Forced reps involve doing an exercise to failure and then continuing to push out an extra rep or two with the help of a training partner. Forced reps are an advanced technique and generate a lot of muscle soreness so there is no reason to exceed three forced reps. If you are able to do more than three forced reps chances are that you didn’t really push yourself to failure.
Forced reps are intended for workouts that use a limited number of work sets. Most often several sets are performed to warm up the muscles before doing a work set. It is only during this top set that failure is reached.
In contrast, this principle can’t be applied to volume programs that are based on a specified number of sets and reps such as the StrongLifts 5 x 5 program, than forced reps won’t work as you are not training to failure on any of these sets.
If you are like me and train alone this is one less tool I have to work with. If you go to a local gym you will always have someone around that can spot you for a set or two. Even one really hard set done with a spotter with a couple forced reps is enough to make serious gains.