Power Grading for Motor Examination
The Medical Research Council (MRC) scale is the standard method for grading muscle power, using a 0-5 scale where each muscle group is tested against gravity and resistance. 1
The MRC Grading Scale
The MRC scale grades muscle strength as follows:
- Grade 0: No visible muscle contraction 2, 1
- Grade 1: Visible muscle contraction without movement 2, 1
- Grade 2: Movement possible with gravity eliminated 2, 1
- Grade 3: Movement against gravity but not against resistance 2, 1
- Grade 4: Movement against gravity with some resistance, but less than normal 2, 1
- Grade 5: Normal strength against full resistance 2, 1
Which Muscle Groups to Test
Test key muscle groups bilaterally in both upper and lower extremities, focusing on clinically relevant segments. 1
Upper Extremity Testing
- Shoulder abduction and flexion 2, 1
- Elbow flexion and extension 2, 1
- Wrist extension and flexion 1
- Long finger flexors 1
- Hand grip strength 1
Lower Extremity Testing
- Hip flexion and extension 2, 1
- Knee flexion and extension 2, 1
- Ankle dorsiflexion and plantarflexion 2, 1
- Hamstrings 1
Testing Technique
Position the patient appropriately for each muscle group, stabilize proximal joints, and apply resistance perpendicular to the limb segment being tested. 1
- Movement against gravity (Grade 3) serves as the critical dividing point in the scale 3, 4
- For cooperative patients, observe functional movements first, then proceed to formal strength testing 2
- Test each muscle group systematically and bilaterally for comparison 1
Important Limitations and Solutions
The MRC scale has significant limitations, particularly in Grade 4, which represents a very wide range of strength. 3, 5
When to Use Alternative Methods
- For muscles grading 3-5 on the MRC scale, consider quantitative myometry or handheld dynamometry for more precise measurements. 2, 1
- The MRC scale is more reliable and accurate for weak muscles (Grades 0-3), while analogue scales or dynamometry are superior for stronger muscles (Grades 4-5) 3
- Dynamometry is more reliable than MRC grading for following individual patients with neuromuscular disorders over time 4
Complementary Assessments
- Supplement strength testing with handgrip dynamometry as a simple, reliable measure of overall muscle function. 1
- Handgrip strength <10 kg at discharge predicts increased mortality risk 1
- Grip and pinch dynamometry can be completed in less than 5 minutes and normative data are available for comparison 2
Special Populations
ICU-Acquired Weakness
In ICU settings, use a composite MRC score across 12 muscle groups (bilateral shoulder abduction, elbow flexion, wrist extension, hip flexion, knee extension, ankle dorsiflexion), with a sum score <48 out of 60 defining ICU-acquired weakness. 2, 1
Duchenne Muscular Dystrophy
- Serial assessment every 6 months in ambulatory patients to identify disease progression 2, 1
- Manual muscle testing using the MRC scale is the primary method 2
- Quantitative myometry is beneficial for muscles grading 3-5 on the MRC scale 2
Stroke Patients
- The MRC scale demonstrates good to very good inter-rater and intra-rater reliability (kappa = 0.70-0.96) for all tested muscle groups in stroke patients 6
- Manual muscle testing is graded on the 0-5 scale and takes less than 5 minutes per extremity 2
Frequency of Assessment
Test upper and lower extremity strength every 6 months in ambulatory patients, and consider more frequent testing (every 4 months) when monitoring disease progression or treatment response. 2, 1
Common Pitfalls to Avoid
- Do not rely solely on MRC Grade 4 as indicating "good" functional recovery—this grade encompasses a very wide range of strength and may not reflect adequate function 5
- Ensure proper patient positioning and joint stabilization to obtain accurate measurements 1
- The MRC scale has limited validity in the Grade 4 range; consider objective dynamometry for these patients 4
- When testing ankle plantarflexors, be aware that reliability may be lower than other muscle groups 6