CHAPTER 4 Additional Biomechanical Principles


Visit Kinesiology in Action and answer the posttest questions to evaluate your understanding of the chapter.


CHAPTER SUMMARY


Biomechanical concepts related to how mass and forces infl uence motion are components of Newton’s laws: the law of inertia, the law of acceleration, and the law of action-reaction. The law of inertia describes how a force is required to start, stop, or change motion. Whether an object is in a state of static or dynamic equilibrium depends on the magnitude and direction of these forces. Acceleration and its infl uence by mass and force describe the law of acceleration. Newton’s law of action–reaction explains how objects respond to actions and forces by reacting to them in equal magnitude and opposite direc- tion if the object is to maintain its static equilibrium. If the reaction is not equal to the initial action and force, the object becomes unstable. Work performed is a function of the force applied over a distance. When work is described over a given amount of time, it describes the power of the force. In terms of human movement, muscles generate forces, and as the forces are increased over shorter periods of time, there is an increase in their power output. The human body relies on muscle power during functional activities to move the entire body weight from a sitting to a standing position or up and down steps. Muscle power provides the ability to jump or throw a ball at increased speeds. Levers and pulleys are mechanical systems that increase the mechanical advantage or effi ciency of forces. As the distance, or moment arm, of a force increases, that force will have a greater mechanical advantage and need to generate less force to perform a given workload. First- class levers are designed for balance, whereas second- class levers allow smaller forces to move against larger resistance. Third-class levers provide a means in human movement for small amounts of muscle force to produce large arcs of range of motion. These biomechanical con- cepts together with the information presented in Chap- ters 1 to 3 provide the foundational concepts to begin studying the kinesiology of individual body sections.


DISCUSSION QUESTIONS


1. Caroline is sitting erect, unsupported, and not moving on a treatment table. What is Caroline’s


state of equilibrium in terms of the trunk and spine?


2. In terms of the law of inertia, what must happen for Caroline to lean forward?


3. Discuss the difference between positive and negative acceleration.


4. How would the quadriceps muscle’s force need to change when going from lifting a 5-lb weight during knee extension to lifting a 10-lb weight if the movement speed remained the same?


5. How does the acceleration of an object change if the force to move it remains constant but the mass of the object decreases?


6. How might ground reaction forces contribute to the formation of tibial stress fractures in a runner?


7. An athlete is in a squat position, preparing to perform a vertical jump. What is the potential energy of the quadriceps and gastrocnemius muscles in terms of being stretched? When does this potential energy change to kinetic energy? In general terms, define the work performed by the lower extremities during the vertical jump. What formula would you use to determine the power generated by the lower extremities during the jump?


8. Jack is strengthening his gastrocnemius muscles by performing closed chain heel raises as he lifts his heels off the ground in standing. What type of lever system do the gastrocnemius muscles represent? Why are these relatively small muscles able to lift his entire body weight?


9. Compare the mechanical advantage and lever system of the gastrocnemius muscles with the deltoid shoulder muscles. Which system allows a greater arc of motion or angular displacement? Explain your answer.


BIBLIOGRAPHY


Angelozzi M, Madama M, Corsica C, et al. Rate of force development as an adjunctive outcome measure for return-to-sport decisions after anterior cruciate ligament reconstruction. J Orthop Sports Phys Ther. 2012;42:772-780.


Henwood TR, Riek S, Taaffe DR. Strength versus muscle power- specifi c resistance training in community-dwelling older adults. J Gerontol. 2008;63:83-91. Houglum PA, Bertoti DB. Brunnstrom’s Clinical Kinesiology. 6th ed. Philadelphia, PA: FA Davis; 2012. Jarrett CD, Brown BT, Schmidt CC. Reverse shoulder arthroplasty. Orthop Clin North Am. 2013;44:389-408. Levangie PK, Norkin CC. Joint Structure and Function: A Comprehensive Analysis. 5th ed. Philadelphia, PA: FA Davis; 2011.


LeVeau BF. Biomechanics of Human Motion Basics and Beyond for the Health Professions. Thorofare, NJ: Slack Incorporated; 2011.


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