Hamstring Strains: Who, What, Where, When…..WHY???
While finishing up my Doctorate in Physical Therapy at Duke University, we were assigned a research project on any topic we found interesting. Coming from an athletic background, I was very interested in researching injuries associated with sport. My thesis was titled “Injury Risk is Altered by Previous Injury: A Systematic Review of the Literature and Presentation of Causative Neuromuscular Factors.” You can read the full article HERE. One of the injuries we investigated were recurrent hamstring strains. The literature was littered with articles looking at proposed mechanisms, biomechanics, strength, and rehabilitation. However, none of them actually proposed to find a link between sprinting technique and hamstring injuries. This ate away at me over the years until I began to take a deeper look into sprinting mechanics, muscle physiology, and rehabilitation. My goal was to find the common link between sprinting technique and strength and conditioning/rehabilitation principles, figure out the root cause behind hamstring stains, and gain better insight into how to prevent them.
What Is A Hamstring Strain, Exactly?
Hamstring Strains are often characterized by a sudden sharp pain or “pop” in the posterior thigh, either high up in the buttocks near your gluteal fold where the muscle originates at the ischial tuberosity, or down near the knee where the muscle inserts onto the tibia or fibula of the lower leg. There are four hamstring muscles: the Semitendinosus, Semimembranosus, and the Biceps Femoris (long and short heads). The Biceps Femoris (long head) is the most commonly injured of the four. Hamstring strains are caused by either an extensive, forceful contraction, or a violent over-lengthening or stretching of the muscle. Strains occur most commonly in sports that require a high amount of dynamic movement like sprinting or jumping. The cause of hamstring strains is usually due to poor sprinting mechanics, loss of posture and position of the midline and lower extremities while changing direction, and poor eccentric strength or control (lengthening under load) of the hamstring muscle itself. The latest research suggests that hamstring strains make up around 12-16% of all injuries in popular team sports (Woods, et al. 2004). Previous hamstring strains have been found to be the number one predictor of a future hamstring strain out of any other risk factor. This tells me injury has a lot to do with how we are sprinting, rather than the act of sprinting itself. Let’s break down each of these to understand more about how the way we train impacts our risk for hamstring injury.
Before we dive deep into the nuts and bolts of this article, let’s get a couple of things out of the way. This article’s focus is on sprinting mechanics and training. However, injuries of the hamstring can and do occur because of improper warm up, inappropriate training loads, playing surface, and fatigue levels. These topics will not be directly referenced, but you should be aware that they play a major role in tissue health and recovery.
Born to Run? Not So Fast…
Sprinting is one of the most complex movement patterns the human body can perform. It involves reciprocal motions of the arms and legs, midline (trunk stability), adequate joint angle range of motion, and high ground reaction forces through the kinetic chain with each foot impact. All of these together are termed Biomechanical Efficiency – when the muscles actually work together in a coordinated manner to control movement. Many athletes, recreational and professional alike, can take running form for granted. They are unaware of what their body is actually doing during a sprint; unaware of any technical breakdowns (unconscious incompetence). Timing is also very important. The nervous system must fire at appropriate times in order to coordinate muscular contractions and display beautiful, visually pleasing sprinting technique. This is typically described as good Neuromuscular Efficiency. The more motor units and muscle fibers involved, the better the muscle works (rate coding and recruitment), leading to improved performance. Simply stepping foot onto the track and going all out will not yield positive results, and can actually lead to a greater risk of injury if you have not established good biomchanical and neuromuscular efficiency.
It All Comes Down to Posture and Position
Sure, individual mobility deficits, preventing the joints and muscles from moving efficiently, can reduce biomechanical efficiency and lead to injury. The same can be said for limited eccentric strength of the hamstring muscles. The hamstrings are predominantly Type 2 muscle fiber in nature, which means they are fast twitch. They essentially act like a loaded spring that recoils with each step to propel you forward. Eccentric training has been found to be a very beneficial training tool for improving performance and decreasing injury rate, but this only gives you a larger buffer zone against injury when something goes awry. Having hamstrings like steel cables means nothing if you sprint like Steven Seagal.
Each human body is different in the sense of varietal anthropometric measurements. However, not one athlete is such a unique snowflake that they don’t need to abide by the principles associated with good sprinting form.
We must start with the chassis of the body, the spine. The spine is our foundation where optimal posture begins. Without a stable base, our arms and legs can not operate efficiently. When it comes to recurrent hamstring strains, there are two areas that seem to have the largest impact on positioning: the cervical spine and the pelvis. Keeping the chin tucked or packed while sprinting is vital. Jutting the chin forward and poking it up towards the sky causes excessive upper cervical extension which can lead to neck pain. Additionally, this posture also causes the athlete to reach and grab with the foot, leading to over-striding and pulling, rather than a pushing motion. Researchers have proposed that most hamstring strains occur in the terminal swing phase of gait. This is where the hamstrings are lengthened quickly with the most amount of hip flexion and knee extension occurring simultaneously (Higashihara et al. 2014). Interestingly, this is also where the Biceps Femoris (most frequently injured) displays it’s greatest peak EMG amplitude and is absorbing the greatest amount of energy. (Higashihara et al. 2014). These facts help explain why the more distal (away from the center of the body) a hamstring injury occur closer to the knee when posture is compromised.
What about proximal hamstring strains? These injuries occur closer to the attachment site of the Biceps Femoris on the ischial tuberosity of the pelvis and are likely the result of a sloppy pelvis (excessive anterior pelvic tilt) while sprinting. This occurs when athletes over extend through the lumbar spine and sprint with a more vertical torso as opposed to a nice forward lean where mechanical advantage is directed towards the hips. This overextension allows the pelvis to tilt anteriorly, providing a “pre-stretch” to the hamstrings at their proximal attachment site. It has been stated in the literature that injured athletes tend to display greater peak anterior pelvic tilt and peak hip flexion than un-injured athletes (Daly, et al. 2015). Greater anterior pelvic tilt with excessive hip flexion leads to greater maximum length under load during sprinting, predisposing that athlete to further injury. This now over-lengthened muscle is assigned the task of decelerating the athlete during a sprint. However, the once “spring-like” nature of the hamstring is now impaired. The actin and myosin cross-bridges within each myofibril are over lengthened, decreasing it’s ability to withstand the ground reaction forces traveling through the lower extremity and up into the pelvis. This force bleed in the system results in strain to the proximal fibers of the hamstring muscles.
Wait A Sec, What About the Middle of My Thigh?
Well, Garrett et al observed in a CT study that the proximal Biceps Femoris tendon and its muscle-tendon junction extend 60% of the muscle length, while the distal tendon and its muscle-tendon junction extend 66% of the muscle length. Thus, the regions of muscle-tendon junction extend the full length of the muscle belly on either the proximal or distal portion. This study highlights that the “springy” tendon of the hamstrings traverse the majority of the muscle belly in either direction, which allows the possibility for strain symptoms to present anywhere along the posterior thigh depending on personal anthropometrics and mechanics.
As you can see, regardless of the area affected, obtaining and utilizing good midline posture is imperative. There is no better way to effectively train this facet than during your warm up with an exercise like the Dead Bug. Becoming proficient with this movement will have transferability not only into your sprinting mechanics, but also with your lifting technique in the gym (where we train to become faster athletes!) If you develop and train good posture and position in the gym, it will carry over to when you need it most – on the playing field.
Do You Walk Like a Duck?
Lastly, I would be remiss if I didn’t talk about foot position. After all, this is our first point of contact with the ground while sprinting (unless you are running on your hands), and efficient foot and ankle mechanics dictate how that ground reaction force is transferred up the chain. I see it every day. Athletes are walking around the gym and training like ducks with their feet turned out. What do you think is going to happen on the field or track, especially under fatigue? The athlete will resort to the neural programming their body is most familiar with, which can be disastrous. Sprinting with the foot spun out causes external tibial and hip rotation when striking the ground. Don’t forget where our hamstrings insert! This imposes a rotational torque force through the distal tendons, adding one more level of stress and shear force which can predispose the tendon to strain. Think about how you tear a piece of tape. You wouldn’t just pull on each end. Instead, you add a rotational force to the point at which you want it to tear. When it is time to cut or change direction, the hamstrings can no longer contract effectively and do their job, also leading to a greater chance of knee valgus and tibial anterior glide – the recipe required for an ACL tear.
This is why I have my athletes train toes forward position with every movement they do in the gym. Building and reinforcing a universal athletic position through neural motor patterning will pay off. Studies show that tibial and femoral internal rotation (created with the foot pointing forward) produces greater medial hamstrings EMG amplitude during a range of different hip extension exercises and movements (Jónasson et al. 2015). We know that the lateral Biceps Femoris is the most commonly injured of the hamstring muscles. Mechanically speaking, this gives insight into why excessive foot turn out creates tibial external rotation, leading to greater incidence of Biceps Femoris hamstring injury than those who sprint toes forward. Going back to midline posture, using the abdominal drawing-in maneuver (short torso, rib cage down) led to increased medial hamstring EMG amplitude (Oh et al. 2007). Therefore, training toes forward with a nice neutral spine is optimal and will have the greatest carryover to improved performance in sport. The gym is our training ground. The way we produce neural adaptation the fastest is through stressing the neuromuscular system with progressive overload. This load should challenge the ideal posture and position we want in sport, not change it just for an arbitrary number on the board.
Maybe we should be focusing less on overall strength and mobility to reduce hamstring strain incidence, and instead look at the quality of movement. I don’t care that you can sprint a 4.4s 40m as much as I care about how you do it. Train good posture and position in the gym and you will see positive carryover onto the track and field.