When you are having acute disc-related symptoms like sciatica and/or severe low back pain, there are some exercises that should be avoided until such time as the major symptoms subside and the sciatica is gone.
The most important thing to avoid, particularly if you have sciatica is any kind of hamstring stretch. Hamstring stretches should be avoided with sciatica and other acute symptoms are present. The reason why you should avoid hamstring stretches is that although tight hamstrings are sometimes a contributing cause to low back problems, if you try to stretch the hamstrings when you are having acute disc-related symptoms, there is a tendency to stretch the sciatic nerve and its component nerve roots as well. If those nerve elements are already irritated, stretching them will only make things worse – often MUCH worse.
You may benefit from hamstring stretches once the nerve irritation subsides as part of a rehabilitation program, but wait until you are feeling better. Another type of exercise to avoid when you are having major symptoms is abdominal strengthening, such as with sit-ups or crunches (partial sit-ups). Again, although weak abdominal muscles do contribute to back problems, the time to start reconditioning the abdominal muscles is not when you have sciatica or major back pain.
Sit-ups and crunches increase pressure in the spine, and may greatly increase pressure on sensitive, already irritated nerves. Wait until your symptoms have subsided before doing any abdominal exercises, and start with the easier exercises discussed later in this article.