A Big 'Fuel Tank', An Energy-Saving Stride

The Cutting-Edge Runner: How to Use the Latest Science and Technology to Run Longer, Stronger, and Faster
by Matt Fitzgerald

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Characteristic #5: A Big "Fuel Tank"

Carbohydrate stored in the muscles and liver as glycogen is the principal fuel for race-pace running for most runners at most distances. Because carbohydrate fuel stores in the body are relatively small, depletion of these stores and the resulting fatigue is an important limiter in longer races (and, for that matter, in some long workouts). Training can greatly increase the amount of glycogen stored in the muscles and thereby substantially increase endurance. The graph on page 14 shows the effect of training on glycogen storage as compared to its effect on three other physiological characteristics important to running. The leg muscles of an elite marathon runner might contain three times as much glycogen as those of a sedentary person of the same weight. Training can also slow the rate at which muscles use carbohydrate at any given running pace by increasing running economy (i.e., lowering the overall energy cost of running) and by increasing the body's capacity to utilize other fuels (fat, amino acids, and lactate).

Training also enhances a runner's ability to create glucose from lactate and amino acids during running. First, training increases the activity of lactate transporters. These bind to lactate molecules and hydrogen ions inside the muscle cell and carry them across the cell membrane into the bloodstream, through which lactate travels to other tissues of the body where it is stored or converted. For example, the liver can convert lactate back into glucose through a process called gluconeogenesis. Training also enhances the liver's capacity to absorb lactate from the bloodstream and convert it to glucose. In addition, training boosts the body's ability to deliver additional carbohydrate fuel to the active leg muscles from the less active muscles of the upper body through a mechanism called the lactate shuttle. In this process, glycogen in the less active muscles is converted into lactate and then carried through the bloodstream to the legs, where it can be metabolized further in the mitochondria. Finally, training also leads to adaptations that increase the efficiency with which muscle proteins are broken down into amino acids, transported to the liver, and converted into glucose through gluconeogenesis.

The best training for most of these adaptations is long runs that deplete the muscle glycogen stores. These workouts begin to increase glycogen storage levels in the muscles almost immediately. By the same token, this adaptation will start reversing itself very quickly in the absence of long runs that continually challenge fuel supply. For this reason, frequent and progressive long runs are essential in training. Long runs also enhance the body's ability to metabolize fats and amino acids after glycogen stores run low, and they reduce the degradation of running economy that develops with fatigue. Very high intensity intervals are probably the strongest enhancers of lactate shuttling capacity.

Certain nutritional practices are also helpful. Maintaining a diet with adequate carbohydrate intake is an important way to maximize carbohydrate storage. Timing is especially important. Consuming carbohydrate before workouts and races tops off liver glycogen stores and helps to prevent hypoglycemia (low blood glucose). Consuming carbohydrate in a sports drink or gel while running decreases reliance on muscle glycogen stores, thus conserving them. And consuming carbohydrate immediately after running results in more rapid glycogen replenishment than does waiting to consume carbohydrate. Also, tapering–that is, drastically reducing training volume–in the days before a long race greatly increases glycogen stores. I'll talk about fueling strategies and the relationship between carbohydrate depletion and fatigue in greater detail in chapter 11.

Characteristic #6: An Energy-Saving Stride

The terms running biomechanics and running technique are often used interchangeably. Both refer to how a runner runs, but the two are not quite the same. Biomechanics refers specifically to the aspects of a runner's stride that are determined by individual body structure. Technique refers to aspects of the stride that are (at least somewhat) independent of body structure. For example, perhaps you have tight hamstrings, and as a result you run with short strides. That's biomechanics. But perhaps you've also fallen into the habit of shuffling, and if you worked at it, you could run with slightly longer strides than you do. That's technique. As you can see by this example, biomechanics and technique are mutually influencing, which is why it's not impractical to use them interchangeably. I'll use the term running form to refer to the combination of biomechanics and technique.

Good running form is efficient running form. A certain minimum rate of energy consumption is required for a runner of a certain weight to run at any given speed. A runner with efficient form will consume energy at a rate that is not much above this minimum, while a runner with inefficient form will consume a lot more energy due to wasteful movements. Various studies have provided evidence of the economical value or cost of various biomechanical and technique characteristics. For example, it has been shown that runners who produce higher impact forces (independent of body weight) tend to be less economical.

Effective ways to improve your stride efficiency include conscious control (using body awareness to practice a specific form alteration with every stride until it's automatic), stretching and resistance training to diminish biomechanical limiters, using orthotics when necessary to reduce the energy waste of poor foot biomechanics, uphill and downhill running, high speed running, and form/power drills. I'll describe the details of "correct running form" and how to achieve it in chapter 7.

Understand, however, that you can't change certain physical characteristics that affect efficiency. Characteristics that are likely to positively influence economy include average or slightly smaller than average height for men and slightly greater than average height for women, an ectomorphic physique, tapering thighs (thicker near the hip, narrow near the knee), a narrow pelvis, and smaller than average feet.

It is also believed that the muscles and tendons of some runners are better able to absorb, store, and return ground impact forces than those of other runners. Such springiness in the soft tissues of the lower extremities is especially beneficial in longer races, when muscle damage resulting from repetitive impact becomes a major performance limiter.

Besides efficiency, the other great advantage of good running form is that it reduces the likelihood of injuries by reducing impact forces and dispersing them better. Most running injuries are caused not by impact forces alone but by the combination of impact forces and joint instability (specifically in the ankle, knee, and hip). When joints lack adequate stability, running impact forces cause them to collapse or move abnormally with each foot strike, creating damage that steadily accumulates.

Superior joint stability is therefore an important characteristic of good running form. Some runners have it naturally, but all runners can enhance it through strength training and making small but crucial gait modifications. Running itself tends to decrease rather than increase joint stability, so it's very important that you make these special efforts to counteract this effect.

© 2005 by Matt Fitzgerald

About the Author

Matt Fitzgerald, runner, triathlete, and coach, is a former editor and current contributor for Triathlete magazine. He writes articles for such national publications as Men's Health, Men's Fitness, Outside, Fitness Runner, and the Runner's World Web site, and serves as managing editor of the sports nutrition Web site, Pioneering Muscles.