The hamstrings group consists of the biceps femoris, semimembranosus, and semitendinosus muscles which all have biarticular function to extend the hip and flex the knee. From the toe-off phase of the contralateral foot through terminal swing of theipsilateral lower extremity during the double float phase in the running gait cycle, the hamstrings act eccentrically to control knee extension while actingconcentrically to extend the hip between terminal swing and initial contact(Sugiura et al., 2008). Because faster running requires greater eccentric demand of the hamstrings, it is oft en associated with higher injury rates in sprinters and sport athletes that require quick bouts of acceleration.
The greatest musculotendon stretch occurs in the biceps femoris, the most lateral hamstring muscle, which has a tendency to be injured more frequently than the other two hamstring muscles. An intramuscular tendon is also injured more commonly with high-speed running due to the deceleration forces of the moving limb whereas a proximal free tendon strain occurs more frequently with fast movements where the hamstrings are fully stretched into hip flexion and knee extension,commonplace in dance and gymnastics movements. An intramuscular tendon lesion usually heals in a shorter period of time in comparison to proximal free tendon injuries (Heiderscheit et al., 2010).
In order to accurately examine for hamstring injuries, hamstring strength should be assessed in prone with the hip stabilized at 0 degrees of hip extension at both 15 and 90 degrees of knee flexion with resisted manual muscle testing. Internal or external rotation can also be applied to bias the medial and lateral hamstrings to better identify the source of the injury. In addition to knee flexion strength,hip extension strength should also be assessed due to the biarticular nature of the hamstrings, which can be done in prone with the knee flexed to 0 and 90 degrees. Range of motion should also be assessed through passive straight leg raise, where the hip should be able to flex to at least 80 degrees, and active knee extension, where the knee should be able to extend to 20 degrees for extensibility to be within normal limits. Palpation of the location of the lesion can also be helpful in gauging the length of the recovery process, with injuries more proximal to the ischial tuberosity requiring a greater length of time to return to preinjury status (Heiderscheit et al., 2010).
While examining lower extremity strength in elite sprinters, Sugiura and colleagues (2008) found no significant differences in quadriceps strength between injured and non injured athletes but measured significantly lower peak torques for the hamstrings working eccentrically as knee flexors and concentrically as hip extensors in sprinters with hamstring injuries. The injuries had to be severe enough to cause the athletes to miss at least one week of training or competition, and were characterized by local tenderness, pain and decreased range of motion with straight leg raise, and pain and decreased strength with resisted knee flexion in prone. The researchers also found significantly smaller knee flexor-quadriceps and hip extensor-quadriceps strength ratios in the involved limbs compared to the non involved limbs in injured sprinters,suggesting an association between hamstring injuries and imbalance between anterior and posterior leg musculature.
Proper rehabilitation post injury is paramount as one-third of hamstring injuries reoccur with greatest risk of re-injury within the first 2 weeks of return to sport (Heiderscheit et al., 2010). The hamstring muscles must be equipped to handle the high eccentric loading that occurs during running as they must lengthen and absorb energy while the leg decelerates to prepare for foot contact with the ground. Lack of hamstring flexibility, strength imbalance between the hamstring and quadriceps, and decreased motor control and coordination of the pelvic and trunk muscles have all been identified as modifiable risk factors for hamstring injury (Heiderscheit et al., 2010).
Research supports successful outcomes when rehabilitation includes strengthening of the hamstrings as both knee flexors and hip extensors, as well as eccentric training, such as the Nordic hamstring curl or Romanian dead lift either performed bilaterally or unilaterally, in order to reduce the incidence of hamstring injuries (Sugiura et al., 2008). Neuromuscular retraining of the lumbopelvic region is also important for preventing injury and improving rehabpotential as muscle activity within the trunk and pelvic core occurs first and provides a stable base of support before limb movement occurs. Deficits in proximal core musculature can lead to increased compressive forces within joints of the lower extremities and subsequent injury. For a comprehensive rehab program, core training should focus on maintaining control throughout frontal, sagittal, and transverse planes to provide three dimensional stability(Wilson et al., 2005).
Kristen Gasnick, SPT
Heiderscheit, BC, Sherry, MA, Silder, A,Chumanov, ES, & Thelen, DG. (2010). Hamstring Strain Injuries:Recommendations for Diagnosis, Rehabilitation, and Injury Prevention. Journalof Orthopedic and Sports Physical Therapy;40(2):67-81.
Sugiura, Y, Saito, T, Sakuraba, K, Sakuma, K, & Suzuki, E. (2008). StrengthDeficits Identified With Concentric Action of the Hip Extensors and EccentricAction of the Hamstrings Predispose to Hamstring Injury in Elite Sprinters.Journal of Orthopedic and Sports Physical Therapy;38(8):457-464.
Willson, JD, Dougherty, CP, Ireland, ML, & McClay-Davis, I. (2005). CoreStability and Its Relationship to Lower Extremity Function and Injury. Journalof the American Academy of Orthopaedic Surgeons;13(5):316-325.