I started using a kettlebell for core strength last year. I weigh 150 lbs. (68 kg) and bought a 20 kg kettlebell. I use it once a week – single arm swings, cleans and presses. I am not trying to get massive biceps or six-pack abs, just a good solid core. The results in my running have been astounding. I do a track workout once or twice a month and run a 1/4 mile (one lap of my local dirt track) in 68 seconds. Two laps and I start to slow, and a mile comes in at 5:30. I have been visiting the track for 4 years and my biggest speed gains came after I started using the kettlebell. Oh yeah, I’m 51 years old.