The intrinsic muscles are listed to the right and are the intended muscles during holding the plank position.  The extrinsic muscles, listed to the left, are the ones recruited, after 10 seconds of static planking, in order to maintain the position for extended periods of time.

The intrinsic muscles are listed to the right and are the intended muscles during holding the plank position.  The extrinsic muscles, listed to the left, are the ones recruited, after 10 seconds of static planking, in order to maintain the position for extended periods of time.

    The exercise referred to as a "Plank" is a fabulous exercise........ If done right! Unfortunately the vast majority of people are doing it WRONG! Whats worse is that many times its the personal trainers and gyms that lead private and/or group workouts who are responsible for improper instruction. Don't get me wrong, I don't believe they are doing it intentionally, Its more of a "if some is good, more is better" mindset. It may also be in part due to a lack of understanding of function and physiology of the muscles targeted by this exercise. So allow me to elaborate and help you to understand a little better:

Plank is designed to target the Transversospinales group (rotatores, interspinalis, semispinalis, intertranversarii and multifidus) which is responsible for inter and intrasegmental stability in the spine. These muscles are very small and aren't designed to hold contraction for more than 6-10 seconds. They are comprised almost entirely of Type II (fast twitch, non-oxidative fibers) which have faster and stronger contraction but fatigue quickly. Once the muscle has fatigue it fails to hold contraction and is no longer functional. 
     

A dynamic plank has been shown to be just as difficult yet allows the intrinsic muscles to adapt, dynamic allowing for alternating contractile and rest periods, to long term holds and of the position resulting in proper stabilization of the lumbar spine.

A dynamic plank has been shown to be just as difficult yet allows the intrinsic muscles to adapt, dynamic allowing for alternating contractile and rest periods, to long term holds and of the position resulting in proper stabilization of the lumbar spine.

     You may ask: "Then how is it I can hold a plank for 1, 2 or even 5 minutes then?". Simple; Your brain subconsciously recruits other muscles to get the job done and as we all have, hopefully, learned at some point in our life, just because you CAN do something doesn't mean you SHOULD! To put it into context: When you hold a plank for longer than 10 sec you are creating "muscle memory" that is incorrect as applies to functional spinal stability. Instead of using the small muscles spanning 1-3 vertebrae, the brain is forced to recruit the large muscle spanning 5, 6, sometimes 10 or more vertebrae to get the job done. You're creating an IMPROPER default pathway for your nervous system (brain) when stabilizing the low back. A pathway that will eventually lead to pain and dysfunction (herniated disc and sciatica are the most prevalent).
So in closing; Im not saying you can't hold planks for longer than 6-10 seconds EVER. Im simply offering an explanation as to why you may reconsider holding them that long. After all, if your gonna take the time out of your busy day to take care of your body; a job worth doing is worth doing right! And if you are one of the unlucky experiencing daily low back pain, perhaps this has helped isolate a contributing factor to your dysfunction.

Cheers
Mark Aylor DC