The Human Paradox

     My husband and I always get a laugh when we get stuck behind a sports car that is going 15 mph below the speed limit.  Its not that we want the driver to go beyond the speed limit, but it seems odd that (s)he owns this beautifully engineered "muscle" car designed for high speeds, yet s(he) chooses to go below what is an acceptable speed.  It is a paradox of sorts.  The potential is there, but the minimum isn't even attempted.

     Where am I going with this?  The human body has evolved to possess specific features that provide us with the potential to be exceptional long distance runners, an ability which is unique to humans when compared to other primates and is a rarity among mammals.  According to Daniel E. Lieberman and Dennis M. Bramble in their article "The Evolution of Marathon Running Capabilities in Humans," this ability emerged about 2 million years ago when food had to be hunted and scavenged by our ancestors for survival.  

     What are these features that our bodies possess to achieve such a feat?  In an earlier article by Dennis Bramble and Daniel Lieberman "Endurance Running and the Evolution of Homo," they describe 4 categories of these specialized features which are outlined below:

Energetics (the metabolic or energy cost of moving the body):

• The tendons (most important of which is the Achilles tendon) in the legs store elastic energy that is used to propel the body into the aerial phase (both feet off the ground) of running, thereby reducing approximately 50% of the energy cost of running.
• The arch and ligaments of the foot function as a spring during the stance phase, reducing approximately 17% of the energy cost of running.
• A smaller lower leg (calf) mass compared to that of the upper leg (thigh) reduces muscular effort to move the lower leg.
• Stride length is increased and stride rate is decreased during endurance running due to the spring-mechanisms of tendons and ligaments and relatively long legs compared to animals.  The decreased stride rate reduces the force that has to be used to move the legs, which account for 30% of total body mass.
• Human forearms are smaller in muscle mass compared to chimpanzees.  The smaller muscle mass reduces the effort needed to keep the elbow flexed during running.

Skeletal Strength:

• The larger joint surfaces found at the femoral head, knee, sacroililac, and lumbar centra joints allow the resultant impact forces from the ground during heel strike to be spread and dissipated over a larger surface area reducing stress placed on the body.

Stabilization:

• The neck and trunk shift slightly forward during running to aid with balance.
• Trunk stabilization is enhanced due to broader surfaces on the sacrum and posterior iliac spine to which the erector spinae muscles are attached.
• An enlarged gluteus maximus aids in stability while running.  This muscle is not active during walking, but is greatly recruited during running.
• The torso is very mobile.  It allows for counter rotation within itself and of the arms to counteract the opposing forces created by the legs when they are swung forward in running.  In walking these forces are counteracted by ground reaction forces (during running there is the aerial phase in which there is no contact with the ground to make use of this form of stabilization).
• The elongated, narrow waist of the body allows the trunk to rotate independently from the hips to a greater degree compared to apes.  This allows for a counter balance of torques generated by the legs while running.
• Humans possess relatively wide shoulders which function to counteract the motion of arm-swinging during running. 

Thermoregulation and Breathing:  

• Running can generate up to 10 times more heat than walking.  Most mammals are not as efficient at dissipating heat as are humans.  Therefore, they have to stop after short distances to avoid hyperthermia.
  
• Humans are proficient sweaters due to an increased number and density of sweat glands.  This allows for heat to be dissipated through evaporation.
• Humans have reduced body hair (fur) compared to animals allowing an increase in convection rates to dissipate heat.
• Although humans mouth breath while running, they do not pant like animals.  Panting is how mammals cool their bodies.  However, it interferes with respiration, decreasing endurance capabilities.  The mouth breathing of humans helps to release excess body heat during expiration.

     According to Lieberman and Bramble, humans have this amazing capability to outrun most other mammals, even horses at times, when it is hot.  In their articles, they hypothesize that these specialized features evolved in humans to give them a hunting and scavenging advantage over other animals.  It is believed that early humans used "persistence hunting" to obtain food.  This tactic involved the human runner to follow an animal for a prolonged period of time until it was driven into hyperthermia, allowing the runner to safely get close to it to kill it.

     Isn't it interesting, and a bit unfortunate, that the years of "fine tuning" our specialized features have been met with modern man's physical inactivity and complacency?  Many of us are like the slow driving owner of a sports car, we are not even attempting what we have been designed to do - run.  Don't you think it is time that we at least try?

Note:  Before beginning an exercise program or increasing the intensity level of a current routine, a physician's approval should be obtained, especially for older adults and those at risk for or who currently have chronic health conditions. 

Resources: 

Sports Med 2007: 37 (4-5): 288-290. "The Evolution of Marathon Running Capabilities in Humans," Lieberman, DE. and Bramble, DM.

Nature 432, 345-352 (18 November 2004). "Endurance Running and the Evolution of Homo," Bramble, DM., Lieberman, DE.

Symposium - What was Normal Human Physical Activity and What is the Biological Basis for Current Daily Exercise? American College of Sports Medicine 2009 Annual Meeting, Seattle Washington. "Endurance Exercise and the Emergence of the Human Body: A Two Million Year Record," Bramble, DM.