What happens when you run (long term)

     If you're a seasoned runner go back to when you first started. Remember how tired you were after only a few miles? Remember those first few weeks when it felt brutal? This author does when he took four months off from running due to a calf injury. 

    So, what happened to get you to the point where you felt that you were getting good at it?

    I'll tell you what happened.... you adapted. Your body went through some internal changes to enable you to get better at running. 

Aerobic adaptations center around two concepts: Delivering oxygen to the muscles and ability of those muscles to utilize oxygen to produce ATP (energy currency.). Or in simpler terms your skeletal muscle gets better at using oxygen and the cardiorespiratory system gets better at delivering oxygen.

Let's start with the cardiorespiratory system. The gold standard for measuring aerobic adaptations is Vo2 max. Which is the maximum amount of oxygen your body can use per unit of time. Higher the Vo2 max then the better you are at aerobic activity. Vo2 max is the product of Q (cardiac output.... more on this later) and Avo2 difference or the difference in oxygen in blood going to and from blood flow. 

 Cardiac output is the product of Heart Rate and Stroke volume (how many times does your heart pump multiplied by the amount of blood per beat). As adaptations occur more blood i.e. more oxygen will be pumped by the heart in fewer beats. At rest or resting heart rate will be lowered with training. 

    With increased blood flow the muscles would have to adapt as well. One of the adaptations is the increased capillaries within each muscle fiber. (Capillaries are the smaller vessels that circulate blood within the body). More capillaries lead to more oxygen within the muscles. Another adaption is the increased mitochondria within the muscle cell. As you recall from 6th grade biology mitochondria is the powerhouse of the cell. In this instance powerhouse means more capacity of muscles to use oxygen for ATP. Increased levels of creatine phosokinase (assists with energy production) takes place as well. Another highlight is that motor units start cycling to keep up with the increased demands of the muscles to work for longer period of time.

    For aerobic training we want the muscles to act like pistons in an internal combustion engine. We want them to alternate to keep the body running. This is opposite for resistance training as we want the motor units to synchronize to ensure maximum force is applied. 

    One final point. Your body will adapt to whichever aerobic event you choose. If you run, then the leg muscles will adapt. If you're a swimmer, then the whole body will adapt. In nerdy fitness terms that's called localized muscle adaptations. This is why a hardcore cyclist may struggle in the pool and a swimmer may struggle with running.