Ask Healthy Living calls this muscle quaking and lists three causes:
As a solution for the last problem, have someone massage the muscles for you or do it yourself. Providing the brain with feedback from the nerves that innervate the skin can make it easier for it to send nerve signals to the muscles there, instead of signalling other ones it has a stronger connection to and has come to rely on. Source: Muscle Activation Techniques therapy. When it works, it only takes one try for you to go from shaky motion (due to stabilizing with incorrect muscles) to smooth motion (like flexing a bicep). It may work better with a therapist testing exactly which muscles are not being recruited correctly.
The massage also helps get blood flowing through the muscles, like a warmup set would if you were actually using those muscles when you did the lift.