How Rattlesnakes Rattle

Rattlesnakes can vibrate their rattles at phenomenal speeds for hours at a time; do their muscles hold secrets that could help humans better perform exercises?

• Peter Friederici
• Feb 01, 2008

PITY THE POOR hummingbird--so small, so slight, so slow. Slow? Not to our eyes, maybe, but even the bird's supercharged wingbeats are on the sluggish side compared to the moves of a true champion, the rattlesnake.

True, the reptiles don't cover ground all that fast. (Good thing.) Rather, it's their trademark noisemaker that qualifies them as the swiftest movers and shakers among terrestrial vertebrates. When a rattler shakes its tail on a hot day, its muscles are lengthening and shortening as many as 90 times per second--twice as fast as the wing muscles of a hummingbird sipping nectar. A pro basketball player sprinting to a breakaway dunk can't manage such muscle movements more than 8 times a second--almost ponderous, by comparison.

You wouldn't think to look to a cold-blooded creature for lessons on how to help pro athletes and others perform better. But that's exactly what some scientists are doing. By studying how rattlesnakes are able to use their muscles so quickly and at a low cost of energy per shake, the researchers are developing new exercise techniques that may allow frail and elderly people to better avoid injury, enable patients in rehab to heal faster, and perhaps even make it possible for some people a lot shorter than basketball players to pull off those gravity-defying dunks.

A rattlesnake rattle is made of dead tissue and its owner shakes it by twitching sets of small muscles on either side of its tail. What scientists could not understand, for a long time, was how the reptile can rattle so vigorously for, in some cases, hours.

"The rattling is extraordinarily fast and can be sustained for a prolonged time," says Kevin Conley, a zoologist and radiologist at the University of Washington. "Human muscles can do one of those things, but not both at the same time. The rattlesnake combines the speed of a sprinter with the endurance of a marathoner."

By placing western diamondback rattlesnakes into a magnetic resonance scanner, Conley and his collaborators, comparative physiologist Stan Lindstedt of Northern Arizona University and his graduate student Paul Schaeffer, discovered that the snakes require surprisingly little energy per shake. They found two reasons for this: First, the muscles produce little force; second, they don't rely exclusively on oxygen for fuel. The researchers also believe--though they've been unable to prove--that rattlesnakes shake not by shortening the muscles on the side of the tail toward which the rattle is moving, as one might expect, but by using their muscles as a brake.

"We thought that the system may function like a slingshot," Lindstedt says. "The energy needed to slow a muscle could be stored in the form of elastic potential energy, like a spring, and it becomes available for movement in the opposite direction."

Although experiments did not support this concept, they did inspire the researchers to further test the lengthening of muscles. Producing force in a lengthening muscle, they found, requires much less energy than in one that is shortening. That's why a backpacker hiking into the Grand Canyon, using most of her muscles as a brake, burns fewer calories than she does coming back out. She's moving the same amount of weight around, but the muscles that do the bulk of the work are primarily lengthening rather than shortening as she descends. Lengthening a muscle also produces at least twice as much force as shortening it. A weightlifter who can bench-press 250 pounds knows that the challenge comes in lifting the barbell up; he can easily let down much more weight than that.

It was this combination of low energy consumption and high force production that really captured the researchers' attention. When muscles produce high force, they grow stronger. Yet most exercise regimens focus on aerobic activity or making muscles stronger through contractions that shorten them. Many people, such as the elderly or heart patients, don't have the energy to expend on such conventional weightlifting and thus cannot avoid muscle wasting.

"We thought, let's take advantage of this in a clinical setting so people who can't meet the energy demands of exercise can still benefit," says Lindstedt. "We could uncouple the energy needs from the exercise benefits. Even if you're elderly your muscles are responsive. You can still build muscle mass."

To do this, they built an exercise bicycle with pedals that are electrically powered to run backwards. The job of the person sitting on it is to resist that motion. Even a few minutes of resistance causes the sitter's muscles to work hard. That causes them to grow not faster, but stronger. When elderly patients tried out the device, their muscle mass increased, as did their balance. That translated into a lower risk of falling. When members of a high school basketball team used the equipment, they increased the extent of their vertical leaps by 8 percent--a couple of inches.

Impending clinical trials for the new equipment may make it available in rehabilitation and physical therapy settings in a few years. In the meantime Lindstedt, a fit bicyclist and hiker who backpacks the Grand Canyon every spring, makes sure to train on the prototype machine before he goes. "I'm really susceptible to getting sore," he says, "but if I sit on this bike two or three times before I go, I won't be."

Writer Peter Friederici is based in northern Arizona.