Abs Training
Also, there are studies out there that show the best sit ups are the bicycle sit ups.
Again, there is no such thing as a best exercise for any bodypart. The reason many people hit plateaus is because they think there is. So there is no variety in their workout.
Also, anybody who knows how to read a study will realize that most researchers do not understand how abdominal recruitment works. Strain in the lower back is due to the lifter failing to recruite their lower abdominals first. Instead they uset their hip flexors first. This pulls the lumbar spine into an arch and puts stres on the spine itself. So using the abdominals to stabalize is a big deal.
1.) If you know how to use your abs as well as understand proper form there are no unsafe exercises.
2.) If you find that you are getting low back pain from an abdominal exercise
A. Your abs a too weak to do it
B. You do not have good form
C. Both
3.) Research dating from today back to the 1950's has found nothing wrong with straight leg situps.
From my own experiance I have done straight leg situps as well as streight leg situps on a glute ham raise using weight and found that it significantly helped to increase my abdominal strength as well as better my ability to stabalize heavy weights in other exercises.
A third time, exercises are all good when done at the proper time and when the lifter is at the proper training level.
Please understand that good information is hard to come by. It took me 8 years of training to really understand how abdominal training actually worked.
Then I was able to increase my squat by 150lbs.
After another 4 years another bell went off, and my squat went up another 200lbs.
I give full credit to my increase in effeciancy of abdominal training for all of these gains. Without stronger abs I would not have been able to focus on other muscle groups.
Either that or I am a slow learner.

. I will pretend that is not the case.
Now I will step down from my soap box.