Muscle Injuries: An Overview

Lawrence V. Gulotta, MD
2009-2010 Sports Medicine Fellow
Hospital for Special Surgery


Overview

Muscle injuries are commonly referred to as “strains” or “pulls.” They are one of the most common reasons for missed playing time in the NFL. But you do not have to be a professional football player to have such an injury. In fact, muscle injuries affect thousands of recreational athletes every year.

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 can be so great that the tissue partially or completely tears.

These injuries commonly occur during eccentric 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 season.

Fortunately, most injuries do not require surgery and a full recovery is expected in most cases.

Normal Muscle Anatomy and Function

Muscles are 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 myosin pull against thin ropes of protein called actin when 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 and is what allows athletes to run, kick, throw, and even walk and breathe, for that matter. 

Muscle Damage and the Body’s Response

Muscle injuries occur when the force in the muscle is so great that the tissue begins to tear. This can occur within the muscle itself, where the tendon attaches to the bone, or most commonly, at the junction between the muscle and tendon.

When muscle is initially injured, there is significant inflammation and swelling that occurs. Following the inflammatory phase, 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 never fully regenerates. This is thought to make muscle prone to a subsequent injury.

Diagnosis

The diagnosis is usually made based on history and physical exam. Patients will report pain in the area of injury that is worse with contraction of that muscle. They may also report sudden pain or the sensation that they have been stabbed. On examination, there is usually swelling and bruising. In severe cases, the examining physician may feel the defect where the muscle had completely torn.

Patients often lose some strength and range of motion with a muscle injury. The severity of the injury can be assessed by how much strength and range of motion they lose, and this can provide an idea as to how long it will take to recover. Muscle injuries can be categorized into three grades, as follows:

Grade 1: Mild damage to individual muscle fibers (less than 5% of fibers) that causes minimal loss of strength and motion. These injuries generally take about 2-3 weeks to improve.

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 2-3 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.  

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. MRI’s can also show collections of blood, called a hematoma, that sometimes occur following severe injuries. 

Treatment and Recovery

The majority of acute muscle injuries are partial thickness tears. These can most often be treated successfully with rest, ice, compression, elevation (also known as RICE), and nonsteroidal anti-inflammatory drugs (NSAIDs), such as ibuprofen. This will be done for the first week, followed by progressive functional physical therapy, as needed.

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 re-injuring the muscle and sustaining a setback. Mild, grade 1 injuries may require only 2-3 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.

Surgery, Platelet-Rich Plasma, and Losartan

Complete muscle injuries can lead to significant functional impairment and lost playing time and may require surgery. This is especially the case for patients who need to be able to run or be sufficiently agile to participate in sport.

There are several new therapies that are currently in the investigational phase. Platelet-rich plasma (or PRP) requires blood to be drawn from the patient which is then spun in a centrifuge in order to concentrate the platelets which are then injected into the site of the injury. These platelets contain several growth factors that may stimulate healing and muscle regeneration, and limit the amount of scar tissue that forms. There are currently no reliable scientific studies that show if this therapy works.

Other drugs, such as the anti-hypertensive medication losartan, have been shown to reduce scar tissue formation and improve healing in experimental animal models(2). Clinical trials in humans are just getting started. 

Conclusions

Muscle injuries are very common in both professional and recreational athletes. They typically occur in muscles that cross two joints, during explosive activities such as sprinting, and when the athlete is suddenly increasing their exercise regimen (i.e., training camp).

These injuries result in pain, swelling, and loss of strength and motion. 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.

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.

1. Levine WN, Bergfeld JA, Tessendorf W, Moorman CT 3rd. Intramuscular corticosteroid injection for hamstring injuries. A 13-year experience in the National Football League. Am J Sports Med. 2000 May-Jun;28(3):297-300.

2. Bedair HS, Karthikeyan T, Quintero A, Li Y, Huard J. Antiotensin II receptor blockade administered after injury improves muscle regeneration and decreases fibrosis in normal skeletal muscle. Am J Sports Med. 2008 Aug;36(8): 1548-54.

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