A pulled hamstring or strain is an injury to one or more of the muscles at the back of the thigh. Hamstring muscle injuries — such as a "pulled hamstring" — occur frequently in athletes, and are especially common in athletes who participate in sports that require sprinting, such as track, soccer, and basketball.  Most hamstring injuries respond well to simple, nonsurgical treatments.

Anatomy

The hamstring muscles run down the back of the thigh. There are three hamstring muscles:        

• Semitendinosus
• Semimembranosus
• Biceps femoris

They start at the bottom of the pelvis at a place called the ischial tuberosity. They cross the knee joint and end at the lower leg. Hamstring muscle fibres join with the tough, connective tissue of the hamstring tendons near the points where the tendons attach to bones.  The hamstring muscle group helps you extend your leg (hip) straight back and to bend your knee.

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Normal hamstring anatomy. The three hamstring muscles start at the bottom of the pelvis and end near the top of the lower leg.

Description

A hamstring strain can be a pull, a partial tear, or a complete tear. Muscle strains are graded according to their severity. A grade 1 strain is mild and usually heals readily; a grade 3 strain is a complete tear of the muscle that may take months to heal.

Most hamstring injuries occur in the thick, central part of the muscle or where the muscle fibres join tendon fibres.

In the most severe hamstring injuries, the tendon tears completely away from the bone, or it may delaminate of its bony insertion, essentially making it dysfunctional. It may even pull a piece of bone away with it. This is called an avulsion injury.

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A severe hamstring injury where the tendon has been torn from the bone.

Cause

Muscle Overload

Muscle overload is the main cause of a hamstring muscle strain. This can happen when the muscle is stretched beyond its capacity or challenged with a sudden load.Hamstring muscle strains often occur when the muscle lengthens or as it contracts and shortens. Although it sounds contradictory, this happens when you extend a muscle while it is weighted, or loaded. This is called an "eccentric contraction."  

During sprinting, the hamstring muscles contract eccentrically as the back leg is straightened and the toes are used to push off and move forward. The hamstring muscles are not only lengthened at this point in the stride, but they are also loaded — with body weight as well as the force required for forward motion.

Like strains, hamstring tendon avulsions are also caused by large, sudden loads.

Risk Factors

Several factors can make it more likely you will have a muscle strain, including:

• Muscle tightness - Tight muscles are vulnerable to strain. Athletes should follow a year-round program of daily stretching exercises.
• Muscle imbalance - When one muscle group is much stronger than its opposing muscle group, the imbalance can lead to a strain. This frequently happens with the hamstring muscles. The quadriceps muscles at the front of the thigh are usually more powerful. During high-speed activities, the hamstring may become fatigued faster than the quadriceps. This fatigue can lead to a strain. Hip flexor weakness is often also a feature of this imbalance.
• Poor conditioning - If your muscles are weak, they are less able to cope with the stress of exercise and are more likely to be injured.
• Muscle fatigue - Fatigue reduces the energy-absorbing capabilities of muscle, making them more susceptible to injury.
• Choice of activity - Anyone can experience hamstring strain, but those especially at risk are:
  * Athletes who participate in sports like football, soccer, basketball
  * Runners or sprinters
  * Dancers
  * Older athletes whose exercise program is primarily walking
  * Adolescent athletes who are still growing - Hamstring strains occur more often in adolescents because bones and muscles do not grow at the same rate. During a growth spurt, a child's bones may grow faster than the muscles. The growing bone pulls the muscle tight. A sudden jump, stretch, or impact can tear the muscle away from its connection to the bone.

Symptoms

If you strain your hamstring while sprinting in full stride, you will notice a sudden, sharp pain in the back of your thigh. It will cause you to come to a quick stop, and either hop on your good leg or fall.  Additional symptoms may include:

• Swelling during the first few hours after injury
• Bruising or discoloration of the back of your leg below the knee over the first few days
• Weakness in your hamstring that can persist for weeks

Diagnosis

People with hamstring strains often see a doctor (or physiotherapist) because of a sudden pain in the back of the thigh that occurred when exercising.  A history of the injury and examination will be undertaken.  If more significant injury, such as a tendon avulsion is suspected, imaging studies may be ordered, including X-rays and MRI scans.

Treatment

Treatment of hamstring strains will vary depending on the type of injury you have, its severity, and your own needs and expectations. The goal of any treatment — nonsurgical or surgical — is to help you return to all the activities you enjoy. Following a treatment plan based on a clear diagnosis will restore your abilities faster, and help you prevent further problems in the future.

Nonsurgical Treatment

Most hamstring strains heal very well with simple, nonsurgical treatment:

1. RICE. The RICE protocol is effective for most sports-related injuries. RICE stands for Rest, Ice, Compression, and Elevation.
   * Rest. Take a break from the activity that caused the strain. Your doctor may recommend that you use crutches to avoid putting weight on your leg.
   * Ice. Use cold packs for 20 minutes at a time, several times a day. Do not apply ice directly to the skin.
   * Compression. To prevent additional swelling and blood loss, wear an elastic compression bandage or Skins.
   * Elevation. To reduce swelling, recline and put your leg up higher than your heart while resting.
2. Immobilization. Occasionally a knee splint is worn for a brief time, and this will keep your leg in a neutral position to help it heal.
3. Physical therapy. Once the initial pain and swelling has settled down, physical therapy can begin. Specific exercises can restore range of motion and strength. A therapy program focuses first on flexibility. Gentle stretches will improve your range of motion. As healing progresses, strengthening exercises will gradually be added to your program. A safe return to sports activity will subsequently be discussed.
4. Platelet-rich plasma (PRP) is a preparation developed from a patient's own blood. It contains a high concentration of proteins called growth factors that are very important in the healing of injuries. PRP injections may be offered for incomplete proximal hamstring tendon tears in some patients. 

Surgical Treatment

Surgery is most often performed for tendon avulsion injuries, where the tendon has pulled completely away from the bone. Tears from the pelvis (proximal tendon avulsions) are more common than tears from the shinbone (distal tendon avulsions).

Surgery may also be needed to repair a complete tear within the muscle.

Procedure - To repair a tendon avulsion, the hamstring must be freed from any scar tissue and then mobilised so it can be advanced back to the bone origin.  The tendon is reattached to the bone using bone anchors and large stitches or staples.

Rehabilitation - After surgery, you may be advised to keep weight off of your leg to protect the repair. Crutches may be needed for a short period.  The physical therapy program will begin with gentle stretches to improve flexibility and range of motion. Strengthening exercises will gradually be added to your plan.  Rehabilitation for a proximal hamstring reattachment typically takes at least 3-6 months, due to the severity of the injury. Distal hamstring reattachments require approximately 3 months of rehabilitation before returning to athletic activities. You will be advised when it is safe to return to sports.

Hamstring Syndrome

Hamstring syndrome occurs when there is a combination of more chronic proximal hamstring tendon tearing/degeneration, with secondary inflammation and scarring in the covering of the tendon (tenosynovitis).  This inflamed tenosynovial covering may form fibrous bands which tether or pull on the adjacent sciatic nerve.

These patients present not only with posterior proximal thigh and buttock pain but may also have sciatica-like nerve symptoms radiating down the leg.

Management is usually initial conservative (non-surgical), but if this fails, interventions will be necessary.  This may range from injections of steroid or hydrodilitation (fluid is injected in and around the tenosynovial sheath), and if these measures give no benefit, surgery is utilised.

Surgery involves repairing or reinforcing the proximal hamstring tendon origin at the pelvis, combined with removal of the inflamed tenosynovial tendon sheath, and a formal sciatic neurolysis (the adjacent sciatic nerve is mobilised free from the fibrous bands tethering it to the tendon sheath – occasionally an anti-adhesion gel is also added to prevent further scarring).