The Hamstrings’ Functional Actions
The hamstring muscles are multifunctional; they are powerful hip extensors second only to the gluteus maximus muscle. In addition they are knee flexors, medial and lateral rotators, and important stabilizers of the knee.
From a biomechanical perspective the hamstrings function in several ways depending on leg position:
- All of the hamstring muscles extend the thigh at the hip joint, flex the leg at the knee, and tilt the pelvis in a posterior direction.
- The semimembranosus and semitendinosus rotate the leg inward (medially rotate).
- The biceps femoris rotates the leg outward (laterally rotates).
- Note: The short head of the biceps femoris is not involved in hip motion directly because it does not cross the hip joint.
Some researchers have placed more emphasis on certain hamstring actions dismissing other actions as secondary or less important functions. For example, some researchers have shown that the hamstrings can act as knee flexors, but only in nonfunctional settings. Consequently, only the so-called primary tasks, such as extension, of the hamstrings are considered in training or rehabilitative routines.
Personally, I see the hamstrings as a multifunctional complex performing numerous tasks, full of neurological receptors that relay information to other muscles.
Running Biomechanics
When running, your hamstrings function as shock absorbers, force generators and stabilizers. Shock absorption is achieved through eccentric contraction of the hamstring muscles, which is an efficient way of absorbing the kinetic energy of foot impact. This mechanism of shock absorption can be extremely effective if the hamstrings are strong, flexible, and not compromised by scar tissue from previous injuries.
The hamstrings are also a key component of force generation because of their synergistic action with the gluteal muscles in hip extension.
The hamstrings act as a stabilizer when running in several ways. On foot strike, the hamstrings act as a dynamic stabilizer. They decelerate the forward movement of the shin bone (tibia) during knee extension. This is similar to the action of the anterior cruciate ligament (ACL), except that the ACL is a passive stabilizer.
After initial contact with the ground, the hamstrings lengthen. This also has the effect of stabilizing the knee. When we push off with the foot, the hamstrings contract (in conjunction with the quadriceps) to provide propulsion.
Hamstring Tears
After the initial tear of the hamstring muscle, there is usually some bleeding which may show up as a small to large bruise over the hamstrings. I personally had a huge bruise last year after doing some interval training for the Paris Marathon.
The bleeding from a hamstring tear is followed by an inflammatory response with an increase in cells called fibroblasts. Fibroblasts are cells that are involved in the synthesis of collagen, which is the structural framework for all tissues. Fibroblasts are also involved in wound healing and the formation of scar tissue.
As the inflammatory process of a hamstring injury begins to resolve, scar tissue is often formed. Scar tissue is a weak, inflexible, easily re-injured tissue that can decrease range of motion and create abnormal motion patterns. It is scar tissue that most likely accounts for the high rate of reoccurrence of hamstring injuries.
From a symptomatic perspective, most hamstring injuries seem to resolve with time. In reality these injuries often do not completely heal, but are the catalyst for a series of injuries due to residual scar tissue and compensations that this tissue creates.
Grading a Hamstring Injury
It is important to evaluate the degree of hamstring injury so that appropriate treatments can be implemented. Hamstring strains are graded from 1 to 3, 3 being the most severe.
Grade 1 Strain
- The injured person can still walk though with some degree of difficulty. There may be minor swelling, stiffness, and pain.
- In a grade 1 strain there is only minor tearing of the hamstring muscle. A common analogy is comparing a muscle to a piece of tissue paper. In a grade one injury there are only minor tears in the tissue.
- There should be only minor pain on hamstring resistance with no significant loss of strength.
Grade 2 Strain
- The injured person may have difficulty walking and could be in considerable pain. There may be swelling, and some degree of bruising.
- In a grade 2 strain there is moderate tearing of the muscle. To use the tissue analogy there would be a significant amount of tears in the tissue, compared to a grade one sprain.
- The patient may not be able to straighten their knee. There also may be considerable pain on hamstring resistance.
Grade 3 Strain
- A grade three strain is a severe or complete tearing of the muscle. To use the tissue analogy, the tissue could now be torn in two pieces. This type of strain may require surgical intervention to reattach the muscle.
- Any action that causes the hamstrings to contract will cause severe pain.
- Walking will be extremely difficult; the injured person will require crutches. There could be a complete loss of function.
In part three of Resolving Hamstring Injuries we will cover Imaging a Hamstring Tear, Treating a Hamstring Injury.
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