Good posture is so important for any workout or athletic endeavor. Exercise of the day: planks!
People often lose their structural integrity by dropping their hips and letting their lower back arch, or by tucking their pelvis, rounding their back, and pushing their shoulders forward.
Left: Poor form marked by dropped hips and arched back. Right: Poor form marked by rounded back and forward shoulders.
It does take more work for your abdominal muscles, particularly your internal obliques, to maintain proper form...but when you're planking, isn't that what you're going for?? Don't fool yourself into thinking you're getting a better workout by doing a longer plank with bad form!
Characteristics of a good plank include an elongated J-spine, an anteverted pelvis, and shoulders that remain back.
Keep in mind that you are a precision instrument — you have intricate parts that have a very particular baseline architecture. When you stray from the baseline and then load the structure in exercise, you may be irritating tendons, bursa, and muscles, stressing bony surfaces in ways that cause bone spurs to form, and blocking circulation. People sometimes look to exercise to fix alignment, but it’s best to focus on alignment directly to fix alignment. This is usually obvious in inanimate objects. If your car has a crooked axle, you know it needs to be fixed directly; you wouldn’t think to drive extra hard to fix the axle! It’s similar in your body — fix your alignment to make your exercise safer. And as a bonus, you’ll get more out of your exercise, too!