Osteoarthritis, the price of being bipedal?
Bipedal walking is not unique to humans (some monkeys are able to walk on two limbs as were hominids as well).
It is actually this exclusive bipedalism which, among other things, contributes to the uniqueness of Homo sapiens sapiens and their direct predecessors or closest relatives (such as Neanderthal Man).
This evolution was accompanied by skeletal adaptations favouring the dynamics of walking, the highest form of which is found in modern humans.
Walking continuously with both lower limbs leads to advantages such as freeing the hands. Another consequence of our less fortunate "upright" position: the considerable increase in weight and pressure on certain joints, such as the hip and spine.
The evolution of our skeleton only enables us to withstand the weight in an incomplete way and the joints of the lower limbs have to continuously bear the weight of the upper body when walking or even when standing still. Carrying loads only adds to the pressure on the hips, knees and spine.
Bipedalism might then in itself be a "mechanical" risk factor of osteoarthritis. What goes against this hypothesis is the fact that some quadrupeds are equally affected which suggests that the mechanisms of this disease are somewhat more complex.
Exclusive bipedalism requires good force distribution.
When standing, the force exerted on each hip is equal to half the body weight reduced by the weight of both lower limbs
In modern humans, the anatomical features of the joint can "relieve" the joint by distributing the force as shown in the sketch above.
This distribution tends to produce a zero kinetic "moment".
- PC = force produced by the weight of the body
- A = force produced by the abductor muscles
- R = reaction force of the joint.
Lucy walked with a very swaying posture when she was not climbing trees.
The most famous of hominids is probably Lucy, the female Australopithecus afarensis discovered in 1973 by Y. Coppens, M. Taieb and B. Johanson in Ethiopia where she lay peacefully for about 3 million years.
According to current thinking, Australopithecines are not our direct ancestors, but more or less distant cousins.
Lucy's pelvis, wider than ours, and the pronounced direction of her femurs towards the median line (such as a modern woman with a femur deformity indicating coxa vara and genu valgum types of deformity) suggests that she had to walk with a heavily swaying posture in fairly brief episodes of bipedalism (a few tens of metres according to the footprints discovered in Laetoli, still in East Africa). Her upper limbs, longer than in humans, and the less restricted movements of her shoulder show that Lucy must, in fact, have spent a lot of time in the trees.
Lucy could therefore settle for a femur head much smaller than ours to perform limited journeys during which she briefly increased weight and pressure on her hips, without increasing her risk of osteoarthritis of the hip of mechanical origin.
Evolution of the axis of the lower limb (in relation to bipedalism)
… 3.6 million years ago two Australopithecus afarensis (one much smaller than the other) walked side by side in a plain of northern Tanzania, leaving their footprints on volcanic ash which the rain transformed into a sort of plaster.
In drying, this material was to take on the properties of a concrete hard enough to preserve these footprints to the present day. These footprints tell us that a step began when the heel struck the ground and ended with a push of the toes. We do not do otherwise.
…and Neanderthals had a bent forward look
Let's casually skip a few million years to take a look at Homo sapiens neanderthalensis, a European who witnessed the arrival of Homo sapiens sapiens and, like him, was descended perhaps from Homo erectus.
Neanderthal had a curved femur with a concavity towards the front. Also adept at shifting from one foot to the other, Neanderthals had to walk very much bent forward. His bipedalism is thus accompanied by a weight distribution very different from ours, without being able to say whether, because of this, he was more or less susceptible than us to osteoarthritis.
In addition, the many fractures observed on Neanderthal skeletons suggest a perilous life, probably too short to reach the age at which this disease begins to manifest itself!