DAVIS, CALIFORNIA--A football heaved all the way from midfield to the end zone is a biomechanical marvel, but the quarterback's arm is actually wasting energy. A new mechanical analysis, however, suggests that such seemingly counterproductive efforts in fact enable the limb to fling the ball farther.
In throwing and other physical activities, the first step forward is often a step back. For example, to jump straight up, a person first crouches toward the ground. The body briefly continues to move downward even after the muscles in the legs and torso begin to pull it upward, and when moving one way and pulling the other, the body does mechanical work against itself and wastes energy. Similarly, in throwing, the forearm momentarily moves backward even as the upper arm pulls it forward, again squandering energy. Biomechanicists have proposed various explanations for such "countermotion." It might stretch the muscles, triggering physiological changes that then allow them to contract more vigorously. Or it might provide a delay in which slowly reacting muscles catch up to one another.
Or it may simply allow the body to expend the rest of its energy more effectively, argues Sam Walcott, a doctoral student in theoretical and applied mechanics at Cornell University in Ithaca, New York. Using a computer simulation, Walcott studied an idealized arm consisting of two straight segments--one representing the upper arm and the other representing the forearm--that hurled a virtual ball. Torques at the "shoulder" and the "elbow" created the throwing motion. Walcott gave the arm a fixed amount of energy to expend, which determined how far the object could possibly travel no matter how it was launched. He then let the computer search for the arm motion that produced the longest throw.If the computer program allowed some countermotion, it threw the object closer to the maximum distance, Walcott reported here 13 September at the 5th International Conference on the Engineering of Sport. The design of a human arm prevents it from chucking an object at just any angle and speed, he explains, but an arm can get closer to the optimal angle and speed if it wastes some energy along the way.It's an interesting argument, says Michele LeBlanc, a biomechanicist at California Lutheran University in Thousand Oaks, but it's likely not the entire story. The specific interactions between muscles and other body parts will also play a role, she says.