What Muscles They Work, How to Do Them, and Variations

The plank is an isometric body-weight exercise that boosts core endurance. (Isometric means that there’s no movement involved.)

The core includes the pelvic floor, obliques, transverse abdominals, rectus abdominals, and the erector spinae — the muscles along the backside of your trunk that help you stand up, as well as straighten your back while sitting.

But even though you’re not moving, your muscles (including ones other than the core) are working.

How Planks Strengthen the Abdominal Muscles

In a plank, you’ll predominantly be using the transverse abdominis and rectus abdominis muscles of the abdominal wall, says Cameron Yuen, DPT, CSCS, a doctor of physical therapy and practicing physical therapist at Bespoke Physical Therapy, a physical therapy practice in New York City, Seattle, and San Diego.

It’s the group of muscles on the front side of your abdomen: “The rectus abdominis is the most superficial muscle and creates the ‘six-pack look,’ while the transverse abdominis is the deepest abdominal muscle,” he explains.

But strong abdominal muscles are for more than aesthetics — they’re important for stabilizing the body, protecting your body’s inner organs, preventing low back pain, and aiding in virtually all body movements.

To see an improvement in abdominal strength, start by holding the plank for 20 seconds several times a week, maintaining good form the whole time, Marcotte says. Gradually increase your hold time as you get stronger.

How Planks Strengthen the Obliques

A plank works the obliques by tapping those muscles to keep the spine and torso stable throughout the move and to prevent rotating and sagging, Marcotte says.

The body has internal and external oblique muscles, which are located above the pelvic floor and run along the sides of the core.

These muscles are key to keeping your trunk steady during daily activities and helping the body twist and turn during rotational movements (just like they’re called upon when you hold a plank position), says Jamie Nuwer, MD, a board-certified family medicine physician with a certificate of added qualification (CAQ) in sports medicine with virtual health platform PlushCare in Dover, New Hampshire.

The obliques are important for full body stabilization, too, since the core muscles attach to the spine and keep the spinal column stable.

One variation in particular — the side plank — targets the obliques. “Planks that include rotation are also highly effective for engaging those side muscles,” Marcotte says. (More on rotational planks below.)

How Planks Strengthen the Back Muscles

Planks strengthen the upper, middle, and lower back muscles as they work to maintain proper spinal alignment and prevent the lower back from sagging or overextending, Marcotte says. “They work with your front (anterior) core muscles to create a solid, stable ‘bridge’ from your shoulders to your glutes,” he says.

Strong back muscles aid in body stability and play a role in every movement. “Strengthening the back muscles helps keep your trunk stable during daily activities and also helps protect your spine from back injury,” Dr. Nuwer says.

And the reverse — strengthening the core — aids the back as well since a weak core contributes to lower back pain. That’s because when the core muscles aren’t pulling their weight, the body turns to other muscle groups, like the back, to compensate.

“Over time the nonmuscular components (like the ligaments, discs, and joints) start to develop degenerative changes that can lead to things like pinched nerves,” Nuwer says.

The best plank variation to help strengthen the back is the forearm plank (more on how to do one below) because you’ll generate more tension through the core and lats. The latissimus dorsi muscles are the large V-shaped muscles connecting your arms to your spine and back. “The key in this position is that the buttocks need to stay slightly elevated, otherwise it will put strain on the joints of the lower back,” Nuwer says.

If you’re currently battling back pain, the best plank for you is one where your body maintains a 45-degree angle, like a modified plank with your knees on the ground.

How Planks Strengthen the Shoulder and Arm Muscles

Think about what’s holding your body up when you’re in the plank position — it’s your shoulder and arm muscles. “Those muscles are the trapezius, rhomboids, latissimus dorsi, pectoralis, serratus anterior, and deltoids,” Nuwer says.

Engaging the large shoulder stabilizing muscles can help protect the shoulder and improve stability, she adds.

Planks are good to add to your arm strength workouts, too. The bicep and tricep muscles are attached to all of these other major arm and shoulder muscles, so they come into play, too, when you plank, Nuwer says.

To maximize upper body engagement, try the up-down plank. (More on how to do it below.)