let's consider these two famous exercises: triceps pull-down, and biceps curl
Both the exercises have the same starting and ending positions, and even the movement is the same: it's basically a forearm flextion/extension.
What causes the first exercise to work out the triceps and the second one the biceps?
My intuitive (possibly wrong) interpretation: biceps is contracting when we pull something towards us (forearm flexion), triceps when we press something from us (forearm extension).
In triceps pull down, the weights of the machine are lifted when we press. Therefore, triceps work out. In the "return" phase of the exercise, we use biceps too: but in this phasethe machine weights are returning to the floor (so, no gravity force) and so we don't feel the biceps so much (as a normal movement of the forearm without weights).
In biceps curl, the weights are lifted when we pull. Therefore, biceps work out. In the "return" phase of the exercise, we use triceps too: but in this phase the weights are returning to the floor (so, no gravity force) and so we don't feel the triceps so much (as a normal movement of the forearm without weights).
Is this description true?
If it is, it's in principle wrong to say that triceps pull - down works only triceps and biceps curl only triceps. They both work both muscles, but each one loads the weights to a specific muscle and leaves "free" (without weights) the other muscle.