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11-chapter 8

# 11-chapter 8 - Chapter 8 Movement and Control in Locomotion...

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Chapter 8: Movement and Control in Locomotion Purpose Understand the concept of dynamic similarity and how the Froude number is applicable to locomotion and gait changes. Understand the synergistic and antagonist actions of muscle activation creating controlled, smooth movement. Understand levers and be able to measure and calculate gear ratios for limbs and jaws Introduction In this lab, you are going to rotate through a series of stations and learn to think like a biomechanist and gain a deeper understanding of the functions of the muscles and bones you have already learned. All activities for this lab will be done in groups of six unless otherwise noted. At the end of class, your TA will review your data and answers. Make sure that you (as an individual) have the answers for all the questions so they can serve as a study aid. Station 1: Dynamic Stability and the Predictable Froude Number What is the Froude number? To understand Froude numbers, consider what happens when you‟re late to an exam and are walking as fast as you can to class. You‟ll notice that you‟re limited by an upper speed, above which you must start jogging or running. This marks the transition from a pendular gait into a spring-mass gait (Fig. 8-1), and is quite literally due t o the inertial or centripetal forces you‟ve created while pushing off the ground with your legs overcoming the gravitational forces holding you down, causing you to fly right off the ground! Why don‟t you just launch right off into space when this happens? Well, gravity is a constant and feisty companion, and the minute you leave the ground, all of that kinetic energy you input into the system gets converted to potential energy and you come crashing back down to Earth where you belong. The take-home message is that some of the basic mechanics of locomotion are driven by potential and kinetic energy exchange, and a ratio of inertial to gravitational forces dictating the gait choice. Since all animals of different sizes and shapes are subject to the same physical laws, it would seem reasonable to expect that these principles should hold true for animals of any size. In other words, irrespective of size, when an animal pushes off the ground with greater force than the gravitational forces holding it down, it should fly off the ground and be effectively forced to transition to a spring-mass style of locomotion. Froude numbers rely on this line of reasoning and are used to compare locomotion between animals that vary greatly in size. Based on a ratio of inertial to gravitational forces , animals that have similar values of the Froude number “magically” move in a dynamically similar manner. Figure 8-1. The pendular and spring-mass models of locomotion.

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The equation for calculating Froude number ( Fr ) is: Fr = u 2 gl for which u is a speed and l is a length characteristic of the motion, and g is gravitational acceleration (~9.81 m/s 2 ). For most locomotion studies, you‟ll usually see that u is defined as the forward speed and l is leg length.
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