Muscle strain or a "pulled muscle" is a partial or complete tear of a muscle.

These injuries typically occur:

• in muscles that cross two joints

• during explosive action, such as sprinting

• during times when athletes suddenly increasing their exercise regimens (such as during training camps)

A muscle is surrounded by an outer sheath that allows it to move smoothly over the surrounding tissues as it contracts. Inside the outer sheath are bundles of muscle fibers known as fascicles, which are further made up of myofibrils. These myofibrils are composed of millions of microscopic units called sarcomeres that are responsible for muscle contraction. In the sarcomere, muscle proteins called myosins pull against thin ropes of protein called actin when they are stimulated by nerves. When this occurs, the sarcomeres shorten, resulting in a contraction. When the myosin proteins relax, the sarcomeres lengthen back to their original position and so does the muscle.

The combination of muscle contraction and relaxation is coordinated through the nervous system. This is what allows athletes to run, kick, throw, and, for that matter, even walk and breathe.

Muscle strains occur when the force on a muscle is so great that the tissue begins to tear. The tear can occur in one of three places

• within the muscle itself

• at the junction between the muscle and tendon (most common)

• in the tendon, where it attaches to the bone

Muscles, by way of their tendon attachments at each end, insert into bones and provide the force required for movement. During activities that require explosive movements, such as pushing off during a sprint or changing directions during racquetball, the force across the musculotendinous unit (the connected muscle and tendon) can be so great that tissues tear, either partially or completely.

These injuries commonly occur during excessive loading of the muscle; that is, when the muscle is contracting while it is elongating. Muscles that cross two joints, such as the hamstrings (the hip and knee joints), the calf (the knee and ankle joints), and the quadriceps (the hip and knee joints) are the most susceptible to injury. The hip adductor muscles are also commonly affected, though they only cross the hip joint.

Factors that can predispose an athlete to injury include older age, previous muscle injury, less flexibility, lack of strength in the muscle, and fatigue. Many athletes sustain muscle injuries when they just begin a training regimen. That is why they are much more common in training camps in the NFL than they are throughout the regular playing season.

The severity of a strain can be assessed by how much strength and range of motion a person loses, and this can also provide an idea as to how long it will take to recover. Muscle strains can be categorized into three grades, based on severity:

• Grade 1: Mild damage to individual muscle fibers (less than 5% of fibers) that causes minimal loss of strength and motion.

• Grade 2: More extensive damage with more muscle fibers involved. However, the muscle is not completely ruptured. These injuries present with significant loss of strength and motion. These injuries may require two to three months before a complete return to athletics.

• Grade 3: Complete rupture of a muscle or tendon. These can present with a palpable defect in the muscle or tendon. However, swelling in the area may make this difficult to appreciate. These injuries sometimes require surgery to reattach the damaged muscle and tendon.

• sudden pain that worsens while contracting the strained muscle

• swelling and bruising

• loss of strength and range of motion

When muscle is initially injured, significant inflammation and swelling occurs. Patients often report the sensation of pain as the feeling of being "stabbed."

After this inflammatory phase, the muscle begins to heal by regenerating muscle fibers from stem cells that live around the area of injury. However, a significant amount of scar tissue also forms where the muscle was injured. Over time, this scar tissue remodels, but the muscle tissue never fully regenerates. It is thought that this makes a strained muscle prone to future injury.

The diagnosis is usually made based on patient history and physical exam. In severe, grade 3 cases, the examining physician may actually be able to feel the defect where the muscle has completely torn.

An X-ray may be helpful to rule out a fracture or dislocation as the cause of pain. Occasionally in young athletes, the tendon can pull off a piece of bone where it attaches, which can be seen on X-rays.

However, pure muscle injuries cannot be seen on regular X-rays. An MRI can sometimes be helpful to determine where the injury has occurred and whether there is complete rupture or not. MRIs can also show collections of blood, called a hematoma, that sometimes occur following severe injuries.

Most muscle strains do not require surgery, and a full recovery is expected.

If there is a partial tear then the athlete can return when they are pain free and have normal strength and motion. This usually occurs following anywhere from a few weeks to a few months of appropriate treatment and therapy. When the muscle is completely ruptured, the athlete may benefit from surgical repair.

The majority of acute muscle injuries are partial thickness tears. These can most often be treated successfully with:

• the RICE protocol:

  • rest

  • ice

  • compression

  • elevation

• NSAIDs - nonsteroidal anti-inflammatory drugs (such as ibuprofen)

These treatements will be done for the first week, followed by progressive functional physical therapy, as needed.

Many athletes are able to return to their previous level of competition, but since scar tissue forms at the site of injury, they may susceptible to another injury at that location.

Surgery and platelet-rich plasma injections

Complete muscle injuries can lead to significant functional impairment and lost playing time and may require surgical repair. This is especially the case for patients who need to be able to run or be sufficiently agile to participate in sport. Platelet-rich plasma (PRP) injections may be able to help accelerate the regeneration of damaged muscle tissue.

Return to full activity is usually allowed when the patient is pain free, has full range of motion, and full strength. If an athlete attempts to return to their sport before these criteria are met, there is a high chance of reinjuring the muscle and sustaining a setback. Mild, grade 1 injuries may require only two to three weeks before an athlete can return. More severe injuries may require significantly more time.

Such a long period of lost playing time is less than ideal for professional and elite athletes, and some have advocated more aggressive treatment in this group. In a study that examined professional football players with severe hamstring tears with palpable defects, an intramuscular cortisone injection lead to a return to full activity time of 7.6 days, and 85% of the players did not miss a single game.1 However, the use of cortisone injections in the recreational athlete should be reserved for chronic or lingering injuries since there is a chance of weakening the remaining muscle and increasing the severity of the injury.