Lay on your back thingies.
Lay on your back and lift your legs up and down. Leg lifts with your gut muscles.
Ahhhh in the Navy we have 2 different leg exercises like that..
1) flutter kicks = knees locked, toes pointed away from you, hands under your butt cheeks, heels are not allowed to touch the ground during the exercise & neither is your head. They are a 4 count exercise
1- right heel way up, left heel just off the ground
2- switch feet positions
3- seemlessly switch back to right heel way up (knee still locked & tees still pointed away from you) & left heel just off the ground
4- switch again
1- that's your 1st count of 40, 4 count flutter kicks
No stopping during this 40 count either.
Then at the end the instructor usually made us hold our feet Rock solid straight for upwards of 2 minutes before he would say "down".
Next is the really stamina inducing leg lift.
Nickname = good morning darlings
These are also 4 counts
Almost the same starting position
1st thing is head & heels off the ground and they stay off the ground during the entire exercise as well; 2nd knees locked & feet have toes pointed straight up; 3rd hands are again under the butt cheeks
Count one is both feet are spread as wide as possible and staying off the ground
Count 2 is return to ankles touching & straight with your spine
Count 3 is repeat of 1 still keeping feet and head off the ground
Count 4 is a return to start and then it's done 40 more times.....
And yup. You guess correctly, the instructor would do the same wait before "down" was uttered and we could take a rest.
Then of course there were the 8 count body builders, sit ups, push ups (4 types= regular, wide arm, tricept & one armed) & of course Superman's too(flutter kicks on your stomach)....
All this was never fun out on the beach for Aircrew or rescue swimmer school!