How to Assess Muscle Strength
Patients who report weakness may mean fatigue, clumsiness, or true muscle weakness. Thus, the examiner must define the precise character of symptoms, including exact location, time of occurrence, precipitating and ameliorating factors, and associated symptoms and signs.
Limbs are inspected for weakness (when extended, a weak limb drifts downward), tremor, and other involuntary movements. The strength of specific muscle groups is tested against resistance, and one side of the body is compared with the other. However, pain may preclude a full effort during strength testing.
With hysterical or factitious weakness, resistance to movement may be initially normal, followed by a sudden giving way, or patients may not use supporting muscles appropriately. For example, patients with true deltoid weakness use accessory muscles that tilt their trunk and neck away from the weak deltoid because they want to prevent the examiner from overcoming their weakness. In contrast, in patients with factitious deltoid weakness (eg, due to malingering), the shoulder and head tilt toward the weak deltoid as the muscle is overcome, indicating their lack of effort.
Subtle weakness may be indicated by decreased arm swing while walking, pronator drift in an outstretched arm, decreased spontaneous use of a limb, an externally rotated leg, slowing of rapid alternating movements, or impairment of fine dexterity (eg, ability to fasten a button, open a safety pin, or remove a match from its box).
Strength should be graded. The following scale, originally developed by The Medical Research Council of the United Kingdom, is now used universally:
The difficulty with this and similar scales is the large range in strength possible between grades 4 and 5.
Distal strength can be semiquantitatively measured with a handgrip ergometer or with an inflated BP cuff squeezed by the patient.
Functional testing often provides a better picture of the relationship between strength and disability. As the patient does various maneuvers, deficiencies are noted and quantified as much as possible (eg, number of squats done or steps climbed). Rising from a squatting position or stepping onto a chair tests proximal leg strength; walking on the heels and on tiptoe tests distal strength. Pushing with the arms to get out of a chair indicates quadriceps weakness. Swinging the body to move the arms indicates shoulder girdle weakness. Rising from the supine position by turning prone, kneeling, and using the hands to climb up the thighs and slowly push erect (Gowers sign) suggests pelvic girdle weakness.