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Hamstring muscle strains are among the most common injuries in sports today (6,8). These injuries can occur without warning and sometimes repeat at the same location several times. In regards to non-contact injuries, they are the most common and frustrating injuries for athletes and coaches alike.
In fact, according to current and previous epidemiology studies, hamstring injury occurrences have not declined over the last three decades (7,9). This suggests that our current approaches to preventing hamstring injuries are largely ineffective.
We know that previous injury is one of the greatest risk factors for future injury, and we know that functional deficits in hamstring strength can last for at least two years after an initial hamstring injury (4,14,15). One way to assess re-injury is to monitor athletes over long periods to document re-injury rates.
Unfortunately, most of the literature defines a “re-injury” as a second hamstring injury to the same location within a two-month period. So if an injury occurs after three months, it would not be classified as a re-injury.
Carling et al. recently tracked muscle strain recurrences in professional soccer players over a four-year period (5). Injury recurrences were classified as “early” if they occurred within two months of returning to full participation; “late” if they occurred between 2 – 12 months after returning to full participation; and “delayed” if they occurred after 12 months from returning to full participation. It was reported that 50% of the players suffered at least one muscle strain recurrence, and hamstring injuries were the most common recurrences.
The authors stated that the risk of re-injury remained strong 12 months after the initial injury. These findings and the findings of other studies suggest that our re-injury prevention approaches have not been effective, and are possibly getting worse (4,12,14).
Fortunately, there is hope. A few studies have reported reductions in hamstring injury occurrences with eccentric training (1,2,11). Despite several methodological flaws in these studies, thousands of athletes have been monitored, and most have reported reductions in hamstring injury occurrences.
Most recently, Peterson et al. conducted a randomized controlled training study on 943 Danish professional soccer players during a full season (11). The experimental training group performed a progressive eccentric exercise (i.e., Nordic hamstring exercise) for 10 weeks followed by a weekly in-seasonal program. The experimental eccentric group suffered a total of 15 hamstring injuries throughout the season, while the control group suffered a total of 52.
The hamstrings are actively lengthened (i.e., eccentric action) with both knee extension and hip flexion. However, it should be noted that the predominant exercises used in hamstring rehabilitation programs for research purposes, eccentrically contract the hamstrings during knee extension only (e.g., Nordic hamstring and eccentric leg curl). None of the studies used an exercise that emphasized hip flexion. So, why is this so important?
First, athletes require multifaceted training to ensure that the hamstrings are loaded in a variety of ways. Variation is a key to smart programming. You cannot simply do the same exercise in every training session, all year long, over several years.
Second, the moment arm of the hamstrings in most movements is much longer at the hip than at the knee. This means that hip flexion will cause much greater strain (i.e., muscle lengthening) than knee extension. This is especially true for the long head of the biceps femoris (LHBF), which is the most commonly injured hamstring muscle. The LHBF has a longer moment arm at the hip than the semimembranosus, and a shorter moment arm at the knee than the semimembranosus and semitendinosus (16).
What this means is that hip flexion will cause greater strain in the LHBF than knee extension. The take-home message is, “we need to train the hamstrings eccentrically at long muscle lengths during hip flexion.” More authors are now coming to this same conclusion (10,13).
So, what are some of the best exercises to train the hamstrings eccentrically with hip flexion? You may think of deadlifts or good mornings; but it is not likely that these exercises cause much strain at the LHBF because they are performed slowly at moderate muscle lengths. Thus, they are not effective for this purpose. A much better approach is performing eccentric box drops (3).
Box height induces the loading (stress) of the muscle and the dropping motion after landing should induce strain at the LHBF if done properly (see Figure1). It should be noted that research is currently in preparation to either confirm or reject this claim. These exercises have also been shown to induce a shift in optimum length (SOPL) to longer lengths of the hamstrings. This SOPL means that the hamstrings could produce force at longer muscle lengths, with such an adaptation potentially mitigating injury or re-injury.
A muscle strain injury occurs when a muscle is lengthened and loaded, and the ability to produce force at long lengths is of utmost importance. Several recent articles have shown that after an initial hamstring injury, the ability to perform an eccentric action at long muscle lengths is compromised (12,14).
Ironically, one of the best ways to prevent a muscle from being injured during loading at long muscle lengths is to train them during loading at long muscle lengths (i.e., eccentric training).
There are several beneficial adaptations to eccentric exercise that should, theoretically, reduce the risk of injury. Eccentric exercise has been shown to increase eccentric strength, shift the optimum length to long muscle lengths, increase tendon stiffness, and increase passive force production. Each of these adaptations, working in synergy, should help to reduce the risk of hamstring injury.
Figure 1. Eccentric Box Drops
Although we know several risk factors for hamstring injury, we still cannot determine the cause of hamstring injury on an individual basis. Hamstring muscle strain injuries have multiple causes. For each athlete, the cause of injury is different (e.g., anterior pelvic rotation, muscle weakness, weak gluteals, overactive hamstrings, tight hip flexors, etc.).
The future may provide more answers through musculoskeletal modeling of individual athletes. But for now, the best approach to general injury prevention is a well-rounded and comprehensive program that includes functional eccentric exercise, gluteal strengthening, trunk stabilization, and neuromuscular control training.
With regard to recommendations, it can be said that the following exercises can be utilized to improve eccentric strength and target the hamstrings. These exercises have been developed from current literature that has shown an increase in optimum length of tension development. Along with these exercise descriptions, there is an off-season, pre-season, and in-season exercise template below (see Table 1) that can be utilized to guide prescription.
The athlete begins by stepping up onto a box (12 to 36 in. high). He or she then steps off the box and lands in a squat position.
Key points: Do not land in a static position. Allow for full flexion of the hips, knees, and ankles upon foot contact. To end the exercise, place the hands on the knees and push up into a standing position; do not jump out of the bottom position. Do not perform more than 12 drops in a set.
The athlete gets into the starting position. One athlete pushes forward while concentrically contracting the muscles in his or her lower body. The other athlete will apply resistance but still allow him or herself to be pushed backward while eccentrically contracting the muscles in his or her lower body.
Key points: Do not allow the athletes to raise their upper or lower backs during this exercise. This exercise should be performed with cleats or performed on a surface that has a lot of friction. Perform approximately 10 to 15 steps with each leg.
The athlete rises up onto his or her toes while taking a lunge stance, with or without resistance. He or she then quickly drops onto the ground with his or her feet landing flat and balanced. Then he or she will resist the downward forces while moving into a deep lunge position, maintaining good posture.
Key points: Do not perform this exercise if good posture cannot be maintained in the bottom position. Do not come upward out of the bottom position; instead, slowly lower to the ground until the back knee is touching the ground, thus allowing the muscles a break from the contraction, before standing up. Do not perform more than 5 drops with each leg.
The athlete gets into the starting position. He or she bends down, while maintaining this position throughout the exercise, and slowly pulls the load backward. This exercise forces the hamstring muscles to be activated, and then it actively stretches the hamstrings with hip flexion and knee extension.
Key points:Do not rise up from the starting position. Do not perform more than 15 steps with each leg.
The athlete performs a Romanian deadlift (RDL) with a barbell to get into the starting position. Then, he or she simply performs a straight-leg deadlift (SLD) during the eccentric phase (to the ground and reset for the RDL) and an RDL during the concentric phase.
Key points: Make sure to perform the straight-leg deadlift to the ground and then reset. Reset by getting into a clean stance and perform the concentric RDL, thereby breaking this exercise into two separate exercises. Maintain good posture with a neutral spine.
The athlete gets into a starting position with a plate or dumbbell and performs the eccentric phase of the single-leg deadlift only. He or she then resets and repeats.
Key points: Perform the eccentric phase of this exercise all the way to the ground and then relax and reset. Maintain good posture with a neutral spine.
The athlete gets into the starting position. This exercise is similar to a good morning variation except the load is in front of the athlete.
Key point: Maintain good posture with a neutral spine.
For photos of these exercises please refer to Brughelli and Cronin, 2008 (3).
Table 1. Sample Exercise Template
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