When training the core muscles for trunk stability, endurance is far more important than absolute strength.
The problem with training the core for endurance is the likelihood of compensations from other muscles (like the hip flexors) getting involved when an exercise is held for too long.
For example, the plank exercise. After about 12 seconds, the target core muscle starts to lose oxygen rapidly, leading to the inclusion of “other” non-target muscle to help out. You may be able to hold the set, but you’re compensating to do it.
The solution? Do MANY sets of 10-12 seconds with about 10 seconds rest in between the mini sets. This small rest period is enough to return oxygen to the muscle, allowing it to operate and fire well.
For a plank or side planking exercise, try 8 sets of 12 seconds with a 10 second rest in between. You’ll have trained your core muscle for endurance while not exhausting your body to the point of compensation.