Let's get into some relevant science right off the bat! The energy currency our cells utilze is adenosine triphosphate or ATP. We have 3 energy systems which create ATP. These are the phosphocreatine or creatine phosphate (CP), anaerobic, and aerobic energy systems.
For our purposes in this course, we can use simple characterizations of each system.
The CP system is our short-term, "sparking" system which we emphasize when we need a rapid burst of energy. It's readily available and easily accessed, but only lasts for a few seconds. We use this to get going and for all-out explosions. Intramuscular CP replenishes in just a few minutes in most cases.
The anaerobic system describes the production of ATP which doesn't require oxygen, and it's used to generate energy when two conditions are present: we need more energy than the aerobic system can supply alone and the duration of output is longer than the CP system can accomodate.
The aerobic system produces a steady supply of energy in the presence of oxygen, and it's on all the time. This is our basic functional support mechanism and it can also be the primary energy system for low intensity, steady-state output.
Even though the systems are unique, they all work together simultaneously during exercise and sports in blended fashion. The aerobic system is always running. If we need a quick spurt of energy, the CP system kicks in. If that demand last more than 5-7 seconds, the body will switch on the anaerobic system and bias it for sustained higher output.
The limiting factor with high intensity exercise/sport/work is related to anaerobic metabolism. The more fit you are, that later that system will ramp up, but it will inevitably do so once you exceed your aerobic limits. That's when the by-product of anaerobic metabolism, or lactic acid rears its (not necessarily ugly) head. The lactic acid is rapidly converted to lactate (which is used as a fuel source and signaling molecule in the body) and free hydrogen ions (H+). This creates an acidic, or lower pH condition in the muscles, and it leads to the familiar burn and lockup (and sometimes nausea) associated with high effort.
Good training program design and consistent application can do 2 things for anaerobic metabolism. It can decrease the total amount of lactic acid produced, and it can increase our tolerance (buffering ability) to the amount that is present.
The preceding information was valuable because it helps us to understand what is going on in our speed workouts. The CNS can only drive max output for 5 or so seconds (give or take), and thus, most of our true speed or short sprints are mainly supplied by the CP system (of course with aerobic metabolism running in the background). But some of our efforts extend a bit into anaerobic territory, and we can get into some breathlessness and burning. Just not too much of it. This is why we try to limit the anerobic exposure when we're training mainly for speed. In this way we are better at managing fatigue and can execute with the higher quality which speed requires.
To clarify that last paragraph, there is a place for strongly anaerobic training, especially when we are seeking acidosis tolerance and VO2max gains. This helps us to get "fitter." But the body only gets truly faster when we train at, or very near, max velocity. Done properly, speed training doesn't "hurt" as much as higher anaerobic training.