Hamstring Curls

Hamstring injuries are a very real danger in team sports. Petersen et. al. (4) observed 374 elite Danish football players over the course of 12 months and reported a total of 54 hamstring injuries, 46 of which were first time occurrances. This roughly translates to one in nine players.

Hamstring curls, in their countless variations, including the nordic hamstring curl exercise (NHE), seem to be an effective way of reducing the incidence of hamstring injuries in running based sports. Van der Horst et. al. (5) investigated the effect of that particular exercise on soft tissue health of amateur soccer players and concluded that

Incorporating the NHE protocol in regular amateur training significantly reduces hamstring injury incidence, but it does not reduce hamstring injury severity.

The fact that amateur athletes were examined in the study might raise the question whether at higher levels, improved running mechanics alone can decrease the prevalence of hamstring injuries and hence, diminish the potential benefits that come from targeted, hamstring specific training. However, Askling, Karlsson and Thorstensson showed that the inclusion of a corresponding pre season strength training resulted in a significant decrease in injuries compared to the control group. Finally, a meta analysis performed by Al Attar et. al. seems to confirm the usefulness of the NHE for soccer players. Conversely, Orchard et. al. claim that hamstring weakness is correlated with a heightened risk of hamstring injuries in Australian Rules Football players.

These considerations justify the conclusion of targeted, knee flexion oriented hamstring work in a strength and conditioning program for team sports athletes. Attention should be paid to proper execution, though.

The gait cycle is broken up into two phases: a support phase and a swing phase. Each is further broken down into an anterior and posterior portion. The swing phase is, by definition, unloaded and hence, force absorption appears to be a non-issue here. However, transitioning from the anterior to the posterior support phase involves moving the center of mass forward over the planted foot. The hamstrings work in an interesting manner here. Although technically being knee flexors, the hamstrings actually extend the knee in a closed kinetic chain by extending the hip. This is known as Lombard’s paradox. I choose to look at the whole thing slightly differently. Rather than saying the hamstrings are actually extending the knee (which they can’t), I prefer to say that they induce a shift in the center of mass that eventually results in a knee extension. That’s probably nitpicking, though.

What can’t be overlooked in any case is the fact that the hamstrings are not the only, let alone primary hip extensor. In acceleration and top speed running, they always work in conjunction with the gluteal muscles. The glutes really don’t go through a lot of flexion during running, at least not under load. By the time the foot is planted on the ground, the hips are only very slightly bent. Everything else would be a result of overstriding and put the athlete at further risk of soft tissue injuries, in addition to simply taking away from efficiency and performance.

Functional training has gotten a bad rap in the last few years, because too many circus tricks involving unstable surfaces, light weights and dubious underpinning principles have been labeled as functional. Fictional training is a term that well describes that phenomenon.

In order to be functional, training needs to fulfill a function. Put differently, there should be a clear reason to everything that is done in the gym. If it can’t be justified, it probably shouldn’t be done. Otherwise it’s just wasted time and effort.

When including hamstring exercises in a workout routine, there needs to be a reason to do so. In our case, that reason is resilience. We don’t want our athletes to sustain hamstring injuries during running activities if it’s preventable. Period.

That goal really dictates technique. If we want a coordinated effort of hamstrings and glutes in running, then a functional training program needs to reflect this. That’s why we always coach hamstring curls with high hips. What we mean by that is that the torso and thigh must stay aligned in a straight line throughout the movement.

When the hips start to flex, the set is over. If athletes can’t do a single repetition this way, we have them regress to a version that they can handle.

Quality before quantity, always.

Nordic hamstring curls and other exercises have been criticized lately for being inherently slow in nature and thus, not specific to the demands of running based sports. This criticism has value and should be taken into consideration. Our solution to this challenge is the inclusion of kettlebell swings into our program. The kettlebell swing was shown by Zebis et. al. (6) to yield up to 115% of maximum voluntary contraction (MVC) in the semitendinosus muscle. While electromyography (EMG) measures certainly do not allow the patient to jump to any conclusions regarding the efficacy of a particular exercise, it stands to reason that the deceleration of a fast moving kettlebell causes beneficial tissue adaptations.

Of course, I left out a very important distinction. All I have said so far concerned acceleration and top speed running. As a matter of fact, many hamstrung injuries are sustained during deceleration. Here, kinetics looks slightly different. A good deceleration involves a lowering of the center of mass, in an effort to increase ground contact times. Also, the foot is planted slightly further forward, which results in a higher horizontal force vector opposing the running direction, ie, braking forces.

The empirical evidence still seems to support the notion that hamstring exercise is beneficial with regards to injury prevention. As for a specific exercise where force is absorbed in a flexed hip position: there’s a squat for that. In our program, the front squat and hamstring curl are always programmed together, so as to optimally complement each other.

Of course, S&C cannot solve everything. Proper running mechanics need to be taught to an athlete from the beginning. Acceleration, top speed running and deceleration each follow a different technical model that needs to be understood by the skill coach. Ultimately, tissue tolerance needs to be built in specific practice, first and foremost. In an athlete-centered model, the skill coach, strength coach, dietician and therapeutic staff will tightly work together to best serve the athlete. Hamstring curls alone will not achieve this, but, performed properly, they can play their part.

References
(1) Al Attar, W. S. A., Soomro, N., Sinclair, P. J., Pappas, E., & Sanders, R. H. (2017). Effect of injury prevention programs that include the nordic hamstring exercise on hamstring injury rates in soccer players: a systematic review and meta-analysis. Sports Medicine, 47(5), 907-916.

2)  Askling, C., Karlsson, J., & Thorstensson, A. (2003). Hamstring injury occurrence in elite soccer players after preseason strength training with eccentric overload. Scandinavian journal of medicine & science in sports, 13(4), 244-250.Askling, C., Karlsson, J., & Thorstensson, A. (2003). Hamstring injury occurrence in elite soccer players after preseason strength training with eccentric overload. Scandinavian journal of medicine & science in sports, 13(4), 244-250.

3) Orchard, J., Marsden, J., Lord, S., & Garlick, D. (1997). Preseason hamstring muscle weakness associated with hamstring muscle injury in Australian footballers. The American Journal of Sports Medicine, 25(1), 81-85.

4) Petersen, J., Thorborg, K., Nielsen, M. B., & Hølmich, P. (2010). Acute hamstring injuries in Danish elite football: a 12‐month prospective registration study among 374 players. Scandinavian journal of medicine & science in sports, 20(4), 588-592.

5) van der Horst, N., Smits, D. W., Petersen, J., Goedhart, E. A., & Backx, F. J. (2015). The preventive effect of the nordic hamstring exercise on hamstring injuries in amateur soccer players: a randomized controlled trial. The American journal of sports medicine, 43(6), 1316-1323.

6) Zebis, M. K., Skotte, J., Andersen, C. H., Mortensen, P., Petersen, H. H., Viskær, T. C., … & Andersen, L. L. (2013). Kettlebell swing targets semitendinosus and supine leg curl targets biceps femoris: an EMG study with rehabilitation implications. Br J Sports Med, 47(18), 1192-1198.