For about 6 months I have been training with bifins doing 2 times 25 meters (50 meters total) dynamic every 90 seconds. I can do this continuously for 60 minutes and find it relaxing and energizing. It takes about 45 secs of swiming and then 45 secs of recovery, I do 6 to 12 quick recovery "half breaths" on surfacing then slow deep relaxed breathing untill swiming again, no puge or hyperventillating breathing before I go. I usually get mild contractions 1/2 way thru last 25 meters. My PB is 4 x 25 meters so 2 x 25 meters is not too hard a push for me. Also these 2 x 25 meter swims always start out hard with more urge to breathe at end and get easier after about 10 minutes. The last two days I have added 3 x 25 meters on 3 minute intervals. I change the breathing to more like 10 to 15 quick recovery half breaths on surfacing then deep slow relaxed breathing then at the end I do 3 purge or quick very deep hyperventillations before swimming. I do 2 lenghts very relaxed and then get mild contractions 1/2 way thru the last length. The thing that is different is that I get most of the time a rush of tingles in my legs on surfacing. It comes and goes quickly similar to the feeling of dizzieness upon standing that sometimes happens though it is different in that is tingly and seems to start in my lower legs and rush upwards above my knees then stops. What is this tingling from and is it a bad sign? Sorry for the long post and thanks!!