Abs circuit 4

This Abs circuit has 3 levels and requires no equipment so can be used anywhere, so long as you have enough space to assume a plank and sit up position.The circuit is designed to target all of the Abdominal area, (abs, seratus, obliques and lower back.)
In this circuit style training there are no rep ranges, instead I want you to do as many reps as you can, with good form in 20 seconds per exercise.

Circuit 1:
Abdominal crunch - 20 seconds
Reverse crunch (knee crunch)  - 20 seconds
Toe touches - 20 seconds (feet in close to bum)
Flutter kicks - 20 seconds
Plank - 20 seconds

Circuit 2:
Sit ups - 20 seconds
Lying leg raises - 20 seconds
Toe touches - 20 seconds (put your feet further out)
Scissor kicks - 20 seconds
Plank with alternate hand raises - 20 seconds

Circuit 3:
V-shaped sit ups - 20 seconds
Lying leg raises lifting hips up as well - 20 seconds
Russian twists - 20 seconds
Alternate scissor kicks and flutter kicks (4 of each) - 20 seconds
Plank with alternate hand and feet raises - 20 seconds

Beginners, I recommend using circuits 1 and 2, perform circuit 1 twice with 30-60 seconds rest between circuits and then finish with circuit 2.
Intermediates, I recommend using all 3 in order with 30-45 seconds rest between circuits
Advanced, I recommend using either all three in order with only 30 seconds rest between circuits and perform each exercise for 30 seconds each, or perform circuit 2 twice and then circuit 3.