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Background Information:

I recently began a running/walking regime that lasts for 30 minutes. I think I run about half the journey and walk the other half of the journey. There are 2 very steep hills, and numerous medium hills on the journey. I tend to walk up the 2 very steep hills.

Question:

On my walk I noticed that some other people walking had very large/bulky (although doesn't look fat) looking legs. My observations of endurance athletes are that they have quite lean legs. I am weather my regime will turn my legs bulky, or if they will remain lean looking. (I would prefer if they don't become bulky, how can I prevent this?)

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I run. Half marathons, 5ks 10k, and trail runs. building legs muscles are a lot like building other muscles. You can develop slow twitch and fast twitch muscle fibers. For running you are going to need both. The short answer is your legs muscles are going to take on your own unique genetic shape. Simple answer, keep up the endurance and run on flat surfaces if you want lean looking legs. Hills are going to make your muscles bulk up because it requires more strength. Eat light and lean and remember to train in your lactate zone so yo burn fat to keep trim. Most important have fun and allow your legs to be.

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a bit short and only based on personal experience. An improvement would be to add some references. –  FredrikD Jan 23 '13 at 6:22
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Also, fat is burned in all intensities of exercise, it's not a magical "Oops, flick over to the fat furnace!". And if you could expand on what a lactate zone is it would make it a better answer. –  JohnP Jan 23 '13 at 14:33
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In general, runners tend to have more slender, leaner looking legs than other athletes. This is because low weight, high repetition exercising such as distance running (pretty much anything other than sprints, so anything above 400 meters, really) does not really lend itself to building bulky muscle, and is in fact counterproductive because of the extra weight. It's the wrong kind of stress in the muscle to produce bulk.

I would venture a guess that a lot of the "bulky" runners you see are doing weight training and simply running/walking for fitness, or they are multisport athletes such as triathletes or duathletes. This segment will have bulkier legs because of the bicycling that they are doing, which will cause greater hypertrophy in the calves and upper legs.

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It's genetic mostly.
Hypertrophy is not only caused by high intensity and low reps. If you do any exercise long enough this will lead to hypertrophy to some extent (the extent is genetic and proportional to the stimulus you are applying).

Those skinny elite distance runners you see that run 100+ miles a week, that's genetic. Their body simply doesn't want to build muscle.
For most white people, if you start doing long distance running (such as marathon training), your legs will almost certainly get a bit bigger; you shed fat but you will build muscle.

I would also like to point out, that when highly trained distance runners suddenly stop running, and continue eating as normal (like before a big race), it's easy for them to put on 15-20lbs. How so? Glycogen. Highly trained runners constantly have their body in "low energy mode", and when they stop running, the body is like "shoot! Don't know when we'll next get this chance to store up on energy, better do it now!"
And as a result muscles start storing starch in the form of glycogen very effectively.

One more thing, sarcoplasmic hypertrophy is when the muscles don't grow bigger, but the muscle adapts to have more water in it (to help muscle get energy and respiration done more effectively), this is less common in distance runners. But again. genetics.

So overall it's mostly genetic (your body frame), but the type of exercise you do, and how often also plays a big role in hypertrophy (gain in muscle mass, fiber size).

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Do you have any credible sources to back up your claims? –  Baarn Jul 9 '13 at 22:17
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And I wonder why you explicitly mention white people, I don't understand how skin color affects muscle development. –  Baarn Jul 9 '13 at 22:19
    
Just because there is a correlation doesn't mean it is causative. Therefore, perhaps he mentions white people because there is a correlation with skin colour and muscle development, rather than skin colour having a causative relationship with muscle development. –  Mew Jul 15 '13 at 10:24
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