Lying Knee - Ups
This is an excellent exercise for the lower abdominal muscles.
Lie on your back with both feet raised, keeping your legs bent at 90 degrees with your knees and ankles together.

Keep your head and hands on the floor throughout the movement, avoid pushing down with your arms to aid the lift.
Exhale and slowly aim to lift both knees upwards, taking your buttocks 1 -2 inches off the floor, whilst lifting the knees directly vertical.
Focus on keeping the legs bent at 90 degrees throughout the movement, concentrating on a upward lift, rather than bringing the knees in towards the chest.
You may feel that there is no physical movement whilst performing this exercise, however the simple internal contraction will help develop the lower abs.

Extending your hands behind your head, with the palms facing upwards will increase the difficulty level of this exercise, likewise, raising the feet higher off the ground will make the exercise easier.
Progress the exercise by holding in the final phase for 3 - 5 seconds, followed by a slow exhale as you lower than repeat.
Aim for 2 -3 sets of 12 plus slow smooth movements. This exercise can be made harder by resting a medicine ball on your upper shins to create resistance, whilst insuring the movement is only vertical.