When considering any exercise, you need to assess your own ability relative to the movement itself. If you are performing an exercise that requires a greater range of motion, you must decide if you are flexible enough to do it properly. You develop flexibility through stretching over time; you do not force it abruptly.
The same can be said for stability. If you are performing an exercise on an unstable surface or one that requires locomotion, you need to first be able to do it on a stable surface while stationary. Too often, people are seen exercising while trying to balance on a ball or Bosu, despite not having mastered the motion on a bench or solid ground. You have to crawl before you walk, as the old saying goes.
This also applies to your training load. An increase in resistance is necessary to continuously progress. While gravity may provide enough resistance initially when performing an abdominal crunch, eventually you will want to add additional weight to the exercise, either by holding a plate or dumbbell on your upper chest or using an abdominal machine.
