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I recently quit the local gym (budget restrictions) and started to workout from home instead.

I bought a little set of 2x30kg adjustable dumbbells, and built a little bench for doing dumbbell bench press. This being quite a light weight, I bought some 10kg plates to add up on those dumbbells.

My big surprise was that it felt incredibly more difficult to press, say only 2x30kg with the following setup:

  • 5|10 === 10|5 x2

than with this one:

  • 2|2|2|4|5 === 5|4|2|2|2 x2

The 10kg plates I bought are indeed quite large, but I didn't expect this to have an impact on the workout difficulty. Can anyone help me understand what's happening?

Thanks!

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2  
Did you try and weigh the plates individually? My question is are the plates really weighing the same '?kg' they meant to be? –  Freakyuser Aug 20 '13 at 4:05
    
Wow, I admit I didn't even question this point, and assumed everything was right. Will check later today. Thanks for the insight. –  So Jacques Aug 20 '13 at 5:03
1  
You say that they feel more difficult to press, but are you sure that is because they feel heavier? Or do they feel more difficult to control? –  Kate Aug 20 '13 at 6:21
    
If I had to guess, I'd say the larger plates are forcing you to hold the dumbbells further away from your body so that you can get the same stretch in your chest before pressing. That's going to change how much vertical force you can generate, possibly require more stabilisation, etc. –  Anthony Grist Aug 20 '13 at 22:03
2  
I disagree, this is on topic as it discusses different configurations of equipment. –  Baarn Aug 21 '13 at 16:11
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1 Answer 1

up vote 3 down vote accepted

The narrower your center of gravity the harder it is to balance the plate. Think about trying to press a relatively light bar that was 30 feet across. It would be very taxing to balance this bar even if it didn't weigh much. The balancing will happen from the use of your muscles, and the wider the weight distribution makes it seems heavier - due to more muscle interaction for the same "weight". Physics of lifting.

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This is very true, more plates means the weight is spread out wider (more difficult to balance). –  RealityDysfunction Aug 20 '13 at 13:58
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What does it mean for a center of gravity to be narrow? The center of gravity is a single point. –  Kate Aug 20 '13 at 14:56
    
Well maybe I should say pivot point but center of gravity works too. See your center of gravity is the space between your hands on the bar. Since you have two access points (given that your hands are centered) the entire space should represent the center of gravity. With an olympic bar most of the weight is very close to the center of gravity or pivot point (I don't like that term because we have two equalized points). The fact is the further away the weight is from the center not only is there greater balancing needed but their is diagonal force put on the axis points. –  DMoore Aug 20 '13 at 17:23
    
This is basic physics. Go in the gym with an olympic bar and do a lift that you have to push and that you can do a fair amount of weight - say 45s on each side. Now take the 45s off and do 9x5s on each side. Trust me the 5s will feel much heavier and you will not be able to do near the amount of reps/weight. –  DMoore Aug 20 '13 at 17:24
    
Your answer seems to totally contradict what the person asking the question has observed (they're saying fewer heavier plates feels more difficult than more lighter ones). Also, what you're saying about centre of mass is totally wrong, assuming the plates actually weigh exactly what they're supposed to. –  Anthony Grist Aug 20 '13 at 18:10
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