I can not hold a plank for a long time, what should I do?

You can increase the length of time you hold a plank by first modifying (regressing) the exercises and then changing other acute variables. The simplest modification for a plank is to perform the exercise on your knees, rather than on your toes, and forearms. If this modification is still too difficult try performing it in a more vertical position. For example, place your forearms on a step, bench, or chair. This will decrease the demand placed on your core and allow you to progress in terms of body position and time under tension. When adjusting the acute variables, you can perform two to three sets of the exercise with 15 repetitions per set, holding the top position for 2-4 seconds and working to expand the time at the top. This will help you establish the muscular endurance necessary to sustain the position for a longer period of time.

Like most things in exercise you may not be able to do much at first but practice will help.  There are seveal ways to achieve at holding a plank longer. The common plank is where you are on your forearms and toes.  If this is too hard for you there are more simple variations.  A good way is either standing or sitting in your chair and drawing your belly button towards your spine.  If this is too easy you can modify the floor version by going on your knees and forearms.  By this time you should be able to do the standard floor plank on your forearms and toes.  All it takes is practice and commitment.

A plank is a great exercise for core strength but can be a very difficult exercise.  There are a few ways you can go about improving this.  1) Time yourself and see how long you can hold, even if it's not a long time the more you practice it the stronger you will get and the longer you should be able to hold.  Ex. if you can only hold for 15 seconds, every week see if you can hold for an additional 5 seconds.  Holding for 1minute straight is a common bench mark trainers use. 2) You can de-gress the exercise by elevating the body slightly.  Try it on your hands, or elevated on a bench, this decreases the load of gravity on the body thus making it easier to hold.  Or you can try it on your knees.  3) Do other exercises that promote core strength but are not as demanding.  One of my favorites I call a "Leg Slide".  Lie on your back with feet up, and knees/hips forming a 90 degree angle.  Keep a slight arch in the low back (just enough to fit the width of your hand underneath), engage you abdominals and slowly straighten the leg so that it is parallel to the floor.  Be sure not to move the low back when doing this.  Come back to the start position and do the same thing with the other leg.