Muscle strength is graded on a 0 to 5 scale used worldwide, the Medical Research Council (MRC) scale. Comparing both sides and looking at the pattern of weakness helps find where the problem lies.
The scale
Grade 0 is no contraction and grade 5 is normal strength. The key cut point is grade 3: full movement against gravity but not against any added resistance.
How to test
Stabilize the joint above, ask the patient to hold a position and push against it, comparing the same muscle on both sides. Judge strength against what you expect for the patient's age and size.
Patterns of weakness
Weakness of one side of the body suggests a brain lesion, weakness of both legs a spinal cord problem, and weakness in one nerve's muscles a peripheral nerve injury. Proximal weakness in both arms and legs suggests a muscle disease.
Upper and lower motor neuron
Upper motor neuron lesions give weakness with brisk reflexes and increased tone. Lower motor neuron lesions give weakness with reduced reflexes, low tone, wasting and twitching (fasciculations).
| Grade | What you see | Meaning |
|---|---|---|
| 0 | No contraction | Paralysis |
| 1 | Flicker of contraction, no movement | Trace |
| 2 | Movement with gravity removed | Poor |
| 3 | Movement against gravity only | Fair |
| 4 | Movement against some resistance | Good |
| 5 | Full strength against full resistance | Normal |
At the bedside
Record strength as a number for each muscle group, for example "right deltoid 4/5", so changes can be tracked between exams. New or worsening weakness after a back injury, stroke or spinal procedure needs urgent escalation.
Related guides
Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).