# A weight lifting question

1. Nov 25, 2010

### douglis

Hi all,
when you try to lift explosively a weight there is a acceleration phase and obviously a deceleration phase in order to stop the weight at the end of range of motion(end of repetition).
Is it alright to assume that at the deceleration phase less force than the weight is applied?

2. Nov 25, 2010

### Ivan Seeking

Staff Emeritus
Why assume?

In which direction is the net acceleration acting if the upward velocity is decreasing?

3. Nov 25, 2010

### douglis

The net acceleration is acting downward so the net force must be negative...so the force that is applied is less than the weight.Right?

4. Nov 25, 2010

### Ivan Seeking

Staff Emeritus
It sounds like you already know but don't trust the math.

5. Nov 25, 2010

### douglis

Yes...for some reason(...long story) I had to play the stupid and ask an obvious question.

6. Nov 25, 2010

### aris1

Positive net force(force applied greater than the weight) causes acceleration and negative net force(force applied less than the weight) causes deceleration.

7. Nov 25, 2010

### Galteeth

When lifting, you generally want to provide resistance to the "deceleration" all the way down, not just at the end.

8. Nov 25, 2010

### lisab

Staff Emeritus
Absolutely. Using this technique can prevent injury. Allowing the weight to so something like a free fall is a good way to mess up a joint.

9. Nov 25, 2010

### douglis

By deceleration phase I meant the ending part of the lifting phase(not the lowering) where you have to decelerate the weight in order to prevent it "fly" from your hands.
For that part the applied force must be less than the weight.

10. Nov 25, 2010

### Galteeth

right. I misunderstood because in your OP you said "at the end of the repetition" and I would consider both the lifting and the lowering to constitute one rep.

11. Nov 25, 2010

### Proton Soup

i try to avoid doing this, though. the trick is to not let go.

studies have shown (not sure i could dig it up on cue) that you lift the most weight when you're attempting to go as fast as possible. at a lower weight, this means the plates will rattle from the sudden deceleration at the top. as weight increases, it smooths out.

but, yeah, inertia is a major part of lifting. without it, you just wouldn't make it through those sticking points without lowering the weight.

12. Nov 26, 2010

### douglis

Sure...if you goal is to lift the heaviest weight as possible you should take advantage of the inertia.But if you just want to train your muscles you should let them do the job and not count on the inertia.Therefore I prefer a more controlled lifting.