Hamstring injuries are the most common injury in sports1 and have been shown to occur 176 times per NFL season or 5-6 per team each season.2 The most common mechanism of hamstring injury in the NFL was non-contact sprinting which caused 68.2 percent of hamstring strains. Hamstring re-injury is fairly common and has been shown to occur in approximately 16 percent of hamstring injuries.2,3
Re-injuries can occur due to a multitude of reasons, but they obviously indicate the hamstring had not healed enough to allow the athlete to return to full play.
The hamstring is composed of three muscles that begin at the hip (sit bone) and end at the knee. Two of these muscles, the semimembranosus and semitendinosus, are medial or inside of the knee and the other; the biceps femoris is lateral or on the outside of the knee. The biceps femoris muscle is the most commonly strained hamstring muscle, occurring in up to 83% of hamstring injuries,3 in part due to movement dysfunction as this muscle is overactive and very tight in many individuals. The same study by Eckstrand et al. found that allhamstring re-injuriesoccurred with the biceps femoris (outside) muscle.
This research certainly does not indicate that a re-injury cannot occur with a medial hamstring strain, but it does indicate that more caution should occur when treating a biceps femoris (outside) strain. This research study also showed that re-injuries did not take longer to return to play than initial injuries; however the re-injury does re-start the entire injury process—meaning the player will miss an extended period of time overall.
Although most muscle strains do not require diagnostic testing, MRI and ultrasound are commonly performed on muscle strains when elite athletes sustain an injury. This assists the medical team in creating a guideline of when the individual may be able to resume activity and return to play. Medical personnel do not treat MRIs or ultrasounds; however, they treat people. Each person will recover differently and the MRI can only be used as a tool to help guide the process.
MRIs have been shown to be predictive of return to play in the NFL, with more mild injuries recovering sooner than more severe injuries. The size (or amount of area) of the muscle strain is also indicative of return to play.4
Once a muscle is injured it undergoes an inflammatory and healing period beginning immediately after the injury and lasting days, weeks, months, and even years after the initial injury. Scar tissue replaces the original muscle tissue permanently altering the strength and flexibility of the muscle. Scar tissue in muscle is similar to what occurs when we scar following a cut or laceration; the laceration heals, but the scar always looks and moves differently than the original skin. MRIs performed on sprinters following a moderate hamstring strain have shown that 20-55 percent of the original injury has not healed 6 weeks later.5
Rehabilitation following a muscle injury is not an exact science, thus some players (16 percent) will encounter a second injury.2,3 In sports medicine, the medical staff is often faced with the difficult decision of when a player can safely and quickly return to a game. If the medical staff waited for full recovery following a muscle strain, most players would not return until the following season. There are various tests available that check a player's functional strength to assist in the return to play decision. Check back for my next post looking at functional tests following hamstring injuries.
1) Ahmad CS, Redler LH, Ciccotti MG, Maffulli N, Longo UG, Bradley J. Evaluation and Management of Hamstring Injuries. Am J Sports Med. 2013;
2) Elliott MC, Zarins B, Powell JW, Kenyon CD. Hamstring muscle strains in professional football players: a 10-year review. Am J Sports Med. 2011;39(4):843-50.
3) Ekstrand J, Healy JC, Waldén M, Lee JC, English B, Hägglund M. Hamstring muscle injuries in professional football: the correlation of MRI findings with return to play. Br J Sports Med. 2012;46(2):112-7.
4) Cohen SB, Towers JD, Zoga A, et al. Hamstring injuries in professional football players: magnetic resonance imaging correlation with return to play. Sports Health. 2011;3(5):423-30.
5) Askling CM, Tengvar M, Saartok T, Thorstensson A. Acute first-time hamstring strains during high-speed running: a longitudinal study including clinical and magnetic resonance imaging findings. Am J Sports Med. 2007;35(2):197-206.