Muscle injury, also known as a muscle strain or pulled muscle, happens when muscle fibers are stretched too far or torn. This common injury can range from mild discomfort to severe pain and loss of function, affecting people during sports, exercise, work, or everyday activities.
Epidemiology
Muscle strains are among the most common injuries affecting both professional athletes and everyday people. They represent one of the most frequent types of soft tissue injuries, which are injuries to muscles, tendons, and other connective tissues rather than bones.[1][2]
In professional sports, muscle strains are a leading cause of missed playing time. They currently rank as the top reason for athletes missing games in the National Football League (NFL). However, you do not need to be a professional athlete to experience this type of injury—muscle strains are quite common among recreational athletes and people who engage in physical activities during their daily lives.[4]
Muscle tissue represents the largest mass in the human body, accounting for approximately 45% of total body weight. Given this substantial proportion, the potential for muscle injury is considerable across all age groups and activity levels.[3]
Causes
Muscle injuries occur when the force applied to a muscle becomes so great that the tissue sustains damage. This damage can happen in several locations: within the muscle itself, at the junction where the muscle meets the tendon (the most common site), or in the tendon where it attaches to bone.[4]
The way a muscle strain develops depends on whether it happens suddenly or gradually over time. Acute strains can be caused by a single event, such as using poor body mechanics to lift something heavy, sudden sprinting, twisting movements, or jumping. These injuries happen suddenly and cause immediate symptoms. In contrast, chronic muscle strains develop gradually from repetitive injuries when you stress a muscle by doing the same motion over and over without giving it adequate time to repair.[1][2]
To understand how muscle strains occur, it helps to know how muscles work. Your muscles are made of thousands of small fibers woven together, similar to a fabric. When you strain a muscle, these strands of fiber are stretched beyond their limit and tear apart. Think of it like an old bungee cord—new bungee cords have plenty of give and stretch, but if you use one for too long or suddenly jerk on it too hard, the fibers will start to pull apart.[2]
Muscle strain occurs when a muscle is stretched too much, too quickly, or works too hard. Often, a sudden or excessive forceful eccentric contraction causes the injury. An eccentric contraction occurs when a muscle lengthens while it contracts, which happens when the muscle is bearing a load. This type of contraction requires the muscle to stretch and lengthen while simultaneously working, which can cause overstretching that results in tearing of the muscle fibers.[5][2]
Risk Factors
Certain groups of people, behaviors, and circumstances increase the risk of experiencing a muscle strain. Participating in contact sports such as soccer, football, hockey, boxing, and wrestling can increase your risk significantly. Different sports place different parts of the body at greater risk—for example, sports involving running and kicking commonly affect the legs and ankles.[1]
Some muscles in the body are naturally more susceptible to strains than others. Muscles that cross more than one joint are at higher risk because they often act to restrain other muscles when you move that joint. If the joint moves with too much force, these muscles feel the impact first. The hamstrings (which cross the hip and knee joints), the calf muscles (which cross the knee and ankle joints), and the quadriceps (which cross the hip and knee joints) are especially vulnerable. The hip adductor muscles, commonly injured in groin strains, are also frequently affected.[2][4]
Several personal factors can predispose someone to muscle injury. Older age is a risk factor, as muscles may lose some of their flexibility and strength over time. Previous muscle injuries make you more vulnerable to re-injury in the same area. Less flexibility and lack of strength in a muscle increase strain risk, as does general fatigue. When muscles are tired, they cannot absorb energy as effectively, making them more prone to injury.[4]
Training habits play a crucial role in muscle strain risk. Overtraining—training or laboring too hard or too long without letting your muscles rest—weakens them. If muscles don’t have a chance to rebuild between sessions, they might break instead of getting stronger. Conversely, undertraining also poses risks. Low flexibility and strength can cause muscles to strain with ordinary use. Not stretching or warming up before exercise can overstress your muscles before they’re ready for activity.[2]
Muscle strains occur more commonly when muscles are not adequately adapted to the demands of the activity, such as when starting a new sport or experiencing significant increases in exercise load. This explains why muscle strains are much more common during training camps in the NFL than during the regular playing season, when athletes’ bodies have adapted to the demands.[4]
Symptoms
The signs and symptoms of a muscle strain vary depending on the severity of the injury. Common symptoms include pain or tenderness in the affected area, redness or bruising on the skin, limited motion in the injured muscle, muscle spasms, swelling, and muscle weakness.[1]
When you have a torn or strained muscle, it feels sore when you try to use it. You can usually locate the pain in one specific spot, and you may be able to connect it to a recent event or activity. If it’s a chronic muscle strain, you may develop pain gradually over a few days. If it’s an acute muscle strain, you’ll feel pain immediately, and it may even feel like tearing. You may also feel your muscle weaken, and you may feel that you can’t use it at all.[2]
In severe strains, you might experience or notice additional signs. You’ll most likely hear an audible pop and see a dent in the outline of the muscle. You’ll feel immediate pain, swelling, tenderness, and weakness, and you might eventually develop bruising. Some people even report feeling a “pop” during the injury or seeing a gap or dent in the shape of the muscle.[2][12]
It can be challenging to distinguish between simple muscle soreness from exercise and an actual muscle strain. Location provides one clue: if the pain is in a large area, such as your entire back, it’s probably due to sore muscles responding to new activity. If the pain is localized to one specific area, that’s more likely a strain. Healing time offers another clue: if the pain eases by day three, you probably just have sore muscles. If the pain intensity has increased by day three, you probably have a strained muscle.[12]
Muscle strains are currently classified based on severity into three grades. Grade I lesions are mild injuries with signs and symptoms of edema (swelling with fluid) and discomfort. Grade II injuries involve more extensive damage with loss of function, gaps in the muscle tissue, and possible ecchymosis (bruising). Grade III injuries represent complete rupture of the muscle or tendon, with severe pain and extensive hematoma (collection of blood outside blood vessels).[3]
More specifically, Grade 1 strains involve stretching and injury to the muscle fibers with minimal structural damage, characterized by fluid in the muscle visible on MRI scans. Grade 2 strains show more extensive damage with more muscle fibers involved, though the muscle is not completely ruptured; these injuries present with significant loss of strength and motion. Grade 3 strains involve complete rupture of a muscle or tendon and can present with a palpable defect in the muscle or tendon, though swelling in the area may make this difficult to detect.[4]
Prevention
Preventing muscle strains involves several strategies related to proper preparation, training, and body awareness. While the sources provided don’t extensively detail prevention strategies, they offer important clues about risk factors that suggest preventive approaches.
Proper warm-up and stretching before exercise are essential. Not warming up or stretching before physical activity can overstress your muscles before they’re ready, increasing injury risk. Taking time to prepare your muscles for activity helps them handle the demands placed on them.[2]
Gradual progression in training intensity and volume helps prevent muscle strains. Muscle strains occur more commonly when muscles are not adequately adapted to the demands of an activity, such as starting a new sport or experiencing significant increases in exercise load too quickly. Allowing your body time to adapt to new demands reduces injury risk.[4]
Adequate rest between training sessions allows muscles time to repair and rebuild. Overtraining—training or laboring too hard or too long without letting your muscles rest—weakens them. If muscles don’t have a chance to rebuild between sessions, they become more vulnerable to injury.[2]
Maintaining good flexibility and muscle strength through regular conditioning helps protect against strains. Low flexibility and strength can cause muscles to strain even with ordinary use, so building and maintaining these qualities provides protection.[2][4]
Pathophysiology
Understanding what happens inside the body during a muscle strain helps explain the symptoms and recovery process. Muscles have a complex structure that allows them to contract and produce movement, and this structure becomes damaged during a strain.
A muscle is surrounded by an outer sheath that allows it to move smoothly over surrounding tissues as it contracts. Inside this outer sheath are bundles of muscle fibers known as fascicles, which are further made up of smaller units called myofibrils. These myofibrils are composed of millions of microscopic units called sarcomeres that are responsible for muscle contraction.[4]
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, allowing us to perform all movements from running and jumping to walking and breathing.[4]
When excessive force is applied to a muscle, this organized structure breaks down. The injury can occur at different levels: within the muscle fibers themselves, at the junction between the muscle and tendon (the most common site), or in the tendon where it attaches to bone. During activities requiring explosive movements, such as pushing off during a sprint or changing directions in cutting sports, the force across the musculotendinous unit (the connected muscle and tendon) can be so great that tissues tear, either partially or completely.[4]
The damage can range from a few broken fibers in mild strains to complete tears through the entire muscle in severe injuries. In a Grade 1 strain, only a few fibers break and there is minimal structural damage, but fluid accumulates in the muscle causing swelling. In Grade 2 strains, more extensive damage occurs with more muscle fibers involved, though the muscle remains partially intact. In Grade 3 strains, the muscle or tendon completely ruptures, sometimes separating into two distinct pieces.[2][4]
These injuries commonly occur during excessive loading of the muscle, particularly when the muscle is contracting while simultaneously elongating—the eccentric contraction mentioned earlier. This type of activity places tremendous stress on the muscle fibers, stretching them beyond their capacity to maintain structural integrity.[4]