Grade I | No appreciable tissue tearing, no loss of function or strength, only a low-grade inflammatory response | Tear of a few muscle fibres, fascia remaining intact | No abnormalities or diffuse bleeding with/without focal fibre rupture less than 5% of the muscle involved | MRI-negative=0% structural damage. Hyperintense oedema with or without hemorrhage |
Grade II | Tissue damage, strength of the musculotendinous unit reduced, some residual function | Tear of a moderate number of fibres, fascia remaining intact | Partial rupture: focal fibre rupture more than 5% of the muscle involved with/without fascial injury | MRI-positive with tearing up to 50% of the muscle fibres. Possible hyperintense focal defect and partial retraction of muscle fibres |
Grade III | Complete tear of musculotendinous unit, complete loss of function | Tear of many fibres with partial tearing of the fascia | Complete muscle rupture with retraction, fascial injury | Muscle rupture=100% structural damage. Complete tearing with or without muscle retraction |
Grade IV | X | Complete tear of the muscle and fascia of the muscle–tendon unit | X | X |