The first assumption is that you are interested in functional muscular atrophy. If that's true, then:

I am unaware of a universally accepted definition of "functional muscle atrophy." In reality, it is a long-lasting loss of muscle functionality due to a reduced ability to contract the muscle. A frequently observed manifestation of this condition is the decrement in the number of fast-twitch motor units, leading to attenuated force production and contraction velocity—i.e., a reduction in the temporal force output. Additionally, functional muscle atrophy can manifest as altered muscle behavior under varying load conditions, including augmented fatigability or the muscle's impaired ability to engage in post-activation potentiation or twitch potentiation.

Another example is aging and the symptomatology associated with loss of mass (atrophy) and loss of muscle strength (functional atrophy): "Due to their differing aetiologies and consequences, it has been proposed that the term "sarcopenia" should revert to its original definition of age-related muscle mass declines, with a separate term, "dynapenia," describing muscle strength and function declines." (DOI 10.1007/s11914-015-0274-z)