why is running not considered "leg day"?

craigmandu

You're not squatting heavy enough. Running is cardio and hurts your gains.

Hurt your gains???? So are we not allowed to do any cardio ever if we lift, according to you?

If your focus is strength only then in the purest sense yes. OP is asking why running doesn't have the same effect as say a lifting routine for the legs would. And that answer is simple....running is an aerobic (relying on oxygen) process that slowly depletes the type I fibers (slow twitch) in the legs, these fibers control sustained endurance and stamina. Lifting is an anerobic (not relying on oxygen) process that uses minimal type I, and more type II (fast twitch) fibers. These fibers control explosive energy, and fatigue quickly...as such they have to store more nutrients in them than type I fibers, as type I fibers are more efficient at using oxygen to create the energy they need.

Depending on the exercises you routinely do, will decide what that ratio of fibers are in each muscle group...the more strength, the more type II, the more cardio, the more type I.

Doing cardio exercises however, is not all about optimizing or creating a higher type I fiber amount in a muscle group..there are side benefits of increased overall cardiovascular health, and creating a better overall system of delivering oxygen to all the muscle groups in your body.

Why you see alot of people decide to forego cardio while they strength train is simply that they don't need it..they are trying to increase type II fibers or create new muscles that are primarily type II.