Ah, the things that people will research. I have to say this kind of research could either really awkward, or really thrilling, depending.
So now, my friends, it is time to stare at some butts. Some very, very athletic butts. Stare carefully now, you're looking for SCIENCE.
Sanchis-Moysi et al. "Iliopsoas and Gluteal Muscles Are Asymmetric in Tennis Players but Not in Soccer Players" PLoS ONE, 2011.
Really, you may be thinking. Sci, you are SO dirty minded. And so are these scientists! WHY would anyone be wanting to stare at atheletes butts all day like dirty people. Get your mind out of the gutter, Sci, and write about...um...
Anyway, there is a VERY good reason for people to be doing all this butt sampling. And the reason is injury. Professional (and non-professional) athletes get injured very commonly. The most common injury types are usually determined by sport (A runner is fairly unlikely to get tennis elbow, for example). Obviously we want to prevent these injuries if possible, they often become severe enough to need surgery, long recovery, or even retirement from your chosen sport. So it's a good idea to study how the practice of various sports impacts the body, what different sports overdevelop which body parts and underdevelop others, to try and find ways to stave off the injuries before they occur. By knowing how the body responds to training in a sport, we can change the training up to reduce the potential for injury.
And this OBVIOUSLY involves staring that athelete's butts. Via MRI. Of course!
In this case we are interested in two main muscle groups of the butt. The first is the Iliopsoas, made of the psoas and iliacus muscles. The psoas (don't even ask me how to pronounce it) comes off the lower portion of the spine at the side, wraps around to the front, and inserts near the top of your femur (that's your thigh bone). The iliacus comes out lower, in the pelvic girdle, and inserts in the same region. Like so:
These together form the strongest of the muscles you commonly think of as your hip flexors. When they contract, your leg lifts to the front.
The gluteal muscle group is on the back side, and contained three muscles, the gluteus minimus, the gluteus medius, and the gluteus maximus. I will leave you all to guess which is smallest, and which largest. These all begin on the back side of the pelvis. The gluteus minimus and medius connect at the top of the thigh, while the gluteus maximus wraps around about halfway to connect on the side (at the iliotibial band, the curse of many long distance runners such as myself).
These do things like abduct the thigh, or pull it away from the midline, they rotate the hip, and when you are standing, they are very important in postural and balance control, particularly for balance on one leg. And the gluteus maximus "forms the prominence of the buttock", so I guess there's that too.
So the interest here was that both soccer and tennis are sports that get rather asymmetrical. With soccer, you use either your right or your left foot, and you use it for kicking pretty much exclusively. For tennis, you use your right or left arm, but the movement around the court means that the dominant foot will also be leading the majority of the time. For soccer, you need those ilipsoas and gluteal muscles for kicking and running at high speeds around the field. For tennis, you need them mostly for lunging and running at high speeds around the court. So they wanted to see how developed these muscles were in soccer and tennis players compared to non-active controls, and if the developments of the muscles were symmetrical or asymmetrical.
So they took a bunch of soccer players (1st division national Spanish team), tennis players (international tennis federation), and some non-athletic guys who might have felt pretty schlubby and inadequate faced with all this athletic prowess. They stuck their butts in an MRI. Unfortunately, there are no comparison images, which is really too bad, even though the MRI anatomical image of a butt is not as sexy as you might think. I still think it'd be cool.
Being super fit as these soccer and tennis players were, it wouldn't surprise you that some of their muscles were hypertrophied, or overly developed compared to non-athletic controls. In both the soccer players and tennis players, they had hypertrophy of the iliopsoas group (DANG that is hard to spell). The soccer players had right and left side hypertrophy of 32 and 35%, or roughly equal, while the tennis players showed hypertrophy of 24 and 36%, which means the non-dominant side was actually BIGGER than the dominant side.
For the gluteal muscles, the soccer players actually showed no hypertrophy compared to normal controls, while the tennis players showed aymmetrical hypertrophy similar to the iliopsoas group, with the gluteal muscles on the non-dominant side 20% bigger than those on the dominant side. That's a pretty big butt discrepancy.
(source. I should like to note that I titled all of these photos various things like 'soccer butt', which to me is some hilarious disease of the butt as a direct result of over-ethusiasm while watching the World Cup)
Keep in mind though, this is all IN COMPARISON to non-athletic controls. The non-athletes are by no means symmetrical. In fact they tend to have high iliopsoas mass in the dominant side, which the tennis players ended up compensating for, while the soccer players just had hypertrophy going in the same direction. In soccer and tennis players, the magnitude of hypertrophy was pretty much the same, they are definitely getting muscles of a similar size, the tennis players just end up asymmetrical on one side.
The authors discuss why the symmetry might be better in soccer players. It seems a little odd, after all they are doing a bunch of kicking. But they are also doing a bunch of RUNNING, and I think (and they agree) that this may compensate and result in more symmetry. The tennis players, on the other hand, though they'll be running plenty, will also be working the entire dominant side of the body more while working the dominant arm. This means there will be more compensation in balance and lift in the non-dominant side, which might result in the asymmetry.
Now, there are a lot of injuries associated with these two muscles, including lower back pain, groin pain, and of course the dreaded iliotibial band grumpiness which makes athletes roll themselves around on foam rollers. Unfortunately, the authors didn't state whether tennis players were more prone to these injuries than soccer players, or vice versa. But if it turns out that one sport gets it more than another, we may be able to compensate and differences in training.
And hey, it's looking at butts for science! Do you need an excuse?
Sanchis-Moysi, J., Idoate, F., Izquierdo, M., Calbet, J., & Dorado, C. (2011). Iliopsoas and Gluteal Muscles Are Asymmetric in Tennis Players but Not in Soccer Players PLoS ONE, 6 (7) DOI: 10.1371/journal.pone.0022858