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Interpreting ACL Injury Prevention Program Results: Understanding Injury Incidence Rates and Athlete-Exposures In The News

Sue Barber-Westin

Since the first publication of a knee ligament injury prevention training program appeared in the sports medicine literature for female high school athletes in 1996 – Sportsmetrics – [1], at least 50 have followed that focused on female athletes. To date, 9 of these studies have reported ACL injury rates in female athletes according to athlete-exposures. An athlete-exposure is usually defined as one athlete participating in a practice or game in which there is the possibility of sustaining an injury. There have been a few studies on intervention programs that did not provide ACL injury rates according to athlete-exposures. Instead, these investigations provided the number of injuries divided by the number of athletes participating, which is not considered accurate according to modern epidemiology methods. One must instead use athlete-exposures to calculate injury incidence rates.

Athlete-exposures are usually determined using one of two methods. The first, which is the most accurate but difficult to collect, is the actual number of hours a player participates in practices and games throughout the season. The second method defines 1 athlete-exposure as one athlete participating in a practice or game. Therefore, if an athlete participates in 3 practices a week for 16 weeks and a total of 10 games during the season, the athlete-exposures would equal 58 for that player.

Injury incidence rates are calculated as the number of injuries divided by the total number of athlete-exposures. They may be calculated separately for practices and games, or all combined for an entire season.

To date, 3 ACL injury prevention programs have significantly reduced the rate of noncontact ACL injuries in female adolescent athletes. These include Sportsmetrics [2], Prevent Injury and Enhance Performance Program (PEP) [3], and Knee Injury Prevention Program (KIPP) [4]. The Sportsmetrics program had 700 trained athletes with 36,724 exposures and 1,120 control athletes with 61,244 exposures. There were 13 noncontact ACL injuries in the control group and just 1 in the trained group, for incidence rates of .21 and .03, respectively (p = .03). These numbers included athletes who participated in the original Sportsmetrics program and those that participated in the more recent sports-specific programs of Sportsmetrics Volleyball, Sportsmetrics Basketball, and Sportsmetrics Soccer.

It is important to note that other programs have been successful in reducing the rate of other types of lower limb injuries. For instance, the HarmoKnee program reduced the incidence of all types of acute knee injuries and all noncontact knee injuries [5]. When noncontact ACL injuries were sorted from the other knee injuries, the data did not reach statistical significance. A program from Sweden significantly reduced the incidence of ACL injuries when contact and noncontact injuries were combined [6] .

So, for medical professionals, when you hear of a claim that a program “reduced the rate of ACL injuries”, it is important to carefully study the methods used to calculate the injury rates and if contact and noncontact injuries were combined. ACL injury prevention programs are designed to prevent noncontact injuries and are especially relevant for high-risk sports such as soccer and basketball.


  1. Hewett, T.E.; Stroupe, A.L.; Nance, T.A.; Noyes, F.R. Plyometric training in female athletes. Decreased impact forces and increased hamstring torques. Am J Sports Med 1996, 24, 765-773.
  2. Noyes, F.R.; Barber-Westin, S.D. Neuromuscular retraining intervention programs: Do they reduce noncontact anterior cruciate ligament injury rates in adolescent female athletes? Arthroscopy 2014, 30, 245-255.
  3. Mandelbaum, B.R.; Silvers, H.J.; Watanabe, D.S.; Knarr, J.F.; Thomas, S.D.; Griffin, L.Y.; Kirkendall, D.T.; Garrett, W., Jr. Effectiveness of a neuromuscular and proprioceptive training program in preventing anterior cruciate ligament injuries in female athletes: 2-year follow-up. Am J Sports Med 2005, 33, 1003-1010.
  4. Labella, C.R.; Huxford, M.R.; Grissom, J.; Kim, K.Y.; Peng, J.; Christoffel, K.K. Effect of neuromuscular warm-up on injuries in female soccer and basketball athletes in urban public high schools: Cluster randomized controlled trial. Arch Pediatr Adolesc Med 2011, 165, 1033-1040.
  5. Kiani, A.; Hellquist, E.; Ahlqvist, K.; Gedeborg, R.; Michaelsson, K.; Byberg, L. Prevention of soccer-related knee injuries in teenaged girls. Arch Intern Med 2010, 170, 43-49.
  6. Walden, M.; Atroshi, I.; Magnusson, H.; Wagner, P.; Hagglund, M. Prevention of acute knee injuries in adolescent female football players: Cluster randomised controlled trial. BMJ 2012, 344, e3042.