Table 2 Summary of muscle injury classification systems

Grade 1 (fatigue-induced muscle disorder)

Negative MRI imaging/US findings

O a/b

MRI imaging normal/MRI imaging normal or patchy edema

Mechanism of injury (M)

I (indirect)

T (direct)

Grade 1B (delayed-onset muscle soreness, DOMS)

Negative MRI imaging/US findings or edema only

1a (small myofascial tear)

Edema at the fascial border, <10% extension into muscle belly; <5% CC length

Location of injury (L)

P (proximal)

M (middle)

D (distal)

Grade 2A (spine-related neuromuscular muscular disorder)

Negative MRI imaging/US findings or edema only

1b (small muscle-tendon junction tear)

< 10% of CSA of muscle the MTJ; < 5 cm CC length; may note fiber disruption of < 1 cm

Grading of severity (G)

0 (negative MRI)

Grade 2B (muscle-related neuromuscular muscle disorder)

Negative MR imaging/US findings or edema only

2a (moderate myofascial tear)

2b (moderate muscle-tendon junction tear)

Edema evident at fascial border with extension into the muscle or edema evident at the MTJ; Edema of CSA between 10–50% at maximal site; CC length 5–15 cm; architectural fiber disruption < 5 cm

1 (edema without intramuscular hemorrhage or architectural distortion)

Grade 3A (minor partial muscle tear)

Fiber disruption on 1.5 or 3-T MR images; intramuscular hematoma

2c (moderate intratendinous tear)

Edema extends into the tendon with longitudinal length of tendon involvement (< 5 cm); < 50% of maximal tendon CSA. No loss of tension or discontinuity within the tendon

2 (edema with minor muscle fiber architectural distortion ± minor intermuscular hemorrhage, but no gap between fibers)

Grade 3B (moderate partial muscle tear)

Significant fiber disruption, probably including some retraction, with fascial injury and intermuscular hematoma

3a (extensive myofascial tear)

3b (extensive muscle-tendon junction tear)

Edema at the fascial border with extension into the muscle or at the muscle-tendon-junction; > 50% CSA at the muscle-tendon-junction; > 15 cm CC distance; architectural fiber disruption > 5 cm

3 (any quantifiable gap between fibers with partial retraction ± intermuscular hemorrhage)

Grade 4 (subtotal/total muscle tear or tendinous avulsion)

Subtotal/complete discontinuity of muscle/tendon; possible wavy tendon morphology and retraction, with fascial injury and intermuscular hematoma

3c (extensive intratendinous tear)

Edema extends into the tendon with longitudinal length of tendon involvement > 5 cm; > 50% of maximal tendon CSA; probably loss of tendon tension, no discontinuity

Tendinous lesion (r)

r (presence or not depends of tendinous involvement)

4 (full thickness tear of muscle)

Complete discontinuity of the muscle with retraction

Number of muscle re-injuries (R)

0: 1st episode

1: 1st re-injury

2: 2nd re-injury