A plank works the front and sides of your trunk
In a 2016 study by Escamilla and colleagues, 18 adults did ten core exercises while sensors on the skin recorded seven muscles. Each reading is a share of that muscle’s maximum effort. The plank on toes reached 40% in the lower abdominals and in the outer obliques, and 34% in the upper abdominals.
The lower back barely took part, at 5%. In a plank the front of your trunk does the work of keeping your hips off the floor. Each plank in the study was a one-second hold, repeated five times, so the numbers show how hard the muscles worked, not what weeks of long holds would do.
- Lower abs40%
- Outer obliques40%
- Upper abs34%
- Inner obliques29%
- Front of thigh20%
- Lats18%
- Lower back5%
Plank against crunch, muscle by muscle
The same study tested the crunch. It led in the upper abdominals, 53% to the plank’s 34%. The lower abdominals came out even at 39% and 40%. The plank led in the outer obliques, 40% to 28%, and the authors found it two to three times stronger than the crunch in the lats and in the rectus femoris, the muscle at the front of the thigh.
That last result has a catch. The authors note that a busy rectus femoris tips the pelvis forward and arches the lower back, which may not suit people with weak abdominals or back problems. The plank from your knees dropped it to 7% in the same study. For the sides of your trunk, the side plank on toes went higher still: 62% in the outer obliques and 29% in the lower back.
| Muscle | Plank | Crunch |
|---|---|---|
| Upper abs | 34% 53% | |
| Lower abs | 40% 39% | |
| Outer obliques | 40% 28% | |
| Inner obliques | 29% 33% | |
| Lats | 18% 8% | |
| Front of thigh | 20% 6% | |
| Lower back | 5% 5% | |
Researchers time the plank until the form breaks
Strand and colleagues timed 471 college students in a forearm plank, held until they tired, chose to stop or lost proper form. Half the men lasted 110 seconds or more, and half the women 72 seconds or more. The authors picked a static hold partly because it is easier to judge than sit-ups, where someone has to decide whether every rep was good enough.
Stuart McGill’s lab used timed holds for the trunk too. In a 1999 study of 75 young adults, people got nearly the same times again on five days in a row and eight weeks later, and the authors offered the times as targets for testing and rehab. To see where your own hold falls, the plank time calculator has the full table.
- 1:50 Men, 194
- 1:12 Women, 277
Half of each group held at least this long. The ring is three minutes.
Why the timer stops when your hips sag
The Strand test ended when form broke, and the timer in Pushgard follows the same idea. The phone lies on the floor to your side and watches two things: the angle at your hips between shoulders and knees, and how high your hips sit above your elbows, in shoulder heights. In recorded planks, clean holds stayed between 171 and 180 degrees. Below 166 degrees, or with the hips under 0.40 shoulder heights, the clock stops and the app says “Don’t let your hips sag”.
You then get 5 seconds to come back into line. Once your hips have stayed up for a quarter of a second, the clock carries on from where it stopped. Miss the 5 seconds and the set ends, because a plank in Pushgard is one attempt. Drag the hips down and watch what the timer does.
Hip height in shoulder heights, as the timer measures it.
Drag right to sag, then back within 5 seconds.
How often to plank, and what the game counts
The WHO 2020 guidelines ask adults to do muscle-strengthening work at moderate or greater effort, for all major muscle groups, on 2 or more days a week. A plank covers your trunk. Legs, chest and arms need other moves, such as squats and push-ups.
For a plan, the 30-day plank challenge builds two daily holds from your best time, and the plank form guide covers elbow position and the straight line. In the Pushgard camera counter, a clean plank hits the monster in front of you, and about three minutes of plank count the same as 30 push-ups.
About three minutes of plank count the same as 30 push-ups in Pushgard.
