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How the Body Deals with Gravity

Howard Glicksman

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Editor’s note: Physicians have a special place among the thinkers who have elaborated the argument for intelligent design. Perhaps that’s because, more than evolutionary biologists, they are familiar with the challenges of maintaining a functioning complex system, the human body. With that in mind, Evolution News is delighted to offer this series, “The Designed Body.” For the complete series, see here. Dr. Glicksman practices palliative medicine for a hospice organization.

the-designed-body4.jpgOur muscles, under the control of our nerves, allow us to breathe, swallow, move around and handle things. The peripheral nerves send sensory information about what is going on outside and inside the body to the spinal cord and the brain and from them send back motor instructions to the muscles to tell them what to do. In a previous article in this series, I described some of the sensors that as transducers convert phenomena into information the body can use. Pressure is detected by sensors in the skin; body motion, particularly of the head, is detected by the vestibular apparatus within the inner ear; and the proprioceptors provide information on the status of the muscles, tendons, and joints.

My last article described some of the reflexes (involuntary pre-programmed automatic motor responses without conscious direction from the brain) the body uses to avoid serious injury and maintain its position. Now let’s look at how the body deals with the law of gravity and what it takes to keep its balance. Remember that when evolutionary biologists tell us about life and the mechanism by which it must have come about, they only deal with how it looks and not how it must actually work within the laws of nature. Ask yourself which is a more plausible explanation for how life arose: chance and the laws of nature alone, or intelligent design?

An object’s center of gravity is a theoretical point about which its weight is evenly distributed. For an object that has a uniform density with a regular and symmetrical shape, such as a square piece of solid wood, the center of gravity is at its geometric center. Place a square solid wooden block on a table and push it more and more off the edge. It will fall to the ground when its center of gravity is no longer on the table.

The human body is made of muscles, organs, fat, and bone, each with a different density. Although the physical outline of the body is symmetrical from side to side, its shape is very irregular. The center of gravity for most people while standing or lying with their arms at their sides is in the midline, near their belly button (umbilicus). To stay standing, the body’s center of gravity must remain between its two feet, both from side to side and back to front, otherwise it falls. Movement of the arms or legs away from the body or bending the spine in any direction changes the body’s center of gravity. Carrying an object, especially at a distance from the body, will also change its center of gravity. For our earliest ancestors to survive within the laws of nature, they not only had to stay balanced while standing, but also walking, with only one foot, and running, with neither foot, in contact with the ground. In other words, the human body is an inherently unstable object that needs to take control to stay balanced.

The neuromuscular system keeps the body in position while balancing itself in relation to gravity. Although the spinal cord provides reflexes that help it maintain its posture, it is largely the brain (particularly the brainstem and the cerebellum) that provides the coordinated motor patterns needed to maintain balance. To make ongoing adjustments, the brain receives sensory data from mainly four different sources: the pressure receptors in the feet, the proprioceptors (particularly of the neck and the rest of the spinal column), the vestibular apparatus within the inner ear, and vision.

The pressure sensors in the feet inform the brain of the body’s weight distribution relative to its center of gravity. Stand up and lean from side to side, and back and forth. Notice the difference in the pressure sensations felt from each foot with these movements, the feeling of imbalance, and the immediate adjustments that must be made to stay standing.

The proprioceptors of the neck and the rest of the spinal column provide the brain with information about the relative position of the head and the rest of the body. Bend your neck forward and backward and then bend from your waist in any direction. Wherever your neck and spinal column go so goes your head and the rest of your body. Notice the feeling of imbalance as your center of gravity moves away from being between your feet and how you quickly have to adjust to avoid falling.

The vestibular apparatus contributes sensory information about the speed and direction of head and neck angular motion and linear and vertical body movement. In addition, it helps to stabilize the retinal image. Look in a mirror, focusing on your eyes, and move your head slowly up and down and from side to side. Notice that your eyes automatically move in the opposite direction, allowing them to remain in focus. You are seeing the effects of the vestibulo-ocular reflex.

Now, continue to focus on your eyes and move your head up and down and from side to side as fast as you can. You cannot consciously control your eyes fast enough to compensate for these movements. It takes place automatically because of your decision to focus on your eyes (or any other object) while your head and body are in motion. Notice also how you felt a bit dizzy and off balance. This is caused by the strong alternating nerve impulses being sent from the vestibular apparatus on each side of the head to the brain due to the speed of your head movements.

The eyes provide the brain with an image of the environment in which the body is located. Clinical experience teaches that with concentration, training, and slow movement, vision can often help maintain the body’s equilibrium without information from the pressure sensors, the proprioceptors, and the vestibular apparatus. Close your eyes and begin to walk, progressively increasing your speed. Notice how difficult it is to maintain your balance. Closing your eyes makes you totally dependent on the pressure sensors in the feet, the proprioceptors of the spine and limbs, and the vestibular apparatus, throwing you slightly off balance. Now do this exercise again, but this time with your eyes open. It is apparent that visual cues greatly contribute to being able to maintain your balance.

One of the first indications that a person may have a problem with their balance is when they inadvertently fall in the shower. While taking a shower, most people close their eyes to shampoo their hair and then quickly turn their head and neck, and often their whole body, to rinse it off. Moving this way with their eyes closed means their brain can no longer use visual cues to maintain their balance. If a person has condition like a sensory neuropathy (common in diabetics), which limits the reception of the sensory data from the feet, or Multiple Sclerosis, which slows the nerve impulse velocity in the brainstem, or degeneration of the cerebellum, which causes poor coordination, then they will come to realize how important their vision is. Without it, it becomes difficult or impossible for them to maintain balance.

All clinical experience teaches that for our earliest ancestors (and the theoretical intermediate organisms that led up to them) to maintain their balance, they would have needed to have an irreducibly complex system with a natural survival capacity similar to our own. This would have had to include different sensors located in strategic places to provide information on the body’s position in space and relationship with gravity, a central nervous system to receive and analyze it, and the ability to access automatic motor reflexes and send voluntary motor messages fast enough to prevent a fall. For the force of gravity waits for no man and is an equal opportunity leveller, of sorts.

Just because similar organisms have similar mechanisms to maintain their balance does not, in and of itself, explain where those mechanisms and their ability to react properly and quickly came from in the first place. Evolutionary biology, as I said, is very good at describing how life looks, but has no capacity to explain how it must work within the laws of nature to survive. My next article will look at how we are able to accomplish purposeful movements and perform goal-directed activities. As everything else in this series has shown, it’s not as simple as evolutionary biologists would have us believe.

Photo credit: NASA, Astronaut Michael Edward Fossum [public domain], via Wikimedia Commons.