Crunches and sit-ups are a waste of time. There, I said it. Yes they have been staples of training for many, many years but that’s over now. Training as a whole has evolved and with that “core/ab/fill in name here” training. No matter who you are your training should be as efficient and effective as possible. Training movement patterns or the old (but not old enough based on what I see regularly) “movements not muscles” is where most people should be focused. Recent research has shown us that repeated flexion such as high repetition crunches or weighted sit-ups can be harmful to the lumbar spine. For further reading on this topic look here and here.
Let’s be honest here. Most of us sit too much whether it’s in the car, on the couch, or behind the computer. Just thinking logically, why would we want to reinforce poor posture by rounding our backs and necks and tightening up our hip flexors by doing sit-ups? The body adapts and adapting to the positions shown below is not advised. Stand on your feet while at the gym. Not to mention crunch type movements are isolated strength exercises which should rarely find their way into training in my opinion.
These above pictures are very much related. Given that many people are horrified when I tell them that crunches and even lumbar extension exercises like supermans are bad choices, here is a video with multiple options to choose from. No they are not meant to be done in succession like in the video. They are each individual exercises.