Yes, about 5% of energy during cardio comes from breaking down muscle, increasing to as much as 15% when glycogen is depleted.
However, adequate protein intake along with strength training can prevent actual muscle loss and can even result in gain. I gave myself as an example in an earlier post.
BTW this video is about Dwayne Chambers being used in sprinting/marathon runner comparisons… it says that people like him only can look that way if they’re taking steroids. The video shows him before and after steroid use.
I don’t see much point in me doing any cardio then. I mean I hardly use my muscles so they might waste away faster than they can be built. Cardio means burning calories but I can do that walking (though it would take longer) - or I can continue my lowish calorie diet. It might be good for the heart though but I’m not worried about that.
I’ve linked the following page a lot:
It says regular people only need to sprint twice a week, work on strength for three days a week and rest on the other two days.
It talks about sprinting for 30 - 200 m with 3 - 6 minute rests. (there are also drills/warm-ups) That is a bit like what I’m doing… short sprints then breaks (walking)
I wonder if it is equal or detrimental to sprint a lot every day.
Well I would disagree with that analysis.
There are competitions where ordinary members of the public must build an impressive physique within a constrained time, and winners are tested for steroid use. It can be done. And you’ve agreed with me that actors can get in shape very quickly using bodybuilding, and I doubt jake gyllenhall is going to do steroids.
For the general point of this thread, like I say, your exercise programme is absolutely going to be helpful compared to nothing, or just walking.
All I’m saying is that if your primary goals are losing some fat and gaining some muscle, it’s far from optimal for either. But the best exercise plan is the one you actually do, so if you hate the gym, don’t go 
It depends what you mean by “optimal”. Perhaps you mean minimizing the number of calendar days or amount of time exercising. I want to minimize the amount of discomfort and boredom. BTW lately I’ve been walking about 1-2 hours a day (it helps me to think, daydream and listen to music) Short sprinting is not uncomfortable for me until after several seconds when it starts to be a challenge with my stamina.
I’ve tried going to the gym in the past but I hated it.
I assumed that ancient Greek athletes (the original games only had one event - sprinting 180 - 240m Ancient Olympic Games - Wikipedia ) only practiced by running/sprinting. But it looks like they did strength training:
Running in Ancient Greece - Wikipedia
They might have done strength training for their wrestling though rather than reasoning that it was good for sprinting.
From that link:
“One practice that developed had athletes exercise with 3-pound (1.4 kg) weights in each hand. This practice helped improve arm strength, which is beneficial for running, throwing the javelin, swimming, and martial arts”
That sounds interesting…
“When training in the gymnasium, his idea was that one should not run or exercise on the stone floor but on sand instead to add difficulty.”
I live near a beach… it is a lot more difficult but at the moment I’m worried that sand will get in my shoes.
I was the skinniest girl and was a great sprinter.
Also, man boobs are often difficult to lose through diet and exercise. Sometimes they are just pockets of fat resistant to losing. Plastic surgery removes them.
Various thoughts and comments connected to this op …
Mijin nails it that the most important aspect is that you are enjoying the exercise you do: the “optimal” program not done or done half assed will work much less well than something you actually do regularly and with some progression.
Adding sprints to your walking program will do more for increasing muscle mass than walking alone does. Make you look like Usain Bolt? No. But indeed sprinting will stimulate more growth of whatever fast twitch muscle fibers you have than low level cardio will.
In general the rule of thumb is to do intense sprints days no more than every other day. Just the rule of thumb though. Concern is not having enough recovery time and increasing the risk of injury.
If you like the shorter bursts, hate gyms, weights, and machines, but still want some more overall muscle preservation or possibly even growth during fat loss, then you may experiment with adding some calisthenics intervals into your program. Walk/jog to warm up, sprint, walk, pushup setsx3 each separated by a sprint, walk, burpee setsx3, walk, sprint, etc. Some people find various body weight progression programs to be lots of fun too.
Wrestlers, like gymnasts, rarely do very much weight training (back in my day we did no weight room work at all). And both have plenty of strength per unit body weight and usually spare on the body fat. And many prefer the wrestler or gymnast physique and functional strength/power to a lifter’s or bodybuilder’s.
“The most reliable way to increase any athlete’s performance is to improve his or her strength-to-weight ratio, which is a fancy way of saying minimizing the amount of bodyfat the athlete carries while maintaining or adding lean body mass.”
So you would have had a minimal amount of bodyfat and perhaps the same amount of muscle as the other girls… on the other hand I had some body fat (I wasn’t sinewy and had a fat bottom) and I didn’t have much fast twitch muscle.
Luckily they’re not too big - I was being picky. They’re pretty perky.
When someone says something like this, my thought is that you should lift. But that’s just my opinion.
Look at the belly of the best lifter in history though:
I think lifters often have a special diet or something.
It’s been 5 days since I did some sprinting for one outing… when I have tried jogging today for a couple of seconds I get strong pain deep in my thighs. I haven’t sprinted for decades though and 5 days ago I did it quite a few times.
It’s normal for a new programme of exercise to ache or be uncomfortable for the first week or two.
Pain is a concern though. Go easy on yourself and gradually ratchet up to figure out your limits safely.