When muscular endurance is increased, if muscles are not getting larger, in what way are they becoming stronger?
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They are getting more blood flow and more mitochondria.
This allows for even more aerobic respiration (burning oxygen) which is sustainable long term unlike anaerobic (making lactic acid) you use when weightlifting. Mitochondria are also where fat is metabolized and burned so they will be able to tap into their fat reserves to a greater extent than a non-endurance athlete would. For blood flow, the muscles will get more vessels supplying them while at the same time the amount of blood volume will increase and there will be more total red blood cells/hemoglobin to deliver the oxygen. Waste products can more rapidly be removed and metabolized.
It’s not about strength, it’s about being able to function for hours without rest.
It’s called “vascularisation”, which is the process of growing additional tiny blood vessels throughout the muscles.
This allows more oxygen and fuel (sugars) to be brought in to the muscle, and for waste products (lactic acid) to be removed.
Every time you exhaust a muscle there are chemical growth signals generated which cause the process of vascularisation to begin.
If you keep doing it, the muscle will go from barely being able to jog a kilometre to being able to run an ultra-marathon without stopping.
In addition to what others have said, improvements in strength without increases in size are also due better communications between brain and muscles.
Muscles get stronger through hypertrophy, which can be sarcoplasmic or myofibrillar — two pretty different ways the muscle can get larger. Yet even without it, strength can increase without size changes in several ways.
The blood vessels feeding the muscles can expand and allow more flow, which means more fuel, oxygen and waste removal. Like when you can’t do something even once more, but an hour later you can do it again.
Also, the central nervous system can ‘adapt’ to doing the same thing, over time; IE., your brain figures out what corners it can cut in driving an action. A single, simple, repeated action you might eventually be able to do all day, yet if you try some other form of endurance it’s much harder. There’s still plenty of debate as to how this works, though.
And finally, the heart itself is a muscle, and can get stronger accordingly. Full body exercises generally do this the best, like swimming or running or bare-knuckle alleyway fighting — anything that raises your heartrate enough according to your age, over longer periods of time. When the heart is stronger, you can do more things longer even though the body is the the same size.
And finally, muscle size is not a static measure of strength. There’s more to them than their size. There’s different types of muscle fibers, and different type of muscle-LIKE fibers, (like tendons), and because of this smaller people can be stronger than larger people some times. …Though there is definitely a limit.
Most combative sports involve weight classes for similar reasons.
an additional element is that muscles are one part of a system.
Exercise not only builds muscle strength but also builds, or for older folks, maintains bones. Use them or lose them.
Then there is the glue that holds it all together, connective tissues and tendons, ligaments etc.
I’m not a biology expert but I attended a few lectures on the science of strength from a current Olympic team trainer. So forgive me my terminology.
Most responses here are looking at a cellular level however we also need to look at how muscles are structured.
Muscle fibres are neatly laid out in strands, side by side. These strands slide by one another to change the length of your muscle, e.g. When you bend your arm your bicep is shorter.
Sliding passed deals with changing length but doesn’t allow for any strength.
Each of these strands has lots of little arms/hands on it, like a natural version of velcro, in this way the strands can lock themselves in place.
Your ability to activate all of these little hands is called recruitment. An unused muscle will have a low recruitment % and a highly trained one will utilise more of them.
This is why when you start training you get a lot stronger before you perceive any change in size of your muscles.
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