurred much less commonly in pitchers (3.3% of all injuries) compared to all other position players (13.7% of all injuries) [1], all of the pitchers were excluded from the analysis. If a player was placed on the DL for a hamstring injury more than one time on non-consecutive occasions during the study period, only the initial injury was included in the analysis.
Height, weight, position played, hand dominance, and age at time of injury were recorded, and each player’s body mass index (BMI) was calculated. Number of days on the DL for the hamstring injury were recorded. Number of MLB game appearances and official at-bats were recorded for each of the 2 seasons prior to injury, the season of injury, and the 2 seasons following the injury.
Totals are reported as means with standard deviation. Analysis of variance was conducted to compare age and BMI with the number of days on the DL. Paired t-tests were used to compare game appearances and official at-bats between seasons before the injury and seasons after injury with the season of injury.
3. Results
During the study period, 75 players were identified on the DL to have a hamstring injury of any kind. Fourteen were pitchers and subsequently excluded. Three players were placed on the DL for hamstring injury on two non-consecutive occasions during the study period and so the second injury was excluded from the analysis. Therefore, 58 position players met the inclusion criteria and form the basis of the analysis. Twenty-six players were outfielders, 25 were infielders, 5 were catchers, and 2 played first base and outfield.
The mean age at time of injury was 30.7 + 4.1 years (range, 21 to 39). Twenty-eight players were left hand dominant and 30 players were right hand dominant. The mean height was 183.5 + 6.0 cms (range, 168 to 201). The mean weight was 90.4 + 7.9 kgs (range, 75 to 105). The mean calculated BMI was 26.9 + 2.6 (range, 22.4 to 38.1). The mean length of stay on the DL was 15.9 + 5.9 days. However, 56 of 58 players were on the DL for 15 days, while the 2 others were on the DL for 23 and 60 days, respectively. Therefore, there was no significant correlation between age or BMI with length of stay on the DL.
The mean number of MLB game appearances during the season of injury was 87.6 + 40.1 games (Figure 1). The mean number of MLB game appearances for one and two seasons prior to injury were 105.1 + 44.1 and 108.6 + 45.0 games, respectively, and significantly greater than for the season of injury (p = 0.002 and p = 0.003 respectively). The mean number of MLB game appearances for one and two seasons after injury were 82.2 + 52.1 and 75.6 + 57.4 games, respectively, and not significantly different than for the season of injury (p = 0.26 and p = 0.08 respectively). Nine players did not play another season after the season of injury, and 8 other players only played one more season after the season of injury.
The mean number of official MLB at-bats during the season of injury was 289 + 163 (Figure 2). The mean number of official MLB at-bats for one and two seasons prior to injury were 348 + 187 and 371 + 187, respectively, and significantly greater than the season of injury (p = 0.01 and p = 0.007 respectively). The mean number of official MLB at-bats for one and two seasons after injury were 269 + 199 and 247 + 212, respectively, and not significantly different than the season of injury (p = 0.26 and p = 0.07 respectively). Nine players did not play another season after the season of injury, and 8 other players only played one more season after the season of injury.
If the 17 players who did not play for 2 seasons after hamstring injury were excluded from the analysis, mean age and BMI of remaining players were not significantly different (30.5 + 4.0 years, and 27.0 + 2.7, respectively). If the 17 players who did not play for 2 seasons after hamstring injury were excluded from the analysis, there was no significant differences between the mean number of MLB game appearances before, during, and after the season of hamstring injury (0.09 < p < 0.42), nor any significant differences in the number of official MLB at-bats before, during, and after the season of hamstring injury (0.28 < p < 0.82). However, for these 17 players, the mean number of game appearances and official at-bats during the season of hamstring injury and in each of the 2 previous seasons did demonstrate significantly lower totals than the other players ( p < 0.006 across all parameters).
4. Discussion
To my knowledge, this is the first descriptive study analyzing hamstring injuries in MLB players. Because the incidence of hamstring injuries is more prevalent in position players[1], pitchers were excluded in the present study. The data shows that the length of stay on the DL is relatively short, with the typical MLB player spending only 15 days on the DL for most hamstring injuries. However, our data demonstrate that the number of MLB games played and official at-bats during the season of injury is significantly reduced. Furthermore, when all injured players are taken into account, our data shows that the reduced number of MLB games played and official at-bats appears to persist for the next 2 seasons after the injury. Certainly, there are numerous factors other than physical well-being that influence the number of games played and the number of official at-bats taken throughout a MLB season (e.g., manager’s decisions, in-game strategies, historical performance against specific opponents, other injuries, etc). Similarly, there are non-physical factors that influence length of stay on the DL (e.g., performance of replacement player, clubhouse social and general managerial issues [15]. However, our data shows that 9 out of 58 players (15.5%) did not play another MLB season following the season with hamstring injury, and that another 8 players only played one more MLB season following the season with hamstring injury, for a total of 17 players (29.3%) who were out of MLB by 2 seasons after hamstring injury.
Only one player demographic variable was associated with the incidence of hamstring injury. The average age of the players who sustained a hamstring injury was 30.7 years - older for a MLB player when considering the data of Witnauer et al that suggests that a normal MLB career lasts until the age of 30 years [16]. However, there was no correlation between age and length of stay on the DL or whether the player continued playing professional baseball after the hamstring injury. The BMI of the players who sustained a hamstring injury is not dissimilar to the average BMI of all MLB players[17, 18]. There was no correlation between BMI and length of stay on the DL or whether the player continued playing professional baseball after the hamstring injury. Hand dominance was not predictive of injury incidence, length of stay on DL, or continuance of playing.
The data seems to suggest that the 17 players (i.e., 29%) who were out of MLB by 2 years after hamstring injury were already on the decline in their career. While their mean age and BMI were the same as the other injured players, their pre-injury levels of performance, as measured by MLB games played and official at-bats, were significantly lower than the other injured players during the 2 seasons leading up to the season of injury. Moreover, the lower statistics of these 17 players directly skew the calculations for games played and official at-bats during the season of injury and the following 2 seasons. The remaining players managed to maintain the same number of games and at-bats throughout their 5 seasons studied. Therefore, a hamstring injury in a player on the decline seems to hasten the end of his career, while a player who is demonstrating more consistency in games played and at bats prior to the injury may not suffer a significant decline in participation upon recovery. Further research is needed to determine what associated factors contribute to the injured players decline in performance and eventual retirement from MLB.
4.1 Limitations
My analysis has several limitations. First, although our data were collected from an official MLB source, it was done retrospectively. Another weakness of this study stems from our use of the disabled list as our source of injury data. Although the likelihood is low, there may have been an untold number of hamstring injuries that were not severe enough to require the baseball player to be placed on the DL. Players with minor injuries can miss up to 2 weeks of the season without being placed on the disabled list. Furthermore, length of stay on the DL cannot be used as a true measure of healing or “return to play,” as 56 of 58 of the injured players were taken off the DL after the first 15 days. Specific treatment modalities and rehabilitation regimens were not examined. Moreover, seasonality of these injuries was not analyzed, such that players who were injured during the month of September may not be placed on the disabled list because of MLB rules permitting teams to expand their rosters to include 40 players. Finally, we did not define a true incidence of hamstring injuries because the changing number of active players on MLB rosters during a season and the varying level of exposure to injury conditions in practice and games were not analyzed. Despite these limitations, the present study is the first to examine hamstring injury patterns among MLB players and these data may provide guidance to medical staffs for players with this type of injury for their long term prognosis.
5. Conclusion
Hamstring injuries mostly occur in older MLB position players. If the injured player had been declining in MLB games played and official at-bats in the 2 previous seasons leading up to the injury, his future career may be in jeopardy of ending after the injury. However, if the injured player had relatively consistent participation in MLB games played and official at bats in the 2 previous seasons leading up to the injury, he has a good chance of returning to and maintaining the same level of participation up to 2 seasons following injury.
The author declares no conflict of interest.
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Figure 1. Mean number (+ standard error) of MLB games played each season (before, during and after the season of hamstring injury).
