Do sprints really make you run faster?
Let’s start at the beginning. The body is made of of two basic types of muscles, slow twitch and fast twitch. Slow twitch muscle fibers are more efficient at using oxygen to generate fuel for continuous, extended muscle contractions over a long time. They fire more slowly than fast twitch fibers and can go for a long time before they fatigue Therefore, slow twitch fibers are great at helping athletes during endurance activities, like distance running.
Fast twitch muscles are further broken down into two categories Type IIa and Type IIb. Fast twitch muscle fibers use anaerobic metabolism to create fuel. They are better at generating short bursts of strength or speed than slow muscles; however, they fatigue more quickly. Fast twitch fibers generally produce the same amount of force per contraction as slow muscles, but they get their name because they are able to fire more rapidly. Having more fast twitch fibers can be an asset to a sprinter since she needs to quickly generate a lot of force.
Type IIa fast twitch muscle fibers are also known as intermediate fast-twitch fibers. They can use both aerobic and anaerobic metabolism almost equally to create energy. In this way, they are a combination of Slow twitch and fast twitch muscle fibers. Type IIb fast twitch fibers use anaerobic metabolism to create energy and are the “classic” fast twitch muscle fibers that excel at producing quick, powerful bursts of speed. This muscle fiber has the highest rate of contraction (rapid firing) of all the muscle fiber types, but it also has a faster rate of fatigue and can’t last as long before it needs rest.
The amount and proportion of fast twitch to slow twitch muscle fibers an individual has is largely determined by genetics. There is some evidence showing that human skeletal muscle may switch fiber types from “fast” to “slow” and from Type IIb to Type IIa fast twitch due to training. During aerobic exercises such as running or swimming, slow-twitch fibers are the first to contract. When the slow-twitch fibers become tired, fast-twitch fibers begin to take over. There are significant benefits to working to the point of temporary fatigue—and therefore making sure fast-twitch fibers have been recruited.
As stated above, fast-twitch fibers have a high threshold and will be recruited or activated only when the force demands are greater than the slow-twitch fibers can meet. Running sprints and plyometric training are both good at recruiting fast twitch muscle fibers. Strength and power training can increase the number of fast-twitch muscle fibers recruited for a specific movement.
Endurance athletes can increase their speed by recruiting and training their fast twitch muscle fibers using sprints, plyometric and strength training. Traditional speed work (think 400m-mile sprints) is concerned with improving your metabolic energy systems (VO2 max, lactate threshold, aerobic capacity). And speed development workouts (think tempo runs) are focused on increasing the maximum amount of fast-twitch muscle fibers recruited for each stride and improving the speed at which your brain sends signals to your muscles to fire. Instead of improving your specific fitness, you’re focusing exclusively on the neuromuscular system. The main goal of speed development workouts is to improve your running economy and efficiency. This means being able to run faster and farther with less effort and while expending less energy. For a marathoner or half marathoner, this means race pace will require less effort (making it feel easier, especially in the latter miles). See more here.