Pooping uses more muscles than most people realize. A bowel movement requires coordinated effort from your diaphragm, abdominal wall muscles, pelvic floor muscles, two anal sphincters, and a sling-shaped muscle called the puborectalis. Some of these muscles actively push, others have to relax at just the right moment, and a few work automatically without any conscious input from you.
The Diaphragm Starts the Push
The process begins with your diaphragm, the dome-shaped muscle at the base of your ribcage that you normally use for breathing. When you bear down, your diaphragm contracts and drops, pressing against your abdominal organs from above. This is the same action that happens when you take a deep breath and hold it, sometimes called the Valsalva maneuver. Your diaphragm essentially acts as a piston, generating downward pressure that helps move stool toward the exit.
Your Abdominal Muscles Build Pressure
Right after the diaphragm drops, your abdominal wall muscles contract. This includes the rectus abdominis (the “six-pack” muscle), the obliques along your sides, and the transversus abdominis, the deepest layer that wraps around your torso like a corset. Together with the diaphragm, these muscles squeeze inward to raise the pressure inside your abdomen. That increased pressure pushes down on your rectum and helps propel stool forward. The harder you strain, the more these muscles work, which is why people with weak abdominal muscles sometimes struggle with constipation.
The Puborectalis: Your Body’s Kink Valve
The puborectalis is a U-shaped muscle that loops around the junction between your rectum and anal canal like a sling. At rest, it pulls the rectum forward, creating a sharp bend called the anorectal angle. When you’re sitting on a standard toilet, this angle sits at roughly 80 to 90 degrees. That kink acts like a bend in a garden hose: it pinches the passageway enough to help keep stool in place between bowel movements.
When you’re ready to go, the puborectalis has to relax. As it loosens, the anorectal angle opens up and the rectum straightens, giving stool a clearer path out. In a squatting position, the angle widens to about 100 to 110 degrees, which is why squatting tends to require less straining. If you’ve seen footstools marketed for toilet use, this is the principle behind them: elevating your knees mimics a partial squat and helps the puborectalis release more fully.
The Pelvic Floor Has to Let Go
Your pelvic floor is a hammock of muscles stretched across the bottom of your pelvis, and the largest group within it is called the levator ani. These muscles support your bladder, rectum, and (in women) the uterus. During a bowel movement, the levator ani relaxes and descends slightly, which lowers the pressure around the anal canal and helps stool pass through. Research in urology has shown that levator ani contraction actually lowers anal pressure, suggesting this muscle group actively assists evacuation rather than simply getting out of the way.
Most of the time, you don’t consciously think about relaxing your pelvic floor. It happens reflexively as part of the coordinated sequence your nervous system runs when you decide to have a bowel movement. But when this coordination breaks down, the pelvic floor can tighten instead of relaxing, creating a condition called dyssynergic defecation.
Two Sphincters With Two Different Jobs
Your anal canal is surrounded by two ring-shaped muscles that work as gatekeepers, but they operate on completely different systems.
The internal anal sphincter is involuntary. You can’t control it any more than you can control your heartbeat. It stays contracted at all times to maintain a baseline seal, and it relaxes automatically when your rectum fills and stretch receptors signal that it’s time to go. This reflex is what triggers the urge you feel when a bowel movement is ready.
The external anal sphincter is under your conscious control. It’s the muscle you squeeze when you need to hold it in until you reach a bathroom. When you’re ready, you voluntarily relax it, opening the final gate. These two sphincters have entirely separate nerve supplies, which is why one responds to your brain’s commands and the other operates on autopilot.
How All These Muscles Work Together
A normal bowel movement is a precisely timed sequence. Your rectum fills and contracts to push stool downward. The internal sphincter relaxes reflexively, giving you the urge to go. Once you’re in position, you bear down: your diaphragm drops, your abdominal muscles contract, and intra-abdominal pressure rises. Simultaneously, your puborectalis relaxes to straighten the rectal angle, your pelvic floor descends, and you voluntarily release your external sphincter. The whole process, when it works well, takes minimal effort and less than a minute.
The key insight is that defecation requires both pushing and releasing. The upper half of the system (diaphragm and abs) generates force, while the lower half (puborectalis, pelvic floor, and sphincters) has to open up and get out of the way. Problems on either side of the equation cause trouble. Weak abdominal muscles mean inadequate pushing force. A pelvic floor that won’t relax means the exit stays closed no matter how hard you push.
When These Muscles Don’t Coordinate
Dyssynergic defecation affects up to half of people with chronic constipation. The core problem is a failure of coordination between the muscles that push and the muscles that should relax. Instead of the pelvic floor and external sphincter releasing when you bear down, they paradoxically tighten. It’s like trying to push toothpaste out of a tube while squeezing the cap end shut.
This condition is considered a learned behavioral problem rather than a structural one, which means it responds well to retraining. Biofeedback therapy teaches you to recognize and correct the misfiring pattern, essentially relearning the proper sequence of contract-here, relax-there. During evaluation, a clinician checks whether the puborectalis relaxes and the perineum descends when you push, while also feeling for adequate abdominal effort. In people with dyssynergia, electrical readings of muscle activity show increased or unchanged pelvic floor tension during pushing, the opposite of normal.
How Posture Changes the Equation
The position you sit in directly affects how hard your muscles have to work. On a standard toilet, your hips are at roughly a 90-degree angle, and the puborectalis maintains a tighter grip on the rectal bend. You can still defecate, but it often requires more straining and more time. Squatting opens the anorectal angle to around 100 to 110 degrees, allowing the rectum to straighten more completely and reducing the muscular effort needed to evacuate.
If you deal with straining, leaning forward on the toilet with your elbows on your knees, or placing your feet on a low stool to raise your knees above hip level, can make a meaningful difference. These adjustments partially replicate the squatting posture and allow the puborectalis and pelvic floor to relax more naturally, so your abdominal muscles don’t have to work as hard to compensate.