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You’d bet good money that the superstar on your rival basketball team was born for the fast break, or that your marathon-loving neighbor has been running for miles since she was old enough to walk.

Some people seem naturally gifted with speed, while others appear destined to go the distance. But the fact is, while a few individuals may be genetically inclined toward certain athletic strengths, the majority of us can excel in both speed and enduranceif we train properly.

So if you were a sprinter in high school, but now you want to run cross country (or vice versa), you can retrain at least some of your muscle fibers to develop the athletic ability you most desire.

Nature vs. Nurture

Most of us are born with a fairly even mix of the muscle fibers that are directly related to sports performance. The human body typically has about 45 to 55 percent of both fast-twitch and slow-twitch fibers (although über-athletes tend to have a more lopsided mix). And within your fast-twitch fibers lies the ability to choose your speed or endurance destiny: There are two types of fast-twitch fibers, one of which can be trained either to fully embrace its fast-twitchiness or, conversely, to evolve into an endurance-loving, slow-twitch-like fiber.

A small study published in the December 2000 edition of the Journal of Sports Medicine and Physical Fitness compared leg-muscle biopsies from endurance-trained and untrained participants. The study found that the trained group had 71 percent slow-twitch fibers, compared with just 38 percent in the untrained group. The trained group had just 25 percent fast-twitch fibers, compared with about 52 percent in the untrained participants. The researchers concluded that endurance training promotes a transition from fast- to slow-twitch fibers, but that this transition occurs at the expense of the fast-twitch-fiber population. Explosive training, meanwhile, encourages your fast-twitch fibers to become even more powerful.

Whether you’ve been unintentionally slanting your training toward speed or endurance, or you are one of the few people who have an uneven balance of fast- and slow-twitch fibers, neither scenario should stop you from participating in the sport of your choice. It’s how you train your body, or don’t, that makes the critical difference in your athletic abilities.

The Fog Factor

With some know-how about muscle-fiber makeup, function and adaptability, you can design a training program that suits your speed or distance interests. As noted, slow-twitch fibers support endurance-based activities, while fast-twitch fibers generally support speed-based activities. But there is a catch: Fast-twitch fibers come in two varieties, known as Fast Glycolytic (FG) and Fast Oxidative Glycolytic (FOG). As with slow-twitch fibers, your allotment of FG fibers is set at birth. FOG fibers, however, have the ability to take on the characteristics of either slow-twitch or FG fast-twitch fibers. In other words, FOG fibers are the ones you develop specifically to improve either your endurance or your speed.

In deciding how you want to train, it’s helpful to have a clear idea of precisely what all these fast- and slow-twitch fibers do:

Fast Oxidative Glycolytic (FOG): FOG fibers are the switch hitters of the muscle-fiber world. With continued endurance training, they pick up slow-twitch traits such as increased capillary density and better oxygen utilization. According to a 2004 issue of Peak Performance newsletter, some endurance athletes’ FOG fibers become as good at producing aerobic energy as the slow-twitch fibers in less active subjects. But when trained for speed, FOG fibers remain true to their fast-twitch nature and act quickly and powerfully, fueled by glycogen (the body’s main source of stored energy). FOG fibers turned slow will still tire more quickly than bona fide slow-twitch fibers, but not as quickly as their FG counterparts.

Fast Glycolytic (FG): Known as “pure” fast-twitch fibers, the FGs leap into action immediately and explosively when you need them – for example, at the start of a swim race or while sprinting a hundred meters. FGs are fueled by glycogen, which is stored in the muscles, and they convert the stored energy into fuel without the use of oxygen. Their available supply of glycogen, however, tends to deplete quickly – within two or three minutes of maximum-effort exertion – so FG fibers need to rest often to replenish their fuel stores.

Slow-Twitch (ST): ST fibers are hard-working, capillary-dense, mitochondria-rich fibers that can keep muscles working for hours before fatigue sets in. Slow-twitch muscles are called upon in time-intensive activities like walking or standing or running long and slow. That kind of extended muscle use requires a constantly available energy source: oxygen. When oxygen mixes with glycogen in the muscle cells, energy is created. And ST fibers are able to replenish and store their supply of energy in a way that keeps the muscles working for long periods of time.

Learn more about your metabolic systems at “All About Your Metabolic Energy Systems”.

Into the Distance

Endurance sports require continuous moderate-intensity activity over an extended period. The actual duration of activity required in endurance sports is generally sport-specific. For instance, distance swimmers might need to be able to swim continually for a half-hour, runners to press on for one to three hours, and cyclists to pedal three to four hours at a stretch.

Although endurance athletes once trained almost exclusively for distance – in effect, training the speed right out of their muscles – experts now agree that some speed training is necessary in order to build the neuromuscular pathways and combined aerobic/ anaerobic capacity responsible for optimizing overall sports performance (generating that last big kick at the end of a long race, for example).

Developing strength and coordination is an often-overlooked component of aerobic efficiency, but the main key to enhancing endurance lies in training your body to absorb more oxygen. That staves off the anaerobic-threshold window in which the body’s anaerobic (sugar-burning) system takes over most of the energy-production process. Although you can train your body to use lactic acid as a fuel up to this point, this fuel switch produces a lactic-acid buildup that soon hampers your ability to continue.

Left untrained, the body will absorb just enough oxygen from each breath to do simple activities around the house. Through dedicated distance training, however, the mitochondria in your slow-twitch muscle fibers increase in size and number. That improves oxygen use, and the capillaries become denser, which further improves the transportation of oxygen.

Training well below your anaerobic threshold for extended periods will improve your body’s oxygen-absorption capacity. To fend off that sluggish feeling of lactic-acid buildup, however, you’ll need to do some interval training. This calls for building brief, two- to three-minute sessions of elevated intensity or speed into your regular workout. According to Rob Sleamaker and Ray Browning, authors of SERIOUS Training for Endurance Athletes, those brief bursts of activity teach your FOG fibers to increase their oxygen capacity before too much lactic acid builds up. Once the fibers learn to deliver oxygen to cells when working at a higher intensity, you’ll be able to train faster, and for longer, without feeling unpleasant effects.

Leave ‘Em in the Dust

Training that emphasizes speed and power over endurance involves recruiting your muscle fibers to work quickly and explosively for short periods of time. Speed- and power-dominant sports like volleyball, golf, baseball and basketball require you to quickly recoil and launch your legs and arms or twist your core.

Training for speed, then, requires practice in making quick and precise movements. Directing those quick-as-a-flash movements is the work of the nervous system, which tells your muscles what to do. By practicing moving quickly, you create a well-worn neuromuscular groove that instructs your muscle fibers to work double-quick, right now.

Speed training is also a matter of developing the elasticity of your muscle fibers. Elasticity is the muscle fibers’ collective ability to stretch slightly in and then shorten quickly, a process known as the stretch-shortening cycle (picture the dynamic involved in landing a jump and then springing up again). So when you’re in the gym contemplating whether to increase your weight by another 10 pounds, hold off. Instead, make sure you’re doing that clean-and-jerk or lunge-with-shoulder-press as precisely, powerfully, quickly and as safely as you can. The lesson is simple: When training for speed, speed is what is counts.

As Richard Diaz, a California-based personal trainer and the founder of Diaz Human Performance ( points out, speed and power play a major role in many sports. “In most situations, the physical demands for a field sport such as basketball or tennis last less than 14 seconds at a stretch,” he says.

For increasing the neuromuscular pathways and elasticity responsible for short-burst speed and power, experts recommend training methods like these:

The Truth is in the Training

Whether you crave speed or endurance, you can tailor your training – and your muscle fibers – to serve that ambition. Spending ample training time well below your anaerobic threshold can help you hang tough with your marathoning neighbor. Speed-training techniques can make you an even match against that fast-break rival in your basketball league. With your FOG smarts and your training know-how, you’re in charge of your muscle-fiber destiny.

This article has been updated. It was originally appeared as the “Fast and Slow of It” published in the May 2006 issue of Experience Life.

Jen Sinkler

Jen Sinkler, PCC, RKC-II, is a fitness writer and personal trainer based in Minneapolis. Her website is

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