For determinism to be true, does this make sense...?

for determinism (environment/genetic making every act predictable if only all could be known) doesn't that suggest that it would have to be true all the time, or it's false, right? an example would be if i were to raise one of my arms, no environment/genetic factor could predict which it would be. or am i missing something? it's true you might perceive the other persons anticipation, but it's possible to block that out if you really wanted to.