Life in Motion


What allows my body to move around, stay balanced and handle things?

It’s your muscles, under the control of your nervous system that allows you to move around, stay balanced and handle things.  Muscle cells (myocytes) are said to be excitable because when they’re adequately stimulated (by motor neurons) this allows certain proteins inside of them to come together and makes the cell contract.  Each skeletal muscle is made up of many myocytes that run parallel to each other and are grouped in bundles.  At each end of the muscle bundle is fibrous tissue called a tendon that attaches the muscle to two different bones making up a joint.

Your musculoskeletal system consists of over two hundred bones with about six hundred muscles attached to them.  Most joints have complementary pairs of muscles that allow it to be moved back and forth in different directions.  For example, while making a fist you can feel the flexor muscles along the inner aspect of your forearm contract and see their tendons pull your fingers inward.  And when you spread them out you can feel the extensor muscles on the outer aspect of your forearm contract and see their tendons pull your fingers back.   This is how the body uses its muscles to perform work.   

It’s important to realize that since muscles work by contraction this means that a given muscle can only move the eyeball or the bones of a joint in one direction.  To move them back requires a complementary muscle which must stay relaxed to allow the given muscle to do its job in the first place, and vice versa.  So, when you make a fist, while the flexor muscles along the inner aspect of your forearm are contracting, the extensor muscles on the outer aspect of your forearm must relax and vice versa.   If this were not the case, there would be a continuous tug of war between the muscles servicing your joints which would make it impossible for them to do what you want them to do.  The main lesson is to realize that to perform a well coordinated action, it’s not only important for a given muscle to contract but that its counterpart relax as well.  So when you decide to make a fist, your brain tells your flexor muscles to fully contract and your extensor muscles to fully relax.  That’s how your body is able to do work through coordinated action.    

To appreciate how your muscles work now would be a good time to test them out.  Slowly move your eyes and eyelids, mouth and jaw, then your neck, all the joints of your upper and lower extremities and then your back, in every possible direction.  Then go back and try it again, except this time go as fast as you can and see which parts of the body you can move the fastest and with more precision and control.  You should have noticed that your eyes and eyelids, mouth and jaw moved very quickly with precision.  Your fingers moved much faster with more precision and control than your toes, your wrists more than your ankles, your elbows more than your knees, your shoulders more than your hips and your neck more than your upper and lower back.

As noted above, each skeletal muscle consists of numerous muscle fibers which when stimulated by a motor neuron makes them contract and the bones to which they are attached move toward each other.  The number of muscle fibers controlled by a given motor neuron is called a motor unit.  The motor units of different muscles contain different numbers of muscle fibers in relation to their function.  For the coarse strong movements of the back, the legs and the arms, there are usually hundreds to several thousand muscle fibers per motor unit.  In contrast, for the fine and precise movements of the eyes and the fingers there are as few as five to ten muscle fibers per motor unit.  In addition, your muscles have stretch sensors called muscle spindles that send information to the brain about their length and therefore the joint angle as well.  The finer and more precise the muscle action the more muscle spindles are present and the coarser and less precise the action the less are present.  After all, how can your brain allow you to move around, stay balanced and handle things if it doesn’t have enough information on what your muscles and joints are doing?   It’s this information that allows your brain to figure out how to perform coordinated and precise actions.

Another very important thing to keep in mind is that your body has different types of nerves that have different conduction velocities.  In other words, some of your nerves send their signals quicker than others.  For example, the nerves servicing the blink reflex that protects your eyes from injury send their messages at about 100 m/sec (200 mph) while the nerves for deep pain send their messages a hundred times slower at 1 m/sec (2 mph).  Experience tells us that if we’re moving around, trying to stay balanced, and handling things there’s this force of nature called gravity that may get in our way.  After all, we know that gravity makes things fall to the ground at 10 m/sec2.  So, the messages going to your brain telling it what you’re doing and the ones going back to your muscles telling them what to do, better move fast enough or else you’re not going to be able to move around, stay balanced and handle things.  As luck would have it, the nerves that send this information back and forth from the spinal cord and the brain do have fast enough conduction velocities (50-100 m/sec).   

Finally, you need to understand that there are several regions in the central nervous system that are responsible for you being able to move around, stay balanced and handle things.  The motor cortex in the cerebrum, the basal ganglia, the cerebellum and the spinal cord all receive sensory information from all over your body which they share with each other and then work together to allow you to perform purposeful precise and well coordinated actions.  

So, it’s your muscles, under the control of your nerves directed by the brain and spinal cord that allows you to move around, stay balanced and handle things.


Three Questions for Mr. Darwin

  1. Where did the information come from to make all of my muscles, attach them to the right bones in a complementary fashion and teach my brain how to control them?

  2. How did my body know what it would need to move around and stay balanced and where did the information come from to make all of the parts to get the job done?

  3. How did my body anticipate its need to have the right amount of muscle spindles and muscles for each motor unit to allow me to handle things precisely and with control?

Also see Dr. Glicksman's Series on

"Beyond Irreducible Complexity"

"Exercise Your Wonder"


Howard Glicksman M. D. graduated from the University of Toronto in 1978. He practiced primary care medicine for almost 25 yrs in Oakville, Ontario and Spring Hill, Florida. He now practices palliative medicine for a Hospice organization in his community. He has a special interest in how the ethos of our culture has been influenced by modern science’s understanding and promotion of what it means to be a human being.

Comments and questions are welcome.

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