MadSci Network: General Biology

Re: Do your muscles only get fatigued during extreme exercise?

Date: Mon Feb 6 11:51:39 2006
Posted By: Keith Anderson, Staff, Vascular Research, Brigham & Womens Hospital
Area of science: General Biology
ID: 1137447685.Gb

Muscles indeed will fatigue during extreme exercise but there are other less obvious ways 
that muscle can fatigue….. let’s look closer.

Muscle fatigue is defined as an exercise-induced reduction in the maximal force capacity of 
the muscle.  This reduction of muscular capacity is largely due to lactic acid and metabolites 
because of anaerobic conditions, that is, the muscle is not getting enough oxygen.  Fatigue 
can occur because of an accumulation and/or depletion of these metabolites….build up of 
lactate and therefore hydrogen ions or depletion of energy in the muscle cells like ATP and 
Undoubtedly in your science classes you have learned how the body creates energy via the 
Krebs Cycle.  However, our body during exercise often cannot get enough oxygen to keep 
our muscles working at 100% efficiency and the muscle begins to use a mechanism of 
anaerobic respiration to gain a sort-of supplemental energy (this is much less efficient but 
prevents the muscle from suddenly not being able to function).  Chemically, this is glucose 
being cut in half to form two lactic acid molecules and energy.  The harder the muscle 
works, the more lactic acid build-up.  The lactic acid interferes with muscle function as it 
increases in amount therefore worsening the fatigue.
There can also be a neuronal component to muscle fatigue where the muscle has the 
capacity to exert a force but cannot by voluntary effort. This can be measured as the 
difference between force exerted voluntarily and force exerted when say an electrical 
stimulus is applied to the muscle causing an involuntary twitch.
Muscle fatigue is not necessarily caused by intense exercise per se.  Muscle fatigue from 
sitting in a particular position for a long period of time can result in fatigue because you are 
causing that muscle to work in a particular way for an extended period of time (and may be 
cutting off circulation which supplies oxygen).  If your muscles are not accustomed to a 
particular motion, even a short duration, fairly low intensity exercise can result in the 
fatigue of the muscle(s) involved (which could be caused by something as simple as carrying 
too many books home from school).

I hope this has answered your question.

Keith Anderson
Brigham & Women's Hospital, Boston, MA

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