From Table Talk about Isometrics:
Given the lack of information regarding the lifter and training program, Dave forms his answer in a general, broadly-applicable way. His first reaction is to ask why the lifter thinks they need isometrics in their program. If you have a solid answer—which means more than "I think they're cool" or "I've never done them before"—then Dave has a few guidelines to follow.
Typically, when Dave helps a lifter incorporate isometrics into their program, it is within a conjugate-based system. They can be placed on either max effort or dynamic effort day, and the point within the movement where the isometric is performed should be at or around the sticking point for the lifter. Dave says to be careful when diagnosing this sticking point, however, because the place where a lifter fails is not necessarily the place where they are weak.
For instance, if a lifter misses their bench press three inches off their chest, it doesn't necessarily mean that the lifter is weak in the position three inches off their chest. It could instead be that the lifter isn't bringing the bar down quickly enough on the eccentric portion. It could also be that the lifter lacks the ability to strain for a sufficiently long period of time. Or maybe the lifter is slow off their chest, which makes them unable to get past the position three inches off the chest once they make it to that point. So when it comes to deciding the point in the lift to place the isometric, you need to take these things into account.
Generally, Dave uses two or three different positions, but never only one. The positions are then waved for three weeks each. This means the isometric at the first position will be used for three weeks and then the isometric at the second position will be used for three weeks. Through these waves, the first position will use no weight, while the second position will be very light loading, as low as 25%. The only purpose of this added weight is to cue the lifter to push harder. Each week will use the same five-second time for the isometric contraction.