47

key concepts

47.1

Interactions of Actin and Myosin Cause Muscles to Contract

47.2

Many Factors Affect Muscle Performance

47.3

Muscles and Skeletal Systems Work Together

Champion Jumpers

The women’s long jump Olympic record is 7.4 meters, set in 1988 by Jackie Joyner-Kersee. Another world-record long jump was set two years earlier by Rosie the Ribeter, who jumped 6.5 meters. Rosie was a frog competing in the Calaveras County Jumping Frog Contest. Rosie’s jump is actually more impressive—it was 20 times her body length, whereas Jackie’s was 5 times her body length. And Rosie jumped from a sitting, crouched position, while Jackie’s jump was propelled by a sprint.

Both jumps were powered by skeletal muscle. The cellular mechanisms of muscle contraction are the same in the frog and the human, so why is the frog’s jump more impressive? The answer involves the concept of leverage, which depends on the muscles and skeletal elements working together. Muscles pull on bones that are connected at joints to make levers. With a lever you can use the same force to move a large mass a small distance or a small mass a large distance. The ratio of leg length to body mass is greater for the frog than for the human. Thus the frog’s long legs can move its small body mass a long distance.

The power for the frog’s jump comes from a single contraction of its leg muscles (there is no sprint), and those muscles contract only a very short distance compared to the length of the jump. One species of frog, Rana pipiens, has a body length of only 11 centimeters. It has much longer legs, but muscles that power its jump are only about 3.5 centimeters long and shorten by just 0.75 centimeters (21%) when the frog jumps. Once again leverage comes into play. The thigh muscles are attached to the lower leg bones close to the knee joint, thus they can move the opposite ends of those bones over a longer distance and more quickly than if they were attached farther away from the joint. There is a trade-off, however, in terms of force and speed. To use a shovel, you have one hand near the end of the shaft and the other hand farther down the shaft. With hands close together, you can move the shovel blade fast, but not pick up a heavy load of dirt. With hands farther apart, you can’t move the blade as fast, but you can pick up a heavier load.

What adaptations optimize the jumping muscles of the frog?