Upper extremity impairment rarely is assessed using grip strength, according to the AMA Guides to the Evaluation of Permanent Impairment (AMA Guides), because results can be influenced by subjective factors that are difficult to control, including sex, age, comorbidities, fatigue, handedness, time of day, pain, and the individual's cooperation. The AMA Guides, Section 16.5b, Impairment Evaluation Methods, discusses the approach used for rating peripheral nerve injuries, but this section applies only to specific nerve lesions with resulting weakness of the muscles supplied or sensory changes. Strength correlates only poorly with performance of the activities of daily living, and grip strength testing using a dynamometer or other types of isometric strength testing has not been shown to reliably discriminate between submaximal and maximal efforts. Grip strength usually is not used in the presence of decreased motion, painful conditions, deformities, or absence of body parts (eg, missing digit), nor is it used to rate weakness from a peripheral nerve lesion. In rare cases, the AMA Guides allows the use of loss of strength (eg, due to a severe muscle tear that healed leaving a palpable muscle defect). Impairment ratings based on objective anatomic findings take precedence, and loss of strength is rarely combined and only if based on unrelated etiologic or pathomechanical causes.

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