Physiology lesson 2.0
Runners with different muscle fibers have different strengths.
What It Means For You: Tailor your training to match your muscle fiber composition.
There are two types of runners – those who have superior speed, whose performance gets better as the race gets shorter, and those who have superior endurance, whose performance gets better as the race gets longer. Despite this, most runners, unless they are individually coached, follow some generic training program. However, those programs don't acknowledge differences in runners' muscle fiber types and their associated metabolic profiles. The types of fibers that make up individual muscles greatly influence your performance.
Humans have three different types of muscle fibers, with gradations between them (see Characteristics of the 3 Muscle Fiber Types). Slow-twitch (ST) fibers are recruited for all of your aerobic runs, while fast-twitch B (FT-B) fibers are only recruited for short anaerobic, high-force production activities, such as sprinting, hurdling, and jumping. Fast-twitch A (FT-A) fibers, which represent a transition between the two extremes of ST and FT-B fibers, are recruited for prolonged anaerobic activities with a relatively high-force output, such as racing 400 meters. It's a given that you have more ST fibers than FT fibers, otherwise you would be a sprinter rather than a distance runner. However, even within a group of distance runners, there is still a disparity in the amount of ST fibers. Some runners may have 90 percent ST and 10 percent FT fibers (marathoners), while others may have 60 percent ST and 40 percent FT fibers (milers).
Understanding your fiber type can help you train smarter. While most runners do the same workouts to focus on a specific race, your training and racing should reflect your physiology. For example, if you have 90 percent ST and 10 percent FT fibers, your best race will likely be the marathon and your training should focus on mileage and tempo runs. If you have 60 percent ST and 40 percent FT fibers, your best race will likely be the 800m or mile, and your training should focus less on mileage and more on interval training. If both runners want to race a 5K or 10K, the former runner should initially do longer intervals, trying to get faster with training, such as 1,200m repeats at 5K race pace, increasing the speed to 3K race pace or decreasing the recovery as training progresses. The latter runner should do shorter intervals, trying to hold the pace for longer with training, such as 800m repeats at 3K race pace, increasing the distance to 1,200 meters or increasing the number of repeats as training progresses. Thus, there can be two paths to meet at the same point.
2.1 What's Your (Muscle) Type?
In lieu of a muscle biopsy to determine your exact muscle fiber type composition, ask yourself the following questions:
1. When you race, a) are you able to hang with your competitors during the middle stages, but get out-kicked in the last quarter to half-mile, or b) do you have a hard time maintaining the pace during the middle stages, but can finish fast and out-kick others?
If you answered (a), you probably have more ST fibers. If you answered (b), you have more FT fibers.
2. Which type of workouts feel easier and more natural – a) long intervals (800m to mile repeats), long runs, and tempo runs, or b) short, fast intervals (200s and 400s)?
If you answered (a), you have more ST fibers. If you answered (b), you have more FT fibers.
3. Which workouts do you look forward to more – a) long intervals and tempo runs, or b) short, fast intervals?
If you answered (a), you have more ST fibers. If you answered (b), you have more FT fibers. (People tend to get excited about tasks at which they excel, while being more anxious about tasks that are difficult.)
Physiology lesson 3.0
Metabolism is tightly regulated by enzymes and oxygen.
What It Means For You: Develop your aerobic base and do sprint training to enhance enzyme activity that maximizes your running.
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 the metabolism toward a greater reliance on aerobic metabolism at a given submaximal speed. Enzymes are also activated or inhibited (e.g., their effectiveness in speeding up chemical reactions can be either increased or decreased), determining which metabolic pathways are functional during certain cellular conditions. Thus, enzymes essentially control metabolism and therefore control the pace at which you fatigue.
A number of studies have documented an increase in enzyme activity in response to aerobic training. One of the first among these was published in 1967 in 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 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.
Similarly, 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 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.
Metabolism is also regulated by its patriarch – oxygen. The availability of oxygen determines which metabolic pathway predominates. For example, at the end of the metabolic pathway that breaks down carbohydrates (glycolysis), there is a fork in the road. When there is adequate oxygen to meet the muscle's needs, the final product of glycolysis – pyruvate – is converted into an important metabolic intermediate that enters the Krebs cycle for oxidation. This irreversible conversion of pyruvate inside your muscles' mitochondria is a decisive reaction in metabolism since it commits the carbohydrates broken down through glycolysis to be oxidized by the Krebs cycle. However, when there is not adequate oxygen to meet the muscle's needs, pyruvate is converted into lactate. An associated consequence of this latter fate is the accumulation of metabolites and the development of acidosis, causing your muscles to fatigue and you to slow down.
The more aerobically developed you are, by focusing on increasing your mileage and doing LT runs, the more you'll steer pyruvate toward the Krebs cycle and away from lactate production at a given pace. That's a good thing, because the amount of energy you get from pyruvate entering the Krebs cycle is 19 times greater than what you get from pyruvate being converted into lactate. While pyruvate will always be converted into lactate given a fast enough speed, the goal of training is to increase the speed at which that occurs.
Physiology lesson 4.0
Carbohydrates are extremely important.
What It Means For You: Don't neglect carbohydrates in your diet, particularly during recovery.
The many proponents of diets like Atkins and South Beach would have the public believe that carbohydrates are some kind of poison. Don't listen to them. Carbohydrates are a runner's best friend. Carbohydrates are stored in our skeletal muscles and liver as glycogen, and are also found as sugar (glucose) in the blood. When we run, our bodies use a combination of blood glucose and glycogen as fuel to regenerate the high-energy chemical compound ATP through a process called glycolysis. Endurance performance is strongly influenced by the amount of pre-exercise muscle glycogen, with intense endurance exercise decreasing muscle glycogen content. Carbohydrates are so important that ingesting them during prolonged exercise can even delay fatigue. With the well-documented decrease in muscle glycogen content that accompanies endurance exercise, an empty-refill cycle becomes evident. Since your muscles prefer carbohydrates as fuel, a metabolic priority of recovering muscle is to replenish muscle glycogen stores. And the more your glycogen tank is emptied, the greater it's refilled. Empty a full glass, and you get a refilled larger glass in its place. It's a lot like college fraternity parties.
Glycogen synthesis is controlled by the hormone insulin and the availability and uptake of glucose from the circulation. Insulin, which is secreted from the pancreas, is the primary signal for glycogen synthesis. Through its effect on proteins that transport glucose, insulin draws glucose from the blood into muscle cells. Glucose is then used to make new glycogen, which is simply a branched chain of glucose molecules. The higher the blood insulin concentration and the greater the availability of glucose, the faster glycogen is synthesized and stored.
How do you increase insulin concentration and make glucose available? Consume carbohydrates.
4.1 Got carbs?
Research has shown that the synthesis of glycogen between training sessions occurs most rapidly if carbohydrates are consumed immediately after exercise. Indeed, delaying carbohydrate ingestion for just two hours after a workout significantly reduces the rate at which muscle glycogen is resynthesized and stored. While research suggests that you should consume 0.7 gram of simple carbohydrates (preferably glucose) per pound of body weight (which equals about 7.5 eight-ounce glasses of Gatorade or 3.5 glasses of chocolate milk for a 150-pound runner) within 30 minutes after you run and every two hours for four to six hours to maximize the rate of glycogen synthesis, you don't have to get it all back right away since glycogen will continue to be resynthesized over the 24 hours between workouts.
Despite the many highly advertised commercial sports drinks, any drink that contains a large amount of glucose is great for recovery. For example, my research published in International Journal of Sport Nutrition and Exercise Metabolism in 2006 showed that chocolate milk is just as good or better than other recovery drinks after exhausting exercise. While some studies have found that consuming carbohydrates and protein together also speeds muscle glycogen storage, others have not found this to be the case. The total amount of calories consumed seems to be more important for recovery than the carbohydrate-protein mix. While immediate post-workout carbohydrate ingestion is the best strategy for optimal performance, it may not be the best strategy for runners specifically preparing for the marathon, a race which requires the largest glycogen storage capacity possible, a very efficient capacity to make new glucose, and a very effective system of fat use. Molecular evidence suggests that the opposite strategy – holding out on the muscles by delaying the consumption of carbohydrates – may be even more beneficial. By "starving" the muscles of carbohydrates, they are forced to use fat more effectively and even more glycogen may be synthesized when carbohydrates are finally introduced.
Low muscle glycogen content has been shown to enhance the transcription of genes involved in protein synthesis. Think of this strategy as creating a threat to the muscles' survival: When you threaten the survival of muscles by depriving them of their preferred fuel, a strong signal is sent to make more of that fuel to combat the threat and to use other sources of fuel more effectively. The downside to training in a low-glycogen state, however, is that it's hard to maintain a high intensity since such high-intensity running is dependent on carbohydrates for fuel. A lot more research needs to be done in this area, but if you're going to try training with low muscle glycogen, make sure you consume lots of carbs before your marathon, so you "train low, race high."