Does long-distance running build leg muscle mass? How?

Define "build". Long distance running builds strength, endurance, and running economy. I would for sure define it as "building" the muscle. It does not usually build size - although this will depend on the genetics of people that are doing the running. People who tend to build muscles whatever activity they do will build some size by running.

P.S. Comparing Olympic marathoners to sprinters is beyond meaningless. There is huge self selection involved - you are comparing people the least likely to gain muscle mass to the people the most likely to gain muscle mass. Marathons don't make your legs thinner - it's the other way around. Runners with thin legs win marathons because thin legs indicate genetic predisposition to good running economy.
What are the most common injuries that occur in gyms?

A workout injury can happen to anyone, no matter your experience or fitness level. Even walking can cause an injury.But you can significantly cut your risk of getting hurt by following certain workout precautions.Common Workout InjuriesPeople hurt themselves in all kinds of ways when they

Do you burn fat or build muscle first?

Depending if you're over 15% body fat or not. Here is a generalization of the two:Building muscle & losing fat are total opposite agendas. You can't simultaneously build muscle while shredding fat (unless you are pharmaceutically enhanced, or a beginner). You can build muscle while keeping fat at a lower body fat percentage, but you do need some

Do Icelanders ride bicycles?

We sure do, bike riding is very popular in Iceland and many people ride their bikes all year long. Mountain bikes are common and many Icelanders think nothing of riding in below 0 degree temps and with snow on the ground.The amount of people