I'm a bit of a couch potato with a sedentary job, who normally gets out of breath walking up a few flights of stairs. I recently walked up Snowdon in North Wales (10 mile round trip, 1100m summit, around 6 hours in total) which I found a struggle at times, being out of breath and my heart exploding out of my chest! There were people of all ages (6-90) passing me and not breaking a sweat.

Recently I saw an article on TV about an 82 year old guy who has been hill running for decades, and still goes out every day (often 20 miles a time).

I'm curious to know what makes him (and the walkers I saw) "fit" - where does his stamina comes from? He was stick thin like me, so I guess muscles don't play a huge role in it. I can only assume it's down to heart and lung efficiency?

  • +1 for the medical condition answer - see a doctor if you haven't ever had good stamina. Also, your DNA will play a part in your maximum fitness level. People don't like to acknowledge that point but it is valid. Not that many people can be elite level tri-athletes but lots and lots of us can get to where we can walk 10 miles without feeling like we are going to die :)
    – user29073
    Commented Jul 25, 2018 at 18:08
  • 3
    Genetic variability is unlikely to be the culprit here. Commented Jul 25, 2018 at 18:58
  • 10
    82 year old guy who has been hill running for decades will generally have more training than someone who never jogged Commented Jul 25, 2018 at 19:54
  • 4
    Please read the question before you answer, and don't guess what the question is based on the first paragraph and title. The question asks about the current differences in fit and unfit person, what's the difference in body that plays a role here, as both look "stick thin" to the outside. The question is not about how to get fit.
    – JiK
    Commented Jul 26, 2018 at 11:57
  • Have you ever played GTA:SA? It works exactly like that. No kidding :P
    – Yates
    Commented Jul 26, 2018 at 13:29

11 Answers 11


Your answer lies in your own question:

I'm a bit of a couch potato with a sedentary job, who normally gets out of breath walking up a few flights of stairs.

Recently I saw an article on TV about an 82 year old guy who has been hill running for decades, and still goes out every day.

Stamina is something that can be trained like everything else. As @JustSnilloc says, the body will adapt to the stresses that are put on it. Since you do basically no aerobic activity (By that I mean regular activity where your heart rate goes up, sweat, work, etc), your body doesn't have the mechanisms to cope with it. The other gentleman has been doing it for decades, for him it's as natural as sitting down is for you.

The good news is that it doesn't take much to get aerobically fit, other than a willingness to dedicate yourself to the goal. Start small, and work your way up. At first you will be tired, sore, can't do much. As you go on, you will get better and fitter, and probably feel much better in everyday life. While running isn't for everyone, if you want to try it, I highly recommend any of the popular Couch to 5k apps/programs. They start out with light jogging and walking, and progress to a 5k run.

For the physiology, there are a couple of training adaptations that are not covered by @nurdyguy's answer, I will add them as well here:

  • Capillary/venous development - Not only do the lungs have capillary growth, this growth also occurs in the muscle tissues as well. This enables more oxygen to get to the muscles.
  • Muscle adaptation - The muscles adapt to be able to both store more glycogen, as well as increased efficiency in oxygen use.
  • Increased mitochondrial density - The mitochondria are where respiration in the cells occur, as well as energy production. More of these means more power to the muscles.
  • Neuromuscular patterning - As you do a thing, your body adapts to be better at doing the thing. This means that your body uses less energy to do the same amount of work.
  • Fiber adaptation - This is iffy and still unproven. Some studies say that fiber type (slow/fast twitch) is fixed at birth, other studies show some adaptation/change in the fibers. Slower is better for endurance, fast is better for sprinting and power.
  • 2
    Second this. 3 months ago I would consider a 5 mile hike, but if you had told me I would consider 10 miles/4,000' elevation gain I would have said it's way too much--but that's my plan for Sunday. I've simply been getting out and hiking. Commented Jul 27, 2018 at 19:39

If you are really interested in the biology behind fitness then I'd suggest reading Dr. Jack Daniel's "Daniel's Running Formula". I'll paraphrase a few of the points you would read there.

The goal is to get oxygen from the air to the muscles that need it. There are several systems involved in this process.

  1. Lung capacity

    Quite simply, how much air can your lungs hold? This value won't change much from training but it can a bit.

  2. Lung capillary development

    This is actually much more important that overall capacity. As you train the capillary system around your lungs will grow and expand. Then result is that you are able to pull more oxygen from each breath.

  3. Heart strength

    Your heart is a muscle. As you train it gets stronger. The result is that you are able to push more blood with each beat. Thus more oxygen, which is carried in the blood, is moved to the muscles.

  4. Muscle oxygen absorbtion

    Ok so your lungs breathe in the oxygen and your heart passes it through the blood stream to the muscles. Now your muscles need to be able to absorb the oxygen from the blood and into their cells for use. The more you train, the more efficient the muscles become at doing this.

If you look at points 1-4 together you get what is sometimes referred to as "VO2 Max". This is basically how much oxygen can you get from the air and into your muscles.

  1. Lactic acid threshold

    Ok so if your muscles are getting enough oxygen then the activity is purely aerobic. However, most exercises are not 100% purely aerobic, there is an anaerobic element. The byproduct of anaerobic activity is lactic acid. The lactic acid begins to accrue in your muscles (which is a big part of why you get the "burning" feeling). Your body is able to clear the lactic acid out in order to alleviate this. Lactic acid threshold is basically the point where your body can just barely clear out as much as you are producing. If the activity intensifies much beyond that point then very soon the muscles will become overwhelmed and you'll have to stop for a breath.

When it comes to training, certain kinds of workouts can target certain systems. A tempo run for example is great for targeting lactic acid threshold. If you are interested in learning more, check out the book, it is FANTASTIC. Then go get some good running shoes and hit the trail!

Edit 1: When I said that lung capillary development is more important that lung capacity I mean from the perspective of training. Lung capacity can change as the result of training but not by all that much. Lung capillary development on the other hand can be increased dramatically.

Edit 2: Above was mostly a general overview of what is involved in "Cardiovascular Fitness" but I'll address a couple of your other questions more directly.

I'm curious to know what makes him (and the walkers I saw) "fit" - where does his stamina comes from?

Exercise in general involves a "stress - recovery" dynamic. When you stress a system in your body, your body reacts by making that system stronger during recovery. This is the same whether the stress be from something like weight lifting or something like walking. Runners and walkers as you saw have great fitness because they stressed their cardiovascular system and their body strengthened it as a response. Above I wrote about specific systems in your body involved in this.

He was stick thin like me, so I guess muscles don't play a huge role in it. I can only assume it's down to heart and lung efficiency?

Actually that is only partially true. If you look back at my descriptions of VO2max and Lactic Acid Threshold, both are at least partially muscle specific. In VO2max, the muscle has to be able to absorb oxygen from the blood stream. You could pump as much oxygen there as you want but if the muscle can't absorb it then it won't help. Similarly, weaker muscles will result in lower Lactic Acid Thresholds which will cause fatigue faster.

A couple of years ago I was running daily, 60-ish miles per week, and had pretty darn good overall fitness. I had to back off from running (due to an injury) but didn't want to sacrifice fitness so I added swimming. When running I could go do a 20-miler at a fairly aggressive pace but when swimming I would be pretty wiped out after only 15-20 minutes because the muscle groups involved were completely different. Sure my lungs and heart were in great shape but my arms were like wet noodles.

One other note: To some extent fitness is "in the eye of the beholder". I know weight lifters (bulk/power lifters) who think they are in great shape but they can't run much at all. Similarly, most runners don't lift. Who is to say one is more "fit" than the other? Clearly my background as a runner exposes my own preference but I just wanted to note that the idea "fitness" can vary.

  • @JohnP My post was certainly not comprehensive. In Dr. Daniel's book he identifies 6 different systems in the body which are involved in the biology of running. For brevity sake I really only talked about 2 of them, VO2 max and lactic acid threshold.
    – nurdyguy
    Commented Jul 25, 2018 at 19:25
  • 1
    Yeah, and I'm not a fan of VO2 being a performance factor. You can increase it just by losing 5 lbs. :p It is a good predictor of potential though. And we still don't fully understand lactic acid, other than we now know it's a fuel.
    – JohnP
    Commented Jul 25, 2018 at 19:34
  • 1
    VO2 max is entirely weight based (O2 uptake measured in ml/kg/min). Unless you are referring to the Fick equation? I really prefer vVO2 max, or how fast you are going when VO2 max is reached.
    – JohnP
    Commented Jul 25, 2018 at 19:59
  • 1
    Finally an answer that actually answers the question. No idea why @JohnP's non answer is so highly upvoted. Commented Jul 27, 2018 at 8:45
  • 1
    @CodesInChaos - It answers the title, maybe the question in the body got missed?
    – user28192
    Commented Jul 27, 2018 at 19:43

Cardiovascular endurance and general conditioning.

Simply put, the body will adapt to whatever your regular activities are. The fuel that you provide your body with also plays a part, but your conditioning is what makes the biggest difference. Muscle size is largely irrelevant to something like walking, but it can help if you were trying to sprint faster. Regardless, the adaptations that you aren’t seeing include the blood vessels providing the muscles with more oxygen, the heart maintaining a steady pace, the muscle memory, the honed form, etc.

  • 5
    +1 for "Muscle size is largely irrelevant to something like walking, but it can help if you were trying to sprint faster" - This is one of my favorite points to make: Look at the physique of a world class sprinter, and a world class marathon runner. They're both runners, but it really highlights the differences between an explosive runner and an endurance runner.
    – Alec
    Commented Jul 25, 2018 at 15:58

I'm curious to know what makes him (and the walkers I saw) "fit" - where does his stamina comes from? He was stick thin like me, so I guess muscles don't play a huge role in it. I can only assume it's down to heart and lung efficiency?

Well, a quick look at that man's body may not have given you an accurate enough picture of his body's composition. He looked stick then, as do you, but how much of his thinness was muscle (particularly leg muscle) and how much of yours is visceral fat? There is a popular term now, "skinny-fat", meaning someone who looks thin but who has a fair bit of body fat anyway and very little muscle mass compared to a trained person. Even a small difference, where your fat mass is his muscle mass, can give him a significant advantage.

Aside from skeletal muscle growth, some of the physical adaptations due to fitness training are:

  • More efficient heart contraction, so ejection of a larger amount of blood with each beat (and usually far fewer beats per minute, since each contraction does so much more work).
  • increased capillary blood supply to muscles
  • increased number of mitochondria, the "energy factories" of cells.
  • increased hemoglobin and myoglobin, the oxygen carriers of the body.

Based on what I know about biology, likely there are a number of other cellular level changes having to do with receptors on cells and even gene expression. These likely affect things like blood sugar handling and many other functions that would be helpful during exertion.


I once saw a Nova documentary -- Marathon Challenge -- in which they trained a group of "average people" to run the Boston marathon, in 40 weeks.

One of the characteristics which they measured was "VO2max" i.e. the rate at which people consume oxygen (measured by wearing a respirator/mask on a treadmill and measuring the difference between oxygen in and oxygen out).

They said that was a good overall/single measure because it measures efficiency of:

  • Lungs transpiring oxygen, and
  • Heart pumping blood, and
  • Muscle tissue using oxygen

They said that, with exercise/training, tissues becomes more heavily "vascularised": i.e. capillaries become larger or denser (or something like that), so blood is more easily delivered to the tissues.

Quoting from the transcript:

When we measure somebody's VO2 max, it's a very interesting number, because it is really complicated and there are a lot of different factors.

So it's how well the heart is beating; it's how well the vessels are expanding, how elastic they are; how many capillaries there are to bring the oxygenated blood to the muscles. So it's one number that shows us an overall good health of the entire cardiovascular system.


So in nine short weeks, what's happened? What's changed?

The runners' hearts are more efficient, filling up faster between beats and pumping more blood with less energy. They might even be slightly bigger.

Certainly the heart is working a bit better. But by far, the majority of the changes are happening with the vessels, the plumbing of the body.

Arteries and veins have become more elastic, easing blood flow. And down at the level of the muscle cell itself, there are more tiny capillaries, meaning faster delivery of oxygen.

Even inside the cell, energy production has been ramped up by mitochondria, the structures that transform fat, carbohydrate and oxygen into energy.

As you become more and more trained, the muscle actually starts making more mitochondria and also making them larger so that they can actually process and break down more fuels for energy.

So in nine weeks, from their hearts to the tiniest enzymes in their cells, these bodies were transformed.

The human body is an amazing organism. And what we see is that when you don't use things, you lose that body tissue.

I think it's not only about the delivery of oxygen (and other nutrients) to the tissues, but also the removal of the spent by-products.

There may be some difference in gross musculature too -- people say that cyclists look as if they have strong legs -- but legs are already the biggest muscles in the body.

Another consideration is that if or when circulation fails (when you're unwell) it tends to fail in the legs first -- causing "edema" or "peripheral neuropathy" -- I think the body privileges e.g. the circulation to the brain, and the legs get the left-overs (also the feet are more distant, and circulation has to work against gravity).

In case it isn't clear, the body adapts to training: e.g. if you move weight[s] the muscles become stronger, and you train aerobically then your aerobic capacity (that "VO2max" again) increases.

I think it's worth reading the whole transcript, by the way.


Depending on how tired you feel walking up a few flights of stairs, it could also be a medical issue. I recently had a friend who had extremely low hemoglobin levels in their blood, but didn't know it. The only "symptoms" were similar endurance and fatigue issues with even moderate exercise. If you're concerned about it, pay a visit to your doctor and have them check you over for overall health and blood levels to rule out issues other than a sedentary lifestyle.


There are lots factors that come into play - blood pressure and cholesterol will impact how well oxygen and glucose get to the muscles, diet will change the way your body metabolises (how "accessible" is the fuel, do the cells have materials to make enough enzymes to process it quickly, etc), and technique will play a large part in efficiency - how much energy is just wasted?

One component mentioned in several other answers referring to VO2Max - however, that is (as the name says) a maximum. How about VO2 Average?

As a quick experiment: Sitting or standing where you are, place one hand in the centre of your chestbone, and the other just above your belly-button. Then leave them there for a couple of minutes.

Now, while you wait, you'll (hopefully) have been breathing. So, which way did this move your hands - and how far? The shallowest breathing would have the bottom hand not moving, and the top hand scarcely moving. If you breath like that when walking, you won't get enough oxygen, and you'll be out of breath.

Now, at the other end of the spectrum: Both hands moving much further, and in opposite directions. Breathe in - top hand out / bottom hand in. Breathe out - top hand in / bottom hand out. This means that the diaphram is pulling down, sucking air right down to the bottom of the lungs and forcing it out again. Every breath provides massively more oxygen. If this is not your "default" breathing pattern, you can train your body to use it instead, and usually see an increase in stamina.

(There is a related technique for recovering after exercise, where you use the abdominal muscles to help suck the diaphram down and squeeze it back up - this helps keep the diaphram under control if it starts spasming, and you'll often see pro athletes doing this with their hands behind their head and elbows pushed up and back to maximise lung capacity after a race. Because it squeezes the diaphram to keep it under control it can, done correctly, stop hiccups.)


The other answers give you a lot of technical details (extremely interesting BTW). I can give you a data point on perceived fitness.

I commute to the office everyday. This is 30 km per day in a "wavy" environment (not flat, but without difficult areas either). I have a quick cadence and I am bathing in sweat when arriving to the office or home, but without being really tired (after a shower I am good to go, full of energy)

I am not very fit (a bit overweight) and used to play volleyball and trained Kung-Fu for years. I have a sedentary work otherwise.

After two or three years of biking I happened to get a puncture. I did not have the time to fix it on the spot so I hid the bike in the forest and realized that the superman I am after all these years of pedaling will easily run the 1 or 2 km separating me from the office.

After maybe 200 meters my lungs were on fire.

I think (and would love to have some real information on that) that one can be relatively performant in a specific activity, and nothing in a similar one (say, running-biking-swimming - not biking-chess or biking-weight lifting)

  • Not sure how helpful the anecdote is, but yes you can be very good at one particular activity and be kind of garbage at everything else. Cycling just doesn't use the same muscle groups that running does (especially for casual cycling there's barely any core activation which is fairly important for running) and the movement pattern is completely different - it's not just how much force your muscles can generate - it's also about learning when to tighten and when to loosen which muscles, without practice you'll be extremely inefficient in a movement you're not used to.
    – Cubic
    Commented Jul 30, 2018 at 9:51
  • @Cubic: what I wanted to convey is that despite biking, my lungs were on fire after running for a short while. I could understand the muscles, the knees, the feet, whatever is engaged differently - but the fact that I had a hard time breathing took me by surprise. And as you are mentioning - I do not know whether the anecdote is helpful, it is just one data point;
    – WoJ
    Commented Jul 30, 2018 at 11:11

I think that it has been properly answered your question regarding the physical issues that lead to having no stamina.

But I will say that even if you compare yourself to another "couch potato" you will find that this other person will endure more physical stress or less physical stress than you. This is because physical stress is well, stress.

In resistance based activities, like running, cycling, swimming long distances, climbing high peaks, and all that kind of things, your mind is an important part of the equation, even as important as your body.

I see this everyday in the gym or when taking my bike for a ride. There is people who can stand better against muscular pain when doing an effort, or who can keep their mind fit when cycling long distances. This depends about too many things. Experiences of your past life, genetics, having achieved something similar so you lose fear knowing you can do it, being motivated to something, etc.


Take a look at some of your other personal habits, what is your diet like? A good diet can have an effect on concentration. How much sleep are you getting? How much exercise? All of this can have negative effects on you. After that look at your medical history, have you ever been tested for diabetes? Low blood sugar is hell on concentration. Have you ever been placed on prescription medication for things like ADD or depression? You might want to talk to your doc next time you get a checkup about it. Don't go into a tizzy trying to self diagnose on Google but rather talk to a real doc.

Good luck.


Well the stamina will be increasing by regular workout and proper training , it is quit simple that a person have better stamina then the other person the reason is difference of healthy diet and daily workout...

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.