As with any exercise it depends on what you are trying to achieve
If a person is new to a side plank I would do isometric only until they are comfortable and confident with their form.
Side Planks can be tough on the shoulder girdle and adding a twist without being fully aware of the movement can sometimes be extremely difficult and could lead to injury.
Hi Fran. “Most effective” depends on a number of factors such as your training goal, any underlying physical limitations, physical condition of the person performing them etc. In the wonderful world of exercise, variety is the spice of life, and so no matter if one exercise is “more effective” than another or not, I will often use a variety of the same exercise to keep things interesting, challenging, and boredom-free!
/agree with LaRue.
It depends on who you are training, what their goals are and how they wish to implement it into their functional training.
I would personally argue that the less they do with more weight would be the best way to go. Most people fall into the “abdominal workout” = more reps.
They are a muscle like any other part of the body and are a mix of type I and II mucles fibers. The best way to stimulate their growth would be aiming for 2-3 sets of 16 or less reps.
For research, see anything by Dr. Stuart McGill, researcher on spine mechanics, here’s his website:
discussion of “core”, Part 1 and Part 2,
http://www.youtube.com/watch?v=tr2xlwXwrWA&feature=related , http://www.youtube.com/watch?v=IyMm9oq2AgY&feature=relmfu