I frequently see runners with overuse injuries. Most of the time; performance declines and injuries happen when muscles are fatigued. If you want to run faster, safer, and smarter, you need to understand how the different muscle types work, when to train them, and when to use them.
Muscle Physiology 101
There are three main types of muscle fibers, Type I, Type IIa and Type IIb. Each fiber has a specific use and strength. Type I fibers are the classic slow twitch fibers. They are full of energy producing organs which convert sugar and fat into energy. The benefit of these fibers is that they can go for hours at a time before fatiguing. In fact, they are so efficient with converting stored fat into energy, that most people could run with their type I fibers for days on just stored fat. The down side to using type I fibers is that they aren’t very strong and their twitch speed is slow which means that they don’t produce a lot of power. To get more endurance out of these fibers, they are also able to generate energy from circulating blood sugar which comes from the food that you ingest during exercise.
At the other end of the spectrum are the Type IIb fibers. These fibers are ten times as strong as the slow-twitch type I fibers. Unfortunately, these fibers aren’t able to produce power for very long. Type IIb fibers fatigue after only fifteen seconds. These fibers are further limited by their inability to regenerate energy. Once the energy that has been previously stored is depleted, they won’t be able to generate any more power.
Type IIa fibers are a combination of both Type I and type IIb fibers. They get their energy from a combination of both circulating nutrients (fat and carbohydrate) and stored carbohydrate. As a hybrid fiber, it is no surprise that they can provide energy for longer than the Type IIb fibers, but not as long as the type I fibers. Most people get about 5 minutes of power from IIb fibers when they are all used.
Now that we know about the different fiber types, let’s look at how they are used. Muscle fibers are recruited to handle different workloads. Imagine that you are sitting on the couch watching the Boston Marathon on TV. You are using very few of your Type I fibers, then your friend calls to invite you out on a run. As you get up to answer the phone, more of your type I fibers are recruited. As you head out the door and start to go faster and faster, eventually more of your Type I fibers are recruited. Eventually, as you run increasingly fast; all of your type I fibers will be recruited. Once you have reached full recruitment of the Type I fibers, the Type IIa fibers start to be recruited. What is important to note is that the transition is not a switch from one fiber type to another, but an addition of different fibers. As you continue to run faster and faster, Type IIb fibers are recruited until you have recruited all of the fibers in a particular muscle group.
Most people don’t run in this sort of increasing speed until failure pattern. At a constant pace, different amounts of each fiber are used. At an easy pace, you may be running with some of your type I fibers. As these fibers fatigue, more of the type I fibers are recruited. At a moderate pace, you may be using all of your type I fibers with a few type IIa fibers for some extra speed. When you are running fast, you are using all of your Type I’s, most of your Type IIa’s and some of your Type IIb’s. When you are sprinting, you are using all of your muscle fiber types maximally.
Ideally, different distances would result in different muscle recruitment. A marathon pace should recruit all of your Type I fibers. A 5k or 10k should recruit all of your type I fibers and some of your type IIa fibers.
Training for a specific event should mimic the fiber recruitment of that event. All you need to know is when each of the fibers is maximally recruited, so that you can train them effectively.
Picture the following scenario. You are on a treadmill and the speed is increasing every minute. At first, you are only using some of your type I fibers. As the speed increases, more type I fibers are being recruited.
All of the Type I fibers are recruited maximally when you are running comfortably, yet still maintaining a sustained pace. When undergoing medical testing, this is the point just as lactic acid starts to develop.
Training the Type IIa fibers happens as you move from difficult, yet maintainable exercise to difficult exercise. Once this second transition happens, your ability to sustain that pace diminishes greatly. When all of the IIa fibers are being used, most people have about 4 to 5 minutes of sustained power left.
When you are barely able to keep up with the treadmill speed, you are using all of the type I, all of the type IIa and most of the type IIb fibers.
Looking back, maximal type I recruitment happens at 65-70% of maximal speed. Type IIa fibers are maximize at about 80-85% of maximal speed. At maximal speed, all of the fibers are being used. Of course, this is largely an estimate, but accurate for most people. Knowing for certain involves breath-by-breath analysis of inhaled oxygen and carbon dioxide.
Training by muscle fiber type is a powerful tool to build the fibers that you need for a particular event and to determine which events may be best suited for you.
Michael Ross, MD is a sports medicine physician who has been treating endurance athletes for over a decade. He has been a team physician for numerous professional cycling teams. He also runs the Rothman Institute Performance Lab, a medical and scientific exercise testing and training facility in Bryn Mawr, Pennsylvania. He has written two books on training and sports medicine for endurance athletes as well as multiple scientific papers. He has been an invited speaker at USA cycling and consulted for several bicycle companies to provide the optimum fit. He is an avid triathlete himself who has qualified for short course triathlon nationals several times. When he is not at work or spending time with his family he can be found on the trails and the roads around Philadelphia. www.rothmaninstitute.com/physicians/michael-j-ross-md