how to grade power in small muscle of hand
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MRC grading scale small muscles of hand clinical examination
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Grading Power in the Small (Intrinsic) Muscles of the Hand
The MRC (Medical Research Council) Scale
All muscle power in the hand - including the small intrinsic muscles - is graded using the MRC scale (0-5), also called the Oxford Grading Scale:
| Grade | Clinical Features |
|---|---|
| M0 | Complete paralysis - no contraction |
| M1 | Flicker of muscle activity only |
| M2 | Movement present but cannot overcome gravity |
| M3 | Movement against gravity through full range |
| M4 | Movement against some resistance (but weaker than normal) |
| M4+ | Strong movement, but not quite normal |
| M5 | Normal, full power |
From: MRC Classification of Motor Nerve Dysfunction - Rockwood and Green's Fractures in Adults, 10th ed.
Important note for the hand: Because the intrinsic muscles work in a gravity-eliminated plane (horizontal), Grade 3 is modified - it means full active range of motion (not "against gravity" as in limb muscles). The examiner's hand feels muscle activity and provides resistance when grading Grade 4 and 5.
The Small Muscles: What to Test and How
The small muscles of the hand are divided by nerve supply, and you test each by the specific movement it performs:
1. Thenar Muscles (Median Nerve - C8, T1)
| Muscle | Movement to Test | How to Grade |
|---|---|---|
| Abductor pollicis brevis (APB) | Thumb abduction (lift thumb perpendicular to palm) | Patient lifts thumb off the palm against your downward resistance. Palpate APB at radial thenar eminence. |
| Opponens pollicis | Opposition (touch thumb tip to little finger tip) | Patient opposes thumb to little finger; try to break the grip. Also tests by rotating the thumb. |
| Flexor pollicis brevis (superficial head) | Flexion of thumb MCP joint | Less commonly tested in isolation |
Clue: If APB is weak/wasted = median nerve palsy (seen in carpal tunnel syndrome).
2. Hypothenar Muscles (Ulnar Nerve - C8, T1)
| Muscle | Movement to Test | How to Grade |
|---|---|---|
| Abductor digiti minimi (ADM) | Little finger abduction | Patient abducts the little finger against your resistance. Palpate the hypothenar eminence. |
| Opponens digiti minimi | Opposition of little finger | Patient opposes little finger to thumb |
| Flexor digiti minimi | Flexion of little finger at MCP | Flex little finger at MCP against resistance |
Clue: Wasting of the hypothenar eminence + weak ADM = ulnar nerve palsy.
3. Dorsal Interossei (Ulnar Nerve - C8, T1)
- Action: Finger abduction (DAB = Dorsal Abduct)
- Test: Ask patient to spread fingers apart against resistance
- Grade each web space individually if needed
- The first dorsal interosseous (index finger abduction) is particularly important and often the first muscle to show wasting in ulnar nerve lesions
4. Palmar Interossei (Ulnar Nerve - C8, T1)
- Action: Finger adduction (PAD = Palmar Adduct)
- Test: Patient squeezes a piece of paper between adjacent fingers; examiner tries to pull it out ("Froment's paper sign" modification)
5. Lumbricals (Mixed: 1st & 2nd = Median; 3rd & 4th = Ulnar)
- Action: Flex MCP joints while keeping IP joints extended (the "intrinsic plus" position)
- Test: Ask patient to flex at knuckles while keeping fingers straight. Apply resistance at the proximal phalanx.
Key Clinical Tips
-
Always palpate the muscle belly or tendon while testing - to confirm which muscle is actually working (some movements are possible by substitution from other muscles).
-
Compare both sides - the dominant hand is often slightly stronger.
-
Grade 4 is broad - it covers a wide range, from barely resisting a touch to almost-normal strength. Hence Grade 4+ (or 4-) is commonly used.
-
Wasting before weakness - in chronic denervation, observe for wasting of thenar or hypothenar eminences and guttering between metacarpals (interossei wasting) before testing power.
-
Froment's sign - a proxy test for adductor pollicis (ulnar nerve): patient pinches paper between thumb and index finger. If the IP joint of the thumb flexes (using FPL as a substitute), adductor pollicis is weak.
-
For formal assessment, a handheld dynamometer (grip dynamometer or pinch gauge) reduces subjectivity, especially at Grade 4-5, and is useful for tracking recovery after nerve repair.
Summary Table: Nerve, Muscle, Test Movement
| Nerve | Muscle | Test |
|---|---|---|
| Median (C8,T1) | Abductor pollicis brevis | Thumb abduction (perpendicular to palm) |
| Median (C8,T1) | Opponens pollicis | Thumb-little finger opposition |
| Ulnar (C8,T1) | Abductor digiti minimi | Little finger abduction |
| Ulnar (C8,T1) | 1st dorsal interosseous | Index finger abduction |
| Ulnar (C8,T1) | Palmar interossei | Finger adduction (paper test) |
| Ulnar (C8,T1) | Adductor pollicis | Thumb adduction/Froment's sign |
| Mixed | Lumbricals | MCP flexion with IP extension |
Sources: Rockwood and Green's Fractures in Adults 10th ed.; Bradley and Daroff's Neurology in Clinical Practice; Schreuders et al., Journal of Hand Therapy 2006
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