Hamstring strains can be one of the most frustrating athletic injuries to deal with. Recovery time of hamstring strains ranges from 8 to 25 days . Once a hamstring is injured, there is a 33.33% risk of re-injury. The greatest risk of re-injury occurs during the first two weeks after returning to sport (Heiderscheit BC,. et al 2005).
Hamstring injuries are especially prevalent in runners, soccer and football players. In a 10 year study conducted by the NFL, hamstring strains were second in prevalence to only knee injuries.
Anatomy of the Hamstrings
The hamstrings are made up of three muscles. Two of which are on the inside and one on the outside of the back of the thigh. The semimembranosus lies underneath the semitendinosus on the inside. While the biceps femoris, the longest of the three hamstring muscles is on the outside of the back of the thigh.
Signs and Symptoms of Hamstring Strains
Hamstrings strains usually present with is a sudden onset of posterior thigh pain. It is also common for athletes to report an audible pop with the onset of pain. Athletes may also report pain in the buttock region while sitting (Heiderscheit BC,. et al 2005).
Predisposing Factors of Hamstring Strains
The fact that there is such a high re-injury rate with hamstring injuries makes a history of previous hamstring strains a major risk factor for future injuries. Other predisposing factors include a strength imbalance between the hamstrings and the quadriceps. Research has found if the quadriceps are 20% stronger (concentric) than the hamstrings (eccentric) there is a four fold increased risk of hamstring injury.
Fatigue is also major predisposing factor of hamstring strains, as the majority of hamstring injuries occur at the end of matches or training sessions (Heiderscheit BC,. Et al 2005).
Studies have also shown that individuals with decreased hamstring strength and length have a greater chance of injury. Whereas, hamstrings that are lengthened and strengthened tend to have the lowest risk of injury (Opar et al, 2016)
Classification of Hamstring Injuries
Hamstring injuries are graded mild, moderate or severe. The grading reflects the extent of muscle or tendon damage. A mild or grade I injury consists of minimal damage, while a grade III injury represents a complete tear. Severe injuries tend to present with greater weakness, bruising, and loss of motion (Heiderscheit BC,. et al 2005).
Advanced imaging (MRI) has shown that hamstring injuries usually occur close to the buttocks. This is because the proximal end of the biceps femoris and semitendinosus share a common tendon insertion into the ischial tuberosity (Heiderscheit BC,. et al 2005).
Biomechanics of Hamstring Strains
The majority of hamstring strains occur while running. Video analysis has shown that injury occurs when a runner is accelerating to, or maintaining their maximal speed. Furthermore, hamstring strains tend to occur the most during the late swing and early stance phase of running (Schache, A, et al, 2009). This is occurs because the hamstring muscle is being lengthened (eccentrically) at both ends of its attachment sites – flexion of the hip and extension of the knee.
Lengthening of the hamstrings (eccentrically) during running corresponds with the activation of the hamstrings. Electrical activation (EMG) of the hamstrings (biceps femoris) is greatest when an athlete is increasing their speed from a slow jog to their maximum.
The biceps femoris is the longest of the three hamstring muscles. As a result it experiences the greatest amount of strain (eccentric loading) and electrical activation. For these reasons, the biceps femoris is the most commonly injured muscle of the hamstrings.
Hamstring Strains and Low Back Pain
Individuals with previous hamstring injuries tend to run with an increased curve of the low back (lumbar spine lordosis) and internal rotation of femur/tibia. Because the hamstrings originate on the ischial tuberosity of the pelvis, individuals with previous hamstring injuries also present with increased extension of the pelvis and hip.
Research has found that after a hamstring injury, flexion of the hip (peak swing ROM) decreases from 92.6 degrees to 35.5. In addition, post-injury peak knee power (eccentric) of the hamstrings decreases by 87% (Schache, et al,).
This article is a two part series on the hamstrings. Part two will look at hamstring rehab exercises and stretches.
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