Research Review By Demetry Assimakopoulos©

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Date Posted:

February 2012

Study Title:

Preventive Effect of Eccentric Training on Acute Hamstring Injuries in Men’s Soccer: A Cluster-Randomized Controlled Trial

Authors:

Petersen J, Thorborg, K, Nielsen MB et al.

Author's Affiliations:

University of Copenhagen, Copenhagen Denmark

Publication Information:

American Journal of Sports Medicine 2011; 39: 2296-2303.

Background Information:

Many athletes are injured at some point in their careers (1). Soccer players are no stranger to this fact. In fact, injury to the hamstring muscle group is the most common injury in soccer, with an incidence of 0.5 – 1.5 injuries per 1000 hours of soccer competition and training (2). This type of injury unfortunately has a high recurrence rate. As such, one of the largest research trends in the sports realm is the prevention of these injuries by increasing the eccentric strength of the hamstring muscles (as eccentric loads are often the mechanism of injury). Put simply, this can be accomplished by lengthening the hamstring muscle complex while it is loaded and contracted (3).

With this in mind, these researchers endeavored to design a cluster-randomized controlled trial, using a 10-week eccentric training exercise program (Nordic exercise – explained below) in an effort to reduce the incidence of new and recurrent acute hamstring injuries. They also attempted to reduce the severity of hamstring injuries when they occur.

Pertinent Results:

  • Fifty-four teams were included in the study. Twenty-six teams were assigned to the intervention group, while 28 teams were assigned to the control. Four teams withdrew from the study (3 intervention; 1 control) – their data was not included in the final analysis.
  • Teams allocated to the intervention group performed an average of 91% of their 27 intended training sessions.
  • A grand total of 67 acute hamstring injuries in 67 players were registered – only 15 of these were registered by the intervention group, while 52 of the reported injuries were registered by the control group.
  • Overall, the teams allocated to the control group showed a higher injury rate in the preseason period, but this relationship was not statistically significant.
  • Overall, hamstring injury rates were significantly lower in the teams in the intervention group – this is qualified by significantly lower new and recurrent injury rates. Additionally, injury severity (judged as number of days absent from competition) was lower in the intervention group; 15 total injuries resulting in 454 lost days compared to the 52 injuries in the control group, which resulted in 1344 lost days.
  • Nine of the 15 injuries in the intervention group occurred within the 10-week training period during the preseason. Twelve of the 52 injuries in the control group occurred within the same preseason period. This finding led the authors to suggest that the intervention group had to complete a number of training sessions to gain the injury preventive effect.
  • No injuries were incurred during the performance of the Nordic Exercise.

Clinical Application & Conclusions:

This study is the first to utilized cluster-randomization in their research methodology. It is also the first to document the possibility of significantly reducing the incidence of hamstring injuries in soccer athletes by integrating eccentric hamstring muscle training. Eccentric training has the potential to decrease the rate of new injuries by approximately 60% and can reduce the rate of recurrence by approximately 85%. However, there is no apparent effect on decreasing the severity of hamstring injuries when they do occur. Further research is needed on athletes in other sports, but for now the exercise utilized in this study (see Study Methods below) is a simple one to implement with your athletic patients as a component of their training programs.

Study Methods:

What is the Nordic Exercise?

This is a partner exercise, where the one athlete begins in the kneeling position, while the training partner applies a manual counter weight to the kneeling athlete’s heels/lower legs to ensure that the feet stay in contact with the ground. The kneeling athlete then attempts to slowly lower their torso toward the ground to maximize hamstring loading eccentrically. The athletes performing the exercise were asked to touch their chest to the floor, while allowing their arms/hands to buffer the fall. Subsequently, the athlete was asked to return back to the starting position by using their hands to minimize loading in the concentric phase. Each athlete assigned to the intervention group performed 27 sessions of the Nordic Exercise over the course of 10 weeks.
nordic_hamstring_exercise
What is the intervention group’s training protocol?

Each athlete assigned to the intervention group performed the following training protocol:
  • week 1 – 1 session per week, 2 sets, 5 reps;
  • week 2 – 2 sessions per week, 2 sets, 6 repetitions;
  • week 3 – 3 sessions per week, 3 sets, 6-8 reps;
  • week 4 – 3 sessions per week, 3 sets, 8-10 reps;
  • weeks 5-10 – 3 sessions per week, 3 sets at 12 reps, 10 reps, and 8 reps, respectively;
  • week 10+ - I session per week, 3 sets, 3 at 12 reps, 10 reps, and 8 reps, respectively.
All teams clustered into the intervention group performed this 10-week routine, on top of their normal training program. When and how the exercise was integrated into the training program was left to the discretion of the staff managing the individual team.

Randomization of treatment groups was done in a cluster-like fashion, whereby individual teams were assigned into a treatment group (those teams who collectively performed the Nordic Exercise) and the control group (those teams who did not perform the Nordic Exercise). All 3 professional divisions in the Dutch soccer league were used as a pool of subjects. The study did not incorporate blinding into its methodology. All players, coaches and investigators were aware of group allocation. The study took place over the course of 10-weeks. The injury registration period started during the midseason break of the 2007-2008 season, and continued until the end of the first half of the 2008-2009 – a full 12 month season. To enhance compliance of injury registration, a free ultrasound investigation was offered to each player who reported an injury. However, this procedure was not used to verify the clinical diagnosis, because not all hamstring injuries can be detected using this imaging modality.

Study Strengths / Weaknesses:

Weaknesses:
  • No imaging modality was used to validate a clinical diagnosis (ie. there was no identification, comparison or utilization of a gold standard diagnostic modality).
  • Lack of long-term follow-up
  • Lack of blinding – although this was not really possible in this study
  • Were the groups homogeneous? The researchers made no effort in their publication as to whether or not they took precautions to create a homogeneous sample in all groups. The researchers used athletes from all 5 divisions within the Dutch league; one could imply that if more division 1 teams were allocated to the intervention group, the fact that they spend more time during a given week training could have a preventative effect.
Strengths:
  • Imaginative cluster-randomization
  • Large number of participants
  • Great compliance

Additional References:

  1. Schmikli SL, Backx FJ, Kemler HJ, van Mechelen W. National survey on sports injuries in the Netherlands: target populations for sports injury prevention programs. Clin J Sport Med. 2009; 19:101-106.
  2. Arnason A, Gudmundsson A, Dahl HA, Johannsson E. Soccer Injuries in Iceland. Scand J Med Sci Sports. 1996; 6:40-45.
  3. Croisier JL. Factors associated with recurrent hamstring injuries. Sports Med. 2004; 34:681-695.