These are actually 2 progressions which you should perform in parallel, in each session. Jointly, they will help you develop a rock solid core and work the muscles in your stomach (the rectus abdominis and the obliques, as well as stabilizing deep core muscles). The plank progression works these muscles isometrically (where the length of the muscle worked does not change) whereas leg raises develop them both concentrically and eccentrically.
These progressions will ultimately lead to such exercises as the dragon flag (a favourite of Bruce Lee), the hanging dragon flag and –from gymnastics- floor L-holds.
Unless otherwise indicated, you should be aiming to complete 6 sets of 6 repetitions of the given exercise within a workout.
The first set is a warm up set: go back two or three levels in the progression and perform that variation for 6 reps.
Perform each following set ‘to failure’ (e.g. 6,6,5,5,4).
In each new workout, you should aim to improve on the number of reps you performed previously by at least 1, e.g.: (6,6,5,5,4) → (6,6,6,5,4)
Once you manage 6 sets of 6 reps, you are ready to move on to the next variation in the progression.
If you find yourself not progressing any longer, take a few rest days, then resume training.
Leg raises progression:
3. Flat leg raises. Lying on the floor, bring your legs vertical, keeping them straight at all times, then return without touching the floor.
With all the plank exercises, start at 30s, then build up to 1 minute, before moving on to the next exercise.