The Motor System

As you assess the motor system, focus on body position, involuntary movements, characteristics of the muscles (bulk, tone, and strength), and coordination. These components are described below in sequence. You may either use this sequence or check each component in the arms, legs, and trunk in turn. If you see an abnormality, identify the muscle(s) involved. Think about whether the abnormality is central or peripheral in origin, and begin to learn which nerves innervate the affected muscles.

Body Position.

and at rest.

Observe the patient's body position during movement

Involuntary Movements. Watch for involuntary movements such as tremors, tics, or fasciculations. Note their location, quality, rate, rhythm, and amplitude, and their relation to posture, activity, fatigue, emotion, and other factors.

Abnormal positions alert you to neurologic deficits such as paralysis.

See Table 16-8, Involuntary Movements (pp. 608-609).

Muscle Bulk. Compare the size and contours of muscles. Do the muscles look flat or concave, suggesting atrophy? If so, is the process unilateral or bilateral? Is it proximal or distal?

When looking for atrophy, pay particular attention to the hands, shoulders, and thighs. The thenar and hypothenar eminences should be full and convex, and the spaces between the metacarpals, where the dorsal in-terosseous muscles lie, should be full or only slightly depressed. Atrophy of hand muscles may occur with normal aging, however, as shown on the right below.

Muscular atrophy refers to a loss of muscle bulk (wasting). It results from diseases of the peripheral nervous system such as diabetic neuropathy, as well as diseases of the muscles themselves. Hypertrophy refers to an increase in bulk with proportionate strength, while increased bulk with diminished strength is called pseudohypertrophy (seen in the Duchenne form of muscular dystrophy).

Atrophy

Hand of a 44-year-old woman

Hand of an 84-year-old woman

Atrophy

Flattening of the thenar and hypothenar eminences and furrowing between the metacarpals suggest atrophy. Localized atrophy of the thenar and hypothenar eminences suggests damage to the median and ulnar nerves, respectively.

Hand of a 44-year-old woman

Hand of an 84-year-old woman

Hypothenar eminence

Flattening of the thenar eminence due to mild atrophy

Hand of an 84-year-old woman

Hypothenar eminence

Flattening of the thenar eminence due to mild atrophy

Other causes of muscular atrophy include motor neuron diseases, disuse of the muscles, rheumatoid arthritis, and protein-calorie malnutrition.

Hand of an 84-year-old woman

Be alert for fasciculations in atrophic muscles. If you see none, a tap on the muscle with a reflex hammer may stimulate them.

Fasciculations suggest lower motor neuron disease as a cause of atrophy.

Muscle Tone. When a normal muscle with an intact nerve supply is relaxed voluntarily, it maintains a slight residual tension known as muscle tone. This can be assessed best by feeling the muscle's resistance to passive stretch. Persuade the patient to relax. Take one hand with yours and, while supporting the elbow, flex and extend the patient's fingers, wrist, and elbow, and put the shoulder through a moderate range of motion. With practice, these actions can be combined into a single smooth movement. On each side, note muscle tone—the resistance offered to your movements. Tense patients may show increased resistance. You will learn the feel of normal resistance only with repeated practice.

Decreased resistance suggests disease of the peripheral nervous system, cerebellar disease, or the acute stages of spinal cord injury. See Table 16-11, Disorders of Muscle Tone (p. 614).

If you suspect decreased resistance, hold the forearm and shake the hand loosely back and forth. Normally the hand moves back and forth freely but is not completely floppy.

Marked floppiness indicates hypotonic or flaccid muscles.

If resistance is increased, determine whether it varies as you move the limb or whether it persists throughout the range of movement and in both directions, for example, during both flexion and extension. Feel for any jerki-ness in the resistance.

To assess muscle tone in the legs, support the patient's thigh with one hand, grasp the foot with the other, and flex and extend the patient's knee and ankle on each side. Note the resistance to your movements.

Muscle Strength. Normal individuals vary widely in their strength, and your standard of normal, while admittedly rough, should allow for such variables as age, sex, and muscular training. A person's dominant side is usually slightly stronger than the other side. Keep this difference in mind when you compare sides.

Increased resistance that varies, commonly worse at the extremes of the range, is called spasticity. Resistance that persists throughout the range and in both directions is called lead-pipe rigidity.

Test muscle strength by asking the patient to move actively against your resistance or to resist your movement. Remember that a muscle is strongest when shortest, and weakest when longest.

If the muscles are too weak to overcome resistance, test them against gravity alone or with gravity eliminated. When the forearm rests in a pronated position, for example, dorsiflexion at the wrist can be tested against gravity alone. When the forearm is midway between pronation and supination, extension at the wrist can be tested with gravity eliminated. Finally, if the patient fails to move the body part, watch or feel for weak muscular contraction.

Impaired strength is called weakness (paresis). Absence of strength is called paralysis (plegia). Hemi-paresis refers to weakness of one half of the body; hemiplegia to paralysis of one half of the body. Paraplegia means paralysis of the legs; quadriplegia, paralysis of all four limbs.

See Table 16-12, Disorders of the Central and Peripheral Nervous Systems (pp. 615-617).

Muscle strength is graded on a 0 to 5 scale:

0—No muscular contraction detected

1—A barely detectable flicker or trace of contraction

2—Active movement of the body part with gravity eliminated

3—Active movement against gravity

4—Active movement against gravity and some resistance

5—Active movement against full resistance without evident fatigue. This is normal muscle strength.

More experienced clinicians make further distinctions by using plus or minus signs toward the stronger end of this scale. Thus 4+ indicates good but not full strength, while 5— means a trace of weakness.

Methods for testing the major muscle groups are described below. The spinal root innervations and the muscles affected are shown in parentheses. To localize lesions in the spinal cord or the peripheral nervous system more precisely, additional testing may be necessary. For these specialized methods, refer to detailed texts of neurology.

Test flexion (C5, C6—biceps) and extension (C6, C7, C8—triceps) at the elbow by having the patient pull and push against your hand.

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