Question About Rotational Forces when Lifting Weights

[FizicsAM]

Hello, apologies if this is the wrong forum to post this.

I have a question related to fitness. When holding a barbell overhead, sometimes it can spin due to poor timing, flexibility, etc. However, I notice it is significantly harder to counter the barbell's spin when holding it with a narrow grip compared to a wider grip, assuming the lower body position is the same. Can anyone explain why this is?

Additionally, with the narrow grip it feels like there is less room for error in the forward-backward direction, holding the lower body position constant. Does this have anything to do with the fact that the barbell is higher off the ground due to the narrower grip? If yes, why does this create more potential for instability?

 

[kuruman]

I think it's a matter of lever arm for the torque you need to apply in order to stop the spin. Your arms can provide a more or less fixed amount of force. However, to stop the spin you need a torque which is the product of the force that one arm applies times the separation between your arms^*. The higher the torque the higher the angular deceleration, the faster the spin will stop.

This idea should not be strange to you. You need a torque to close or open a door. You do so with least effort if you push or pull as far away from the hinge (relatively large lever arm) as possible. If you try to push or pull near a hinge, the door will not budge.

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^*This assumes that your hands exert equal forces in opposite directions, i.e. you have a couple.

 

[FizicsAM]

Thanks for your explanation.

So in this case, your shoulder is the door hinge and the weight's rotation is like the rotational for to open or close a door, correct?

So in terms of levers:

1) What is the distance between you left and right hands called?
2) Is the distance between your right hand and the end of the barbell the resistance arm?

What about the shoulders? They are not able to move despite the hand spacing but it feels more difficult on the shoulders to resist the spin with a narrow grip. Thanks for your patience with this question :)

 

[kuruman]

My assumption is that you are talking about stopping rotation of the barbells in a horizontal plane, that is the height of each weight about the floor remains the same. In this picture, the job of the shoulders is to hold the weight up and keep it from falling on your head. The also act as a secondary pivot to stop the spin (see below).

The distance between your hands is called the lever arm or the moment arm. As mentioned above, the larger it is, the easier it is to stop the spin by applying a force with your hands. I would assume that the wrists are locked and you don't try to stop the rotation with them. The hands themselves can exert a force as you pivot each arm about the shoulder by contracting or relaxing the appropriate muscles in each arm. Thus, the overall torque to stop the rotation is borne by the shoulders as pivots and the arm muscles between the shoulders and the hands as force generators. Disclaimer: I am not a physical therapist or athletic trainer; I am only guessing here.

The distance between the hand and the end of the barbell has no name that I know. Sorry I cannot be of more help.

 

[FizicsAM]

Thank you that's very clear about the rotation aspect.

In terms of height, the barbell is higher with a narrow grip than a wide grip because the arms are more vertical with a narrow grip. Would this height difference affect stability in the forward-backward direction? Or would it reduce the room for error before the system loses balance?

 

[A.T.]

Would this height difference affect stability in the forward-backward direction? Or would it reduce the room for error before the system loses balance?

To answer this you have to isolate the height effect from the width effect. So let's say you have different arm lengths and hold the weight always with vertical arms, but at different heights.

If your feet are fixed to the ground, the higher weight is more difficult to balance. If you are allowed to make steps, the higher weight is easier to balance

 

[FizicsAM]

How do you isolate the width effect and height effect?

Yes, assume the feet are on the ground as in the photos. My question is why is the higher with more difficult to balance?

 

[FizicsAM]

Another question, what is the resistance arm in this scenario? I thought it would the lever formed from the hand to the end of the barbell...

 

[A.T.]

Yes, assume the feet are on the ground as in the photos. My question is why is the higher with more difficult to balance?

If the feet are fixed: For the same angular disturbance from the vertical a higher weight has a longer horizontal lever arm, so it requires more countering moment from the body.

If the feet can move: A higher weight also falls over slower, so you have more time to adjust the base of support.