Run up some stairs or something.
Btw when you do the sit ups do u come right back up??if you do.....you might want to try stopping mid way up and then going back down really low until you almost touch the ground then back up to the mid way point.
Also you might want to lay flat on a table with your legs off of it. your behind should be at the edge of the table and your legs should be dangling off the table. Oh yeah make sure your feet don't touch the ground. And then you can raise you legs up with your knees bent....until your hind legs make a ninety degree angle with your back....................and bring back down your legs slowly until they almost touch the ground and right before they touch the ground bring the legs back up to the ninety degree.
(you probadly do these already so I don't know why I bothered)
This is suppose to be down slowly it really cuts you up when you do it slow and controlled

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Well I hope I explained it right and you understand it.
How many push ups can you guys do at once?? 30? 50? 100??