Many clients rejoice when they step on the scale and see that they have lost weight. The numbers on the scale, as well as the changes in their physical appearances, are great indicators that their bodies are changing.


Many external changes take place while engaging in exercise, but several changes often go unnoticed because they take place within the body. The body changes internally, physiologically, just as it does externally. There are several very significant changes that happen inside of the body during exercise that enable a person to exercise and continuously increase their proficiency. These changes take place when stress is placed upon the body and the body adapts.

What is SAID?

The acronym “SAID” stands for “Specific Adaptations to Imposed Demands.” This principle states that the body will adjust to the stresses placed upon it by becoming stronger and physiologically adapting to whatever exercises a person does. “When the body is subjected to stress and overloads of varying intensities, it will gradually adapt over time to overcome whatever demands are placed on it”.3 Just as the muscles respond to lifting weights by becoming stronger, different internal systems also adapt to exercise by physiologically changing to become more efficient. These changes include:

  • A delayed onset of reaching the lactate threshold
  • An increase in the number of red blood cells, to help deliver oxygen
  • An increase in the efficiency of the heart to pump out blood

The first way that the body can adapt is by increasing the amount of time the body can work before reaching the lactate threshold. When exercising, the body can remove this waste product efficiently at first, but after a while, and depending on the fitness level, the body produces lactic acid faster than it can be removed. This is the point where the body begins to build up lactic acid in the bloodstream, and this point is known as the lactate threshold. Performing exercise causes a person’s body to become more efficient at removing the lactic acid. This is because the body adapts to the new imposed demands. A client will often notice that he or she can endure longer cardio and lifting sessions as his or her body adapts. This is due to the fact that the lactate threshold has been pushed back further during a workout session.

The next way that the body changes in order to respond to stresses placed upon it while exercising is by increasing the number of red blood cells in the bloodstream. Red blood cells work as a taxi to deliver oxygen to the muscles. When the body is working out hard and the muscles are not receiving the desired amount of oxygen, it sends a message to the kidneys telling them to produce more of these taxis. “Erythrocyte (red blood cell) production varies with the oxygen concentration of the blood in a negative feedback control mechanism. If blood oxygen concentration is low…the kidneys release erythropoietin, a hormone that stimulates red bone marrow to produce more RBCs”.1 The bone marrow produces more red blood cells and this, in turn, helps the body to deliver more oxygen to the working muscles. The increase in oxygen delivery to the muscles helps a person to be able to exercise longer because the body is able to withstand reaching the lactate threshold as quickly. These two adaptations work hand in hand.

Lastly, the heart adapts to the imposed demands placed upon the body by becoming a more efficient pump. The heart, like any muscle in the body, will become stronger when it is worked. “Aerobic training induces alterations in pulmonary dynamics during exercise. Such changes contribute to a more effective ventilatory response to the stress of physical activity”.2 The heart has the ability to increase the stroke volume, or the amount of blood that it pumps out with each stroke. The heart, as it becomes stronger, will increase the stroke volume as a response to imposed demands being placed upon the body. In addition to being able to pump more blood out with each contraction, the cardiac output also increases as a person’s body becomes more adept to exercise. This means that the heart has the ability to pump more blood through the body’s arterial system per minute. The heart changes physiologically, just as the red blood cells and lactate threshold do.


These three major physiological changes all work synergistically to help a person’s body become more efficient while exercising. The heart changes to increase cardiac output and stroke volume. The blood changes, by way of negative feedback loop, to contain more red blood cells. The increase in red blood cells allows for the muscles to receive more oxygen, and thus delays the onset of the lactate threshold.

While a client may not rejoice at the news that his lactate threshold could increase, or his cardiac output could be greater, or he could have more red blood cells, these are very significant changes that his body goes through. These internal changes happen in the body while working out and while they are less obvious than the changes in physical appearance, they are very foundational to help a person get results.


Gunstream, Stanley. “Anatomy and Physiology with integrated study guide”. 3rd. McGraw-Hill companies 2006. Print.

McArdle, William, Frank Katch, and Victor Katch .Essentials of Exercise Physiology. 3rd. Philadelphia: Lippincott Williams and Wilkins, 2006. Print.

Prentice, William. Principles of Athletic Training, a competency-based approach. 13th. New York: McGraw-Hill, 2009. Print.

About the Author

Aimee Teuscher graduated with her bachelor’s degree in Health and Human Kinetics from Warner Pacific College in Portland, Oregon. Aimee loves the outdoors and can be found outside jogging or hiking on her days off. She currently runs a holistic, in home personal training business: “Aspire. Train. Emerge.”


These resources are for the purpose of personal trainer growth and development through Continuing Education which advances the knowledge of fitness professionals. This article is written for NFPT Certified Personal Trainers to receive Continuing Education Credit (CEC). Please contact NFPT at 800.729.6378 or with questions or for more information.