Weak. Atrophy (muscle wasting) is often sneaky and painless. The wasted muscle feels weak, and often 'untrustworthy'. A knee may buckle if the quad is atrophied, or grip may fail and cause dropping of objects with hand atrophy. Sometimes patients feel an aching due to fatigue, or even trembling. In some cases, twitching or spasms can occur.
Weakness.... ..The muscle generally atrophies from lack of use.