How to Assess the Motor System
The limbs and shoulder girdle should be fully exposed, then inspected for the following:
Passive flexion and extension of the limbs in a relaxed patient provide information about muscle tone.
Atrophy is indicated by decreased muscle bulk, but bilateral atrophy or atrophy in large or concealed muscles, unless advanced, may not be obvious. In the elderly, loss of some muscle mass is common.
Hypertrophy occurs when one muscle must work harder to compensate for weakness in another; pseudohypertrophy occurs when muscle tissue is replaced by excessive connective tissue or nonfunctional material (eg, amyloid).
Fasciculations (brief, fine, irregular twitches of the muscle visible under the skin) are relatively common. Although they can occur in normal muscle, particularly in calf muscles of the elderly, fasciculations frequently indicate lesions of the lower motor neuron (eg, nerve degeneration or injury and regeneration).
Myotonia (slowed relaxation of muscle after a sustained contraction or direct percussion of the muscle) indicates myotonic dystrophy and may be demonstrated by inability to quickly open a clenched hand.
Increased resistance followed by relaxation (clasp-knife phenomenon) and spasticity indicates upper motor neuron lesions.
Lead-pipe rigidity (uniform rigidity throughout the range of motion), often with cogwheeling, suggests a basal ganglia disorder.