Thursday, April 26, 2012

Geek numbers

So how does bell size affect power output on the swing? It depends. To a certain extent increasing the bell size can increase power output. The magic number where increasing bell size actually decreases power output is around 30% of our body weight. Above that, and the need to stabilize robs you from applying all your strength to the task at had - projecting the kb forward. Using myself as an example (100kg body weight) my "sweet-spot" bell is the 32kg bell (32% of my bw). Just going from the 24kg bell up to the 32kg bell, my power output jumps 14%. Going from 32kg up to 40kg results in a 2.7% drop in force, and going from 40kg up to 48kg drops my force another 7.6%. 
Force production per kettlebell (in newtons)

Can I swing the 48? Yes. If my goal is maximal power, which bell will get me to my goal the quickest?  The 32kg. For strength, heavier is better. For power, speed is king and the load is the court jester - not really fully understood.

Force production and force:body weight ratio
For me my highest force production comes with the  32kg  (1.866 x BW) on the concentric phase - force production.  However, as the bell size increases, the eccentric loads -force absorption- continue to increase as the bell size increases.  But this is NOT force production - it deceleration.  But, it opens the door to a question Lexus asks on one of their TV ads :"What determines performance? The time it takes to go from 0-60, or the distance it takes to go from 60-0?"  So, again, depending on the goal and the sport/activity going heavy does have it's benefit.

changes in load vs. changes in force

Finally when comparing everything to my snatch weight bell,  increasing the bell size always results in me allays generating more force than I can with the24kg bell.  The grain of salt in this is that increase in force production isn't proportional to the increase in size of the bell.  Meaning, while the 48kg gives me 5.9% more force production than the 24kg bell this comes from increasing my load by 50%.  So a 50% increase in load for a 5.9% increase in performance, not good numbers.  BUT, going up to the 32kg bell (a 25% increase in load) results in performance increase of 14.8%.  Much better.

To wrap it up, increasing the load can only contribute to the power out point to a certain extent.  Once above about 30% of body weight the increase in power output is very nominal - so if power output is the goal heavy isn't better.  Speed is king.  Now, if power is your ULTIMATE GOAL - the swing might not be the best choice in exercise.  WHAT?!?!?!  The swing isn't the best tool - sorry, but no.  There is another KB drill that with the same load that results in significantly more power.  If you are at Level II right now, you are about to be introduced to the drill - if not, go with what you've got, the swing.


  1. What kettlebell drill exceeds the swing in terms of force production?

    Double swing?
    Triple extension swing?

  2. Interesting and thanks for doing the research, but we really need a sample size of more than 1 to draw any meaningful conclusions. Otherwise, the research is basically saying that, for *you* a KB of 30% bodyweight gives the greatest results. That number may drift higher or lower with other study participants.

  3. we do have a sample of more than one - just presenting what I could. the range tends to be 28-30% of bodyweight.

  4. So what's the KB drill that results in more power than the swing?

    The suspense is killing me here...4+ year cliff hanger...

  5. I would be interested to know what was the bell speed at 48 when compared to 32. Additionally, if you got the bell speed of both the 32 and 48 the same would you still see more force production from the 32.

    1. Don't know about the bell speed of the 48 compared to the 32 - I couldn't look at that then, but I'm able to now. Maybe I'll look at that....
      If the bell speed were the same for the 48k and the 32k, then there would be more force production from the 48. Mass x acceleration = force. If acceleration is constant the mass will dictate the force - higher mass will result in higher force. BUT - moving the 48k at the same speed as the 32 would require significantly more force.