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Do you have a fitness store nearby that you can try them out first? You can use a dumbbell as a kettle bell to get a feel for how heavy you should start. Tim says the goal is to get to 75 reps without stopping. I just went for it and bought a 40 lbs kettle bell without trying anything else first (ego thing). First try resulted in 15 reps. And I could not get to 75 even with rests. Now I can go over 100 reps at 40 lbs and just got a 55 lbs kettle bell. For you, it depends on your current strength and also fitness (kettle bell swings are aerobic as well as anaerobic).
I agree with th's advice to try different kettlebell weights in a store if you can. I planned to get a heavy one, then when I tried it decided to go lighter. The one I bought is 12 kg. I am 5'7, 140 lb and I can do 40 swings MAX with 12kg so I do 2 sets to reach 75.
Start slow and work on your form. Do them in a mirror or have someone watch if you can. If you're doing them right you shouldn't strain your back. It's all in the hips.
It's worth doing. Ketllebell swings do show results fast!
I started with a 25 lb. It seems perfect for me to start with. I do sometimes have shoulder issues with front raises so I went light. Even though I know this is coming from the hips and not the shoulders I did not want to take any chances. I have really had to be careful about my form because my left knee hurts sometimes when I do them. My back is totally fine with this weight and did not hurt at all even after the first time I did them.
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