Typically, if my client can hold a plank for one minute in good form, it is time to add challenge. Because the plank is a static move, the body will adapt quickly and once you reach a certain point it may not be as effective. I’d rather progress my client with some movement to add challenge, rather than lengthen the time they can hold the plank.
One way to challenge the core and upper body is to either move the feet closer together (which lessens the base of support and makes the core work harder), or add dynamic movement. There are numerous ways to do this–add a pushup, alternate extending one arm, alternate a small leg lift,”saw” back and forth, and so on.
You may get different answers here, but that is how I progress. Hope this helps.
Muscles are typically more functional in positions where they can contract and relax, contract and relax, so I don’t train my clients to hold any pose “for time.” Frasier Quelch does a great job explaining this in more detail in some of the TRX workshops, as does Michol Dalcourt in his loaded movement training workshops (both accessible through IDEA conventions and online content).
As with others’ posts, if a client can hold a plank with good posture for about a minute, they have enough stability to a variety of other non-plank activities with relative safety.
I really like Christine’s answer.
We do a lot of core work in yoga. The way we would refer to the use of intraabdominal pressure, as in an isometric, is mulha bandha. Plank is great for getting this, but then keeping it while adding the dynamic (the flow) is better than just holding it for longer. One type of progression I sometimes use is the half sway…. there is a position called vasisthasana, or side plank…. I will do a half plank on the elbows and keeping the feet together as a unit roll slightly and then work to keep the hips straight, and then add lifting one arm, and then move to straight arms, and then to rolling to full side plank and lift the upper leg as well as the upper arm. I also will sometimes cue to imagine the feet are tied together and say to alternate straightening each foot and pointing the toe without moving the leg, keeping the ankles together.
Similar to what Nancy, planks are more functional and effective when you contract and relax by having them rest for a time period in between their plank.
Personally after a client can hold a good solid plank for 30 seconds I begin to progress the plank by adding limb movements, object transfers, stability rollouts, etc. and I still break up the time of the planks (contract and relax), 10-15 seconds and rest for 5 seconds and repeat for how many reps (about 2-3x). Breaking up the plank also helps the client refocus each time.