I would like to know if doing sit-ups from the floor (lying on my back) produces different results from doing sit-ups using a sit-up bench? Are there any other instruments that are better than using the regular sit-ups?
The key difference between a floor and a bench sit-up is that the latter affords us the ability to alter our load profile.
In a sit-up from the floor, the greatest load on the hip flexors and abdominals occurs at the very bottom of the lift. The load decreases progressively thereafter, with the centre-of-upper-body-mass drawing closer to the fulcrum, and the resultant load being a function of the cosine of the angle of the torso from the floor. Depending on the angle of knee-bend and range-of-motion through which the sit-ups are performed, the hip flexors and abdominals may be unloaded completely by the top of the lift. Thus, the greatest load occurs when the hip flexors are only partially shortened, and when the abdominals are at resting length, hence being in their strongest position to develop tension.
The disadvantages of floor sit-ups are twofold: (1) we have limited control of the loading profile across the range-of-motion, and (2) our muscles are completely unloaded at the top of the lift. Their advantage lies in the fact that the greatest load occurs when the muscles can develop the greatest tension.
Provided that we can anchor our feet somewhere, we can load a floor sit-up as much as we wish by holding an additional mass to our chest or behind our head. (The latter shifts the combined centre-of-mass away from the fulcrum, thereby increasing the load geometrically.) And of course, we can simply limit our range-of-motion in order to ensure that the muscles are never unloaded completely.
In a bench sit-up, we adjust the starting angle of the bench, thereby altering the relationship between the angle of our body and the tension that can be developed by the hip flexors and abdominals. The greatest load does not occur at the bottom of the movement, but rather when the torso reaches horizontal (0°). Thus, the capacity of the muscles to develop tension is reduced when the load is at its greatest, and they remain loaded even at the very top of the lift.
The advantages of bench sit-ups are that we have considerable control of the loading profile—that is, where in the range-of-motion peak load is developed—and that we can ensure that the muscles are loaded throughout. Their key disadvantage is that the loading pattern produced is unnatural for most real-world functional movements, with peak loads being developed when the muscles are already significantly shortened.
Neither version is ‘better’ per se, and neither produces greater hypertrophy. However, bench sit-ups will naturally develop strength throughout the heavily shortened range-of-motion, while floor sit-ups will develop greater strength through a more general range. The combination of the two will likely maximise hypertrophy. The better choice, therefore, will depend on our training goals.
I hope that is helpful.
Learn how to bat hang... It's basically when you grip a pull up bar with your feet.
Hold on the bar with your feet and then do sit ups upside down, alternatively you can grip the bar with your thighs by curling the leg over the bar and then squishing it really tight.
Another option is to make a roman-chair. Basically sit on a chair and find a way to block your feet on something so you don't fall when you try to do sit ups on air.
You can have a partnener holding your feet or even using a bed and achoring your toes under it or maybe a table.
Is there any specific reason you do sit ups? If you must be good at doing sit ups for some physical test or competition, I suggest the Roman-chair version because it's literally a normal sit up but way harder. So normal sit ups will feel easy after that.
If you do them for hip strength then there are better exercises, try learning the L-sit pose and then the V-sit pose...you may think they are flexibility limited exercises, but in reality they are just about hip strength.
If you do sit ups just for abdominal strength, then I suggest switching them with curl ups and then progress with other exercises.
Once you are able to do 100 slow and good form curl ups, switch to deficit-curl ups they are basically the same exercise but on the edge of a bed so you can go down with your spine and bend it backwards just to then flex it up again.... Make sure to have something holding your feet or use a roman chair with a pillow on it.
After you are able to do 100 slow and controlled deficit curl ups in a row, there's isn't anymore you can do to train your abdomen without weights.
You might want to train the front lever or dragon press which are the next step in therms of abdominal strength in the bodyweight realm, but they are a great jump in difficulty. A good option to make more strength gains for your abdomen is to use weighted versions for the exercises I listed.