Hamstring strain is one of the most common athletic injuries. It accounts for 12-16 percent of all injuries and has one of the highest re-injury rates of any muscle injury—after a hamstring strain occurs, 30 percent of those people will reinjure it within the next year. Furthermore, those who re-injure their hamstring will require more time away from sport than they did after the initial injury. Those at greatest risk for this form of injury are men, who are 62 percent more likely to sustain a hamstring injury than women, and this is more common in field sports than in court sports. This type of injury is common and we often hear the word ‘strain’ in reference to sports injuries, but never really in a context deep enough to define what the injury really is or how it differs from a ‘sprain’.
Sprains occur in ligaments and strains occur in muscles. Typically strains don’t involve the muscle tearing away from the tendon. In reality the damaged tissue is adjacent to this junction. Upon injury there is an immediate and acute inflammatory response that is followed by regeneration of the muscle and collagen. Fibrous scar tissue can form if the injury is not cared for properly. There is also a degree of edema and hemorrhage that can be observed on MRI to determine the severity of the strain. In order to avoid this type of injury and the resulting detrimental physiological processes it’s important to know what will predispose you to hamstring injury.
History of hamstring injury is a significant risk factor for future injury. Other risk factors include older age, decreased quadriceps flexibility, poor lumbar posture, decreased flexibility, inadequate warm-up, fatigue, and muscle imbalances of the thigh. Greater risk of injury is incurred if the individual has these risk factors and also participates in activities where hamstring strain is most likely to be suffered.
Hamstring strains are most likely to occur when running. Sprinting alone accounts for 57% of all hamstring strains. In order to understand why this is we need to break down the body mechanics of running. Terminal swing phase of running is when the leg swings forward and the hamstrings are activating eccentrically (muscle produces force as it lengthens) to slow down the leg before it makes contact with the ground, it is at this moment researchers believe the hamstrings are most susceptible to injury. This hypothesis is explained by three reasons. First, peak hamstring stretch at the junction of the muscle and tendon occurs during this phase. Second, data shows the hamstrings are active at this time. Third, the active lengthening that occurs in eccentric contractions produces the ideal conditions for a strain injury to occur. On top of this, hamstring strains are more likely to occur at the end of activity when fatigue has decreased eccentric hamstring strength. But as in most things physical, the changes we make to improve performance are also the same changes we make to prevent injury.
In running, performance is enhanced through the stretch-shortening cycle (SSC). In the SSC eccentric contraction precedes concentric contraction (force produced as muscle shortens). The hamstrings contract eccentrically in terminal swing phase and then have to contract concentrically just before foot contact, this increases power output of the concentric contraction. The negative work done by the hamstrings to eccentrically contract during this phase is substantial and only increases with running speed. It’s easiest to think of the hamstrings and their tendons as springs that work on a cycle to absorb and recover elastic energy throughout the running cycle prior to foot contact. The recovery of this energy must happen prior to foot contact or the energy will be lost as heat and running efficiency will decline. This crucial point when the hamstrings activate during terminal swing phase is referred to as the optimum length of tension.
The injured hamstring will produce peak tension on average at 12.7 degrees less than an uninjured hamstring, which is to say the optimum length of tension of the injured side was shorter than the uninjured side. Eccentric exercise is the only form of training that has been shown to increase the optimum length of tension, making it a crucial exercise for anyone who’s sport involves running to improve running efficiency, and therefore sport performance, before or after injury. When performing this type of training it is important to exercise at the intensity appropriate for your skill level.
Eccentric exercise should be performed at the following weight intensities for athletes of different skill levels. Novice to intermediate individuals should train with 60-70% of 1 rep max loads. Advanced individuals should use loads between 80-100% of 1 rep max, to maximize strength. After 6 weeks of training twice a week (2 x 8 repetitions) hamstring strength increased by 29% in the eccentric exercise group and by 19% in the concentric group. Consistent eccentric strengthening of the hamstring muscles is also shown to increase their flexibility. So, if you have tight hamstrings like so many of us do, this form of training will have dual benefits. While eccentric training is an important form of training, a well-rounded approach to prevention and recovery will ensure the best results.
Self-management of hamstring strain will often only involve stretching and strengthening, we now know these alone are insufficient to completely rehabilitate from hamstring strain. A stretch and strengthen approach will result in a re-injury rate of up to 54.5% within just two weeks of returning to sport. An injury prevention program for hamstring strain should account for the following elements: core stability, range of motion, strength, fatigue, and neuromuscular control. Stabilization training can improve motor control and functional ability. Furthermore, balance training has been shown to decrease lower limb injury. A 7% recurrence rate of injury was found when progressive agility and trunk stabilization exercises were part of rehab, as compared to a 70% recurrence over one year when rehab consisted only of stretching and strengthening. In the agility and stabilization group the average return to sport was after 22 days, the return for the stretching and strengthening group was 37 days. In order to be on the shorter end of return to sport it is important to have the right equipment to help you along the way.
There is a lot of balance exercise equipment on the market and you need the right balance board trainer to get you back in the game. The TherRexTM balance board trainer is the only piece of equpment of it’s kind built to mimic your body’s natural motion and adjust intensity as you improve. Below are targeted exercises using the TherRexTM balance board trainer to help you prevent and recover from hamstring strain.
Single Leg Deadlifts
Keep your back straight and bend at the hips as you reach down toward the floor. Change your foot positioning on the balance board trainer forward or backward on the board to mimic ankle positioning as you run.
Concentric contraction of the hamstrings is done in the descent phase. Eccentric contraction of the hamstrings is made at the end of ascent phase to counteract hip flexion. Be sure to contract your abs to stabilize your core throughout this exercise.
Slide Leg Curl
Bend your knees and slide the balance board trainer toward you, while keeping your hips off the ground. After you attain the bridge position, control the motion of the board as you slide back to the start position.
Standing on the balance board trainer, lift your knee up and kick your foot out. Contract your hamstrings to catch your foot before you reach full extension.
Rest one arm on the balance board trainer as shown. Start with your hips in the down position and lift them in the air until your body is straight.
Chad Franche, PT, DPT
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