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July 29, 2011

Endurance Exercise: Metabolic Responses

RunningMost realize that exercise, or at least physical activity, promotes good health and improves quality of life.  But, what really happens to our bodies and how do they change in response to exercise?  We’ll focus on metabolic changes that occur from endurance exercise in this article.

First, it’s important to know that adaptations to exercise primarily occur after the exercise is over.  So a certain amount of rest is needed between workouts for these physiologic adaptations to occur.  This is especially true after a bout of endurance exercise performed at a relatively high level of intensity, or for any significant length of time (more than 45 minutes).

Physiology 101

Let’s review a few basic points.  Energy our bodies need is produced in the mitochondria of our cells.  ATP is the energy source our bodies use.  Blood delivers oxygen and nutrients (fuel) to the cells.  Many enzymes are involved in the conversion of food to energy.  Anything that can increase the number of mitochondria, improve delivery of oxygen and nutrients to the cells, and increase enzyme production will lead to more energy production.   And, that’s exactly what endurance exercise does.

Metabolic Responses to Endurance Exercise

  • Increased resting ATP and CP levels in muscles.
  • Increased number of mitochondria per cell.
  • Increased muscle glycogen (carbohydrate fuel source) stores.
  • Increased aerobic enzymes.
  • Increased percentage of slow twitch (endurance) muscle fibers.
  • Increased number of capillaries (tiny blood vessels).
  • Increased extraction of oxygen from the blood by the cells.
  • Increased myoglobin (iron protein that stores and transports oxygen in muscles).
  • Decreased body fat.
  • Improved insulin sensitivity.

All of these positive adaptations improve the overall efficiency at which our bodies function and enable us to perform at a higher level of function thereby improving quality of life.

Metabolic adaptations not only occur, but significant changes involving the cardiovascular, pulmonary, and neuro-endocrine systems occur too.  These include increase cardiac output, increase stroke volume (amount of blood pumped with each heart beat), decreased resting heart rate, decrease blood pressure, and increase tidal volume (lung volume).

In future articles we’ll look at hormonal changes that occur with endurance exercise and the metabolic and hormonal adaptations that develop in response to resistance training.  For optimal health and functional improvement it is wise to engage in both endurance and resistance exercises.  And, don’t forget flexibility, as you can only build strength and endurance within the range that you move your joints and muscles.

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Dr. Joe Jacko


Dr. Joe is board certified in internal medicine and sports medicine with additional training in hormone replacement therapy and regenerative medicine. He has trained or practiced at leading institutions including the Hughston Clinic, Cooper Clinic, Steadman-Hawkins Clinic of the Carolinas, and Cenegenics. He currently practices in Columbus, Ohio. Read more about Dr. Joe Jacko

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