Which of the following best represents the average speed of a fast runner?

Impulses from the receptors travel along the fibers of nerve cells found in the part of the peripheral nervous system known as the dorsal-root ganglion. MUSCLES IN THE BODY are regulated by the feedback-control system of the reflexes: commands in the form of electrochemical impulses are sent to the muscles along the axon, or long fiber, of the specialized nerve cells in the spinal cord called alpha motoneurons information about muscle activity is sent back to the central nervous system by stimulus-sensitive receptors. The computer analysis for this illustration was performed in the Gait Analysis Laboratory of the ChildrenвЂ™s Hospital Medical Center in Boston. A very hard surface, however, is not the fastest The authors found that there is a range of intermediate track stiffnesses at which running speed is enhanced. The sequences show that a runnerвЂ™s step length and ground-contact time are both increased on a soft surface, with the net result that his running speed is decreased. Running speed can be viewed as the ratio of the runnerвЂ™s step length (the distance the body travels while one foot is on the ground) to his ground-contact time Ref. The same runner is shown on a hard, or stiff, surface ( top) and on a soft, or springy, one ( bottom). In the sequences shown here the points mark the runnerвЂ™s right hip, shoulder, ear, elbow, wrist, knees, ankles and big toes. The results of these tests, which are discussed below, are in good agreement with the theoretical predictions.Д¬OMPUTER SEQUENCES showing running positions are obtained by digitizing the points marking a runnerвЂ™s major limb joints in a motion picture of running and then connecting those points on a digital plotter. The ultimate test was provided by the new Harvard track, whose wood substructure is designed to achieve a compliance in the range we found to be optimum. Our final step was to test the model predictions with runners and experimental tracks. In this way we were able to predict the effect various track compliances would have on running speed. With the physiological simplifications introduced in the model it was possible to derive mathematical expressions for those parameters as functions of track stiffness. Running speed is determined by several parameters. Hence our first step in designing such a track was to develop a model of the mechanics of running, one that was complex enough to be realistic but simple enough to be mathematically manageable. A track whose compliance is in the theoretically optimum range can be called a tuned track.ДЄ tuned track is special only in that its properties are particularly compatible with those of the runner.

At that optimum compliance the runner stepping down onto the track stores elastic energy in its surface, much as a pole vaulter stores elastic energy in bending a fiberglass pole, which is recovered as the pole propels him upward. In other words, there is a specific intermediate track compliance at which running speed is optimized. A compliant surface acts as a spring, and we found that if the stiffness of the spring is closely tuned to the mechanical properties of the human runner, the runnerвЂ™s speed can be increased. One might therefore suppose the hardest track surfaces are the fastest, but that is not the case. Hence if a running track is to be fast, its compliance must be limited.

Intuition also suggests, however, that on a springy track the time spent rebounding from the surface is increased, so that a runner is slowed down. Indeed, if comfort were the only consideration, all tracks would be extremely springy, perhaps as springy as a diving board. It is intuitively clear, however, that running on a compliant surface such as grass is less conducive to injury than running on a hard surface such as asphalt. At that time there was no information available concerning the effect of a given track surface on running speed and comfort. Just how compliant-or, alternatively, how stiff-should a track be? In 1976, when the new track was being designed, the Harvard track coach, Bill McCurdy, and the Harvard Planning Office sought our advice on this question. One of the most important parameters of track design is compliance, and it is mainly in this respect that the new track differs from other indoor tracks.

Which of the following best represents the average speed of a fast runner?