That type of muscle, called fast-twitch, is essential to sports and balance.
"You look at any athlete who's not a marathoner or triathlete or cyclist," Holbrook says. "They don't do long-distance work."
Rather, he says, they exercise in intervals interspersed with periods of rest.
Holbrook trains older adults at his Salt Lake City studio, AgePerformance, where the average age of his clients seeking higher levels of fitness is 68. He agrees with researcher Mark J. Smith, a former professional rugby player and interval-training expert who believes elders should not limit themselves to light or moderate exercise.
More than any other age group, Smith says in a 2008 study, "the elderly need vigorous exercise." And by that he means a conditioning method called sprint-interval training or high-intensity interval training, commonly shortened to their acronyms SIT and HIIT.
The training focuses on the ability to move quickly with a burst of power, which comes naturally to us when we're young.
But around age 50, we start to lose muscle mass, especially fast-twitch muscle. If those muscles aren't strong, bursts of power become just about impossible. Even everyday needs such as climbing stairs, recovering when we stumble, moving quickly to get out of the way or standing up from a chair become difficult.
Run for it
Salt Lake City resident Ted Belknap, 90, one of Holbrook's clients, is training to run the 50-meter dash at the Huntsman World Senior Games this October in St. George.
Athletic in high school, and intermittently fit throughout his life, Belknap saw his strength ebb away with age. His spinal discs degenerated and osteoarthritis set in, making his back hurt, so he stopped his daily walks, he says, "and became a couch potato."
Though he started coming to AgePerformance a while back, for the first two years, Belknap says, he just "lazed about." Predictably, his mood plummeted. Then his gallbladder ruptured last year, sending him to the hospital for 10 days.
"When I came out," he says, "I was a basket case. My balance was gone."
That's when Holbrook took over training Belknap. Using a combination of high-intensity intervals, strength tests that include hauling a sled with a 25-pound weight on it and lifting "weights" on Keiser machines that employ air pressure instead of iron to provide fine-tuned resistance, Belknap has improved his strength and balance.
Holbrook suggested he treat the 50-meter dash at the Huntsman games as a goal, a prospect Belknap says bothered his daughters. "They don't even want me to drive," he says.
Belknap, a realist, acknowledges he might not achieve the goal, but that's all right. "Even if I don't make the games," he says, "I will be fit."
A CIA agent for 26 years, Belknap was a physical education major before World War II. "I studied all kinds of anatomy and kinesiology but they never talked about fast-twitch," he says. "We never went scientific. You just had the body and did with it what you could."
Technically, the term "high-intensity" means activity that requires effort ranging from 85 percent to 250 percent of maximum oxygen uptake, or VO2 max. The intensity is relative to an individual's ability; a very fit person will run up more stairs in 60 seconds than an out-of-shape counterpart; VO2 max can be 40 percent to 60 percent higher in men than in women.
The science has long been accepted but has been overshadowed by the notion that cardio - or aerobic - exercise three times a week for 20 to 60 minutes (at 60 to 90 percent intensity), and training to run distance, are the best ways to lose weight and stay healthy.
Smith says this conventional wisdom, which he calls "cardio dogma," came about through coincidental events.
Nike started engineering shoes for middle- and long-distance runner Steve Prefontaine while he attended University of Oregon in the early 1970s. The shoes helped ignite the jogging craze of that decade. Researchers at the time were still allowed to use students as their subjects; students were in class for 50 minutes.
Smith points out that for elders, leg power is key to staying independent. "Lower-limb explosive power," he says, "is the most important factor in fall prevention."
Watch the kids
Endurance exercise works slow-twitch muscles. High-intensity interval training works fast-twitch muscles. Both types, fast and slow, are important to physical health, and both are natural to humans. All anyone has to do is watch children at play to see short bursts of intense activity amid episodes of low- and moderate-intensity tearing about.
Born to Run insists endurance running was necessary to early human hunters, but Smith is convinced that because humans running at high speed are energetically inefficient, they were more likely to walk - the most efficient gait - and deploy short bursts of energy to hunt, gather, carry, dig and escape danger.
The discussion continues. But the fact that aerobics remain king really bothers Holbrook.
He recently did a web search for the number of long-distance races from 5 kilometers to ultra-marathons across the nation and found 14,837 were scheduled for the year, equivalent to 40 per day.
"Why, why and why," he says, "are we so stuck in this paradigm?"
Get out of the long-distance rut
One of the main reasons people give for not being in shape is that they don't have time. But high-intensity intervals, which many trainers and physiologists say are the best fitness exercise, aren't time-consuming and show better fitness and weight-loss results than low- or mid-intensity aerobic activity.
Try this • Use the stairwells in your office building.
Start by sprinting up the stairs as fast as you can for 20 seconds eight times per week. That has been shown to improve heart health.
When you improve, do six 10-second sprints with 10 seconds of recovery between each sprint; three 20-second sprints with 20 seconds of recovery; two 30-second sprints with 30 seconds recovery; or one 60-second sprint. Do these in four to eight mini-sessions a day, two or three days of the week with a full day of rest in between.
On rest days, engage in other types of exercise or play.
Try this • Use the treadmill at your gym for real high-intensity workouts.
Set the treadmill at 15 degrees.
Depending on your fitness level, set the speed from 2 to 6 miles per hour (or more).
Without holding onto the grasp bar, go all out, walking or running, for 60 seconds, then stop the machine. You should be out of breath. Recover for at least four minutes. Do four intervals, and work out your upper body if you like. Do this two or three times per week.