Difference in weight while pulling weights Let's say you are using a standard pulley gym machine to lift some amount of weight. If while you exercise you are standing on a weighing scale. What variations in weight if any would occur?
Does it depend on the position of the weights? For example would you increase in weight until it reaches what would essentially be your center of mass?
I think your weight increases while while the weights are going up (you are pulling) and decreases back to your original weight on it's weight down. Probably some scale jitter when you start pulling.
 A: It is actually the opposite, the weight in the balance will go down in the same amount than the force applied to the bar attached to the pulley. This is because the force this bar makes on the person is upwards. The only way the weight in the scale will mark more is if you are pushing something up (but not if slowly releasing it up, because you are still making a down force in such a case)
A: It depends which type of machine it is, quite simply.
If it is the type that are pulled upwards (which have multiple pulleys), then it will add the amount of force you are applying to your weight.
If it is the type that are pulled downwards (which usually have only one pulley and therefore the force required is equal to the weight), then it will reduce your scale reading accordingly.
In regard as to when it adds how much, I am not sure. I would assume that as soon as the weights are relying entirely upon you to hold them aloft, their weight would be added/taken away, but as my old gym teacher used to say, to assume makes an ass out of you (u) and me...
A: There are several possible scenarios as pointed out by other users, but I'll focus on the following one:
Consider the case where the weights are pulled downwards. For the sake of simplicity, let's assume we're just holding the weight (so we can ignore the shifting center of mass, since a human is not a rigid body).
Observe the diagram, where a human of mass $M$ is holding a weight of mass $m$ using 1 (assumed massless and frictionless) pulley:

Note that the magnitude of the normal force $\mathbf N$ is what the scale will read. As you can see, the upwards tension will decrease the required normal force for the human to remain stationary.
Edit: yes, the normal force will be exerted at the person's feet (so in two spots), but the way I drew it makes the magnitude of the normal force clearer.
