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Archive for the 'Biomechanics' Category

Did we evolve to run?

StacyHackner5 January 2015

By Stacy Hackner

A few years ago, spurred by my research on just how deleterious the sedentary lifestyle of a student can be on one’s health, I decided to start running. Slowly at first, then building up longer distances with greater efficiency. A few months ago, I ran a half-marathon. At the end, exhausted and depleted, I wondered: why can we do this? Why do we do this? What makes humans want to run ridiculous distances? A half-marathon isn’t even the start – there are people who do full marathons back-to-back, ultra-marathons of 50 miles or more, and occasionally one amazing individual like Zoe Romano, who surpassed all expectations and ran across the US and then ran the Tour de France.[i] Yes, ran is the correct verb – not cycled.

I’ve met so many people who tell me they can’t run. They’re too ungainly, their bums are wobbly, they’re worried about their knees, they’re too out of shape. Evolution argues otherwise. There are a number of researchers investigating the evolutionary trends for humans to be efficient runners, arguing that we are all biomechanically equipped to run (wobbly bums or not). If you have any question whether you can or can not run, just check out the categories of races in the Paralympic Games. For example, the T-35 athletics classification is for athletes with impairments in ability to control their muscles; in 2012, Iiuri Tsaruk set a world record for the 200m at 25.86s, which is only 6 seconds off Bolt’s world record at 19.19 and 4 seconds off Flo-Jo’s womens record (doping aside). 2012 also saw the world record for an athlete with visual impairment: Assia El Hannouni ran 200m in 24.46.[ii] You try running that fast. Now try running with significant difficulty controlling your limbs or seeing. If you’re impressed, think about these athletes the next time you say you can’t run.


Paralympian Scott Rearden. Wikimedia Commons.

Let’s think about bipedalism for a bit. Which other animals walk on two legs besides us? Birds, for a start, although flight is usually the primary mode of transport for all except penguins and ostriches. On the ground, birds are more likely to hop quickly than to walk or run. Kangaroos also hop. Apes are able to walk bipedally, but normally use their arms as well. Cockroaches and lizards can get some speed over short distances by running on their back legs. However, humans are different as we always walk on two legs, keep the trunk erect rather than bending forward as apes do, keep the entire body relatively still, and use less energy due to stored kinetic energy in the tendons during the gait.[iii] Apparently we can group our species of strange hairless apes into the category “really weird sorts of locomotion” along with kangaroos and ostriches.

Following this logic, Lieberman et al point out that a human could be bested in a fight with a chimp based on pure strength and agility, can easily be outrun by a horse or a cheetah in a 100m race, and have no claws or sharp teeth: “we are weak, slow, and awkward creatures.”[iv] We do have two strokes in our favor, though – enhanced cognitive capabilities and the ability to run really long distances. Our being awkwardly bipedal naked apes actually helps more than one would think. First, bipedalism decouples breathing from stride. Imagine a quadruped running – as the legs come together in a gallop, the back arches and forces the lungs to exhale like a bellows. Since humans are upright, the motion of our legs doesn’t necessarily affect our breathing pattern. Second, we sweat in order to cool down during physical exertion. (In particular, I sweat loads.) Panting is the most effective way for a hairy animal to cool down, as hair or fur traps sweat and doesn’t allow for effective convection (imagine standing in a cool breeze while covered in sweat – this doesn’t work for a dog.) But it’s impossible to pant while running. So not only are humans able to regulate breathing at speed, but we can cool down without stopping for breath.

From a purely skeletal perspective, there is more evidence for the evolution of running. Human heads are stabilized via the nuchal ligament in the neck, which is present only in species that run (and some with particularly large heads), and we have a complex vestibular system that becomes immediately activated to ensure stability while running. The insertion on the calcaneus (heel bone) for the Achilles tendon is long in humans, increasing the spring action of the Achilles.[v] Humans have relatively long legs and a huge gluteus maximus muscle (the source of the wobbly bum). All of these changes are seen in Homo erectus, which evolved 1.9 million years ago.[vi]

H. erectus skeleton with adaptations for running (r) and walking (w). From Lieberman 2010.

H. erectus skeleton with adaptations for running (r) and walking (w). From Lieberman 2010.

The evolutionary explanation for this is the concept of endurance or persistence hunting. In a hot climate, ancient Homo could theoretically run an animal to death by inducing hyperthermia. This is also where we come full circle and bring in the cognitive capabilities of group work. A single individual can’t chase an antelope until it expires from heat stroke because it’ll keep going back into the herd and then the herd will scatter. But a team of persistence hunters can. If persistence hunting is how humans (or other Homo species) evolved to be great at long distance running, that’s also the why humans developed larger brains: the calories in meat generated an excess of calories that allowed nourishment of the great energy-suck that is the brain. However, persistence hunting is a skill that mostly went by the wayside as soon as projectile weapons (arrowheads and spears) were invented, possibly around 300,000 years ago. Why? Because humans, due to our large brains, are very inventive, but also very lazy. Any expenditure of energy must be made up for by calories consumed later, at least in a hunting and gathering environment – so less energy output means less energy input; a metabolic balance. Thus we have the reason why humans can run, but also why we don’t really want to. (As an aside, some groups such as the Kalahari Bushmen practiced persistence hunting until recently, although they had projectile weapon technology, probably because of skill traditions and retaining cultural practices. Humans are always confounding like that.)

Which brings up another point: gathering. As I’ve written before, contemporary hunter-gatherers like the Hadza rely much more on gathering than hunting. Additionally, it is possible that the first meat eaten by Homo species was scavenged rather than hunted. There is no such evolutionary argument as endurance gathering. If ancient humans spent much more time gathering, why would we evolve these particular running mechanisms? As with many queries into human evolution, these questions have yet to be answered. Either way, it’s clear that humans have a unique ability. Your wobbly bum is, in fact, the key to your running. Another remaining question is why we still have the desire to continue running these ridiculous distances – a topic for a future post, perhaps.


[i] http://www.zoegoesrunning.com

[ii] Check out all the records at http://www.paralympic.org/results/historical

[iii] Alexander, RM. Bipedal Animals, and their differences from humans. J Anat, May 2004: 204(5), 321-330.

[iv] Lieberman, DE, Bramble, DM, Raichlen, DA, Shea, JJ. 2009. Brains, Brawn, and the Evolution of Human Endurance Running Capabilities. In The First Humans – Origins and Early Evolution of the Genus Homo (Grine, FE, Fleagle, JG, Leakey, RE, eds.) New York: Springer, pp 77-98.

[v] Raichlen, DA, Armstrong, H, Lieberman, DE. 2011. Calcaneus length determines running economy: implications for endurance running performance in modern humans and Neandertals. J Human Evol 60(3): 299-308.

[vi] Lieberman, DE. 2010. Four Legs Good, Two Legs Fortuitous: Brains, Brawn, and the Evolution of Human Bipedalism. In In the Light of Evolution (Jonathan B Losos, ed.) Greenwood Village, CO: Roberts & Co, pp 55-71.

Movement Taster – Movement in Premodern Societies

StacyHackner14 May 2014


The following is a taster for the Student Engagers’ Movement event taking place at UCL on Friday 23 May. Stacy, a researcher in Archaeology, will be discussing movement through the lens of biomechanics.

by Stacy Hackner

Imagine you’re in the grocery store. You start in the produce section, taking small steps between items. You hover by the bananas, decide you won’t take them, and walk a few steps further for apples, carrots, and cabbage. You then take a longer walk, carefully avoiding the ice cream on your way to the dairy fridge for some milk. You hover, picking out the semi-skimmed and some yogurt, before taking another long walk to the bakery. This pattern repeats until you’re at the checkout.

What you may not realize is that this pattern of stops and starts with long strides in between may be intrinsic to human movement, if not common to many foraging animals. A recent study of the Hadza, a hunting and gathering group in Tanzania, shows that they practice this type of movement known as the Lévy walk (or Lévy flight in birds and bumblebees). It makes sense on a gathering level: you’ve exhausted all your resources in one area, so you move to another locale further afield, then another, before returning to your base. When the Hadza have finished all the resources in an area, they’ll move camp, allowing them to regrow (for us, this is the shelves being restocked). This study links us with the Hadza, and the Hadza with what we can loosely term “ancient humans and their ancestors”.

Diagram of a Levy walk.

Diagram of a Levy walk. Credit Leif Svalgaard.

It’s unsurprising that the Hadza were used to examine the Lévy walk and probabilistic foraging strategies. As they are one of the few remaining hunter-gatherer groups on the planet, they are often used in scientific studies aiming to find out how humans lived, ate, and moved thousands of years ago, before the invention of agriculture. The Hadza have been remarkably amenable to being studied by researchers investigating concepts including female waist-to-hip ratios, the gut microbiome, botanical surveys, and body fat percentage. Tracking their movement around the landscape using GPS units is one of the most ingenious!

Much of the theoretical background to my work is based on human movement around the landscape. The more an individual moves, the more his or her leg bones will adapt to that type of movement. Thus it is important to examine how much movement cultures practicing different subsistence strategies perform. The oft-cited hypothesis is that hunter-gatherers perform the most walking or running activity, and the transition to agriculture decreased movement. An implicit assumption in this is that males, no matter the society, always performed more work requiring mobility than females. This has been upheld in a number of archaeological studies: between the Italian Late Upper Paleolithic and the Italian Neolithic, individuals’ overall femoral strength decreased, but the males decreased more; over the course of the Classical Maya period (350-900 AD), the difference in leg strength between males and females decreased, solely due a reduction in strength of the males. The authors posit that this is due to an economic shift allowing the males to be free from hard physical labour.

However, I take issue with the hypothesis that females always performed less work. The prevailing idea of a hunting man settling down to farm work while the gathering woman retains her adherence to household chores and finding local vegetables is not borne out by the Hadza. First, both Hadza men and women gather. Their resources and methods differ – men gather alone and hunt small game while women and children gather in groups – but another GPS study found that Hadza women walk up to 15 km per day on a gathering excursion (men get up to 18 km). 15 km is not exactly sitting around the camp peeling tubers. Another discrepancy from bone research is the effect of testosterone: given similar levels of activity, a man is likely to build more bone than a woman, leading archaeologists to believe he did more work. Finally, hunting for big game – at least for the Hadza – occurs rarely (about once every 30 hunter-days, according to one researcher) and may be of more social significance than biomechanical, and berries gathered account for as many calories as meat; perhaps we should rethink our nomenclature and call pre-agricultural groups gatherer-gatherers or just foragers.

For a video of Hadza foraging techniques, click here.

For a National Geographic photo article, click here.



Marchi, D. 2008. Relationships between lower limb cross-sectional geometry and mobility: the case of a Neolithic sample from Italy. AJPA 137, 188-200.

Marlowe, FW. 2010. The Hadza: Hunter-Gatherers of Tanzania. Berkeley: Univ. California Press.

O’Connell, J and Hawkes, K. 1998. Grandmothers, gathering, and the evolution of human diets. 14th International Congress of Anthropological and Ethnological Sciences.

Raichlen, DA, Gordon, AD, AZP Mabulla, FW Marlowe, and H Pontzer. 2014. Evidence of Lévy walk foraging patterns in human hunter–gatherers. PNAS 111:2, 728-733.

Wanner, IS, T Sierra Sosa, KW Alt, and VT Blos. 2007. Lifestyle, occupation, and whole bone morphology of the pre-Hispanic Maya coastal population from Xcambó, Yucatan, Mexico. IJO 17, 253-268.

The Biomechanics of Breasts

StacyHackner24 February 2014

Stacy Hackner_ThumbnailBy Stacy Hackner

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.

The breast in three dimensions. Zhou et al 2012.

The breast in three dimensions. Zhou et al 2012.

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.

The figure-8 movement of breasts in a digital reconstruction. Image via ShockAbsorber website.

The figure-8 movement of breasts
in a digital reconstruction.
Image via ShockAbsorber website.

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. 

How are Ancient Nubians Like Astronauts?

StacyHackner6 January 2014

Stacy Hackner_Thumbnail By Stacy Hackner

Some respected individuals (supervisors, mentors, parents) have advised me to not get distracted by the primrose paths that crop up during a PhD. These primrose paths are always deliciously exciting, offering opportunities to study wonderful new topics that one can justify as marginally related to one’s thesis and therefore potentially of use. Of all the primrose paths I’ve followed, I never expected the most relevant one to be about astronauts.

Credit: Wikipedia Commons

Sudanese pyramids. (Wikimedia commons.)

My thesis explores ancient Nubia, the region that is now northern Sudan, from roughly 3000 years ago to medieval times. Unlike their contemporaries, the Egyptians, the Nubians didn’t have a system of writing until the Meroitic period (300 BCE-400 CE), a time of Egyptianizing influence. They built small pyramids and imported Egyptian goods, attesting to the influence of their famous northern neighbors. In the absence of writing (and even in the presence of texts, as humans tend to play with the truth), archaeologists try to build a picture of the ancient society using physical evidence, including human remains. Fortunately, the dry climate and sandy soil usually result in excellent bone preservation, allowing me to identify differences in bone shape. But wait – let me back up a little.

We aren’t entirely sure how bone works. There are two types of cell responsible for bone maintenance – osteoblasts and osteoclasts. Osteoblasts build bone, and osteoclasts take it away. The body is highly responsive to changes in activity, and bone is constantly updating itself accordingly. The general principle is that your body thinks what’s happening now will happen forever. Think about when you’re running a race: it’s hard to start because your body’s been used to standing still and needs some time to amp up your heart rate and muscle contractions. When you finish the race, your heart keeps pounding for a minute or two because it hasn’t quite got the signal to stop running yet. Bone works in a similar way. In response to physical stress, bone will accumulate more osteoblasts to strengthen itself. Each step makes tiny microfractures, which tells the bone “Come on, I’m breakin’ here! Give me more strength!” and the osteoblasts pile on. In the absence of activity – during periods of prolonged sitting or lying down – the osteoclasts come in to take away unnecessary bone. “You’re not using this one, right? Then we can send the calcium somewhere else.” The thing is, scientists don’t know all the signals involved in this process. We know what happens, but not the channels of communication. I like to imagine bone cells having little conversations with each other, but clearly it’s all on a neurochemical level we haven’t yet discovered.

The concept of bone building in necessary areas is keenly presented in studies of elite athletes. In a study by Haapsalo et al (1998) of young female tennis players, the players gained significant bone mineral content in the bones of their dominant (forehand and serving) arm. When the authors looked at a control sample (girls who did not play tennis), there was minimal or no difference between their arms; there was also minimal difference between the nondominant arms of the tennis players and the controls.

Another study, by Shaw & Stock (2009), examined differences between university athletes who competed in either hockey or long-distance running. They found significant differences in the actual shape of the tibia (shin bone) due to the physical stress of these activities. The tibias of the long-distance runners were more elongated front to back while the tibias of the hockey players were more even side-to-side, showing a distinct difference in the direction of activity in these sports. Clearly, osteoclasts were being sent to the bone locations these athletes needed them most: for runners, the front, and for hockey players, the sides. It is important to point out that many of the studies investigating activity and bone growth look at adolescents, since their bones develop until the end of puberty. After that, it seems to take a lot more effort to alter bone shape and density.

Credit: wikipedia

Her bones are losing mineral content by the minute! (Wikimedia commons.)

And what about the other side of the cycle? The osteoclasts? That’s where the astronauts (and cosmonauts) come in. The constant pounding of our feet against the floor keeps our bones as strong and dense as they need to be for everyday use. In zero gravity, though, there’s no pounding, just the occasional soft push off the wall of the space station. The osteoblasts don’t have any stress to react to, and the osteoclasts assume the extra bone is useless, so it starts to be resorbed. It helps that astronauts are some of the most-studied individuals on our planet (and definitely the most studied off the planet!). During spaceflight, urine calcium output is found to increase, indicating that bone is being sapped of minerals, and post-spaceflight bone scans reveal a condition called “spaceflight osteoporosis”, similar to the osteoporosis experienced on earth – but the bone density is only lost from the legs, feet, and hips, all weight-bearing regions, including an 8% loss in four months (compared to 1% loss per year for earth-bound sufferers of osteoporosis). The upper body and head generally remain unaffected (unless one of the astronauts was a tennis player, of course). One study found that after a “long-duration” spaceflight of 4-6 months, it took up to three years for astronauts to recover the bone density they’d lost in space (Sibonga et al, 2007). It makes one really appreciate gravity.

So how do I apply this to ancient populations? The data from astronauts indicates that most of the density lost is from trabecular bone, from the internal core, rather than from the outside. This means that even if ancient bones have lost density due to age, either before death or after burial, it’s likely to have happened from the inside out and thus the external shape should remain intact. This gives me more confidence in figuring out what kinds of activities they performed during adolescence, which in most cultures was when young people started to take up adult cultural roles. I hope to compare the shape of the bones of Nubians to those of athletes and to other populations whose activities are known in order to draw a better picture of their society.


For an amusing (but factual) look at the craziness that is astronaut and cosmonaut research, check out Mary Roach’s “Packing for Mars: The Curious Science of Life in the Void” (Norton & Co, 2010).

Haapasalo, H, P Kannus, H Sievänen, M Pasanen, K Uusi-Rasi, A Heinonen, P Oja, and I Vuori. 1998. Effect of long-term unilateral activity on bone mineral density of female junior tennis players. Journal of Bone and Mineral Research 13/2, 310-319.

Shaw, CN and JT Stock. 2009. Intensity, Repetitiveness, and Directionality of Habitual Adolescent Mobility Patterns Influence the Tibial Diaphysis Morphology of Athletes. AJPA 140, 149-159.

Sibonga, JD, HJ Evans, HG Sung, ER Spector, TF Lang, VS Oganov, AV Baulkin, LC Shackelford, and AD LeBlanc. 2007. Recovery of spaceflight-induced bone loss: bone mineral density after long-duration missions as fitted with an exponential function. Bone 41, 973-978.