When I do planks at the gym, I can ask someone else to check my form and make sure I'm not sagging or lifting my butt up to high. But I can't always go to the gym, and then I end up doing planks at home. I don't seem to have a good feel for weather I'm doing it correctly even at the gym, and since I'm often not sore after doing planks at home I think I'm doing them wrong. Is there some way to tell if you're doing the plank correctly without having someone else watch you?
Use a small foam block that is just the right height so that if you are barely are touching it with your stomach, your hips are neither sagging nor lifted too high.
The foam block should be very soft, so that it's not actually capable of supporting you at all.
How to do the plank correctly
Looking from a distance, your upper body can seem to be parallel to the floor, but it is not what really makes the difference. The important thing is to engage the right muscles. Firstly, make sure you are not collapsing in the upper back. Broaden and depress your shoulders, and lift between the shoulder blades. Second, make sure you don't stick your tailbone up. In the plank position slightly tilt the pelvis: move the top of the hips sligthly up, the tailbone down. These will lengthen your spine and activate the right muscles around your shoulders and core. Make sure you can breathe smoothly and effortlessly into your belly.
How to make sure you're doing it right
The easiest is to have someone correct you, by touching the spots where you have problems, and gently adjusting you so that you get a feel for what the verbal instructions mean. If no-one is available, you can try to get the feeling in the following way:
How to progress
Once you can do 2-3 minutes, progress to more dynamic versions (eg. lifting one hand-leg, or doing it on unstable surfaces, balls etc. so you need to stabilize more). There's no need to do hours of it. If you can do a few minutes, move on to something more challenging.