Tread Drill: If Treading is an Issue - Try Different Kicks: Scissor, Breast, Egg-beater kick
Here is a quick treading drill you can do as a rest set in between fast pace swim sets. Treading could be a mobility issue:
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I usually do a 3 minute tread with these three kicks (1 min each): NO HANDS
Start off with 1 min of scissor kick - alternating left / right leg kicks.
Then do 1 min breast stroke kick - vertical swimming
Then try to start alternating your breast stroke kicks into left / right kicks which is a way to start your egg-beater motion.
Then start your swim set again - resting with tread with no hands.