Windshield wiper is a good exercise for the lower part of abs and the whole core

Ground windshield wiper without weights

Lie on the floor with your arms in T position for stability and you both of your legs 90 degrees up and straight. Move your legs slowly from right to left in a wind shield wiper motion.

You can also do the tight ground shield wiper by keeping you hands close to your hips – not in T position

Ground windshield wiper with plate

Lie face up on the floor and lift both of your legs 90 degrees . Hold a plate with your arms extended above your face. While keeping your legs together, lower them to either direction (left or right) and simultaneously lower the weight to the opposite direction. Repeat to the other side.

Ground windshield wiper with barbell

Bar windshield wiper

Circular windshield wiper

Starting from a hang lift your legs, ascend and descend in as large an arc sideways as possible