Table 1. Summary of 17 currently existing muscle classification models, divided between systems based on clinical and imaging findings, systems based on image evaluation, and systems based on clinical presentation ¹¹ .

1. Classifications based on clinical and imaging findings
Author	Description
Lopes, A. 1993.	Classification based on etiology and ultrasound findings Type I: muscle injury caused by extrinsic factors: muscle contusion Type II: muscle injury caused by intrinsic factors without muscle rupture Type III: muscle injury caused by intrinsic factors with muscle rupture
Verrall, J. 2003.	Clinical parameters				Image findings - MRI Classification of the lesion
	Beginning		Insidious	Abrupt
	Circumstance		Playing	Training	Positive		Negative
	Pain		(0–10) visual analog scale
Malliaropoulos, N. 2010.	Clinical Grade - ROM Deficit				Image findings (US)
	I		< 10 th		Degree 0 to 3 (based on Peetrons)		Injury area:
	II		10 th –19 th				< 25%
	III		20 th –29 th				25–50%
	IV		> 30 th				> 50%
Pollock, N. 15 (British athletics muscle injury classification)	Degree of injury			Description		MRI
	Grade 0: referred pain
	0a			Local pain		Normal
	0b			Generalized muscle pain		normal or with signs of delayed pain
	Grade 1: small muscle injuries (< 5 cm or < 10% of total muscle area)
	1a			Fascial pain		Intermuscular fluid
	1b			Muscle or JMT pain		Intermuscular fluid
	Grade 2: moderate muscle injuries (5–15 cm or 10–50% of total muscle area)
	2a			Fascial pain		high perspherical signal
	2b			Muscle or JMT pain		high signal in JMT
	2c			Tendon pain		high sign on tendon
	Grade 3: extensive muscle injuries (> 15 cm or > 50% of total muscle area)
	3a			Fascial pain		high perspherical signal
	3b			Muscle or JMT pain		high signal in JMT
	3c			Tendon pain		high sign on tendon
	Grade 4: complete muscle injuries
	4a			Fascial pain		high perspherical signal
	4b			Muscle or JMT pain		high signal in JMT
	4c			Tendon pain		high sign on tendon
Mueller-Wohlfahrt, H. 8 (The Munich consensus statment)	A. Indirect muscle injury
	Functional muscle injury Type 1: Overload-related muscle disorder  Type 1A: Fatigue-induced muscle disorder  Type 1B: Late-onset muscle pain (DMIT) Type 2: Neuromuscular disorder  Type 2A: Related to the spine  Type 2B: Related to muscles
	Structural muscle injury Type 3: Partial muscle injury  Type 3A: Minimal partial muscle injury  Type 3B: Moderate partial muscle injury Type 4: Injury (sub)total  Subtotal or complete muscle injury  Tendinous avulsion
	B. Direct muscle injury    Bruise  Laceration
Maffulli, N. 16	- Direct muscle injury   Bruise  Laceration
	- Indirect muscle injury   Nonstructural muscle injury Type 1: Fatigue muscle injury  Type 1A: Fatigue-induced muscle disorder  Type 1B: Late-onset muscle pain (DMIT) Type 2: Neuromuscular disorder  Type 2A: Related to the spine  Type 2B: Related to muscles
	- Indirect muscle injury   Structural muscle injury Type 3: Partial muscle injury  Type 3A: Minimal partial muscle injury  Type 3B: Moderate partial muscle injury (< 50%) Type 4: Injury (sub)total  Subtotal or complete muscle injury  Tendinous avulsion Structural lesions can be proximal (P), middle (M), and distal (D)
Valle, X. 7	Clinical findings
	Injury mechanism (M)		Location of the lesion (L)			Degree of injury (G)	Rescan number (R)
	T - Direct lesion of the hamstrings		P Lesion located in the proximal third of the muscle belly M Lesion located in the middle third of the muscular belly D Lesion located in the third of the muscular belly			0–3	0: 1 st episode 1 s t reinjury 2: 2 nd reinjury
	I - Indirect injury of the hamstrings, plus index s if it is by stretching (stretching), or index p if it is run.		P Lesion located in the proximal third of the muscle belly. The second letter is index p or d, describing whether the lesion is proximal or distal to JMT, respectively M Lesion located in the middle third of the muscle belly, plus the corresponding index D Lesion located in the middle third of the muscle belly, plus the corresponding index			0–3
	N - Negative MRI injury		N p Lesion in the proximal third N m Injury in the middle third N d Lesion in the distal third			0–3
	Magnetic resonance findings
	Grade 0		Negative MRI
	Grade 1		Hyperintense muscle fiber edema without intramuscular hemorrhage or change in architecture
	Grade 2		Hyperintense edema of muscle fiber and/or paratendon with minimal intramuscular hemorrhage without gaps or minimal alteration in muscle architecture.
	Grade 3		Any gap between muscle fibers in the craniocaudal or axial plane. Hyperintense focal defect with partial retraction of muscle fibers ± intermuscular hemorrhage.
	(r) code overwrite		Used when there is intratendinous injury or affecting JMT or intramuscular injury with retraction or loss of normal tension.
2. Classifications based on image findings
Pomeranz, S. 1993.	MRI assessment
	Muscle group involved		Injury area		Location		Superficial Involvement
	Semimembranosus		< 50%		Tendineous		Yes
	Semitendinosus		> 50%		JMT		No
	Femoris biceps		Total
	Femoris square
Takebayashi, S. 12	US findings
	Type 1				Normal
	Type 2				Hyperecoic infiltration
	Type 3				Mass
	Type 4				Complete lesion (Infiltration + mass)
Peetrons, P. 13	US findings
	Grade 0		Normal
	Grade 1		Hyperecoic area, < 15 mm on the longest axis; < 5% of muscle.
	Grade 2		5–50% of muscle. Partial muscle rupture.
	Grade 3		Complete muscle or fascia injury, with collection extravasation from the injured muscle.
Slavotinek, J. 2002.	MRI image of hamstring injury
	Affected muscle			Location		Total area of the lesion
	Femoris biceps			Proximal to short biceps head		0–100%
	Semitendinosus			Distal to short biceps head
	Semimembranosus
Bordalo-Rodrigues, M. 2005	MRI image of Proximal Rectus Femoris - anatomical location
	Avulsion injury of the apophysis Musculotendinous junction injury (JMT) Muscle contusion and laceration
Cohen, S. 2011.	MRI-based graduation system
	Item	Description		0 points	1 point	2 points	3 points
	1	N° of muscles involved		No	1	2	3
	2	Location		−	Proximal	Middle	Distal
	3	Insertion		No	−	Yes	−
	4	Total area of injury in % of the muscle involved		0%	25%	50%	≥ 75%
	5	Retraction		No	−	> 2 cm	−
	6	Longitudinal axis involvement		0 cm	1–5 cm	6–10 cm	> 10 cm
Chan, O. 2012	Graduation based on imaging findings and lesion site
	Degree		MRI		US		Local
	I (distension)		< 5% fiber rupture;		Normal; without distortion of architecture		. Proximal to JMT
	II (Partial loom)		< 5% fiber rupture; high intramuscular signal; edema and bleeding of the muscle or JMT extending to the fascial planes between the muscle groups		Discontinuity of muscle fibers		Muscle A. Proximal B. Medium C. Distal
	III (Complete loom)		Complete discontinuity of muscle fibers, hematoma, and muscle retraction		Comparable with MRI		Distal to JMT
Corazza, A. 2013.	Combined US-MRI assessment
	Degree	MRI			US
	0	No pathological findings			No pathological findings
	I	Muscle edema without tissue alteration			Altered echotexture at the site of pain, without rupture
	Ii	Partial muscle injury			Lesion with associated hematoma
	Iii	Complete muscle injury			Complete muscle injury
3. Classifications based on clinical findings
Bass, A. 1969.	Classifies muscle injuries by etiology and location
	Type	Etiology			Location
	I	Direct external contact			Intramuscular
	Ii	Twitch			Intermuscular
Wise, D. 1977	Classification based on cause, severity, and location of leg muscle injury
	Indirect lesions - inflammation
	Direct injuries - trauma
	Degree	Pain	Circumference difference	Arc of motion	During contraction
					Pain	Loss of strength	Function disorder
	I	Minimum;	< 6 mm	100%	Minimum	No	Moderate
	Ii	Substantial	6–12 mm	50%	Middle	Middle	Important
	Iii	Intractable	> 12 mm	<5 0%	Serious	almost total	Don't step

Abbreviations: JMT, myotendinous junction; MRI, magnectic resonance imaging; US, ultrasound.