Improved Athletic Performance
According to a 2004 article published in "Strength and Conditioning Journal," the ability to rapidly change positions without losing speed is something athletes learn to do at a specific speed with a specific set of movements. For example, you might have good agility while jogging, but if you switch to running or jogging downhill, your agility could suffer. By increasing your speed, you improve your athletic performance at different speeds and with the different movement patterns that are necessary. The cardiorespiratory fitness gains you make as a result of speed training may also help boost your athletic performance by enabling you to run faster and for longer periods without becoming exhausted.
Gained Cardiorespiratory Fitness
Running at high speed can increase your V02 max, which is a measure of how well your body uses oxygen to fuel your movements. According to a study published in the "Journal of Applied Physiology," a two-week speed-training regimen consisting of ten four-minute bouts of high-speed training helped boost V02 max in women. Even if you can't sprint or run for long periods, the study show that short bursts of activity can help you achieve better fitness.
When you increase your speed, you increase the energy demands you place on your body. This means your body has to burn more calories to keep you moving forward, and burning more calories means you can burn more fat and lose more weight. According to Harvard Health Publications, a 155-pound person who runs at 5 mph will burn about 300 calories in 30 minutes. By increasing speed to 8.6 mph, that same person can burn over 500 calories in the same period. Of course, this calculation evaluates the calories burned when running in a steady state, whereas speed training generally consists of short, high-intensity bursts. Another boon to your calorie burn, though, comes from post-exercise oxygen consumption, which keeps you burning calories even after your workout. Sustained, high-intensity exercise increases post-exercise oxygen consumption the most, and is therefore more likely to burn the most calories, according to a 2011 study published in "Medicine and Science in Sports and Exercise."
Higher Running Speed
Unsurprisingly, speed training increases your running speed. According to ExRx, several elements of speed control how quickly you run -- starting speed, stride length, speed endurance and stride rate. Regularly performing speed training can help increase each of these speed measures, thus improving your overall speed. A combination of a boost in your cardiorespiratory endurance, better muscular endurance and increased muscle strength all play roles in this increase in speed.