I've been using planks to increase my trunk strength, and am getting OK at holding the plank for 1 1/2 minutes. Will I get more benefit by starting to increase that time, or adding a different exercise (and which one do you recommend)?
Dr. Stuart McGill suggests everybody should be able to hold the plank position for two minutes.
Once you are comfortable with this, I vary my routine with
- elevated planks
- single leg planks
- stability ball planks
- side planks,
- elevated side planks
- planks with feet on wall
- planks with feet on wall raising one leg to chest alternatively (I find this very tough).
One more plank variation you can try is to do them with one arm: get into the normal position and then raise one hand and keep it in front of you (or anywhere else, just not flailing about). The idea is to keep the same position like usual, not to shift your body weight so that your center of mass is above the arm you use. Holding your body in such a way is sure to activate some stabilizer muscles you don't normally use. As with any unilateral exercise, though, don't forget to exercise both arms evenly. You can combine these with "side planks" for a very complete core workout. You can also try to spice up your planks by balancing on a medicine ball (or a basketball).
Beyond this, you can add weight or increase the length of the hold. I wouldn't recommend adding weight as it can cause injuries down the line (you have to be very careful not to overexert some small muscle somewhere and you'll probably need a partner to help you); if you really must add weight use something like a weighted vest (ie. something that distributes the weight evenly, don't just plop it on your back). Increasing the hold length is a worthy goal, but if your goal is just general fitness then holding one of the advanced variations mentioned above for 1-2 minutes is more than enough.
Finally, planks are a great exercise but you'll want to mix it up. Check out V-ups, straight leg raises (lying, hanging) or the lying hip swing. These are not isometric exercises and so will probably be a different kind of challenge to you. Remember, variety is the spice of life. :)
In general, it is a good idea to mix up the type of exercise to give your body some new stimulus every few weeks (8-10). So, do crunches or planks on the side or other variations, you should be able to find a ton of exercises online.
Oh, and try to increase your plank time to over three minutes :)
You should always mix up exercise types, as each different exercise targets different areas and muscles and stresses/strengthens different tendons and joints. After a period of doing the same exercise with the same resistance, you will no longer be gaining strength, but rather endurance. Increasing time will gain you additional endurance, but I doubt seriously that it will benefit you significantly in raw usable strength.
Rippetoe (of Starting Strength) has book of essays entitled Strong Enough? . Maybe you are for your current needs. I gave all the answers an up, they're all good. One thing I might add is that the plank is very easy to do kind of wrong. Has anyone put a yardstick on you to check your 'plankiness'?