Principle #1: Don't Forget to Breathe!
It is common for people to intentionally or unintentionally hold their breath while performing an exercise. Holding one’s breath is not a good thing because your muscles require oxygen, and if you are holding your breath, they will not be able to work as effectively, and as a result, you will not be as effective in strengthening them.
Holding your breath is also an indicator that you are recruiting the stronger, more superficial abdominal muscles rather than the deep ones. As a result, holding your breath can contribute to muscle imbalance and undue pressure through the abdominal cavity.
If you are performing an exercise in which you are holding a contraction for a specific period, as you would in a planking exercise, for example, be sure to keep your breathing pattern steady and controlled. When doing an activity that requires you to contract and relax your core, as you would in a sit-up, make sure you breathe out when contracting the targeted muscle and breathe in when returning to the start position.
Principle #2: Bulging is Bad
Suppose you notice that your belly sticks out or bulges while performing a core exercise. In that case, you are likely recruiting the superficial abdominal muscles and leaving the deeper ones under-utilized. Not only that, but it is also likely that you are not performing the exercise at an appropriate difficulty level. Consider decreasing the exercise intensity by lessening the time you are holding the contraction, reducing the number of sets/repetitions, or choosing a more accessible version of an exercise to ensure you can perform the activity effectively and with good technique.