What adaptations optimize the jumping muscles of the frog?

Rosie the Ribeter was an exceptional jumper judged by the length of time her record remains unbeaten, but frogs in general are remarkable jumpers. This ability is not due to the molecular structure of their muscles, as the arrangement of sarcomeres is very similar across a wide cross section of animals, and in all cases, the sarcomeres and therefore the muscles only contract by about 25–30 percent of their length. So how can a muscle only a few centimeters long produce a jump of many meters? One answer, which is considered in the experiments described in Investigating Life: What Is the Optimal Resting Position for the Jumping Muscle of the Frog?, involves maximizing force and power. First, you saw that the resting posture of the frog maintains the sarcomere length where it can generate the maximum force when it contracts. Then by using the measured force/velocity curve, you saw that the velocity of the jump was at the level that maximized the power of the jump. This rather fast velocity of contraction is possible because the jumping muscle contains mostly fast-twitch fibers. As mentioned in the opening story, and further explained in Key Concept 47.3, the legs operate as class 3 levers to move the body mass. Because the legs of the frog are long, the load arms of those levers are long in comparison with the force arms—that is, the length ratio of the load and force arms is high. A frog’s long legs and big feet have another advantage as well. Muscles must have something to exert force against to maximize power, and once the frog leaves the ground, its muscles can no longer generate power. Long legs and feet ensure that as the jumping muscles contract, the feet remain in contact with the ground for a longer time, thus maximizing the power of the jump.

Future directions

Understanding how muscles and skeletal structures work together to generate movements of different types is essential in the growing field of robotics. Robots that repeat the same movement over and over are quite common in manufacturing, but more and more robotic systems are being created to do more variable and complex tasks that mimic and frequently augment the abilities of humans and other animals. For example, robots have been developed to travel over rough terrain, even while carrying heavy loads. Robots have been developed to do household chores such as washing dishes and folding laundry. Robotic systems are improving the functionality of prosthetic limbs for amputees. And melding of robotic systems with normal human functionality is leading to technologies called “exoskeletons” that greatly enhance the physical abilities of humans. The starting point in all of these engineering efforts is a thorough characterization of how the biological musculoskeletal system works.