Table 1

Overview of previous muscle injury classification systems

O'Donoghue 1962Ryan 1969 (initially for quadriceps)Takebayashi 1995, Peetrons 2002 (Ultrasound-based)Stoller 2007 (MRI-based)
Grade INo appreciable tissue tearing, no loss of function or strength, only a low-grade inflammatory responseTear of a few muscle fibres, fascia remaining intactNo abnormalities or diffuse bleeding with/without focal fibre rupture less than 5% of the muscle involvedMRI-negative=0% structural damage. Hyperintense oedema with or without hemorrhage
Grade IITissue damage, strength of the musculotendinous unit reduced, some residual functionTear of a moderate number of fibres, fascia remaining intactPartial rupture: focal fibre rupture more than 5% of the muscle involved with/without fascial injuryMRI-positive with tearing up to 50% of the muscle fibres. Possible hyperintense focal defect and partial retraction of muscle fibres
Grade IIIComplete tear of musculotendinous unit, complete loss of functionTear of many fibres with partial tearing of the fasciaComplete muscle rupture with retraction, fascial injuryMuscle rupture=100% structural damage. Complete tearing with or without muscle retraction
Grade IVXComplete tear of the muscle and fascia of the muscle–tendon unitXX