Hey, I am a Personal Trainer and I injured my back a few years ago, so have had to deal with and fix my own lower back pain.
First thing I would say is dont do sit ups at all if u have pain, you can maybe do crunchs if you really want, but wise to stay away from them as well. These will just cause more pain and limit recovery.
The "abs" or core is made up of differant muscles; Abdominos rectus(6 pack) Internal Obliques(side), External Obliques(side), Tranverse Abdominus-TVA(deep muscle that wraps around like a natural weight belt) and the deep back muscle group erector spinae( of which there a few differant muscles). This is NOT every muscles involved in the core, just trying to make it easy to understand!!
A very important muscle in the core is the TVA. It is a muscle that is deep to your other muscles, it wraps around your back and it looks similar to a weight belt you would have seen weightlifters use. This muscle is extreamly important in stabilising your spine and taking pressure of your back. It needs to be used in EVERY exercise you do. When i started working out I had trouble activating that muscle, which is very common. To help activate this muscle during exercise you need to suck your belly button in, think that u are sucking it all the way to your back. Also you need to breath out on the HARD part of the exercise. This will help activate various core muscles. I used to struggle doing both at the same time, so I would practise, just suck ur belly button in and then breath out....keep doing it until u can do it properly.
Back pain can be caused by a million differant things, but one way to fix it is to strenghten your core as a whole, not just focusing on your "abs". As i suggested I would stay away from sit-ups and crunchs also you mentioned that leg raises hurt, well dont do them! If it hurts, dont do it...simple! Also make sure your technique is good, so many back injuries happen by incorrect lifting techniques
While we all want an awesome 6 pack, training for one is largely misunderstood. We do a million sit ups because we think we will get huge abs, it just wont happen.As our abs cant really increase in size very much. Also its not very functional. Diet is key to a 6 pack!
To help strengthen your back I would suggest doing some Pilates classes. You can start with 1 a week then increase to 2 or 3 if you want. Alternatively go to a Pilates qualified physiotherapist that can take you for a session and give you some exercises to do at home. Pilates is so good for you, and core strenght is its main focus. You could also do exercises like front bridges(plank, prone hold/hover) and side bridges as well
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I had this problem also a while back and a trainer gave me two great tips.
1. Stretch your quads before working out your abs - it sounds weird but it helps.
2. Try exhaling when you are contracting the muscle. This also helps take some pressure off your back and concentrates it on the abdominal muscle.
Fender, you had a smart trainer, the reason he said stretch your quads is that beacause alot of the muscles of the hip/leg start further up the body in the core, and are therefore involved in some contractions. Also exhaling will automatically activate (turn on) your core muscles, therefore taking pressure off your spine.
I hope that all makes sense, i tryed to explain it well, but if you dont understand any of that or just have any questions just ask and I will try to clear it for you! Hope that helps.[/quote]