Muscles make up over half of the weight of a human body and they are required to make even the smallest of movements such as nodding your head or tapping your toe. If too much stretch is put through one of your muscles you may end up with a painful muscle strain. If the similar type of injury occurred to one of the ligaments in your body, it is termed a sprain.
Muscles are composed of many fibers bundled together; the bigger, more frequently used muscles have more fibers than the smaller, lesser-used ones. Among the muscles are voluntary and involuntary muscles. Voluntary, or striated muscles, are those that we move by choice (for example, the muscles in your arms and legs). Involuntary muscles, or smooth muscles, are the ones that move on their own (for example, the muscles that control your diaphragm and help you breathe). The muscles in your heart are called involuntary cardiac muscles.
Voluntary muscles are attached to bones by tendons, a sinewy type of tissue. The area where the muscle attaches to the tendon is called the musculotendinous junction.
A muscle strain, or a muscle pull occurs when a muscle in your body is overstretched or overworked. Even if the injury from overstretching or overworking occurs more to the attaching tendon it can also be classified under the term muscle strain. A muscle strain can occur in any of your voluntary muscles (or tendons which attach to the muscle), but they are most common in the low back, the calves, the front and back of the thighs, the pectoral muscles, and the muscles of the neck and the shoulder. Muscle strains occur more often in muscles that cross two joints (such as the thigh or calf muscles) and often occur when the muscles are working eccentrically (working while under a stretch). Most often a strain occurs at the musculotendinous junction but can occur anywhere along the muscle.
A muscle strain can occur due to a one-time overstretching or overworking of a muscle (acute injury) or can occur from repetitive use of a muscle over time (overuse injury).
There are several classification systems developed and in use regarding muscle strains but the most commonly used system includes three grades. All muscle strains include tearing of some muscle fibers:
Grade I (mild): Very few muscle fibers have been injured. Pain may not be felt until the following day after the instigating activity. Strength and range of motion of the muscle remains full but pain can be felt when engaging the muscle often when it is at its end range of stretching. No swelling or bruising is noted.
Grade II (moderate): A large category including all strains between a grade I and grade III. Being that this category is so large, it is sometimes further divided into having a mild, moderate or severe grade II strain. With this category many muscle fibers are torn which results in a decrease in strength and often a limited range of motion. Some muscle fibers remain uninjured and intact. Pain is present both when stretching the muscle and on muscle strength testing. Swelling and bruising may be noted.
Grade III (severe): All fibers of the muscle are completely torn. This means that the muscle is completely torn into two parts or the muscle belly has torn from its attachment to the tendon. Severe swelling, pain, and bruising accompany a grade III strain. There is generally limited ability to generate any force on strength testing of the muscle due to the tear (however other muscles may compensate to initiate some strength) and the range of motion is either severely limited due to pain, or the range of motion testing may show excessive range due to the torn muscle not providing any limitation as it is stretched.
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