The hamstring (also known as the biceps femoris, semitendinosus, and semimembranosus muscles if you want to sound fancy) is notorious for its variability in how injuries affect players. Some are out for the season, others might grab it slightly while walking off the pitch to justify a leisurely, clock-burning pace for an 80th minute substitution (as has been demonstrated beautifully by the consummate professional, Clint Dempsey, independent of his March 2015 injury). Considering it’s the most common injury in soccer players, there are a lot of questions about it. Like, what’s the hamstring do anyway? Why do soccer players seem so prone to injuring it in the first place? And why such variability in when a player comes back?
The number one job of the hamstring is to control deceleration. Most people think of muscle contraction as occurring only while a muscle shortens, but the hamstring contracts while it lengthens, known as eccentric contraction. Think of a bungee cord pulling back more as it gets longer and you’ll get the general idea. These deceleration actions are particularly key for control during sprinting, cutting, and kicking, so you can imagine how essential the hamstring is to a soccer player. It’s less important for others, and the semitendinosus can even be removed (along with the gracilis muscle), doubled over on itself, and used as a replacement for the ACL. Having been on the receiving end of this exact surgery I can tell you that its absence has an undetectable impact on the performance of someone who never remotely approaches the speed of a professional athlete.
So if the rest of us can have 1/3 of our hamstring removed and never miss it, then what are elite soccer players doing that they end up tearing them? They do the same thing that other athletes (track and field, outfielders in baseball, wide receivers/cornerbacks) with a relatively high rate of hamstring injury do. In short, sprinting really, really fast. The faster and farther you sprint in a given sport/position, the higher the likelihood of injury. In fact, in 2004 a study compared rates of injury between the EPL and Championship level, and EPL players had more frequent hamstring injuries corresponding to EPL’s faster play overall. Other factors definitely influence the rate of injury as well. Age is clearly key, with risk of injury increasing by about 30% per year over the course of a career. Also, other weak spots in the “kinetic chain” will have an impact, whether simply due to an imbalance of forces (e.g. the dominant leg is too much stronger leading to speeds the weaker leg can’t handle) or any pre-existing injury that changes gait mechanics.
Certainly prior hamstring injury plays a significant role, with up to three times the risk during the first year after an injury (most of which is within the first two months). However, the problem with studies looking at reinjury rates is that they can’t account for whether the athlete was truly ready to return. So when is it safe to put a player back out on the pitch? It doesn’t take a doctor to know that the answer is highly variable. A player might push through a little twinge in which just a few fibers were torn, but would be out for the season with a complete tear. The size of injury on MRI is one of the best indicators for how long rehabilitation will take. Complete rupture requires surgery, and surgery is even potentially beneficial if the partial tear is in the lower portion of the hamstring.
Because imbalance in strength increases the risk for reinjury the return to action depends on how long it takes to eliminate the imbalance caused by the injury itself. Very minor tears might mean just a couple of days away, but those were likely treated with a wrap and magic spray at the time of injury and the player never even left the match. Players just shy of a moderate tear can be back as early as two weeks, but some argue that these players remain at higher risk for reinjury. Moderate size tears that don’t meet criteria for surgery, depending on the exact severity and the rehabilitation protocol, take an average of about four weeks to return to unrestricted activity. Poor physical therapy protocols can put that closer to 6 or 7 weeks, but at the professional level it is reasonably safe to expect that the best studied protocols are used.
So enough with the biology lesson - what we really want to know is when will Flaco be back? The mere fact that he was a game-time decision for the Houston match means that he is on that two week timeline. Barring re-injury in training (and the fact that we really missed him Wednesday night) there is a high likelihood that he’ll be back on the pitch Sunday. That said, he might be better suited as a sub, since 90 minutes of play a mere two weeks from the injury is a high risk proposition. A reinjury at this point would likely mean being out for the season, playoffs included (and I have faith they will be).