Hi, I do lots of work with short scuba fins. I found using a small kick float (arms stretched out in front) helpfull as it helps by pulling ever so slightly to the surface , which means you can work your upper back more on the down wards part of the undulation.... breath every 2 to 3 kicks and once it feels familliar enough then discard the float. another one i do is 25m staying close to the bottom of the pool with just enough space so as my knees don't hit the bottom of the pool, this one helps me regulate the undulations so that they all become fairly uniform.
keep your head down and go slowww.
my basic fin training is only about 1000m at the moment but I am starting to build it up now.bulk of the work is done in 200m slots on the surface, alternating between front and back plus about 16 x 25m dynamic, thats it so far apart from land training ...sit ups (all sorts) and lower back work and of course good stretching.
I think its probably important to switch over to doing more of the training in the mono rather than the fins ...after all thats what i will be using out there in the big blue
