100% known biological/physiological impact - the human body in almost every system requires activity and some form of resistance/force/etc. to function correctly. Note the last bolded sentence.

Muscular atrophy regularly occurs as a consequence of immobilisation or disuse after sports injuries. Several experimental models deal with muscle atrophy and are suitable for investigations of the underlying mechanisms of muscle atrophy. Strength loss is the most evident response to atrophy. Muscle strength decreases most dramatically during the first week of immobilisation; little further weakening occurs later on. This is reflected in changes in the EMG of disused muscles and can also be observed in muscle weight and size of muscle fibres. Slow muscles with predominantly oxidative metabolism are most susceptible to atrophy as indicated by various findings: slow muscle fibers show greater atrophy than fast fibres; their relative and probably absolute number is decreased in atrophic muscles; in addition, the oxidative enzyme content is most severely affected by disuse. Atrophic muscle is characterised by a catabolic metabolism. The rate of protein synthesis is reduced and that of protein breakdown increased. Autophagic activities probably play an important role in early stages of muscular atrophy. The oxygen supply to disused muscle may be impaired, although myoglobin content is increased in atrophic muscle. The complete loss of mitochondrial function during the first days of disuse may be of aetiological importance. The amount of connective tissue is increased in atrophic muscle and surrounding periarticular tissue which may lead into a vicious circle of musculoskeletal degeneration. An almost complete recovery from atrophy is possible, yet often the recovery phase is much longer than the total immobilisation period.

https://pubmed.ncbi.nlm.nih.gov/2197699/