If a child was kept immobile at a certain age would this have an effect on their motor functions? Bewick GS?

reading a journal Postnatal emergence of mature release properties in terminals of rat fast- and slow-twitch muscles by Bewick GS, Reid B, Jawaid S, Hatcher T, Shanley L.
There is a turning point when the infant starts to use their limbs where the muscles develop into their mature adult pattern. If the child is kept immobile, as in some cases of child abuse, would they miss a vital stage in muscle development and exhibit poor motor function/coordination?

Yes they would. Its like a child not receiving visual stimulation during the vital pathway forming months, they would not see!
I have come across a case in my nursing career. A young girl came in with "problems" with her knees. Mother was suspected of Munchousens by proxy. Child was treated for minor problems and sent home. The child came in regularly for treatment, every time there was a noticable loss of function. The child was not beling allowed to walk at all when she was home with the mother. Outcome, the child ended up in a wheelchair without any use in her legs at all...;

Every human need some kind of Range of Motion so their muscle will not weaken.

yes they can miss the vital stage and may never learn it i believe its from a very very early age though and not sure if it can be relearnt i believe it may be through stimulation of the nerve endings, but as a fetus moves in the womb not sure if this is a viable answer thinking about it hope u know what i am getting at its hard to write it down

Yes,.. overall they would probably still be able to function,.. although they would be somewhat behind many who grew up under,.. say "normal" (although what is normal these days) conditions...

Thats why parents are encouraged to buy toys for babies that have various objects and such that can be grabbed at and manipulated to assist the body in development of motor skills and encourage usage of the limbs that leads to a strengthening of synapses in the brain that ultiamtely assist inthe development of motor skills

If you kept a baby immobilized,.. by injury,. or even say,.. by just holding it still (in theory) it would definatly slow that process down