There is a lot of myths and misinformation regarding disc herniations.  The Science on disc herniations shows us that:

• Repeated flexion-bending of the spine is necessary to cause herniation.  In fact herniation of the disc seems impossible without full flexion.  This would seemingly de-emphasize flexion stretching, sit-ups, some exercise machines and activities that promote prolonged sitting.
• Thousands of cycles of flexion are needed to herniate a healthy disc.
• Prolonged sitting exacerbates the risk of herniation.
• Herniations tend to occur in younger spines, because of the higher water content and more hydraulic characteristics of the disc.
• The greatest risk of herniation of the disc occurs within the first hour after getting out of bed when the disc is fully hydrated.  (It’s like trying to balance yourself on a ball)

Reference: Ultimate Back Fitness and Performance, 4^th Edition, Stuart McGill, PhD