Mitochondrial Biogenesis and Exercisehttp://carrcellularfitness.com/products/l-arginine

What Does it Mean to Train?

The primary long term response that muscles make to increasing metabolic demands is to increase the number of mitochondria, the energy factories in each cell. This mitochondrial biogenesis occurs when exercise-induced cellular stress activates a substance known as  PGC-1alpha. This very important signal causes  reproduction of mitochondria, thus allowing the athlete to increase endurance and exercise performance. Mice with mutations allowing their cells to produce large amounts of PGC-1alpha become the Rich Fronings of the mice world, with little effort or training. 

Another very important cellular messenger  that stimulates mitochondrial biogenesis is nitric oxide (NO), produced when L-arginine combines with oxygen. NO production is increased dramatically in exercising muscle, resulting in increase blood flow by relaxing the smooth muscle in arteries. NO also has been shown to directly stimulate glucose uptake into muscle, the primary fuel burned during intense exercise. NO  has also been shown to be a direct and potent stimulator of PGC-1 alpha, the signal that causes more mitochondria to be synthesized.

In summary, when you train many things happen to improve your performance, but the most important adaptation, especially with endurance training (for example, running 100 meters or more) is increasing the number of mitochondria, a process facilitated by L-arginine.