Hamstring strains or pulls are very common in sports that require sprinting and rapid changes in running direction. Injury to the hamstring muscle can leave the athlete unable to play for weeks, even months. Elsewhere on this site, information has been posted regarding the relationship between hip and knee range of motion (hamstring flexibility) and the risk of injury. Studies show increasing hamstring flexibility can lower injury risk. In a study soon to be published in the American Journal of Sports Medicine, Jean-Louis Croisier and colleges suggest that hamstring strength may also be an important factor. Specifically, they report that strength imbalances between the right and left hamstrings as well as deficits between the hamstring and quadriceps muscles are associated with increased risk of hamstring injury. The authors also report that correcting the imbalances through strength training can markedly lower the risk of injury.
The investigators tested and monitored 462 Belgian, Brazilian and French professional players over a five-year period. Isokinetic strength measurements of the hamstring and quadriceps muscles were made on each player at the start of each season. Strength imbalances were defined as a player having more than a 15% difference in hamstring strength between the right and left limb (bilateral deficit) or hamstring strength less than 45% of quadriceps strength (H/Q deficit). Injuries were reported over the course of the season and were defined based on physical and diagnostic examination by the team’s medical staff and a least 4 weeks of missed playing time for each player.
Of the 462 players, 216 were identified with hamstring imbalance. 70 of these players were given a specific hamstring strengthening program designed to eliminate the imbalance. In group of players with no imbalance, only 4% suffered a hamstring injury. In players with an imbalance that was not corrected, more than 16% were injured. Thus, hamstring strength imbalance increased the risk of injury by more than 4-fold. In the group of players that received training, only 6% were injured, reducing their risk to near normal.
The authors of the study conclude that hamstring strength imbalances increase the risk of hamstring injury. That is, weak hamstrings, relative to each other or relative to the quadriceps, can lead to hamstring muscle pulls. The authors also suggest that correcting these strength imbalances by training nearly eliminates the increased susceptibility of injury.
There are a few limitations to the study. All of the measurements and training programs used isokinetic devices such as the Cybex or Biodex machines. As for youth athletes, it is probably too expensive and too time consuming to utilize these devices for testing or training. However, the authors are quick to point out that is it reasonable to suggest that either weight training or manual training using exercises aimed at hamstring strengthening could reduce the risk of injury. Based on other research, add to that program, stretching exercises designed to improve hamstring flexibility. By combining strength and flexibility training, it may be possible to dramatically lower the incidence of hamstring injuries.
Croisier J-L, GAnteaume S, Binet J, Gentry M, Ferret J-M (2008) Strength imbalances and prevention of hamstring injury in professional soccer players: A prospective study. American Journal of Sports Medicine, DOI:10.1177/0363546508316764.