Anatomy TUESDAY – Tonic v. Phasic – Part 3 – Quadratus Lumborum v. Rectus Abdominus

Lie on your back and try to do a sit-up without your legs anchored. Is this difficult? You may think this is a weakness of the abdominal muscles, but for many, the inability to do a sit-up is a prime example of the tonic v. phasic relationship of the quadratus lumborum and the rectus abdominus.

As a tonic muscle, the quadratus lumborum is prone to shortening (tightness), creating an extended resting position of the lumbar spine. This places the rectus abdominus in a long/weak position, making anti-gravity trunk flexion more difficult.

If your abdominal muscles feel weak, try this routine:

Stretch the low back muscles using child’s pose or double knee to chest. When you do the latter, actively pull your belly button into your spine. This stretches quadratus lumborum.
Power pelvic tilt. Lie on your back with your feet flat on the floor. Tighten your abdominal and glute muscles to press your back flat into the floor. This is using the concept of activating an agonist (in this case the abdominal muscle) to inhibit the antagonist (tight quadratus lumborum)
THEN try to curl up into a sitting position. Activating the phasic abdominal muscles should feel easier once you have stretched out the tonic quadratus lumborum

Why does this matter?

If a muscle feels weak, don’t blame that muscle first. Phasic muscles may appear weak when in fact it is the antagonist tonic muscles that are overriding activation of the prime mover, in this case, the rectus abdominus. Trying to strengthen a phasic muscle without first lengthening the tonic muscle that opposes it may just cause frustration.

Optimal length tension is 1.2x normal resting length. A tight quadratus lumborum places the abdominal muscles in a long/weak position.

I’m not a fan of doing sit-ups as it isn’t the most functional activation of the abdominal muscles. But one SHOULD be able to do a sit up. Stretch the TONIC before you work the PHASIC.

Because nobody has time to be in pain.

Until next time…