While these are usually performed at speed, as a cardio/core exercise, I recommend you do them slowly, to ensure safer and more effective execution. They will support your back, preventing future niggles, and help improve mobility in all four limbs. If you struggle to hold a regular push-up position, put your hands on to a chair.
a) Come into a push-up position.
b) Slowly bring your right knee towards your right elbow.
c) Extend your right leg back and repeat using your left leg.
d) Keep your core tight throughout, and avoid letting your hips sink towards the floor.
e) Repeat 30 times (15 each side).