This rectus abdominis exercise is a killer.
There is a lot going on as you move backward into the position, working your balance and your back, as well as your sit-up muscle, the rectus abdominis.
The rectus abdominis is a very difficult muscle to work with. I think these muscles tend to be too long due to people habitually leaning backward.
This shortens the back of the body and lengthens the front leaving the rectus abdominis longer than it should be.
It is also stronger than we would like in relation to the other abdominal muscles.
There are four sets of paired abdominal muscles and though they are separate muscles all are bound together through fascia and tendons.
Ideally, these muscles have equal tone so they can function together like a team.
Instead, rectus abdominis tends to dominate, working too often and too hard. In general, the brain calls on any available muscle to accomplish a task and in the case of the abdominal muscles, the rectus abdominis answers the call far too often.
When assessing the strength of a client, the first thing I look at is the transverse abdominis—abdominal muscles that wrap from the back to the front meeting at the linea alba, the center line of the stomach.
This muscle is the first stop when it comes to core support.
A stable and supported spine is dependent on good tone in the transverse abdominis. Everyone interested in building a new body would be smart to start with this muscle.
This complicates things a bit. We don’t want to shorten the rectus abdominis unless the transverse abdominis is strong (and the psoas released but that is a whole other story).
But to bring lasting changes to your posture you will eventually have to shorten your rectus abdominis.
As I am wont to say, changing your body is like undertaking a science project. It won’t happen by osmosis and you need to understand why you are doing the exercises you are choosing to do.
On that note here is:
  
Rectus Abdominis Exercise: Six Pack on a Block
- Sit on a block with your legs either outstretched or slightly bent at the knees.
- Interlace your fingers behind the head. If your shoulders are tight have the fingertips touching instead of interlaced. The elbows should be open as wide as possible.
- Lean backwards and round slightly forwards shortening the distance between the rib cage and the pelvis. It is like a counter intuitive crunch. The rectus abdominis muscle between the pelvis and the ribcage should shorten and engage.
- Keep your heels on the floor. Bend your knees a little if the backs of the legs feel tight.
- If possible keep the elbows wide as you round forward. This works the latissimus dorsi, a large back muscle that connects the arm to the spine and pelvis.
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