News broke on Wednesday that Darren Waller is being placed on injured reserve due to a hamstring injury suffered during Monday night's game against the Arizona Cardinals. While no one from the Ravens has discussed the specifics of his injury, his season is likely over due to a tear of the muscle. In this article we'll examine the anatomy of the hamstring muscle group and why hamstring injuries are so prevalent amongst NFL players and athletes in general.
The hamstrings are located on the posterior surface of the femur (thigh bone) and are comprised of three individual muscles: the semimembranosus, semitendinosus, and biceps femoris. This muscle group originates at the ischial tuberosity (sit bone) of the pelvis and its distal tendons insert just past the knee joint. The semimembranosus and semitendinosus run along the medial (inside) aspect of the knee while the biceps femoris is located at the lateral (outside) aspect.
A hamstring strain occurs when fibers of the muscle tear. Like ligament sprains, muscle strains are also categorized with a grading system:
Grade I muscle strain: mild tearing of individual muscle fibers. Typically requires 2 to 3 weeks to recover.
Grade II muscle strain: moderate tearing with more muscle fibers involved. Typically requires 3 to 6 weeks to recover.
Grade III muscle strain: complete tearing/rupture of the muscle. Requires surgery and months of rehabilitation to recover.
MECHANISM OF INJURY
Hamstring injuries can potentially occur any time an athlete quickly accelerates, decelerates, or changes direction. However, a 2011 study found that non-contact sprinting was the most common mechanism of hamstring injury in NFL players; accounting for over 68 percent of all hamstring strains. This can likely be explained through biomechanics and the role the hamstrings play with sprinting.
Research demonstrates that the muscles of the posterior thigh (hamstrings, gluteals) are under the most strain during terminal swing of the swing phase of sprinting (see below). It's at this moment that the hamstrings reach maximal length and must fire rapidly to eccentrically control the limb as the foot prepares to strike the ground (initial contact). This action is vital and, if someone lacks this control, their ability to effectively push off from stance phase will be inhibited as well.
Another study examined NFL hamstring strains by player position over a ten year period from 1989-1998. It found that the overwhelming majority of these injuries occurred in defensive backs (23.1%), wide receivers (20.8%), and running backs (12.3%) while offensive and defensive linemen had the lowest rates. Clearly, those who play positions that require intense sprinting are at higher risk of experiencing hamstring strains.
BUT WHY DO THESE INJURIES OCCUR SO FREQUENTLY?
Hamstring strains are one of the most common injuries in all of sports. In fact, statistics show that they occur in the NFL, on average, 176 times per season. That's nearly six injuries per team. They also have a reputation for being injuries that linger and are difficult to fully recover from. This notion is pretty evident considering that the re-injury rate for hamstring strains exceeds 16% in NFL athletes. With that said, these numbers bring about some pretty important questions: why do these injuries occur so frequently and, if we're rehabilitating our athletes appropriately, then why is there such a high re-injury rate?
There are obviously several potential causes for hamstring strains. Traditionally they are believed to be due to a lack of strength and/or extensibility of the hamstring muscle group and treatment for these individuals would consist of various hamstring stretching and strengthening exercises. However, in my experience, this is seldom the case.
Hamstring strain injuries are often attributable to muscle imbalances that result in weak or inactive gluteal muscles. The glutes are the largest muscle group in the body and are where athletes generate the majority of their power. The hamstrings act as a synergist to the glutes, meaning that they assist them in performing certain joint movements; notably hip extension. Therefore, if the glutes are inhibited in some way the hamstrings are forced to pick the slack. This is termed synergistic dominance.
So rather than hamstring strains being due to a lack of hamstring strength, these injuries actually occur because the hamstrings are being overused. By correcting dysfunction to the gluteals and improving their recruitment the hamstrings no longer have to strain and overcompensate. I hardly ever focus on strengthening the hamstring with my patients. To me, and others, it’s all about taking pressure off the hamstring and letting the muscle (the glute) that is supposed to be doing the work, do the work.
- Bobby Esbrandt, PT, DPT, PES