Biceps and Triceps exercises: are both involved?

Question

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.

Answer 1

You are almost there.

I will use just one exercise as an example to keep it simple, but the explanation can be extrapolated to other muscle groups.

When curling the weight you are right to say our biceps contract to pull the weight up, as this happens our triceps lengthen to a stretched position, but there is no load upon it.

When we lower the weight, the resistance hasn't changed, the weight is still pulling towards the ground so as we lower the curl and extend our biceps we are still fighting against a downward force in a controlled decent, keeping load no the biceps. Since our triceps are responsible for extension of our arms you are right to assume that at this point they start to contract. But they still have no load on them at all, thus playing no part in either the concentric part or the eccentric part of the movement.

Your triceps will receive a comparable load during a curl as they would by just tensing your tricep in the mirror.