The penis itself doesn’t contain any skeletal muscle you can flex or control. What it does contain is smooth muscle, found inside the two columns of erectile tissue called the corpora cavernosa. These smooth muscle cells are the ones responsible for erections, and they work automatically without conscious effort. The muscles most people associate with the penis are actually located at its base: the bulbospongiosus and the ischiocavernosus, both of which play direct roles in sexual function.
Smooth Muscle Inside the Penis
The penis is built around three columns of spongy erectile tissue wrapped in connective tissue and covered with skin. The two upper columns are the corpora cavernosa, and the single lower column, which surrounds the urethra, is the corpus spongiosum. Woven throughout the corpora cavernosa are smooth muscle cells. Unlike the biceps or quads, smooth muscle can’t be voluntarily contracted. It operates on chemical signals from the nervous system.
During an erection, nerve endings and blood vessel cells in the corpora cavernosa release a signaling molecule called nitric oxide. This triggers a chain reaction that relaxes the smooth muscle, allowing blood to rush in and fill the erectile tissue. Surrounding the corpora cavernosa is a tough fibrous sheath called the tunica albuginea, which is normally about 2 mm thick but stretches to as thin as 0.25 mm during an erection. This sheath traps the blood inside, creating the pressure and firmness of an erection. It can withstand internal pressures up to 1,500 mm Hg before rupturing.
So if someone asks “what muscle makes the penis hard,” the answer is the smooth muscle lining the corpora cavernosa. When it relaxes, blood flows in. When it contracts again, blood drains out and the erection subsides.
The Two Muscles at the Base of the Penis
The skeletal muscles connected to the penis sit at its root, in the pelvic floor. These are voluntary muscles, meaning you can learn to contract them on purpose. Two are especially important for sexual function.
The ischiocavernosus muscle wraps around the base of each corpus cavernosum. Its job is to compress the erectile tissue and push internal blood pressure above what the heart alone can generate. This is what takes an erection from partial to fully rigid. Without the ischiocavernosus contracting during arousal, erections would be softer.
The bulbospongiosus muscle surrounds the base of the corpus spongiosum and the bulb of the penis. Its primary role is ejaculation: it produces the rhythmic contractions that propel semen through the urethra. It also helps empty the urethra after urination.
Both of these are part of the superficial layer of the pelvic floor, a dome-shaped sheet of mostly striated (voluntary) muscle that supports the bladder, bowel, and sexual organs.
Why Smooth Muscle Health Matters
The condition of the smooth muscle inside the corpora cavernosa has a direct impact on erectile function. Over time, that smooth muscle can undergo atrophy (wasting) or fibrosis (replacement with scar-like tissue). When this happens, the smooth muscle can no longer relax fully, which means less blood gets trapped inside the penis during arousal. The result is difficulty maintaining a firm erection.
This process is sometimes called “venous leak,” but the problem isn’t actually with the veins. It’s a malfunction of the smooth muscle surrounding them. According to Johns Hopkins Medicine, this is now considered an early sign of atherosclerosis and vascular disease. The same processes that stiffen and damage blood vessels elsewhere in the body also affect the delicate smooth muscle tissue in the penis. That’s one reason erectile difficulties can sometimes appear years before other cardiovascular symptoms.
Strengthening the Pelvic Floor Muscles
You can’t exercise the smooth muscle inside the penis, but you can strengthen the ischiocavernosus and bulbospongiosus through pelvic floor exercises, commonly called Kegels. These involve repeatedly contracting and relaxing the muscles you’d use to stop the flow of urine midstream.
Consistent pelvic floor training supports bladder and bowel control and can improve sexual function. Stronger pelvic floor muscles help the ischiocavernosus generate more pressure during erections and give the bulbospongiosus more force during ejaculation. The exercises are simple, require no equipment, and can be done anywhere. Most programs suggest several sets of 10-second holds per day, gradually increasing over weeks as the muscles get stronger.