Medicine Neurology · Clinical · High Yield

MRC Muscle Grading Scale

The Medical Research Council (MRC) scale grades muscle power from 0 to 5. It is the universal bedside tool for documenting and tracking motor deficits in neurological and neurosurgical patients.

✦ The Mnemonic

"No Flicker, Active Against, Full Power"

0 = nothing → 5 = full normal power — six steps

0 No contraction Complete paralysis — no visible or palpable muscle activity

1 Flicker / trace Visible or palpable contraction but no movement

2 Active movement — gravity eliminated Full range of movement when gravity is removed (limb supported)

3 Active movement against gravity Can lift against gravity but not against any added resistance

4 Active movement against resistance Reduced power but moves against examiner's resistance; subdivided 4−/4/4+

5 Normal power Full power against full resistance — age and sex appropriate

📚 Clinical Breakdown

The MRC scale was developed in World War II for documenting peripheral nerve injuries and remains the standard motor grading system worldwide. Grades 0–2 represent severe weakness where gravity alone prevents movement. Grade 3 is the critical threshold — the patient can move the limb against gravity, suggesting partial reinnervation or incomplete cord lesion.

Grade 4 is deliberately broad and is often subdivided into 4− (barely resists), 4 (moderate resistance), and 4+ (strong but not full). This subdivision improves sensitivity for documenting subtle progression in conditions like Guillain-Barré syndrome or myasthenia gravis.

In clinical documentation, power is recorded per muscle group per limb — e.g. 'shoulder abduction 4/5 right, 5/5 left'. A pattern of proximal weakness (grades 3–4 proximally, 5 distally) suggests myopathy; distal weakness suggests peripheral neuropathy; a clear dermatomal or myotomal pattern suggests root compression.

Grade 3 significance Can lift against gravity — key functional threshold

Grade 4 subdivisions 4−, 4, 4+ — used in detailed neurological follow-up

Used in Neurology, spinal surgery, ICU, stroke, Guillain-Barré

Normal = 5 Full resistance — always compare to contralateral side

⭐ Clinical Pearl

MRC Sum Score: in Guillain-Barré syndrome, the MRC sum score (testing 6 muscle groups bilaterally — shoulder abductors, elbow flexors, wrist extensors, hip flexors, knee extensors, foot dorsiflexors) gives a total out of 60. A score <48 indicates significant weakness; it is used to monitor progression and guide decisions about ventilatory support.

⚠ Exam Trap

Grade 4/5 is NOT normal — it is reduced power. A common error is documenting grade 4 as 'satisfactory'. Always compare to the contralateral side and to baseline. Subtle grade 4 weakness in a single myotome can be the only sign of early cord compression.

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