You've Gotta Learn to Walk Before You March

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Saturday, April 22, 2017, is Earth Day. So it is appropriate that organizers have scheduled a large-scale march on Washington, DC (and over 500 other satellite marches planned), in support of evidence-based policy decisions. It is interesting that in the year 2017 we require marches to get across the point (hopefully) that scientific evidence should be a critical aspect of policy development. But alas, here we are.

So, in the spirit of marching along with our fellow scientists and science enthusiasts who understand the importance of scientific inquiry, and who want to advocate for sustained and predictable federal funding for science, we at THE 'SCOPE thought it would be fun to take a closer look at the funny (dare I say, silly?) way that humans walk...er, march: on two legs, or bipedally as it is called.

We briefly covered the evolution of bipedal walking in an earlier post, so we will not rehash the anatomical adaptations that allow for this type of locomotion. Suffice it to say that our skeleton has undergone several changes to the spine, pelvis, and lower limbs, that allow us to keep our knees under our center of mass while standing and walking. Instead, we will focus on HOW we accomplish walking along on two legs.

The phases of the gait cycle. From Anatomy Reference Center.

When walking, our lower limbs alternate between being planted on the ground and swinging through the air in order to be planted as the next step. We refer to these two different positions as the two phases of the "gait cycle": stance phase, or when the foot is planted; and swing phase, or when the foot is.... wait for it.... swinging.

Transfer of the weight across the foot during stance phase.

Let's now put the gait cycle "under the scope" for a moment. The gait cycle is defined as the period from stance phase of one limb to the next stance phase of the same limb - never mind for now that the other limb is in its own gait cycle! The stance phase begins when the heel strikes the ground, at which point the body weight is transferred from the heel to the lateral side of the foot, across the ball of the foot, and toward the base and ultimately distal end of the big toe (referred to as "push off" or "toe off"). At push off, the limb enters the swing phase, which itself ends when the heel strikes the ground again.

So as you can see, the stance phase accounts for a higher proportion of the gait cycle (about 60%) than does the swing phase. But remember this is for one limb only... the other limb is also in one of the two phases of the gait cycle. Over the combined gait cycles of both limbs, the two limbs together contact the ground (double support) only about 25% of the time. So roughly 75% of the combined gait cycles involves support over a single limb. As walking speed increases, the percentage of the gait cycle spent in single support also increases. During running, for example, there is no period of double support!

Even with the silly type of locomotion that humans employ, it is highly energetically efficient because bipedal walking is essentially a series of controlled falls over the supported limb (the limb in stance phase). In other words, the lower limb acts like an inverted pendulum which uses the force of gravity and the principles of angular momentum to propel the body forward. Combine the low input energy required to sustain level walking with the fact that 60-70% of the input energy is recovered through anatomical function of the lower limb that reduces the vertical and lateral displacement of the center of mass during the gait cycle, and what you have is a recipe for efficient locomotion on two legs.

So, to those who are marching on behalf of the scientific enterprise, remember this: you can stride a great deal before you tire out simply because you are the product of 6-7 million years of evolution for bipedal locomotion.

Jason Organ is Assistant Professor of Anatomy & Cell Biology at Indiana University School of Medicine. Follow Jason on Twitter.

References:
Mochon S, & McMahon TA (1980). Ballistic walking. Journal of biomechanics, 13 (1), 49-57 PMID: 7354094

Lovejoy, C. (1988). Evolution of Human Walking Scientific American, 259 (5), 118-125 DOI: 10.1038/scientificamerican1188-118

Gait cycle illustrations from: Organ JM. 2017. Gait. Amirsys Anatomy Reference Center, Elsevier: Salt Lake City, UT. https://app.anatomyreferencecenter.com/