A strain to the muscle or muscle tendon is the equivalent of a sprain to ligaments. A muscle strain occurs when muscle fibres cannot cope with the demands placed on them by exercise overload and leads to tearing of the fibres. It is a contraction-induced injury in which muscle fibres tear due to extensive mechanical stress. This mostly occurs as a result of a powerful eccentric contraction or over-stretching of the muscle. Therefore, it is typical for non-contact sports with dynamic
characteristics such as sprinting and jumping.

Muscle lesions are classified as Grade I, II, and III based on the number of fibres disrupted according to clinical and imaging investigations.

Grade I: (mild) strains affect only a limited number of fibres in the muscle. There is no decrease in strength and there is a fully active and passive range of motion. Pain and tenderness are often delayed to the next day.

Grade II: (moderate) strains have nearly half of muscle fibres torn. Acute and significant pain is accompanied by swelling and a minor decrease in muscle strength.

Grade III: (severe) strains represent the complete rupture of the muscle. This means either the tendon is separated from the muscle belly or the muscle belly is actually torn in 2 parts. Severe swelling and pain and a complete loss of function are characteristic of this type of strain.