Definition of Strain in Medical Terms
A strain is a stretching or tearing injury to a muscle-tendon unit (musculotendinous unit) that results from excessive stretch or stretch while the muscle is being activated, typically occurring near the muscle-tendon junction. 1, 2
Anatomical Location and Mechanism
- Strain injuries specifically affect the musculotendinous unit—the combined structure of muscle tissue and its tendon attachment—rather than ligaments (which connect bone to bone). 1
- The damage localizes very near the muscle-tendon junction, not within the muscle belly itself. 2
- Strains result from excessive stretch or stretch during muscle activation, not from muscle contraction alone. 2
- These injuries commonly occur during eccentric contractions (when the muscle lengthens while contracting), often affecting muscles that cross two joints and contain high percentages of fast-twitch fibers. 3, 2
Classification System
Strains are graded by severity into three degrees: 1
- First-degree strain: Minimal stretching of the musculotendinous unit without permanent structural damage
- Second-degree strain: Partial tearing of the musculotendinous unit with some fiber disruption
- Third-degree strain: Complete disruption of a portion of the musculotendinous unit
Clinical Presentation
- Strains cause acute pain that occurs during strenuous activity, accompanied by swelling, bleeding, and localized discomfort. 1, 3
- The degree of disability depends on the injury location, severity, and the patient's functional demands. 1
- After injury, the affected muscle becomes weaker and remains at increased risk for reinjury during the recovery phase. 2
Distinction from Sprains
A critical distinction: strains affect muscle-tendon units, while sprains affect ligaments. 4 The American Heart Association guidelines specifically differentiate these injuries, noting that both benefit from similar initial management (cold application, compression, activity restriction) but represent fundamentally different tissue injuries. 4
Commonly Affected Muscles
- Muscles frequently strained include the hamstring, rectus femoris, gastrocnemius, and adductor longus. 2
- These muscles share characteristics: they cross multiple joints, have complex architecture, and are susceptible to injury during activities involving acceleration, repetitive movements, and pivoting. 4, 2