So plank is a really awesome exercise, or posture that has you working every part of your body and is seen in any type of workout at an entry level. Why is such a compound exercise used in beginner levels? Because it's a kick ass workout and can be used to intensify any other workout movement (ie: think of marching in place, or jogging with high knees. Now do that in plank; you've got mountain climbers. Which is harder: marching in place or mountain climbers?) The trick is getting yourself ready for plank. Like I said it's a total body movement so your upper and lower bodies need to be strong enough to support you as well as your core. This was prepared to help get you out of your modified plank and into a great solid plank that will stay with you a lifetime.
First thing is first; don't be afraid to modify! I can't stress this enough. When your body is telling you, you've had enough, listen to it, and pull back. Don't try to push through and compromise your form and your body. One of the biggest mistakes I see in form when teaching plank is the high booty.
You want to make sure that you aren't over arching the back or tilting the pelvis so that your rear end goes to the sky of course, but make sure you aren't pulling back in your lower body more than your pulling in your upper body. (In a plank your head should be reaching for the front of the room, while our heels reach for the back. It's these opposing forces in your body that keep you straight in your plank. If your legs are stronger that your upper body and you allow them to pull your entire body too far back so that your hands are out in front some and your hips are set back some, your hips will naturally go up.) This is what I see most often. Make sure that your head is always pulling you forward, and if your arms have had enough, modify them.
The first thing you need to check is your arms. Are they under your shoulders? Let's look at how to modify your arms to help build the upper body strength needed to get the hands and arms in the right position.
One way to modify the arms is to come to the forearms. This is a great option for people suffering from arthritis or pain in the wrists, forearms or elbows. This also shortens the lever of your arms so that you can continue to pull your head towards the front for more time than when your arms were extended.
Another option to help you find upper body strength is to use a strap or band. Tie off your strap tightly, or place a small band around your arms between your elbow and shoulder. Allow your arms to press into the band or strap. This will add support to your arms to that you are using less effort from your upper body, which will in turn allow you to build more strength in plank.
Once you have a good handle on the strength needed to hold your upper body up, start to lift yourself out of the joints. What I mean is don't allow your elbows, wrists and shoulders be what hold you up. Allow the muscles to lift you. One way to start to notice this is to slightly bend the elbows a little bit. As you bend them, pull the elbows towards you so that they bend back, instead of out. This is a tiny bend which allows you to really target the muscles activating.
When you plank you should feel everything in your body light up, including the energy flowing in and out of your finger tips. To help strengthen your fingers, and hand strength, you can try holding plank on your finger tips. Start on your knees, hands below your shoulders. One hand at a time, find your finger tips, and slowly shit the weight off your hips and distribute to the upper body. As this becomes easier, lift the knees and approach this from a full plank.
Ok so we've looked at the upper body, now let's look at the lower. You may have started reading this and say to yourself "my hands are fine, my upper body is fine, but my butt still sticks up." So let's look at the lower half of the body. We know that the upper and lower bodies are pulling away from each other, and we know now how to modify our arms, but if your legs are still out-powering your arms, then its time to modify the legs.
The easiest way to modify your legs is to drop the knees (but this is when a lot of people mess up). When you drop the knees, come from your plank and let your knees come to the ground directly below where they would have been in plank. Do not bend the knees and drop them under the hips. This is not a plank, this is a different movement know as a table top. When you set your knees up under your hips, it pushes the hips up higher in the air and reroutes the opposing forces of heels and head, to tailbone to head. By keeping the knees further back the line from heels to head stays intact.
Keeping the knees further back also allows you to come to the forearms