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What is the difference for my abs between raising my legs, and raising my body?

I've heard that the whole lower abs/upper abs training is BS. It's like trying to train the lower or upper biceps.

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First let's see how the exercises square up on the whole.

Studies have been performed regarding the EMG (electromyography) activity for various abdominal muscles in a variety of exercises. You will find the results for the exercises in stimulation of the rectus abdominis and obliques, relative to the traditional crunch, in this Wikipedia section: https://en.wikipedia.org/wiki/Abdominal_exercise#Types_of_abdominal_exercises_and_effectiveness

For the durability of this answer I'll reproduce the table here:

Activity in rectus abdominis                      Activity in obliques
Exercise                  Mean activity      Exercise                  Mean activity

Bicycle crunch            248%               Captain's chair           310%
Captain's chair           212%               Bicycle crunch            290%
Exercise ball             139%               Reverse crunch            240%
Vertical leg crunch       129%               Hover                     230%
Torso track               127%               Vertical leg crunch       216%
Long arm crunch           119%               Exercise ball             147%
Reverse crunch            109%               Torso track               145%
Crunch with heel push     107%               Crunch with heel push     126%
Ab roller                 105%               Long arm crunch           126%
Hover                     100%               Ab roller                 101%
Traditional crunch        100%               Traditional crunch        100%
Exercise tubing pull      92%                Exercise tubing pull      77%
Ab rocker                 21%                Ab rocker                 74%

From this study, the highest mean activity is to be found in the captain's chair exercise (also known as power tower or knee raise station) and the bicycle crunch maneuver, which exceed the other activities rather dramatically on the rectus abdominis.

There has also been some tests done by Bret Contreras, who looked at a larger number of exercises, but did all tests himself meaning there's far less data points to extrapolate from. Here is the article: https://www.t-nation.com/training/inside-the-muscles-best-ab-exercises

The conclusion here was that chin-ups, hanging leg raises and the ab wheel had the highest mean activity for the rectus abdominis, ab wheel (from feet and from knees) and body saw for internal obliques and ab wheel (from feet), hanging leg raises and bodysaw for external obliques.

As you can see there doesn't seem to be a consensus amongst there articles, although I'd say the actual study would be the most reliable. The hanging leg raise and captain's chair are very similar and an excellent workout provided you focus on using your abdominal muscles and not just the hip flexors.

From the data it seems like the reverse crunch (raising your legs) is a better option than the traditional crunch (especially for the obliques), and leg raises or captain's chair an all-around superior option.

Now as for upper/lower muscle groups, the Wikipedia article claims that a crunch with the arms stretched out over your head (effectively lengthening the moment arm) will result in increased activation of the upper abs, although I'm a bit wary of the reference.

The Bret Contreras article references previous tests of his that claim that different exercises do vary between the ratio of upper/lower ab recruitment.

A study referenced in this article shows activation of lower and upper rectus abdominis: https://www.acefitness.org/prosourcearticle/4934/can-you-train-upper-and-lower-abs-separately

There are big differences between the general activation for various exercises but insignificant differences between upper and lower ab activation.

All in all, there doesn't appear to be much evidence that exercise choice will greatly affect upper/lower activity ratio, but there is evidence that there's a big difference between exercises in general utility.

EDIT: I've done some more digging and am going to have to revise my above answer. I've found two studies (which are very similar) that deal with this question:

  • Journal of Sports Science and Medicine, 2011, 10, 322-327. "Selective activation of the rectus abdominis muscle uring low-intensity and fatiguing tasks"
  • "Muscle Activity in Upper and Lower Rectus Abdominus During Abdominal Exercises"

Links, in order:

In these studies there is a comparison between a supine trunk raise (or curl up, a crunch where the chest is raised towards the knees while maintaining leg position) and a posterior pelvic tilt exercise (a lying leg raise, where the pelvis is tilted towards the chest while maintaining upper torso position). The conclusions seem to be that separate portions (upper/lower) of the rectus abdominis can, at least in part, be individually activated. Furthermore, the ratio of the upper rectus abdominis activity to that of the lower, was much greater for the curl up exercise than for the leg raise.

So not only do certain exercises seem much more effective than the traditional crunch, it does appear that you can indeed train specifically for upper or lower abdominals, at least where the rectus abdominis is concerned. It seems possible that any exercise where posterior pelvic tilt is favored (which is the case for the captain's chair exercise and reverse crunch) will put emphasis on the lower RA, while exercises favoring the upper trunk (like the traditional crunch) would emphasize the upper RA. I should hasten to say that the studies can't be generalized like that (it's speculation on my part) and also that this concerns relative recruitment of the upper and lower portions. The leg raises still worked the upper RA considerably.

  • Screw my answer, this is the best! – Gunge Apr 21 '16 at 15:19
  • @JJosaur Actually, it wasn't. I was too hasty because I had to leave. I've searched some more and found studies that speak against my first conclusion, which are a better reference for the specific question. I've updated my answer with references and conclusions. It appears some upper/lower rectus abdominis focus in training is in fact possible. – G_H Apr 21 '16 at 20:20
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In terms of developing the strength of your abdominals, both will work.

Different exercises activate different muscles more or less. Generally, you cannot split out upper and lower abdominals by exercise and both mentioned exercises will activate both.

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From personal experience, albeit without professional experience or references to back it up, crunches tend to feel like they're only really working the front of the abdomen, while leg lifts always feel like they engage more of the sides, possibly due to differences of stability and how far off of straight legs can go versus the torso.

  • +! Agree completely, leg lifts require you to hold your leg weight central and work your obliques (sp?) harder than rolling up into a crunch. I have heard crunches are bad for backs but cannot find peer reviewed evidence to support this. – Gunge Apr 21 '16 at 13:45
  • @JJosaur: FWIW, I asked after that at fitness.stackexchange.com/questions/19327/… – Sean Duggan Apr 21 '16 at 13:50
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Anything that either causes your ribcage to curl down towards your pelvis, or or pelvis to curl up towards the ribcage will work primarily the abdominals, with support from the obliques and hip flexors. If the spinal column doesn't flex, you're not really using the abdominals except as in support.

By that I mean that if by leg raises you mean keeping your legs straight and just lifting them (as seen in this picture), then you are only minimally using the abdominals as support, and the primary motivation for the exercise comes from the hip flexors.

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