Weakness is an objective loss of muscle strength. This is conveniently quantified or rated using the MRC grading system:
5 = normal power
4 = active movement against gravity and resistance 3 = active movement against gravity
2 = active movement with gravity eliminated 1 = flicker or trace of contraction
0 = no contraction (paralysis).
However, this is not a linear scale; grade 4 often becomes subdivided into 4−, 4, and 4+ (or even 5−) according to the increasing degree of resistance which the examiner must apply to overcome activity. It is also important to assess what effort the patient is making to comply with the testing; "apparent weakness" or "pseudoparesis" may be shorthand for lack of patient effort. Sudden "giving way" of muscle contraction may be an indicator of this. Nonuniform resistance may also be due to pain (algesic pseudoparesis). Testing records only the best forced maximal contraction, and should not develop into an unseemly trial of strength between patient and examiner. Accepting all these difficulties, it should be acknowledged that the grading of weakness, like all clinical observations, is subject to some degree of observer bias.
The pattern of muscle weakness may suggest its anatomical origin. So-called "pyramidal weakness" (i.e., affecting upper limb extensors more than flexors, and lower limb flexors more than extensors), suggests an upper motor neurone lesion (corticospinal pathways). However, there is no evidence that pure lesions of the pyramidal tracts produce this picture: pyramidotomy in the monkey results in a deficit in fine finger movements, but without weakness. Moreover, a similar pattern of weakness may be observed in lower motor neurone disorders, such as Guillain-Barré syndrome. Coexistent wasting suggests muscle weakness is of lower motor neurone origin, especially if acute, although wasting may occur in long-standing upper motor neurone lesions. Weakness with minimal or no muscle wasting may be nonorganic, but may be seen in conditions, such as multifocal motor neuropathy with conduction block.
Aids to the Examination of the Peripheral Nervous System. London: HMSO, 1976