# The Physics of Exercise – Kettlebell Swing I thought I’d post something fun today. Today I was wondering what the energy needs to some of the exercises were that I was doing, and I amused myself running the numbers on some of the exercises I was doing. Here the numbers on the kettlebell swing, the others will follow when I get a chance writing them down

## Kettlebell Swing

First we compute the physical energy of lifting the kettlebell, and of lowering the body, giving 600J per rep, or 78kJ = 19kcal for 130 reps when I am plugging in my approximate stats

• Bodyweight 100kg, body height 2m
• Kettlebell weight 20kg; reps 10×13=130
• Movement of kettlebell ~ 100% of body height = 2m; energy = 20[kg] x 2[m] x 10 = 400J
• Movement of body ~ 10% of body height = 20cm; energy = 100[kg] x 0.2[m] x 10 = 200J
• Total energy for one rep = 400J + 200J = 600J
• Total energy for 130 reps = 130 x 600J = 78,000J = 78kJ = 18.8kcal

All of the above is science (apart from the 100% and 10% estimates, but those are probably not too far off). Now it gets fuzzy however: the body is rather inefficient, so in order to work the equivalent of 19kcal it needs to expend much more energy than that (this is why you get warm working out). I have randomly assigned a number of 3 to this inefficiency multiplier, a choice that I will have to review when I am further through the exercises.

• Total energy expended = 19kcal x 3 = 60kcal
• Protein or carb equivalent = 60[kcal] / 4 = 15g
• Fat equivalent = 60[kcal] / 9 = 7g

So to summarise: the work (in the physical sense) that I have done in my 10x30s@20kg kettlebell swings today was about 19kcal. In order to achieve this, my body needed to expend 60kcal of energy, which is the equivalent of 15g of carbs.

## Appendix – Fundamental Relationships

• Earth acceleration ca 10 m/s^2
• Energy of lifting 1kg by 1m = 10J
• 1kcal = 4.19 kJoule
• 1kcal = 4g carbs = 4g protein = 9g fat
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