Back pain and bipedalism
The World Health Organisation notes that in 2020, 619 million people worldwide suffered from lower back pain. Globally, lower back pain is the primary cause of disability.
I am one of these 619 million people.
I have often wondered why low back pain is so common among humans but have learned that the primary cause of this widespread incapacity is bipedalism (‘from the Latin ‘bis’ meaning twice and ‘pes’ meaning foot). Bipedalism is walking upright on two legs and it is a defining characteristic that sets humans apart from other species, because we cannot walk on four limbs – we are obligate bipeds.
Some animals have limited bipedalism, such as squirrels and raccoons, which crouch on their hind limbs to hold and eat their food but use all four limbs to move. Most carnivorous dinosaurs, such as Tyrannosaurus rex and Allosaurus, were bipedal but some also walked on four limbs. Kangaroos hop bipedally but they use their hefty tails as a fifth leg for balance. Ostriches are bipedal, as they are too big to fly.
Figure 1. Comparison of the skeletons of a gorilla (left) and human (right)
Why did humans become obligate bipeds when our close relatives – the apes – are not?
There is a plethora of theories about why humans became bipedal. The savannah hypothesis is based on the knowledge of major climate shifts (a seasonal cooling and drying trend) during the Miocene epoch (23-5 million years ago), which resulted in the shrinking of forests and the sweeping spread of open savannah (grasslands with intermittent trees) landscapes. Early members of the human lineage stood up to look for danger and found that it was also easier to move across these grasslands while upright. Another hypothesis is the postural feeding hypothesis, which is based on the need to reach for food in trees with the fore limbs, while standing on the back limbs (like chimpanzees) and this evolved into moving on the two back limbs. Yet another theory is that there was a need to free the front limbs to carry food.
Figure 2. The human spine
What happened to the skeleton when humans became obligate bipeds?
Figure 1 is relevant to all the following.
With the advent of bipedalism, the skeletons of our ancient ancestors underwent significant changes. One of the most notable changes was the alteration in the shape of the pelvis, which became shorter and broader, providing a stable base for the upper body when compared to apes and other primates and for transferring the weight to the back limbs. The back limbs – particularly the thigh bones – became longer than the front limbs, facilitating better balance, increasing stride length and transferring weight. Feet – no longer used to grasp branches – experienced a dramatic change and changed, forming platforms that supported the entire body weight. Toes were no longer needed for grasping, so they became short and the ankle bones also changed to bear weight. Each foot contained an arch to distribute weight, much like the base of the Eiffel Tower supports the rest of it. The back limbs – our legs – also became slanted inwards, with the hips wide apart but knees and feet close together, under the body’s centre of gravity, maintaining balance.
The vertebral column adapted to have two S-shaped curves, with one bend at the chest and another near the lower back, to accommodate weight-bearing. Without the second curve, the vertebral column would always lean forward, tipping the person headlong. Vertebrae in the region of the second curve became large to bear the upper body weight.
The skull was now balanced on top of the vertebral column, unlike in the apes and with it came changes to the first few neck vertebrae that now supported and rotated the skull.
Therefore, the human body became arranged so that the head, torso, legs and feet and their respective links in the neck, spine, pelvis, knees, and ankles were aligned in a vertical pole-like structure.
Bipedalism freed our hands to adapt to use tools, carry food and other needs and for defence. This set off a series of anatomical, physiological, and behavioural changes that dramatically improved brain power and eventually resulted in the development of complex social structures, sophisticated communication and the ability to alter the environment in which we lived. Bipedalism, as scientist Frank Jakobus noted, was ‘the evolutionary leap that shaped humanity’.
Why then, is bipedalism considered the root cause of back pain and other disabilities?
Firstly, the way parts of the human body are arranged vertically violates some principles of mechanical engineering. In objects constructed by engineers, most of the weight is situated low to ensure stability. For example, the keel of a boat is filled with lead, so any sudden sideways force will result in only minimal sideways movement. In humans, the weight is concentrated in the upper part of the body, so our structure is inherently unstable. Even just standing requires a great deal of subconscious concentration and coordination. Each time we take a step, we teeter on the edge of falling over, because when we step forward, our entire body weight is on a single foot 40% of the time. Contrast this to four-legged animals, which are supported, when walking, by at least two, but usually three limbs, maintaining high stability.
As we grow older, the chances of losing our balance and falling become significant, as the natural ageing process retards our senses and reflexes, weakens our muscles and wastes our bones. With the wear and tear of age, as well as changes in bone density and muscle weakness, the curves in our spines change: the lower curve decreases and the upper curve increases, leading to a characteristic hunched and forwardly bent posture of the elderly.
The WHO notes that ‘falls are the second leading cause (after traffic accidents) of unintentional injury-deaths worldwide’ and ‘adults older than 60 years of age suffer the greatest number of fatal falls’.
Next, the second curve of the spine, which allowed us to stand without keeling over, also had to bear the increased pressure of keeping us upright. The lumbar vertebrae, within this curve, although the biggest in the whole spine, suffer the effect of this largely unrelenting support for the upper body’s weight. The consequence is chronic pain in the lower back and more seriously, slipped disks, slipped lumbar vertebrae and unstable spines, all of which can result in some degree of disability.
Thirdly, our neck regions are not exempt from the stresses of bipedalism. Balanced on our spines, our skulls do not need strong and large neck muscles to hold them up, in contrast to apes, which have huge neck vertebrae and neck muscles, which hold their heads horizontal. Like the spine, the stress of keeping our head balanced, coupled with small neck muscles, leaves us very vulnerable to damaging the neck region. Worldwide, neck pain is the second most common disability, after low back pain.
In four-legged animals, the spine is like a suspension bridge hanging between four ‘towers’ – the four limbs – and girdled by the pelvis, along which weight is distributed. In humans, the entire body weight is transferred from the hips to the two legs – to the thighs, knees and feet. Because of this, at all these points – the hips, knees and feet – problems can ensue, most often presented as osteoarthritis. In fact, osteoarthritis is known to have evolved in tandem with the evolution of the complex human knee.
Today, layered on top of these existing vulnerabilities, is the change in our lifestyles: we are much less active than our ancestors (because of technology and automation) and our choices often include fast food, junk food, or high-caloric foods that lead to being overweight or obese, putting additional stress and strain on the aforementioned joints. COVID-19 led to the regularisation of working from home, further increasing sitting in one place. With our smartphones, computers, tablets and smart TVs, we spend a quarter of 24 hours in front of screens. Further juxtaposed on all these issues, is our poor posture: we slouch when using our devices, subjecting our necks and spines to stresses and strains not experienced by our ancestors.
Anthropologist Jeremy De Silva encapsulates this perfectly, with his quote that “evolution does not create the best ‘design’ out of scratch. Evolution does not create perfection. It moulds previous structures to produce anatomies just good enough to survive”.
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