The American Journal of Sports Medicine http://ajs.sagepub.com/
Major and Minor League Baseball Hamstring Injuries: Epidemiologic Findings From the Major League Baseball Injury Surveillance System Christopher S. Ahmad, Randall W. Dick, Edward Snell, Nick D. Kenney, Frank C. Curriero, Keshia Pollack, John P. Albright and Bert R. Mandelbaum Am J Sports Med 2014 42: 1464 originally published online April 11, 2014 DOI: 10.1177/0363546514529083 The online version of this article can be found at: http://ajs.sagepub.com/content/42/6/1464
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Major and Minor League Baseball Hamstring Injuries Epidemiologic Findings From the Major League Baseball Injury Surveillance System Christopher S. Ahmad,*y MD, Randall W. Dick,z FACSM, Edward Snell,§ MD, Nick D. Kenney,ll ATC, CSCS, Frank C. Curriero,{ PhD, Keshia Pollack,{ PhD, MPH, John P. Albright,# MD, and Bert R. Mandelbaum,** MD, DHL Investigation performed at Johns Hopkins Bloomberg, Baltimore, Maryland, USA Background: Hamstring strains are a recognized cause of disability for athletes in many sports, but no study exists that reports the incidence and circumstances surrounding these injuries in professional baseball. Hypothesis: Professional baseball players have a high incidence of hamstring strains, and these injuries are influenced by multiple factors including history of hamstring injury, time period within the season, and activity of base running. Study Design: Descriptive epidemiologic study. Methods: For the 2011 season, injury data were prospectively collected for every Major League Baseball (MLB) major and minor league team and recorded in the MLB’s Injury Surveillance System. Data collected for this study included date of injury, activity in which the player was engaged at the time of injury, and time loss. Injury rates were reported in injuries per athlete-exposure (A-E). Athlete-exposures were defined as the average number of players on a team who were participating in a game multiplied by the number of games. Results: In the major leagues, 50 hamstring strains were reported for an injury rate (IR) of 0.7 per 1000 A-Es and averaged 24 days missed. In the minor leagues, 218 hamstring strains were reported for an IR of 0.7 per 1000 A-Es and averaged 27 days missed. Base running, specifically running to first base, was the top activity for sustaining a hamstring strain in both major and minor leagues, associated with almost two-thirds of hamstring strains. Approximately two-thirds of these injuries in both the major and minor leagues resulted in more than 7 days of time loss. Approximately 25% of these injuries kept the player out for 1 month or longer. History of a previous hamstring strain in the prior year, 2010, was found in 20% of the major league players and 8% of the minor league players. In the major leagues, the month of May had a statistically significant higher frequency of hamstring injuries than any other month in the season (P = .0153). Conclusion: Hamstring strains are a considerable cause of disability in professional baseball and are affected by history of hamstring strain, seasonal timing, and running to first base. Keywords: Major League Baseball; MLB; injury; muscle strain; hamstring
occurrence and impairment rates in baseball, few epidemiologic studies have been conducted in the sport of baseball and none in professional baseball. In collegiate baseball, research using 16 years of data from the US National Collegiate Athletic Association (NCAA) Injury Surveillance System (ISS) determined that upper leg injuries (including but not limited to hamstrings) accounted for 12% of all reported game time-loss injuries, with an injury rate of 1 upper leg injury approximately every 96 games for a specific team of 15 active players in each game.9 In 2010, Major League Baseball (MLB), its players union, and minor league affiliates reached an agreement on a medical records and injury tracking system to create an efficient system to track medical histories of players
Hamstring injuries are extremely prevalent and account for up to 29% of all injuries reported in various sports.3,5,13,22,24,29 Hamstring strains also result in prolonged impairment and a reinjury risk of 12% to 31%.5,10,29 Reductions in the incidence or severity of these injuries have obvious potential to reduce medical costs and time lost from sport. Despite the tremendous negative effect of hamstring injuries, injury and reinjury rates have not improved over the past 3 decades.21 While anecdotally, hamstring injuries are well recognized for high
The American Journal of Sports Medicine, Vol. 42, No. 6 DOI: 10.1177/0363546514529083 Ó 2014 The Author(s)
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longitudinally and as players moved across minor and major league organizations. A second goal of this system was to catalog injury data, analyze injury trends, identify areas of specific concern, and incorporate targeted research studies to better understand participation risk and optimize player health and safety. Prior injury history for each athlete was not incorporated into the system. Such systems exist and have been successful in the NCAA and other professional sport organizations.8,11 The purpose of this study is to describe the extent of hamstring strains in 2011 major and minor league baseball and to better understand the factors influencing these injuries. A better understanding of the incidence and characteristics associated with these injuries is the first step in improving outcomes and addressing prevention.
METHODS Our purpose was to investigate 2011 time-loss game injury data in the major and minor leagues related to hamstring strains. All game-related time-loss hamstring injuries (and, where applicable, other body part injuries) reported in 2011 were included in this analysis. Non–baseball related injuries of any kind and non–time loss injuries associated with baseball games were not analyzed.
Electronic Medical Records The professional baseball electronic medical records (EMR) system standardizes and streamlines tracking and transfer of player medical records across MLB. The records for all professional baseball players in both the major and minor leagues (.7000 players associated with .200 teams) are kept in the system. All major and minor league players provide their consent at the beginning of the season. The core component of the system is the entry of player events. Team athletic trainers and secondarily team physicians enter data on all injuries, illnesses, and preventive events for each player on their roster. For each event, the athletic trainer records a response using a list of established answers as well as the options of ‘‘not applicable’’ and ‘‘other.’’ Once an event has been created, various physician notes, treatment notes, and diagnostic information (eg, surgical notes, magnetic resonance imaging [MRI], radiographs) can be entered into the system and linked to the applicable event. The system is the primary medical recording system for each major and minor league club.
The HITS Injury Analytics Overview The MLB Health and Injury Tracking System (HITS) is a centralized database containing a subset of the deidentified medical data from the MLB EMR system. The data extracted from the HITS system are related only to injuries that result in lost time for a player. The data items for these injuries consist of basic information gathered at the time of injury (eg, diagnosis, body part, activity, location, and date of injury).
Hamstring Data Extraction The HITS system generates data regarding all game-related time-loss injuries during the regular season. This study reviewed 2011 game-related time-loss injury data in the major and minor leagues related to hamstring strains. To analyze hamstring strains in the context of other injuries, the HITS data were also analyzed for all injuries recorded for 2011. To compare the recurrence rate of hamstring strains relative to other injuries with high recurrence rates, 2010 injury surveillance data on hamstring strains, oblique and groin strains, and concussions also were analyzed.
Institutional Review Board Approval The institutional review board at Johns Hopkins University reviewed the study and granted approval for the investigators to conduct research using the HITS data, in accordance with federal guidelines.
Definitions Several terms are defined below for study purposes: Major League Baseball (MLB): Major League Baseball is a professional baseball organization composed of 30 major league clubs and their minor league affiliates in the United States and Canada. Major leagues: Major League Baseball consists of 30 separate clubs divided into 2 leagues, the American League and the National League. Each club plays 162 games over the course of a 180- to 183-day season. In 2011, the season lasted 182 days, beginning on March 31 and ending on September 28. The majority of the major league clubs have 3 athletic trainers, 1 head athletic trainer, and 2 assistants. Some clubs have only 1 assistant. All athletic trainers travel with the club to every game throughout the season. Each major league club
*Address correspondence to Christopher S. Ahmad, MD, Department of Orthopaedic Surgery, Center for Shoulder, Elbow, and Sports Medicine, Columbia University, 622 W 168th Street, PH11-1118, New York, NY 10032, USA (e-mail: [email protected]). y Department of Orthopaedic Surgery, Center for Shoulder, Elbow, and Sports Medicine, Columbia University, New York, New York, USA. z Health and Safety Sports Consultants LLC, Carmel, Indiana, USA. § Allegheny General Hospital, Pittsburgh, Pennsylvania, USA. ll Kauffman Stadium, Kansas City, Missouri, USA. { Johns Hopkins Bloomberg, Baltimore, Maryland, USA. # Department of Orthopaedics & Rehabilitation, Iowa College of Medicine, Iowa City, Iowa, USA. **Santa Monica Orthopaedic and Sports Medicine Group, Santa Monica, California, USA. One or more of the authors has declared the following potential conflict of interest or source of funding: Funding for this research was provided by Major League Baseball and the Major League Baseball Players Association. C.S.A. is a consultant to Accumed LLC and Arthrex Inc and receives research support from Zimmer and Major League Baseball. B.R.M. is an associate editor of The American Journal of Sports Medicine and director of research for Major League Baseball.
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TABLE 1 Time-Loss Injuries for Major and Minor Leagues, 2011
Average No. of players per game No. of athlete-exposures per gameb No. of players with injuries Total reported game injuries Injury ratec (per 1000 athlete-exposures)
Major League
Minor League
Total
NCAA (1998-2003)a
14 69,076 541 871 12.5
13 302,744 2558 3697 12.0
13 366,886 3099 4568 12.1
14 819,070 Not reported Not reported 5.8
a
Relevant comparable NCAA game-related time-loss injury data reported for comparison.8 Calculated as total team games multiplied by average game participants per team. c Calculated as game injuries divided by athlete-exposures (A-Es) and reported per 1000 A-Es. Year 2011 major and minor league baseball game-related time-loss injury rates were significantly higher (P \ .0001) than those associated with NCAA baseball game injuries a decade earlier. b
also has a physician available at each game. The physicians do not always go on the road with the club, but the players always have access to the opposing team’s physician when on the road. Minor leagues: Minor league baseball is a vast network of more than 200 teams located across the United States, the Dominican Republic, and Venezuela. Each minor league team is affiliated with a major league parent club that controls all of the players on its affiliates. The minor league teams are split up into a number of different leagues based on both geography and skill level. Season length in the minor leagues differs depending on the league. For example, the low-level short season leagues begin in mid-June and end in early September, lasting 75 to 80 days. In contrast, the highest level AAA leagues begin in early April and end in early September, lasting 152 days. Each minor league team has 1 athletic trainer who travels with the team throughout the season. Each affiliate has a physician associated with the team, but the physician does not travel with the team and is not required to be on site at the ballpark during games. Each city has a physician on call but not necessarily in attendance at the games. Recordable events: In the MLB EMR, athletic trainers record all injuries, illnesses, and preventive visits for both baseball-related and non–baseball related events for all players on their respective team as their medical-legal record. In this study, only game-related timeloss injuries (occurring in spring training, regular season, or post season) associated with the HITS data subset were analyzed. Injury: An injury is any physical complaint sustained by a player that affects or limits participation in any aspect of baseball-related activity (eg, game, practice, warm-up, conditioning/weight training). Time loss: Each event contains an injury event date and a medical clearance date (date athlete is medically cleared to play a game in any capacity). Time loss in this study was the number of days from the injury event date to the medical clearance date. Diagnosis: The system permits the user to select 1 of 17 body regions (eg, thigh) and then an injury type (eg, strain) to narrow the specific diagnosis search. A general search by term function or selection by International
Classification of Diseases, 9th Revision (ICD-9) diagnosis code is also available. An ‘‘other’’ option can be used if the specific diagnosis is not found. Exposure: The average number of players per team game was based on analysis of game participation via box scores. This number, multiplied by the number of team games, was used as an estimate of game exposures, to provide rates comparable to those in the NCAA Injury Surveillance System. Activity: The activity being performed at the time of injury is recorded. Examples of activity options include running, pitching, hitting, and fielding. Temperature: The game start time temperature was recorded at every major league game and used in the seasonal hamstring injury analysis. No such information was available for minor league games.
Statistical Analysis Standard statistical analyses were applied to further complement the descriptive nature of the article and to test specific hypotheses related to hamstring strains. Chi-square analysis was used to test differences in rates, compare the rates of major and minor league time-loss injuries to those reported by the NCAA, and identify seasonal trends in hamstring strains per games played. Two sample tests of proportions were used to compare the recurrence rates of hamstring strains to other common recurrent time-loss injuries. Significance for all hypothesis tests was gauged against the probability of a type I error rate of .05.
RESULTS Overview Table 1 includes characteristics for time-loss injuries for all injuries reported in the MLB HITS system for all major league teams and minor league affiliates for the 2011 season. This includes at least 1 time-loss injury event reported from 100% (30/30) of all major league teams and 91% (220/ 245) of all minor league teams. The average number of players per game for major and minor league teams was determined by box score analysis.
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TABLE 2 Top 10 Time-Loss Injuries by Body Part and Injury Type, Major and Minor Leagues, 2011a
TABLE 3 Time-Loss Hamstring Strains by Activity, Major and Minor Leagues, 2011a
Major League
Minor League
Injury Diagnosis
No.
%
No.
%
Hamstring strain Adductor groin strain Oblique muscle strain Hand contusion Leg contusion Knee contusion Quadriceps strain Foot contusion Concussion Paralumbar muscle strain Other shoulder injury Elbow contusion
50 36 36 32 26 26 25 22 18 18
5.7 4.1 4.1 3.7 3.0 3.0 2.9 2.5 2.1 2.1
218 85 88 203 124 88
5.9 2.3 2.4 5.5 3.4 2.4
104 101
2.8 2.7
78 73
2.1 2.0
a
Frequencies are given in descending order by major league. Empty cells indicate an injury occurrence did not reach the top 10 in that league.
Injury Incidence by Body Region The MLB EMR and HITS system offers 19 categories of body regions injured. The top 3 body regions (thigh, shoulder/clavicle, and hand/fingers) accounted for approximately 38% of both major and minor league injuries. The thigh accounted for 15.5% of major league injuries and was the body region most frequently injured. The thigh accounted for 12.7% of minor league injuries and was only marginally exceeded by hand/fingers (13.1%) and shoulder/clavicle (12.8%).
Top 10 Time-Loss Injuries by Body Part and Injury Type Table 2 presents the top 10 game time-loss injuries by body part and injury type across major and minor leagues. Hamstring strains were the top reported value in both major and minor leagues, accounting for approximately 6% of all injuries. In the major leagues, 50 hamstring strains were reported for an injury rate (IR) of 0.7 per 1000 athlete-exposures (A-Es). In the minor leagues, 218 hamstring strains were reported for an IR of 0.7 per 1000 A-Es. Adductor groin strains and quadriceps strains were other primary thigh body part/injury types. Additional reported hamstring injury types included spasms and contusions (totaling 10 in the major league and 56 in the minor league).
Activity Table 3 shows the game activity reported at the time of time-loss hamstring injury. Base running was the top activity in both major and minor leagues, associated with almost two-thirds of hamstring strains. Pitching, another activity with an important upper leg activity component,
Major League
Base running Fielding Unknown Pitching Other Catching Hitting
Minor League
No.
%
No.
%
31 10 4 2 1 1 1
62.0 20.0 8.0 4.0 2.0 2.0 2.0
154 29 8 14 2 3 6
70.6 13.3 3.7 6.4 0.9 1.4 2.8
a
Frequencies are given in descending order by major league.
accounted for only 4% to 6% of all reported hamstring strains. The majority of base running hamstring strains in both leagues were associated with running to first base. The majority of players who were injured while running to first base in both leagues were right-handed batters (farther from first base); however, this study did not analyze how many right- and left-handed batters actually put the ball in play.
Time Loss Table 4 presents time-loss categories with hamstring strains. Approximately two-thirds of these injuries in both the major and minor leagues resulted in more than 7 days of time loss. Approximately 25% of these injuries kept the player out for a month or more. Average time loss was 24 days in the major leagues and 27 days in the minor leagues, which ranged from 1 day missed to the remainder of the season missed for both major and minor leagues. Relative to other body part/injury type combinations, hamstring injuries were one of the top time-loss injuries in the time-loss categories of 4-7 days, 8-15 days, 16-30 days, and more than 30 days.
Recurrent Hamstring Injuries Table 5 presents major and minor league 2011 recurrent hamstring, oblique, and adductor groin injuries. The nonhamstring injuries were chosen to provide a perspective on the hamstring recurrence rate compared with other often-reported recurrent injuries. A recurrence was noted if a time-loss body part/injury type to the same player and limb (where relevant) was reported in both the 2010 (with subsequent established medical clearance) and 2011 seasons. In this study, 20% (10/50) of the reported major league and 8% (18/218) of the reported minor league 2011 hamstring strains were recurrences from year 2010. The major league recurrence frequency for hamstring injury was significantly greater than the recurrence frequency for oblique and adductor groin strains (P \ .05). For the minor league, the recurrence frequency for hamstring injury was significantly greater than that for
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TABLE 4 Hamstring Strains by Time-Loss Category, Major and Minor Leagues, 2011 Major League
TABLE 5 Recurrent Time-Loss Hamstring, Oblique, and Groin Injuries, Major and Minor Leagues, 2011
Minor League
Time Loss
No.
%
No.
%
1-3 days 4-7 days 8-15 days 16-30 days .30 days
6 11 15 5 13
12.0 22.0 30.0 10.0 26.0
10 45 58 54 51
4.6 20.6 26.6 24.8 23.4
Major League
Hamstring strains Oblique strains Abductor groin strains
Minor League
Total in 2010
Recurrent in 2011a
Total 2010
Recurrent in 2011a
82 44 34
10 0 2
375 148 100
18 4 1
a
Injury recurrent from 2010 season.
adductor groin strains (P \ .05) and comparable with the recurrence frequency for oblique strains.
Hamstring Injury Seasonal Timing Table 6 shows the distribution of hamstring strains by month and the average game time temperature for the regular season (April through September) for major league games; these data were not available at the minor league level. Eleven major league and 20 minor league hamstring injuries analyzed in the other tables were not reported in this table as they occurred in spring training or postseason games. In the major leagues, almost 40% of all reported hamstring injuries for the regular season occurred in April and May. May was the highest single regular season month for hamstring injuries both in absolute numbers and relative to games played. The 13 reported hamstring injuries for May per the 420 games played represented a significantly higher injury rate than hamstring injuries per games played observed in any other month (P = .0153). April and May also had the lowest average temperature, and the average combined temperature of April and May (65.5°F) was 11° F cooler than that for the months of June to October (76.8°F). In the minor leagues, more than one-third of all reported regular season hamstring injuries occurred in April and May. May, July, and August had similar high prevalence of hamstring strains; however, relative to games played each month, April and May were the months of highest risk and combined were significantly higher than the other months (P \ .0001).
DISCUSSION To our knowledge, this is the first study reporting on the epidemiologic patterns of hamstring injuries in professional baseball. Furthermore, this is the first study to use the MLB injury surveillance system. The surveillance system was designed for both longitudinal tracking of medical histories of the more than 7000 participants in major and minor league baseball and standardized collection of data with descriptors of occurrence and features associated with injury for the purpose of research. This initial report demonstrates the high incidence of hamstring injuries in 2011 in professional baseball overall when compared with
other injuries recorded in the sport. When compared with NCAA baseball game time-loss injury data published a decade earlier9 using similar injury and exposure definitions, 2011 major and minor league baseball game-related time-loss injury rates were significantly higher (P \ .0001). Many factors may explain this higher injury rate, including older age of professional players and higher intensity and volume of play at the professional level. Base running was the top activity associated with a hamstring strain in both the major and minor leagues and was associated with almost two-thirds of hamstring strains, whereas pitching accounted for only 4% to 6%. The majority of base-running hamstring strains in both leagues were associated with running to first base. Biomechanical studies of sprint mechanics suggest that the posterior thigh is most susceptible to such strains near the end of the swing phase, when the hamstrings reach maximal length and undergo eccentric contraction just before heel strike.16,26,30 The acceleration from a stationary, but rotating swinging action required for running to first base (and often with no planned deceleration leading into the base) is likely the most complex and frequent acceleration baseball activity compared with running other bases or fielding, catching, or pitching. More work in understanding the unique biomechanical requirements of this action including the possible relevance of starting from the right versus left side of the plate is essential to develop appropriate prevention strategies. Hamstring strains are a debilitating injury in terms of time loss. Approximately two-thirds of hamstring injuries in both the major and minor leagues resulted in more than 7 days of time loss. Approximately 25% of these injuries required a month or more for return to play. Relative to other injuries, hamstring strains are among the top debilitating injuries in terms of time loss. This amount of time to return may be greater than what is observed in other sports. In the National Football League (NFL), hamstring strains entailed a mean of 13.2 and a median of 9 days lost per injury.10 This contrasts with our observed findings of 24 to 27 average days missed. Professional baseball at both the major and minor leagues is played almost every day and with a variety of dynamic movements that are often initiated from a static position. These features may mean that baseball hamstring strains require more complete recovery of injury before return to play to avoid
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TABLE 6 Regular Season Time-Loss Hamstring Strains by Month, Major and Minor Leagues, 2011 No. (% of Total)
Total No. of Games
Injury Rate (1 Hamstring Injury Every n Games)
Average Temperature, °F 63 68 75 82 78 72
a
Major league April May June July August September Minor leagueb April May June July August September
6 13 6 8
(12) (27) (12) (16) 3 (6) 3 (6)
392 420 400 395 420 117
65 32 67 49 140 39
33 44 28 45 41
(15) (20) (13) (21) (19) 7 (3)
1370 1892 2506 3161 2855 322
42 43 89 70 70 46
a
This analysis excludes 11 hamstring injuries that occurred in spring training or postseason games. This analysis excludes 20 hamstring injuries that occurred in spring training or postseason games
b
exacerbation of the injury. Given the high reported recurrent hamstring injury rate (20% in major league), the time to return to play requires further research. This study identified a 20% recurrence rate of hamstring strain injury from the previous year in the major league (10% with combined major/minor leagues). This recurrence rate is significantly higher than recurrence rates for other muscle strains (oblique, groin) that occurred in 2011 MLB. These data are consistent with other sports that report hamstring recurrence rates of 12% to 31%.5,10,12,15,23,29 Recurrence rates have been reported as high as 34% in Australian Rules Football.23 A recent study using NFL injury surveillance in professional football players identified hamstring injury recurrence of 16.5%.10 In this study we used the identification of a time-loss injury in the prior season (2010) to determine a recurrent injury in 2011. Other studies have used patients’ verbal history of a prior hamstring injury in their career to identify a recurrence as opposed to historical data in the EMR.10 Therefore, it is possible that our study underreports true recurrence as defined as any history of a hamstring strain. We observed interesting seasonal timing for hamstring strains. In both the major and minor leagues, the months of April and May had the highest incidence of regular season hamstring strains relative to games played in each month. Seasonal timing of hamstring strains has been shown in other sports. In NFL football, Elliot et al10 showed that in the preseason, athletes are at particular risk for hamstring injury. Numerous published studies in various sports have implicated factors contributing to the risk of sustaining a hamstring strain, including lower extremity flexibility,2,17,28 core stability,4,18,20,27 muscle weakness,6,7,22 and fatigue.19,29,31 In addition, neuromuscular activation may be altered by fatigue and further contribute to hamstring strain risk.14,19,25 While this study does not allow us to determine the reasons for seasonal timing of hamstring strains, it is possible that during the
first 2 months of the regular season, players have relative inflexibility, muscle weakness, and fatigability that make them vulnerable to hamstring strains. In addition, the adjustment from a stable spring training environment based in warm climates of Arizona and Florida to the more dynamic (in terms of both weather/temperature and travel) environments of the regular season may be a contributing factor. The average game start time temperature in April and May was 11°F cooler than in months later in the season. This area deserves further prospective research to identify injury factors and injury prevention strategies. There are several limitations to this study. The study does not include longitudinal data to analyze trends of hamstring injuries with time. In particular, recurrent injuries are not based on a medical history beyond the previous year (when the system began), and it is possible that recurrent injuries are underreported. The amount of data acquired over a single season did not allow for in-depth analysis of detailed features that may be associated with hamstring strains, such as player age or left- or righthanded batting. Grade of hamstring injuries or other diagnostic features such as physical examination findings or MRI findings are not provided in this study. Therefore, specific features of hamstring strains that may influence recovery duration and recurrence risk are not determined in this study. These could include player contract language that may influence behavior. In addition, we did not analyze treatment for hamstring injuries and factors that affect return to play, such as differences between individual players and variability in medical support/ rehabilitation between the major and minor leagues. Exposures (which are currently being evaluated for MLB EMR system enhancement) were calculated indirectly using box scores and therefore do not currently differentiate between a player who played 1 inning versus another who played 9. While more than 90% of major and minor league clubs
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contributed at least 1 injury in the 2011 season, underreporting of injuries may have occurred as club athletic trainers adjusted to the new system. Finally, these data are limited to professional baseball players and may not be extrapolated to amateur players or to other sports. Despite these limitations, this study is the first in professional baseball to highlight the large incidence of hamstring injuries; most occurred during base running, and they entailed an average time loss of 24 to 27 days, twice that of the NFL. Early season appears to be the time of highest risk. These findings were consistent across both major and minor leagues and provide the foundation for further research in hamstring strain injury and prevention. These efforts will be aided by an ongoing robust injury surveillance system that will continue to capture information from the entire population of MLB players with the capability to link to additional rehabilitation and performance parameters. Future work, based on a projected multistep program currently being developed, will include standardized severity assessments based on MRI results and a preventive hamstring exercise program. Arnason et al1 showed that an eccentric strength training program with Nordic hamstring exercises could reduce the incidence of hamstring strains in elite soccer players; a similar program tailored for MLB is being developed and the results of this study have shaped that work.
ACKNOWLEDGMENT The authors acknowledge the Major League Baseball Players Association for its support and review of this article.
REFERENCES 1. Arnason A, Andersen TE, Holme I, Engebretsen L, Bahr R. Prevention of hamstring strains in elite soccer: an intervention study. Scand J Med Sci Sports. 2008;18:40-48. 2. Bradley PS, Portas MD. The relationship between preseason range of motion and muscle strain injury in elite soccer players. J Strength Cond Res. 2007;21:1155-1159. 3. Brooks JH, Fuller CW, Kemp SP, Reddin DB. Incidence, risk, and prevention of hamstring muscle injuries in professional rugby union. Am J Sports Med. 2006;34:1297-1306. 4. Chumanov ES, Heiderscheit BC, Thelen DG. The effect of speed and influence of individual muscles on hamstring mechanics during the swing phase of sprinting. J Biomech. 2007;40:3555-3562. 5. Croisier JL. Factors associated with recurrent hamstring injuries. Sports Med. 2004;34:681-695. 6. Croisier JL, Forthomme B, Namurois MH, Vanderthommen M, Crielaard JM. Hamstring muscle strain recurrence and strength performance disorders. Am J Sports Med. 2002;30:199-203. 7. Croisier JL, Ganteaume S, Binet J, Genty M, Ferret JM. Strength imbalances and prevention of hamstring injury in professional soccer players: a prospective study. Am J Sports Med. 2008; 36:1469-1475. 8. Dick R, Agel J, Marshall SW. National Collegiate Athletic Association Injury Surveillance System commentaries: introduction and methods. J Athl Train. 2009;44:173-182.
9. Dick R, Sauers EL, Agel J, et al. Descriptive epidemiology of collegiate men’s baseball injuries: National Collegiate Athletic Association Injury Surveillance System, 1988-1989 through 2003-2004. J Athl Train. 2007;42:183-193. 10. Elliott MC, Zarins B, Powell JW, Kenyon CD. Hamstring muscle strains in professional football players: a 10-year review. Am J Sports Med. 2011;39:843-850. 11. Fuller CW, Junge A, Dvorak J. A six year prospective study of the incidence and causes of head and neck injuries in international football. Br J Sports Med. 2005;39(Suppl 1):i3-i9. 12. Gabbe BJ, Bennell KL, Finch CF, Wajswelner H, Orchard JW. Predictors of hamstring injury at the elite level of Australian football. Scand J Med Sci Sports. 2006;16:7-13. 13. Garrett WE Jr. Muscle strain injuries. Am J Sports Med. 1996;24:S2-S8. 14. Gehring D, Melnyk M, Gollhofer A. Gender and fatigue have influence on knee joint control strategies during landing. Clin Biomech (Bristol, Avon). 2009;24:82-87. 15. Hagglund M, Walden M, Ekstrand J. Previous injury as a risk factor for injury in elite football: a prospective study over two consecutive seasons. Br J Sports Med. 2006;40:767-772. 16. Heiderscheit BC, Hoerth DM, Chumanov ES, Swanson SC, Thelen BJ, Thelen DG. Identifying the time of occurrence of a hamstring strain injury during treadmill running: a case study. Clin Biomech (Bristol, Avon). 2005;20:1072-1078. 17. Henderson G, Barnes CA, Portas MD. Factors associated with increased propensity for hamstring injury in English Premier League soccer players. J Sci Med Sport. 2010;13:397-402. 18. Hoskins W, Pollard H. The management of hamstring injury—part 1: issues in diagnosis. Man Ther. 2005;10:96-107. 19. Mair SD, Seaber AV, Glisson RR, Garrett WE Jr. The role of fatigue in susceptibility to acute muscle strain injury. Am J Sports Med. 1996;24:137-143. 20. Mason DL, Dickens V, Vail A. Rehabilitation for hamstring injuries. Cochrane Database Syst Rev. 2007;(12):CD004575. 21. Mendiguchia J, Alentorn-Geli E, Brughelli M. Hamstring strain injuries: are we heading in the right direction? Br J Sports Med. 2012;46(2):81-85. 22. Orchard J, Marsden J, Lord S, Garlick D. Preseason hamstring muscle weakness associated with hamstring muscle injury in Australian footballers. Am J Sports Med. 1997;25:81-85. 23. Orchard J, Seward H. Epidemiology of injuries in the Australian Football League, seasons 1997-2000. Br J Sports Med. 2002;36:39-44. 24. Orchard J, Steet E, Walker C, Ibrahim A, Rigney L, Houang M. Hamstring muscle strain injury caused by isokinetic testing. Clin J Sport Med. 2001;11:274-276. 25. Pinniger GJ, Steele JR, Groeller H. Does fatigue induced by repeated dynamic efforts affect hamstring muscle function? Med Sci Sports Exerc. 2000;32:647-653. 26. Schache AG, Wrigley TV, Baker R, Pandy MG. Biomechanical response to hamstring muscle strain injury. Gait Posture. 2009;29:332-338. 27. Sherry MA, Best TM. A comparison of 2 rehabilitation programs in the treatment of acute hamstring strains. J Orthop Sports Phys Ther. 2004;34:116-125. 28. Witvrouw E, Danneels L, Asselman P, D’Have T, Cambier D. Muscle flexibility as a risk factor for developing muscle injuries in male professional soccer players: a prospective study. Am J Sports Med. 2003;31:41-46. 29. Woods C, Hawkins RD, Maltby S, Hulse M, Thomas A, Hodson A. The Football Association Medical Research Programme: an audit of injuries in professional football—analysis of hamstring injuries. Br J Sports Med. 2004;38:36-41. 30. Yu B, Queen RM, Abbey AN, Liu Y, Moorman CT, Garrett WE. Hamstring muscle kinematics and activation during overground sprinting. J Biomech. 2008;41:3121-3126. 31. Zarins B, Ciullo JV. Acute muscle and tendon injuries in athletes. Clin Sports Med. 1983;2:167-182.
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Major and Minor League Baseball Hamstring Injuries: Epidemiologic Findings From the Major League Baseball Injury Surveillance System Christopher S. Ahmad, Randall W. Dick, Edward Snell, Nick D. Kenney, Frank C. Curriero, Keshia Pollack, John P. Albright and Bert R. Mandelbaum Am J Sports Med 2014 42: 1464 originally published online April 11, 2014 DOI: 10.1177/0363546514529083 The online version of this article can be found at: http://ajs.sagepub.com/content/42/6/1464
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Major and Minor League Baseball Hamstring Injuries Epidemiologic Findings From the Major League Baseball Injury Surveillance System Christopher S. Ahmad,*y MD, Randall W. Dick,z FACSM, Edward Snell,§ MD, Nick D. Kenney,ll ATC, CSCS, Frank C. Curriero,{ PhD, Keshia Pollack,{ PhD, MPH, John P. Albright,# MD, and Bert R. Mandelbaum,** MD, DHL Investigation performed at Johns Hopkins Bloomberg, Baltimore, Maryland, USA Background: Hamstring strains are a recognized cause of disability for athletes in many sports, but no study exists that reports the incidence and circumstances surrounding these injuries in professional baseball. Hypothesis: Professional baseball players have a high incidence of hamstring strains, and these injuries are influenced by multiple factors including history of hamstring injury, time period within the season, and activity of base running. Study Design: Descriptive epidemiologic study. Methods: For the 2011 season, injury data were prospectively collected for every Major League Baseball (MLB) major and minor league team and recorded in the MLB’s Injury Surveillance System. Data collected for this study included date of injury, activity in which the player was engaged at the time of injury, and time loss. Injury rates were reported in injuries per athlete-exposure (A-E). Athlete-exposures were defined as the average number of players on a team who were participating in a game multiplied by the number of games. Results: In the major leagues, 50 hamstring strains were reported for an injury rate (IR) of 0.7 per 1000 A-Es and averaged 24 days missed. In the minor leagues, 218 hamstring strains were reported for an IR of 0.7 per 1000 A-Es and averaged 27 days missed. Base running, specifically running to first base, was the top activity for sustaining a hamstring strain in both major and minor leagues, associated with almost two-thirds of hamstring strains. Approximately two-thirds of these injuries in both the major and minor leagues resulted in more than 7 days of time loss. Approximately 25% of these injuries kept the player out for 1 month or longer. History of a previous hamstring strain in the prior year, 2010, was found in 20% of the major league players and 8% of the minor league players. In the major leagues, the month of May had a statistically significant higher frequency of hamstring injuries than any other month in the season (P = .0153). Conclusion: Hamstring strains are a considerable cause of disability in professional baseball and are affected by history of hamstring strain, seasonal timing, and running to first base. Keywords: Major League Baseball; MLB; injury; muscle strain; hamstring
occurrence and impairment rates in baseball, few epidemiologic studies have been conducted in the sport of baseball and none in professional baseball. In collegiate baseball, research using 16 years of data from the US National Collegiate Athletic Association (NCAA) Injury Surveillance System (ISS) determined that upper leg injuries (including but not limited to hamstrings) accounted for 12% of all reported game time-loss injuries, with an injury rate of 1 upper leg injury approximately every 96 games for a specific team of 15 active players in each game.9 In 2010, Major League Baseball (MLB), its players union, and minor league affiliates reached an agreement on a medical records and injury tracking system to create an efficient system to track medical histories of players
Hamstring injuries are extremely prevalent and account for up to 29% of all injuries reported in various sports.3,5,13,22,24,29 Hamstring strains also result in prolonged impairment and a reinjury risk of 12% to 31%.5,10,29 Reductions in the incidence or severity of these injuries have obvious potential to reduce medical costs and time lost from sport. Despite the tremendous negative effect of hamstring injuries, injury and reinjury rates have not improved over the past 3 decades.21 While anecdotally, hamstring injuries are well recognized for high
The American Journal of Sports Medicine, Vol. 42, No. 6 DOI: 10.1177/0363546514529083 Ó 2014 The Author(s)
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longitudinally and as players moved across minor and major league organizations. A second goal of this system was to catalog injury data, analyze injury trends, identify areas of specific concern, and incorporate targeted research studies to better understand participation risk and optimize player health and safety. Prior injury history for each athlete was not incorporated into the system. Such systems exist and have been successful in the NCAA and other professional sport organizations.8,11 The purpose of this study is to describe the extent of hamstring strains in 2011 major and minor league baseball and to better understand the factors influencing these injuries. A better understanding of the incidence and characteristics associated with these injuries is the first step in improving outcomes and addressing prevention.
METHODS Our purpose was to investigate 2011 time-loss game injury data in the major and minor leagues related to hamstring strains. All game-related time-loss hamstring injuries (and, where applicable, other body part injuries) reported in 2011 were included in this analysis. Non–baseball related injuries of any kind and non–time loss injuries associated with baseball games were not analyzed.
Electronic Medical Records The professional baseball electronic medical records (EMR) system standardizes and streamlines tracking and transfer of player medical records across MLB. The records for all professional baseball players in both the major and minor leagues (.7000 players associated with .200 teams) are kept in the system. All major and minor league players provide their consent at the beginning of the season. The core component of the system is the entry of player events. Team athletic trainers and secondarily team physicians enter data on all injuries, illnesses, and preventive events for each player on their roster. For each event, the athletic trainer records a response using a list of established answers as well as the options of ‘‘not applicable’’ and ‘‘other.’’ Once an event has been created, various physician notes, treatment notes, and diagnostic information (eg, surgical notes, magnetic resonance imaging [MRI], radiographs) can be entered into the system and linked to the applicable event. The system is the primary medical recording system for each major and minor league club.
The HITS Injury Analytics Overview The MLB Health and Injury Tracking System (HITS) is a centralized database containing a subset of the deidentified medical data from the MLB EMR system. The data extracted from the HITS system are related only to injuries that result in lost time for a player. The data items for these injuries consist of basic information gathered at the time of injury (eg, diagnosis, body part, activity, location, and date of injury).
Hamstring Data Extraction The HITS system generates data regarding all game-related time-loss injuries during the regular season. This study reviewed 2011 game-related time-loss injury data in the major and minor leagues related to hamstring strains. To analyze hamstring strains in the context of other injuries, the HITS data were also analyzed for all injuries recorded for 2011. To compare the recurrence rate of hamstring strains relative to other injuries with high recurrence rates, 2010 injury surveillance data on hamstring strains, oblique and groin strains, and concussions also were analyzed.
Institutional Review Board Approval The institutional review board at Johns Hopkins University reviewed the study and granted approval for the investigators to conduct research using the HITS data, in accordance with federal guidelines.
Definitions Several terms are defined below for study purposes: Major League Baseball (MLB): Major League Baseball is a professional baseball organization composed of 30 major league clubs and their minor league affiliates in the United States and Canada. Major leagues: Major League Baseball consists of 30 separate clubs divided into 2 leagues, the American League and the National League. Each club plays 162 games over the course of a 180- to 183-day season. In 2011, the season lasted 182 days, beginning on March 31 and ending on September 28. The majority of the major league clubs have 3 athletic trainers, 1 head athletic trainer, and 2 assistants. Some clubs have only 1 assistant. All athletic trainers travel with the club to every game throughout the season. Each major league club
*Address correspondence to Christopher S. Ahmad, MD, Department of Orthopaedic Surgery, Center for Shoulder, Elbow, and Sports Medicine, Columbia University, 622 W 168th Street, PH11-1118, New York, NY 10032, USA (e-mail: [email protected]). y Department of Orthopaedic Surgery, Center for Shoulder, Elbow, and Sports Medicine, Columbia University, New York, New York, USA. z Health and Safety Sports Consultants LLC, Carmel, Indiana, USA. § Allegheny General Hospital, Pittsburgh, Pennsylvania, USA. ll Kauffman Stadium, Kansas City, Missouri, USA. { Johns Hopkins Bloomberg, Baltimore, Maryland, USA. # Department of Orthopaedics & Rehabilitation, Iowa College of Medicine, Iowa City, Iowa, USA. **Santa Monica Orthopaedic and Sports Medicine Group, Santa Monica, California, USA. One or more of the authors has declared the following potential conflict of interest or source of funding: Funding for this research was provided by Major League Baseball and the Major League Baseball Players Association. C.S.A. is a consultant to Accumed LLC and Arthrex Inc and receives research support from Zimmer and Major League Baseball. B.R.M. is an associate editor of The American Journal of Sports Medicine and director of research for Major League Baseball.
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TABLE 1 Time-Loss Injuries for Major and Minor Leagues, 2011
Average No. of players per game No. of athlete-exposures per gameb No. of players with injuries Total reported game injuries Injury ratec (per 1000 athlete-exposures)
Major League
Minor League
Total
NCAA (1998-2003)a
14 69,076 541 871 12.5
13 302,744 2558 3697 12.0
13 366,886 3099 4568 12.1
14 819,070 Not reported Not reported 5.8
a
Relevant comparable NCAA game-related time-loss injury data reported for comparison.8 Calculated as total team games multiplied by average game participants per team. c Calculated as game injuries divided by athlete-exposures (A-Es) and reported per 1000 A-Es. Year 2011 major and minor league baseball game-related time-loss injury rates were significantly higher (P \ .0001) than those associated with NCAA baseball game injuries a decade earlier. b
also has a physician available at each game. The physicians do not always go on the road with the club, but the players always have access to the opposing team’s physician when on the road. Minor leagues: Minor league baseball is a vast network of more than 200 teams located across the United States, the Dominican Republic, and Venezuela. Each minor league team is affiliated with a major league parent club that controls all of the players on its affiliates. The minor league teams are split up into a number of different leagues based on both geography and skill level. Season length in the minor leagues differs depending on the league. For example, the low-level short season leagues begin in mid-June and end in early September, lasting 75 to 80 days. In contrast, the highest level AAA leagues begin in early April and end in early September, lasting 152 days. Each minor league team has 1 athletic trainer who travels with the team throughout the season. Each affiliate has a physician associated with the team, but the physician does not travel with the team and is not required to be on site at the ballpark during games. Each city has a physician on call but not necessarily in attendance at the games. Recordable events: In the MLB EMR, athletic trainers record all injuries, illnesses, and preventive visits for both baseball-related and non–baseball related events for all players on their respective team as their medical-legal record. In this study, only game-related timeloss injuries (occurring in spring training, regular season, or post season) associated with the HITS data subset were analyzed. Injury: An injury is any physical complaint sustained by a player that affects or limits participation in any aspect of baseball-related activity (eg, game, practice, warm-up, conditioning/weight training). Time loss: Each event contains an injury event date and a medical clearance date (date athlete is medically cleared to play a game in any capacity). Time loss in this study was the number of days from the injury event date to the medical clearance date. Diagnosis: The system permits the user to select 1 of 17 body regions (eg, thigh) and then an injury type (eg, strain) to narrow the specific diagnosis search. A general search by term function or selection by International
Classification of Diseases, 9th Revision (ICD-9) diagnosis code is also available. An ‘‘other’’ option can be used if the specific diagnosis is not found. Exposure: The average number of players per team game was based on analysis of game participation via box scores. This number, multiplied by the number of team games, was used as an estimate of game exposures, to provide rates comparable to those in the NCAA Injury Surveillance System. Activity: The activity being performed at the time of injury is recorded. Examples of activity options include running, pitching, hitting, and fielding. Temperature: The game start time temperature was recorded at every major league game and used in the seasonal hamstring injury analysis. No such information was available for minor league games.
Statistical Analysis Standard statistical analyses were applied to further complement the descriptive nature of the article and to test specific hypotheses related to hamstring strains. Chi-square analysis was used to test differences in rates, compare the rates of major and minor league time-loss injuries to those reported by the NCAA, and identify seasonal trends in hamstring strains per games played. Two sample tests of proportions were used to compare the recurrence rates of hamstring strains to other common recurrent time-loss injuries. Significance for all hypothesis tests was gauged against the probability of a type I error rate of .05.
RESULTS Overview Table 1 includes characteristics for time-loss injuries for all injuries reported in the MLB HITS system for all major league teams and minor league affiliates for the 2011 season. This includes at least 1 time-loss injury event reported from 100% (30/30) of all major league teams and 91% (220/ 245) of all minor league teams. The average number of players per game for major and minor league teams was determined by box score analysis.
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TABLE 2 Top 10 Time-Loss Injuries by Body Part and Injury Type, Major and Minor Leagues, 2011a
TABLE 3 Time-Loss Hamstring Strains by Activity, Major and Minor Leagues, 2011a
Major League
Minor League
Injury Diagnosis
No.
%
No.
%
Hamstring strain Adductor groin strain Oblique muscle strain Hand contusion Leg contusion Knee contusion Quadriceps strain Foot contusion Concussion Paralumbar muscle strain Other shoulder injury Elbow contusion
50 36 36 32 26 26 25 22 18 18
5.7 4.1 4.1 3.7 3.0 3.0 2.9 2.5 2.1 2.1
218 85 88 203 124 88
5.9 2.3 2.4 5.5 3.4 2.4
104 101
2.8 2.7
78 73
2.1 2.0
a
Frequencies are given in descending order by major league. Empty cells indicate an injury occurrence did not reach the top 10 in that league.
Injury Incidence by Body Region The MLB EMR and HITS system offers 19 categories of body regions injured. The top 3 body regions (thigh, shoulder/clavicle, and hand/fingers) accounted for approximately 38% of both major and minor league injuries. The thigh accounted for 15.5% of major league injuries and was the body region most frequently injured. The thigh accounted for 12.7% of minor league injuries and was only marginally exceeded by hand/fingers (13.1%) and shoulder/clavicle (12.8%).
Top 10 Time-Loss Injuries by Body Part and Injury Type Table 2 presents the top 10 game time-loss injuries by body part and injury type across major and minor leagues. Hamstring strains were the top reported value in both major and minor leagues, accounting for approximately 6% of all injuries. In the major leagues, 50 hamstring strains were reported for an injury rate (IR) of 0.7 per 1000 athlete-exposures (A-Es). In the minor leagues, 218 hamstring strains were reported for an IR of 0.7 per 1000 A-Es. Adductor groin strains and quadriceps strains were other primary thigh body part/injury types. Additional reported hamstring injury types included spasms and contusions (totaling 10 in the major league and 56 in the minor league).
Activity Table 3 shows the game activity reported at the time of time-loss hamstring injury. Base running was the top activity in both major and minor leagues, associated with almost two-thirds of hamstring strains. Pitching, another activity with an important upper leg activity component,
Major League
Base running Fielding Unknown Pitching Other Catching Hitting
Minor League
No.
%
No.
%
31 10 4 2 1 1 1
62.0 20.0 8.0 4.0 2.0 2.0 2.0
154 29 8 14 2 3 6
70.6 13.3 3.7 6.4 0.9 1.4 2.8
a
Frequencies are given in descending order by major league.
accounted for only 4% to 6% of all reported hamstring strains. The majority of base running hamstring strains in both leagues were associated with running to first base. The majority of players who were injured while running to first base in both leagues were right-handed batters (farther from first base); however, this study did not analyze how many right- and left-handed batters actually put the ball in play.
Time Loss Table 4 presents time-loss categories with hamstring strains. Approximately two-thirds of these injuries in both the major and minor leagues resulted in more than 7 days of time loss. Approximately 25% of these injuries kept the player out for a month or more. Average time loss was 24 days in the major leagues and 27 days in the minor leagues, which ranged from 1 day missed to the remainder of the season missed for both major and minor leagues. Relative to other body part/injury type combinations, hamstring injuries were one of the top time-loss injuries in the time-loss categories of 4-7 days, 8-15 days, 16-30 days, and more than 30 days.
Recurrent Hamstring Injuries Table 5 presents major and minor league 2011 recurrent hamstring, oblique, and adductor groin injuries. The nonhamstring injuries were chosen to provide a perspective on the hamstring recurrence rate compared with other often-reported recurrent injuries. A recurrence was noted if a time-loss body part/injury type to the same player and limb (where relevant) was reported in both the 2010 (with subsequent established medical clearance) and 2011 seasons. In this study, 20% (10/50) of the reported major league and 8% (18/218) of the reported minor league 2011 hamstring strains were recurrences from year 2010. The major league recurrence frequency for hamstring injury was significantly greater than the recurrence frequency for oblique and adductor groin strains (P \ .05). For the minor league, the recurrence frequency for hamstring injury was significantly greater than that for
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TABLE 4 Hamstring Strains by Time-Loss Category, Major and Minor Leagues, 2011 Major League
TABLE 5 Recurrent Time-Loss Hamstring, Oblique, and Groin Injuries, Major and Minor Leagues, 2011
Minor League
Time Loss
No.
%
No.
%
1-3 days 4-7 days 8-15 days 16-30 days .30 days
6 11 15 5 13
12.0 22.0 30.0 10.0 26.0
10 45 58 54 51
4.6 20.6 26.6 24.8 23.4
Major League
Hamstring strains Oblique strains Abductor groin strains
Minor League
Total in 2010
Recurrent in 2011a
Total 2010
Recurrent in 2011a
82 44 34
10 0 2
375 148 100
18 4 1
a
Injury recurrent from 2010 season.
adductor groin strains (P \ .05) and comparable with the recurrence frequency for oblique strains.
Hamstring Injury Seasonal Timing Table 6 shows the distribution of hamstring strains by month and the average game time temperature for the regular season (April through September) for major league games; these data were not available at the minor league level. Eleven major league and 20 minor league hamstring injuries analyzed in the other tables were not reported in this table as they occurred in spring training or postseason games. In the major leagues, almost 40% of all reported hamstring injuries for the regular season occurred in April and May. May was the highest single regular season month for hamstring injuries both in absolute numbers and relative to games played. The 13 reported hamstring injuries for May per the 420 games played represented a significantly higher injury rate than hamstring injuries per games played observed in any other month (P = .0153). April and May also had the lowest average temperature, and the average combined temperature of April and May (65.5°F) was 11° F cooler than that for the months of June to October (76.8°F). In the minor leagues, more than one-third of all reported regular season hamstring injuries occurred in April and May. May, July, and August had similar high prevalence of hamstring strains; however, relative to games played each month, April and May were the months of highest risk and combined were significantly higher than the other months (P \ .0001).
DISCUSSION To our knowledge, this is the first study reporting on the epidemiologic patterns of hamstring injuries in professional baseball. Furthermore, this is the first study to use the MLB injury surveillance system. The surveillance system was designed for both longitudinal tracking of medical histories of the more than 7000 participants in major and minor league baseball and standardized collection of data with descriptors of occurrence and features associated with injury for the purpose of research. This initial report demonstrates the high incidence of hamstring injuries in 2011 in professional baseball overall when compared with
other injuries recorded in the sport. When compared with NCAA baseball game time-loss injury data published a decade earlier9 using similar injury and exposure definitions, 2011 major and minor league baseball game-related time-loss injury rates were significantly higher (P \ .0001). Many factors may explain this higher injury rate, including older age of professional players and higher intensity and volume of play at the professional level. Base running was the top activity associated with a hamstring strain in both the major and minor leagues and was associated with almost two-thirds of hamstring strains, whereas pitching accounted for only 4% to 6%. The majority of base-running hamstring strains in both leagues were associated with running to first base. Biomechanical studies of sprint mechanics suggest that the posterior thigh is most susceptible to such strains near the end of the swing phase, when the hamstrings reach maximal length and undergo eccentric contraction just before heel strike.16,26,30 The acceleration from a stationary, but rotating swinging action required for running to first base (and often with no planned deceleration leading into the base) is likely the most complex and frequent acceleration baseball activity compared with running other bases or fielding, catching, or pitching. More work in understanding the unique biomechanical requirements of this action including the possible relevance of starting from the right versus left side of the plate is essential to develop appropriate prevention strategies. Hamstring strains are a debilitating injury in terms of time loss. Approximately two-thirds of hamstring injuries in both the major and minor leagues resulted in more than 7 days of time loss. Approximately 25% of these injuries required a month or more for return to play. Relative to other injuries, hamstring strains are among the top debilitating injuries in terms of time loss. This amount of time to return may be greater than what is observed in other sports. In the National Football League (NFL), hamstring strains entailed a mean of 13.2 and a median of 9 days lost per injury.10 This contrasts with our observed findings of 24 to 27 average days missed. Professional baseball at both the major and minor leagues is played almost every day and with a variety of dynamic movements that are often initiated from a static position. These features may mean that baseball hamstring strains require more complete recovery of injury before return to play to avoid
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TABLE 6 Regular Season Time-Loss Hamstring Strains by Month, Major and Minor Leagues, 2011 No. (% of Total)
Total No. of Games
Injury Rate (1 Hamstring Injury Every n Games)
Average Temperature, °F 63 68 75 82 78 72
a
Major league April May June July August September Minor leagueb April May June July August September
6 13 6 8
(12) (27) (12) (16) 3 (6) 3 (6)
392 420 400 395 420 117
65 32 67 49 140 39
33 44 28 45 41
(15) (20) (13) (21) (19) 7 (3)
1370 1892 2506 3161 2855 322
42 43 89 70 70 46
a
This analysis excludes 11 hamstring injuries that occurred in spring training or postseason games. This analysis excludes 20 hamstring injuries that occurred in spring training or postseason games
b
exacerbation of the injury. Given the high reported recurrent hamstring injury rate (20% in major league), the time to return to play requires further research. This study identified a 20% recurrence rate of hamstring strain injury from the previous year in the major league (10% with combined major/minor leagues). This recurrence rate is significantly higher than recurrence rates for other muscle strains (oblique, groin) that occurred in 2011 MLB. These data are consistent with other sports that report hamstring recurrence rates of 12% to 31%.5,10,12,15,23,29 Recurrence rates have been reported as high as 34% in Australian Rules Football.23 A recent study using NFL injury surveillance in professional football players identified hamstring injury recurrence of 16.5%.10 In this study we used the identification of a time-loss injury in the prior season (2010) to determine a recurrent injury in 2011. Other studies have used patients’ verbal history of a prior hamstring injury in their career to identify a recurrence as opposed to historical data in the EMR.10 Therefore, it is possible that our study underreports true recurrence as defined as any history of a hamstring strain. We observed interesting seasonal timing for hamstring strains. In both the major and minor leagues, the months of April and May had the highest incidence of regular season hamstring strains relative to games played in each month. Seasonal timing of hamstring strains has been shown in other sports. In NFL football, Elliot et al10 showed that in the preseason, athletes are at particular risk for hamstring injury. Numerous published studies in various sports have implicated factors contributing to the risk of sustaining a hamstring strain, including lower extremity flexibility,2,17,28 core stability,4,18,20,27 muscle weakness,6,7,22 and fatigue.19,29,31 In addition, neuromuscular activation may be altered by fatigue and further contribute to hamstring strain risk.14,19,25 While this study does not allow us to determine the reasons for seasonal timing of hamstring strains, it is possible that during the
first 2 months of the regular season, players have relative inflexibility, muscle weakness, and fatigability that make them vulnerable to hamstring strains. In addition, the adjustment from a stable spring training environment based in warm climates of Arizona and Florida to the more dynamic (in terms of both weather/temperature and travel) environments of the regular season may be a contributing factor. The average game start time temperature in April and May was 11°F cooler than in months later in the season. This area deserves further prospective research to identify injury factors and injury prevention strategies. There are several limitations to this study. The study does not include longitudinal data to analyze trends of hamstring injuries with time. In particular, recurrent injuries are not based on a medical history beyond the previous year (when the system began), and it is possible that recurrent injuries are underreported. The amount of data acquired over a single season did not allow for in-depth analysis of detailed features that may be associated with hamstring strains, such as player age or left- or righthanded batting. Grade of hamstring injuries or other diagnostic features such as physical examination findings or MRI findings are not provided in this study. Therefore, specific features of hamstring strains that may influence recovery duration and recurrence risk are not determined in this study. These could include player contract language that may influence behavior. In addition, we did not analyze treatment for hamstring injuries and factors that affect return to play, such as differences between individual players and variability in medical support/ rehabilitation between the major and minor leagues. Exposures (which are currently being evaluated for MLB EMR system enhancement) were calculated indirectly using box scores and therefore do not currently differentiate between a player who played 1 inning versus another who played 9. While more than 90% of major and minor league clubs
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1470 Ahmad et al
The American Journal of Sports Medicine
contributed at least 1 injury in the 2011 season, underreporting of injuries may have occurred as club athletic trainers adjusted to the new system. Finally, these data are limited to professional baseball players and may not be extrapolated to amateur players or to other sports. Despite these limitations, this study is the first in professional baseball to highlight the large incidence of hamstring injuries; most occurred during base running, and they entailed an average time loss of 24 to 27 days, twice that of the NFL. Early season appears to be the time of highest risk. These findings were consistent across both major and minor leagues and provide the foundation for further research in hamstring strain injury and prevention. These efforts will be aided by an ongoing robust injury surveillance system that will continue to capture information from the entire population of MLB players with the capability to link to additional rehabilitation and performance parameters. Future work, based on a projected multistep program currently being developed, will include standardized severity assessments based on MRI results and a preventive hamstring exercise program. Arnason et al1 showed that an eccentric strength training program with Nordic hamstring exercises could reduce the incidence of hamstring strains in elite soccer players; a similar program tailored for MLB is being developed and the results of this study have shaped that work.
ACKNOWLEDGMENT The authors acknowledge the Major League Baseball Players Association for its support and review of this article.
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