Grounded by mystery of bounce

Jump: Scientists may have the raw mechanics down, but they don't know exactly how humans defy gravity - and how some do it so well.

Medicine & Science

April 05, 2004|By David Kohn | David Kohn,SUN STAFF

On the city's playgrounds, they call it "bounce," "mad hops" or simply "ups."

Most of the players in tonight's NCAA men's championship have it. So does Darnell Harris.

At 6 feet, 1 inch and 150 pounds, the slender 17-year-old can spring from the earth with astonishing ease, soaring higher than seems possible.

"You take off, it feels like you're flying a little bit," said the St. Frances Academy senior, who is being recruited by several Division I basketball programs.

Though Harris' jump has never been measured, St. Frances coach William Wells estimates it at 38 inches, as good as many pros. In other words, he can jump onto your kitchen counter without bending his legs in flight. Assuming your ceiling is high enough, of course.

Valuable in many sports, the ability to elevate is critical in basketball, an intrinsically vertical game that smiles on those who can get closest to the 10-foot-high hoop. Even beyond its utility, jumping has a special charisma. High-flying plays - a trampoline-esque dunk or an acrobatic block - grab a fan's attention like no other feat of basketball skill.

Despite its seeming simplicity - almost any toddler can muster a little hop - sending one's self skyward remains a riddle. Scientists have yet to solve the puzzle of exactly how humans defy gravity, and particularly how a few, like Harris, can do it so well.

Researchers do have a handle on the raw mechanics. A standard two-footed leap begins with the contraction of the gluteus muscles, which rotate the hips backward, bringing the torso upright. Now oriented vertically, the jumper is ready for takeoff. Next, the quadriceps - the large muscles at the front of the thighs - extend the knee joints, straightening the legs.

At this point, the jumper's feet are still flat on the ground. To raise the heels, the calf muscles contract, followed by the toe extensors, which push the toes into the air. This completes the movement, sending the jumper airborne. The entire sequence takes about a quarter of a second.

Beyond this, jumping is mostly a mystery. "With all the studies we've done, we still don't have a great grasp of what's going on," said Lawrence Weiss, an exercise scientist at the University of Memphis.

Researchers have been stymied by the sheer number of variables involved. To accomplish even a brief flight, humans must perform an intricate sequence of actions requiring coordination of bones, tendons, muscles, nerves and the brain.

"It's a very intricate process," said Marcus Pandy, a bioengineering professor at the University of Texas. So intricate, in fact, that researchers are unsure exactly how many muscles actually participate.

Some argue that the hamstrings, the muscles at the back of the thighs, play a key role, moving energy from the legs to the hips. According to this theory, the transferred force helps propel the jumper upward.

Pandy disagrees. By creating computer simulations of humans jumping, he has been able to test how individual muscles contribute to leaping ability. His models show that hamstrings make little difference.

Another point of contention: tendons, the elastic bands that connect muscles to bones. Some see tendons as a likely source of springiness. "I would bet that really good jumpers have really good tendons," said Oregon State University biomechanist Tom Roberts.

To investigate how leaping works, Roberts has studied bullfrogs. Bullfrogs can hop up to 3 feet, an impressive distance considering the creatures are only about 3 inches long.

The frogs have another astonishing trait: They jump farther than their muscular force alone can explain. Roberts found that the animals gained extra distance by storing energy in their tendons - using them as a sort of slingshot.

"They use the muscles to stretch the tendons, and then release that energy really quickly, so they get this brief burst of power," said Roberts.

Frogs are not the only animals who rely on tendons for mad hops. Kangaroos, as well as some species of fleas and locusts, all power themselves via this windup method. Roberts suspects that humans use a similar process: Those who can elevate best might have springier tendons, he says.

Pandy's computer models tell a different story. When he examined tendons' role in human jumping, he found they did not add much. He argues that jumping well is basically a matter of strength - the ability to produce force. This attribute, he says, outweighs speed, which comes from the ability to contract muscles quickly. Pandy's digital jumps show that while both are important, strength plays a larger role.

But not everyone thinks speed takes a back seat. "If you can't do it quickly, it doesn't matter how much force you can generate or how strong you are. You're not going to jump well," said Weiss. He argues that for someone like Harris, who is quick but not bulkily muscled, speed may be more important than strength.

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