The Biomechanics of Breasts
By Stacy Hackner, on 24 February 2014
Have you ever wondered what biomechanics has ever done for you? Well, if you’re a runner, it can tell you a lot about your gait and efficiency. It tells us why people with long legs are good at running and people with long arms are good at swimming, and the forces they use per stride or stroke. It can teach us proper runner techniques. If you’re a female runner, you may have encountered a problem biomechanical researchers are actively working to solve: bouncing breasts.
I’ve only been a runner since I started my PhD. As you may remember from my last post, I learned from my research that it’s very important for your bone strength to practice weight-bearing activity (sorry, astronauts), which includes running. As professional running goes, for some reason marathon organizers decided to exclude women from participating until the mid 1980s, when just a few women snuck into the Boston and London marathons and achieved quite good times (see Heminsley’s book for an exciting run-down of the sneaking). Since then, women have been participating in most major sports, including (very recently) American football; 36.5% of 2012 London Marathon finishers were female (Brown et al 2013). But sports equipment for women is still catching up, and biomechanics – long applied to gait and stride, torso and head movements – is now being brought in to design a better bra. Most biomechanical studies of breasts involve attaching markers to women on treadmills in clothed and unclothed conditions and filming them with an infrared camera – a slightly awkward study for the volunteers, but it’s worth it for the results.
First, let’s look at a breast from an anatomical perspective. Breasts are composed of milk glands and ducts, fat, connective tissue, and Cooper’s ligaments. The latter are fibers that attach the breast to the underlying fascia and pectoral muscles; throughout life, and in vigorous exercise, they can stretch or break and cause sagging and breast pain. Imagine a laundry line with wet clothes hanging on it – now shake it: that’s what happens during intense exercise. Of course, these forces have been measured, which can be difficult as unlike bones and muscles, they squish and deform, and each of the above types of tissue reacts to running forces differently. During the beginning of a running stride, the breasts are found to accelerate at up to 3 G (where G is the force of gravity – at that point in time, it’s like the breast weighs 3 times as much). This is considerably more than the rest of the trunk, and puts strain on Cooper’s ligaments. You can then imagine that each breast goes through a cycle of acceleration, stasis, and deceleration for each stride, like your head when stopping and starting in a car. For each stride, the breasts move forward into the air and backward into the ribcage, in what is called anterior displacement.
Now it gets more serious. In addition to being displaced anteriorly, breasts move in three dimensions. When running, the goal is generally to move forward. In order to do this, you need to move up as well. And with each step, you also sway side to side. Breasts respond to this combination of forces by actually moving in a figure-8, experiencing additional vertical and horizontal displacement. Studies show that it’s these three directions of displacement rather than acceleration that cause breast discomfort and pain; the worst seems to be vertical displacement, which peaks at mid-flight. At this point, the Cooper’s ligaments are basically floating upwards and then being tugged back down during deceleration (not to mention the fat and glands moving about internally). This is important to know for bra design, as many sports bras take the approach of “flattening everything is best” – however, as we’ve now learned, flattening will only reduce anterior displacement! Flattening bras can also cause breast pain, so it’s lose-lose situation.
Now let’s discuss what a bra actually does. The everyday padded bra is an attempt to compromise comfort, sexuality, and stabilization, often emphasizing one to the detriment of the other two. The goal is to hold the breasts in an uplifted position so they appear firm and don’t jiggle around too much while walking or climbing stairs. Sports bras, on the other hand, prioritize stabilization, as they’re to be worn in high-impact activities. Many sports bras take the approach that flatter is better, which as I’ve shown is not quite the case, but they do prevent one kind of displacement. Regular, everyday support bras lift the breasts up, reducing strain on the Cooper’s ligaments, but in tests of treadmill running do little to prevent any kind of displacement. Running bare-chested causes the most displacement, and – in the case of marathons – can lead to a breast injury experienced by men as well, where the nipples chafe against the fabric of the shirt. (This is actually very common, and there are marathoner web forums devoted to sharing prevention tips.) A newer kind of sports bra attempts to encapsulate rather than flatten, and researchers from biomechanics and textile manufacturing have been collaborating on new design. This bra holds each breast separately and matches the figure-8 to the overall movement of the torso, reducing displacement in all three directions.
As more women get into sports (which is particularly important for the prevention of osteoporosis), making us comfortable and keeping us engaged should be a high priority for sports equipment manufacturers. Most runners can find shoes that fit, as shoes have been tested and re-designed for the last thirty years, and have a high profile in the press. Despite the increase in women running, 75% of female London marathoners still reported a problem with their sports bra, with the prevalence higher among larger-breasted women. However, proper fitting technical sports bras receive significantly lower press coverage than proper running shoes. Clearly, there is more work to be done!
Brown, N., J. White, A. Brasher, and J. Scurr. 2013. An investigation into breast support and sports bra use in female runners of the 2012 London Marathon. Journal of Sports Sciences 2013:1-9.
Heminsley, A. 2013. Running Like a Girl. London: Huntchinson.
Scurr, J., J. White, and W. Hedger. 2010. The effect of breast support on the kinematics of the breast during the running gait cycle. Journal of Sports Sciences 28(10): 1103-1109.
Zhou, J., W. Yu, and S.P. Ng. 2012. Studies of three-dimensional trajectories of breast movement for better bra design. Textile Research Journal 82(3): 242-254.
Update: The post originally stated that Cooper’s ligaments connect breast glands to the clavicle; this was incorrect.