Why do we do L-Sits? Here’s an explanation. I know its long but take a minute to read it.

The L-Sit
This exercise is remarkable from several perspectives.
It is isometric, functional, and highly effective. Relatively
unknown outside of the gymnastics community this
exercise may be the most effective abdominal exercise
we know of!
The L-sit is performed by supporting the body entirely
by the arms and holding the legs straight out in front.
The body forms an “L” thus the name L-sit. The exercise
(we can hardly call it a movement) is isometric, i.e., it
involves no joint movement. Being isometric, we quantify its performance not in reps but by time.
We not only contend that the L-sit is functional
but that it is the most functional of all abdominal exercises.

Our justification for this contention lies in our view that
the dominant role of the abdominals is midline stabilization not trunk flexion. Though trunk flexion is certainly
important, midline stabilization is more important both
to everyday living and athletic movement. The leg’s
posture in the L-sit places an enormous, if not unbearable, moment or torque about the hip that must be
counteracted by the abdominals to keep both the legs
up and the spine from hyperextending.
As for efficacy, the L-sit may have no peer among
abdominal exercises. We make this claim not on the
basis of our position on abdominal muscle functionality
but on the simple observation that athletes who have
developed their L-sit to the point where they can hold
it for three minutes subsequently find all other ab work
easy. The gymnasts’ unrivaled capacity at hip and trunk
flexion is in large part due to their constant training and
practice of this exercise.
We mentioned early the ubiquitous phenomenon ofthe ab class instructor with the lower abdominal pooch
– they cannot hold an L-sit. In fact, if you test the ab
class instructor with the lower abdominal pooch for hip
flexion strength you’ll find they are super deficient in this
regard. You can perform a simple hip flexion strength
test by asking the subject being tested to stand on one
leg and raise the other knee to hip level while you press
down on the knee to see how much, or little, force
it takes to push the knee back down. Individuals with
the lower abdominal pooch always have super weak hip
flexors. We can drive their knee down with one finger.
Try this test with someone who has developed the L-sit
and you’ll find that they will tip over before the knee will
drop. You will not find a three minute L-sit and a lower
abdominal pooch in the same person, yet the world
abounds with people who can perform thousands of
crunches and sit-ups and still keep the pooch. It’s that
Practice of the L-sit is for some very tough – they just
can’t seem to find the muscles that raise and hold the
legs. The key is to keep trying. Two successful approaches
for working up to the L-sit include hanging from a pullup bar and raising locked legs as far as possible and holding or working the L-sit by holding one leg at a time
alternately in the L posture.
Though the L-sit can be performed from nearly any
horizontal surface we recommend parallel bars, parallettes, and the floor as platforms for this exercise.
The L-sit is hardest from the floor because the floor
comes up quickly as the legs sag even a little bit. We
use the parallettes for the very reason that it allows
practice at less than perfectly horizontal leg position for
the beginner, but measuring and competing at the L-sit
should be done from the floor.
Measure your progress in the L-sit in 15-second increments. Give your self one point for every fifteen seconds
you can hold the “L”. Twelve points is your goal and with
regular training and practice you should be able to get to
12 points, or three minutes, within six months. During
warm-up and cool-down is the natural place to play
with this movement although the dedicated gymnast
will find uncountable surfaces and opportunities to play
with this superb exercise.

For time:
135 pound Thruster, 15 reps
Run 200 meters
95 pound Thruster, 20 reps
Run 400 meters
65 pound Thruster, 30 reps
Run 800 meters

B) 120 second cumulative L-sit