You make an excellent point. Normally, exercise will increase stamina and people are likely to feel more energy and less "sluggish." Mitochondria are the "energy centers" or "powerhouses" of our cells. The more mitochondria we have, the more energy we will have.

The only way to increase mitochondria is through exercise.

"Mitochondria are intracellular organelles found in almost all human cells. They are responsible for aerobic metabolism through oxidative phosphorylation, which leads to energy production in the form of adenosine triphosphate (ATP)." ( What are mitochondria? | How The Metabolic Process Works - Sharecare)

In some conditions, especially those caused by aberrant mitochondrial processes, exercise may or may not increase stamina. (This topic can be complex and I do not see a current benefit in going into this any more than I have/am writing below.)

With Chronic Fatigue Syndrome (CFS/CFIDS/ME), the increased energy effects are often grossly delayed. People with these conditions tend to become too fatigued to continue a regular regimen of exercise. They take much longer to recover from exertion and cannot participate in daily physical training.

The relatively new diagnostic label for CFS/ME in the U.S. is SEID or Systemic Exertional Intolerance Disorder. This new name reflects the core diagnostic criteria for CFS/ME, now SEID. The exertion referred to in this diagnostic label refers to an intolerance to both physical and mental exertion.

So while those of us living with CFS/SEID can benefit from exercise, we need to accept the fact that we need to be kind to ourselves and slow down the frequencies of our workouts. We often benefit from altering our schedule of physical activities in general. Instead of daily physical exertion, one might plan exercise sessions M, W, F or a similar schedule, according to what works for each individual.

If a person swims one day, for example, and swims again the very next day, s/he would not "qualify" for a "CFS/ME/SEID" diagnosis. The "recovery time," which is the amount of time it takes to recover enough to exercise again, is too quick in the example I have given.

Even so, it is extremely important that everyone exercise, including those with CFS/SEID. Although the training involved in people with SEID must go at a much slower pace, the person does reap benefits over time, including the increase of mitochondria, which leads to increased energy production.

If/when a person is without these (and other) types of limitations, it is very important to exercise on a regular schedule in order to keep/increase the number of mitochondria in their cells/bodies.

Again, mitochondria are the powerhouses of the cells (and of the body). We all experience more energy when we have more mitochondria. The only way to increase mitochondria is through exercise.

Fern's comment in her post above had triggered my remembering and my sharing a little bit of knowledge re: mitochondria and energy.

Thanks, fern!