I have said in the past that traditional situps in a general sense and especially weighted situps on one of those benches that you lock your legs into are even worse. This is because it places an incredible amount of strain on your lower back and more importantly, your spine in a way that your body and spine was not meant to handle. Repeatedly doing these exercises will make your abs stronger and if you are young enough and have a good enough diet, give you a 6-pack that everyone wants. However, the downside is potential long-term damage as well as back or perhaps even spinal problems later in life.
I recently found out, and honestly this didn't surprise me, that other forms of sit up exercises that we seen done all the time are also just as dangerous if not worse and one of those is the "twist" situp.
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The sad reality of situps in a general sense is that they are terrible for your back and I know I know, when you are young you don't worry about that sort of stuff and to be fair, neither did I when I was in my teens and 20's but a lot of this had to do with the fact that we didn't know about the damage we were doing with this sort of stuff until many years later.
The only safe kind of "crunch" are ones where your back stays flat on the ground. IF you don't think that you can get an ab burn using this method, I say that you have never tried it. Sure, it takes longer to "feel the burn" but you will get there.
When you do a situp of any sort where your lower back leaves the ground, you are doing something to your spine called spinal-flexion in medical terms. The problem with this is that you are putting a LOT more stress on your vertebrae than the human body has evolved to be capable of handling repeatedly. Now when you introduce a "twist" motion into this, which is meant to engage the obliques as well - which it does. Unfortunately it also does something else that you wont see or notice until it is too late.
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There is a jelly-like substance that exists inside vertebrae that are engaged when you do a twisting motion with your sit ups called nucleus pulposus (I'm sure you will remember that - the name is not important, just knowledge that it exists) and this is a very important part of your spine to keep healthy and unharmed by repeated damage because it is where the disks tend to herniate.
I don't know if you have ever met anyone that has suffered a herniated disk, but it aint pretty, nor is it something that people generally tend to recover from. They are, in some way or another, negatively impacted by this injury for the rest of their lives.
When you twist with your back off the floor, and especially if you are doing weighted versions of this (sometimes referred to as "Russian twists") you are constantly engaging this very delicate and important part of your anatomy that enable you to do just about anything in a standing way.
This might sound like something I am just making up, but this study has been done dozens of times and all come to the same result. Dr. Stuart McGill, a professor of spine biomechanics (yeah, that's a real job) has done extensive research on this and his findings resulted in the Canadian military eliminating situps and twists from their physical training.
It is estimated that more than 50% of all chronic pain from retired US military (not including combat injuries, of course) can be traced back to situp and other ab training exercises. If that isn't worrying enough for you, then I don't know what can possibly scare you into changing your ab exercise ways.
So what can you do as an alternative? Well basically anything that involves keeping your back straight such as planks, or crunches where you are careful to keep your lower back on the floor. Pilate balls can also be used but even with this "back protector" twists should still be avoided. You can also do something that is referred to by many names but I call "all 4s" and I think the below image is kind of self explanatory.
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For me, I tend to use a ab-roller, but then again, if this machine is used improperly, it can also cause back harm.
The sad reality of the entire ab thing is that a lot of the common exercises are actually quite dangerous and we are only just in the past decade figuring a lot of this out.