MitochondriaMitochondrion
The organelles that produce most of the cell's ATP using oxygen.
Mitochondria are the compartments inside your cells where most of your energy is actually made. They take the fuel you eat, fats and carbohydrates broken down into their component parts, combine it with the oxygen you breathe, and use the resulting flow of electrons to regenerate ATP, the molecule every cell spends when it does anything at all.
The textbook line calls them the powerhouse of the cell, which undersells them. They also help regulate calcium signalling, generate heat, produce reactive oxygen species that act as signals rather than only as damage, and decide when a damaged cell should quietly kill itself. They carry their own small loop of DNA, inherited down the maternal line, which is why mitochondrial diseases follow an unusual pattern of inheritance.
For training, the number and quality of mitochondria in a muscle is much of what endurance means. A trained muscle has more of them, with more internal surface area and more of the enzymes that run the reactions, so it can produce more energy aerobically at any given pace, and lean less on the anaerobic routes that fatigue quickly.
That adaptation is one of the reasons a beginner improves so fast. Mitochondrial content responds to endurance training within weeks, well before the heart's stroke volume has finished adapting, so early aerobic gains often come from the muscle rather than from the pump.
Both ends of the intensity range build them. Large volumes of easy work drive the adaptation through sheer accumulated time, and hard intervals drive it through a different signalling route, often strongly. The claim that only low intensity work builds mitochondria is one of the most repeated errors in endurance folklore.
The supplement aisle has noticed. Products marketed as mitochondrial support, mitochondrial biogenesis boosters or cellular energy formulas generally rest on cell culture or rodent data, and human evidence that any of them improves how a person actually performs or feels is thin. Training remains the intervention with the effect size.
In practice, if you want more mitochondria the prescription is unglamorous: train aerobically, consistently, for months, with most of the volume easy and some of it genuinely hard.