When caused by MS, weakness is a result of damaged nerve impulse flow, preventing instructions from reaching the extremities. This type of weakness does not result from any type of loss in muscle strength.
The most common symptom of weakness is one leg feeling heavy and being difficult to lift, especially at the end of the day. Those who have had MS for a long time could well have both legs affected. Such weakness may be particularly evident when stepping up onto a stair or curb. Although less common, some may experience similar sensations of heaviness or clumsiness in one or both of their arms and hands. Losing the ability to grip, push, or lift is often very frustrating.