Using 4+ to grade near-normal muscle strength does not improve agreement

Area of Science:

• Neurology
• Clinical Assessment
• Musculoskeletal Research

Background:

• Manual muscle strength assessment commonly uses the ordinal Medical Research Council (MRC) scale.
• The standard MRC scale exhibits weaknesses, including poorly defined grade limits and large strength variations between grades.
• Modifications, such as intermediate grades, have been proposed to enhance the MRC scale's reliability.

Purpose of the Study:

• To evaluate if incorporating an intermediate grade ('4+') between MRC grades '4' and '5' improves inter-rater agreement and reduces rating variability.
• To test the hypothesis that a modified MRC scale with a '4+' option enhances clinician agreement compared to the standard MRC scale.

Main Methods:

• A questionnaire with five clinical cases describing muscle weakness was distributed to chiropractors in Northern Europe.
• Respondents graded muscle strength using either the standard MRC scale or a modified scale including a '4+' grade.
• Cases were specifically designed to represent muscle weakness in the ambiguous range between MRC grades '4' and '5'.

Main Results:

• A total of 225 questionnaires were returned, yielding a 7% response rate.
• Average percentage agreement was 64% in the standard MRC group versus 48% in the modified MRC group.
• The mean average deviation analogue was higher in the modified MRC group (0.51) compared to the standard group (0.34), indicating greater score dispersion.

Conclusions:

• Contrary to the hypothesis, the '4+' grade did not demonstrably improve agreement or reduce rating variability.
• Even with written case descriptions designed to target the '4+' grade, the modified scale did not enhance assessment consistency.
• The study suggests that the introduction of intermediate grades may not effectively resolve the inherent limitations of the MRC scale.