To reach the next level in your running performance requires a quick, basic biology lesson. When you understand how enzymes and oxygen tightly regulate metabolism, you'll see that increasing your aerobic capacity and including sprint work should be the goal of your training. Doing so will help you run stronger for longer.
Where We Get the Energy to Run
Enzymes function as biological catalysts that speed up chemical reactions. In the absence of enzymes, chemical reactions would not occur quickly enough to generate the energy needed to run. The amount of an enzyme also controls which metabolic pathway is used. For example, having more aerobic enzymes will steer metabolism toward a greater reliance on aerobic metabolism (Krebs cycle and electron transport chain) at a given submaximal speed.
The Krebs cycle is a series of chemical reactions that produce carbon dioxide and Adenosine triphosphate (ATP), a compound rich in energy. Carbon dioxide stimulates breathing, while ATP provides cells with the energy required for protein synthesis and the replication of DNA—processes vital for growth, energy and recovery.
Enzymes essentially control metabolism and therefore control the pace at which you fatigue.
How Sprint Training Increases Enzyme Activity
A number of studies document an increase in enzyme activity in response to training. One of the first among these was published in 1967 in the Journal of Biological Chemistry, in which aerobically trained rats increased mitochondrial enzyme activity, increasing the mitochondria's capacity to consume oxygen. More recently, a study published in the Journal of Applied Physiology in 2006 found that citrate synthase (a key aerobic enzyme) activity significantly increased by 37 percent in novice runners after 13 weeks of training during which weekly mileage increased from 15 to 36.
Sprint training induces changes in the anaerobic enzyme profile of muscles and also increases aerobic enzyme activity, particularly when long sprints or short recovery between short sprints are used. For example, a study published in the Journal of Applied Physiology in 1998 found that sprint cycle training three times per week for seven weeks using 30-second maximum-effort intervals significantly increased both anaerobic and aerobic enzyme activity. Research on changes in enzyme activity with sprint running is currently lacking.