MRC muscle strength grading scale
/ M-R-C /
When a clinician writes that a muscle is '3 out of 5', they are using a shared shorthand that any rehabilitation professional will instantly understand. That shorthand is the MRC muscle strength grading scale, named for the British Medical Research Council that popularized it. It is the ruler that turns the hands-on judgment of manual muscle testing into a single, communicable number from 0 to 5.
The six grades climb in a logical staircase. Grade 0 is no contraction at all. Grade 1 is a flicker or trace of contraction with no movement of the limb. Grade 2 is movement, but only when gravity is removed — for example, sliding the leg sideways on a bed. Grade 3 is the pivotal one: the muscle can move the joint through its full range against gravity, but breaks the moment any extra resistance is added; lifting the straight leg off the bed is grade 3 at the hip. Grade 4 holds against some examiner resistance but not full resistance, and is often split into 4-minus, 4, and 4-plus to capture the wide gap up to grade 5, which is normal strength.
The scale's importance is that it is a common language. A grade written in one hospital means the same in another, lets a team track change over weeks, and feeds into bigger systems such as the ASIA examination used in spinal cord injury, where individual muscle grades are summed into a motor score. Its weakness is the same as manual muscle testing's: grades 4 and 5 are crowded and subjective, so a person can lose meaningful strength while still being graded 5. Knowing where the scale is precise (the lower grades) and where it is blunt (the top) is part of using it honestly.
A patient's hip flexor is graded by asking them to lift the straight leg off the bed. They can lift it but it collapses the instant the examiner presses down — that is grade 3. A week later they hold against light pressure: grade 4-minus. The single-digit change tells the whole team the leg is getting stronger.
Grade 3 — full range against gravity, but no extra resistance — is the scale's hinge point.
The scale is non-linear: the jump from 4 to 5 covers a far wider range of real strength than the jump from 0 to 1, which is why grade 4 is so often split and why 'normal 5/5' can hide genuine weakness.