I know quite a few competitive breaststrokers who developed knee problems in their later teens, so, yeah. Flutter or dolphin kick, with or without fins ("Zoome

I know quite a few competitive breaststrokers who developed knee problems in their later teens, so, yeah. Flutter or dolphin kick, with or without fins ("Zoomers" are particularly useful) should be safer, and actually is a reasonably good lower-body workout, particularly in conjunction with strength training (squats & deads notably).