Everyone needs to start somewhere with core exercises! The most important question you must ask yourself when prescribing an exercise is why - why did you choose that exercise with those specific parameters and modifications? Finding just the right challenge and determining your target tissue should always be considered.
When targeting the core, you could argue this should always be engaged in every exercise. However, when people are trying to bounce back from an injury or surgery, the body needs to re-learn how to move. Here are some modifications and progressions of lower level core exercises for patients and clients you may be having difficulty with in regards to activating their core, and finding just the right challenge. Try out these exercise progressions in order...
1) Knee bend to chest using a swiss ball *push feet into ball*
2) Alternating marches *cue person to push other leg into table*
3) 90/90 hold with arm press *push hands into ball*
4) Hand-ball-knee holds with knee extension *don’t arch your back*
*Cues to use with the individual that may help improve the movement*
In my opinion, the most important thing the person must learn (and do well) is breathing while exercising. Forcefully breathing out may help with rectus abdominis activity!