Researchers tell us that people who sit for long periods develop weaker lower abdominals. This means the abdominal wall below navel level – particularly the internal oblique muscles and transversus abdominis – is under-active, giving an unsightly soft, pouchy underbelly below the belt-line.

Unfortunately the gym culture doesn’t quite get this and continues to prescribe regimes of sit ups, crunches, the plank and the nothing-more-disastrous double-leg lifts. Sadly, these sorts of abdominal workouts cause back problems - even if you didn't have a back problem.

Excessive upper abdominal exercising bears down on the pelvic floor, stretching it and weakening it, and making it less competent in its roles of both sphincter control and dynamic abdominal support. A weaker pelvic floor can also lead to prolapse. Conversely, the best abs exercises recruit the lower abdominals, below navel level and also the all-important PC muscle.

Another major disadvantage of massive abdominal strengthening is that over-active upper abdominals pull the upper body forward, in front of the line of gravity. In so doing, they create a slightly stooped posture that increases loading of the spinal base.

There are 4 Particularly Bad Abs Exercises for a bad back:

• Crunches
• Sit ups
• Dubble leg lifts
• Plank

Believe it or not, breathing difficulties may arise from the worst abs exercises

There's a strong correlation with the increased incidence of breathing problems – asthma, blocked nose, sleep apnoea and even panic attacks – and the obsession with keeping the belly cinched in. Breathing difficulties may also be related to weak pelvic floors.

Free function of the diaphragm is impaired by over-activity of the upper abdominals. The diaphragm is the flange-like breathing muscle that separates the thoracic from the abdominal cavity. To activate an in-breath this huge dome-shaped muscle contracts and flattens. As it descends into the abdomen - rather like drawing down a syringe - it pulls air in through your nose. The bases of the lungs inflating with air pushes your belly out.

If you are madly trying to hold your belly in you are inhibiting the downward excursion of your diaphragm and perforce you take a shallower breath. If your upper abdominals have actually become adaptively shortened through over-use, then you don’t even have to hold your belly in because your abdominal wall has shrunk and is in anyway.

Reference: ://www.simplebackpain.com/the-worst-abs-exercises.html