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Theory for the Evolution of Short Right Leg Syndrome

I am frequently asked why is it always the right leg that is the short one? I propose it is an evolutionary adaptation increasing the chance of survival for mother and child in prehistoric times.

When early primates began the process of evolving from walking on all fours to upright bipedal motion, there are three options for leg length. Either they are even or have a short left leg or a short right leg. Even leg length is ideal for a level foundation for the spine, long term comfort and freedom from pain and injury. However, the body does have some natural coping mechanisms to manage uneven leg lengths...often not developing difficulties until later in life when stiffening with age and reduced mobility

Signature traits of a short right leg are an elevated left hip and elevated right shoulder. Mothers always carry baby on the left is more secure and easily held in place compared to the opposite hip where baby tends to slide off. Many are aware they always carry a bag on the right is more secure whereas it feels like it is always about to drop off the left is the left bra strap that always drops off...the "off the shoulder look" is always to the left.

Imagine early hominid species living a hunter gatherer existence. Their survival is reliant on being nimble, able to care for their young, gather food and defend themselves from threats. The adaptation of Short Right Leg Syndrome (SRLS) for mothers enabled them to more securely carry baby on the left hip, while over the right shoulder is their bag of charms, implements and food gathered in the forest, leaving their dominant right arm free for work and defence. While it may not seem like a great advantage, the small gains in efficiency collecting food and in defence, do increase the chances of survival in the wild and dangerous prehistoric environment. It was the most successful model. Hence, through the evolutionary process for the selection of the species, the adaptation of SRLS has become a dominant trait now hardwired into our genetics.

While SRLS is still useful today for baby and bag carrying purposes, our survival is more assured. We have baby carriages, shopping trolleys, bicycles and automobiles for mobility and carrying what we need. Living well beyond the life expectancy of our early hominid species we reach an age where we now bemoan the effects of SRLS when our bodies lose the flexibility of youth...we no longer walk like a cat...we begin to feel the stress of this asymmetry causing pain and injury, and wonder "Why is the right leg always the short one?"

By simple logical deduction we know our early hominid ancestors were predominantly right handed for this outcome to occur. Had they been left dominant we would be seeing short left legs. We do not see short left legs. I have only sighted three short left leg cases. All due to injury in their youth or accident at birth. Even or near even leg length is rare.

First published 23 February 2021

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