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1.3 SCOPE OF THE TEXTBOOK
The best way to outline the scope of any scientific text is to describe the topics covered. In this text, the biomechanics of human movement has been defined as the mechanics and biophysics of the musculoskeletal system as it pertains to the performance of any movement skill. The neural system is also involved, but it is limited to electromyography and its relationship to the mechanics of the muscle. The variables that are used in the description and analysis of any movement can be categorized as follows: kinematics, kinetics, anthropometry, muscle mechanics, and electromyography. A summary of these variables and how they interrelate now follows.
In any scientific text, describing the topics covered is the best way to outline the scope. In this text, the biomechanics of human movement is defined as the mechanics and biophysics of the musculoskeletal system relevant to the performance of any motor skill. The nervous system is also involved, but limited to muscle mechanics and the relationship between muscle mechanics and muscle mechanics. The variables used to describe and analyze any movement can be categorized as follows: kinematics, kinetics, anthropometry, muscle mechanics, and electromyography. Below is a summary of these variables and their interrelationships.
1.3.1 Signal Processing
A major addition to this fourth edition is a chapter on signal processing. Some aspects of signal processing were contained in previous additions; it was decided that all aspects should be combined in one chapter and be given a more rigorous presentation. Why signal processing? Virtually all the variables we measure or analyze come to us in the time domain: EMG, forces, displacements, accelerations, energies, powers, moments, and so on. Thus, they are signals and must be treated like any other signal. We can analyze their frequency content, digitize them, analog or digitally filter them, and correlate or average their waveforms. Based on their signal characteristics, we can make decisions as to sampling rate, minimum length of data files, and filter cutoff frequencies. Also, there are correlation and covariance techniques that allow us to explore more complex total limb and total body motor patterns.
An important addition to the fourth edition is the chapter on signal processing. Some coverage of signal processing was covered in previous editions, but it was decided to consolidate all aspects into one chapter with a more rigorous introduction. Why do we need signal processing? Almost everything we measure or analyze is in the time domain: EMG, force, displacement, acceleration, energy, power, torque, and more. Therefore, they are all signals and must be treated like any other signal. We can analyze their frequency domain content, digitize them, process them with analog or digital filtering, and correlate and average their waveforms. According to the signal characteristics, we can decide the sampling rate, the minimum length of the data file and the filter cut-off frequency. In addition, there are correlation and covariance techniques that can help us explore more complex whole-limb and whole-body motion patterns.
1.3.2 Kinematics
Kinematic variables are involved in the description of the movement, independent of forces that cause that movement. They include linear and angular displacements, velocities, and accelerations. The displacement data are taken from any anatomical landmark: center of gravity of body segments, centers of rotation of joints, extremes of limb segments, or key anatomical prominances. The spatial reference system can be either relative or absolute. The former requires that all coordinates be reported relative to an anatomical coordinate system that changes from segment to segment. An absolute system means that the coordinates are referred to an external spatial reference system. The same applies to angular data. Relative angles mean joint angles; absolute angles are referred to the external spatial reference. For example, in a two-dimensional (2D) system, horizontal to the right is 0◦, and counterclockwise is a positive angular displacement.
In describing motion, kinematic variables refer to something independent of the forces that cause that motion. They include linear and angular displacement, velocity and acceleration. Displacement data can be acquired from any anatomical landmark: the center of gravity of a body part, the center of rotation of a joint, the extreme points of a limb segment, or key anatomical prominence points. Spatial reference systems can be relative or absolute. Relative systems require that all coordinates be reported relative to an anatomical coordinate system that varies from section to section. An absolute system means that the coordinates are referenced to an external spatial reference system. The same goes for angular data. The relative angle refers to the joint angle; the absolute angle refers to the angle relative to the external spatial reference. For example, in a two-dimensional system, horizontally to the right is 0°, and counterclockwise is a positive angular displacement.
The basic kinematic concepts are taught on a 2D basis in one plane. All kinematic displacement and rotational variables are vectors. However, in any given direction or rotation, they are considered scalar signals and can be processed and analyzed as such. In three-dimensional (3D) analysis, we add an additional vector direction, but we now have three planes to analyze. Each segment in 3D analyses has its own axis system; thus, the 3D orientation of the planes for one segment is not necessarily the same as those for the adjacent segments.
Basic kinematic concepts are taught on a two-dimensional plane. All kinematic displacement and rotation variables are vectors. However, in any given orientation or rotation, they are treated as scalar signals and can be processed and analyzed as such. In 3D analysis, we added an additional vector direction, but now we have three planes to analyze. In a 3D analysis, each segment has its own coordinate system; therefore, the orientation of the 3D plane of one segment is not necessarily the same as that of adjacent segments.
1.3.3 Kinetics
The general term given to the forces that cause the movement is kinetics. Both internal and external forces are included. Internal forces come from muscle activity, ligaments, or the friction in the muscles and joints. External forces come from the ground or from external loads, from active bodies (e.g., those forces exerted by a tackler in football), or from passive sources (e.g., wind resistance). A wide variety of kinetic analyses can be done. The moments of force produced by muscles crossing a joint, the mechanical power flowing to or from those same muscles, and the energy changes of the body that result from this power flow are all considered part of kinetics. It is here that a major focus of the book is made, because it is in the kinetics that we can really get at the cause of the movement and, therefore, get some insight into the mechanisms involved and into movement strategies and compensations of the neural system. A large part of the future of biomechanics lies in kinetic analyses, because the information present permits us to make very definitive assessments and interpretations.
The totality of the forces produced by the cause of motion is called kinetics. It includes internal and external forces. Internal forces come from muscular activity, ligaments, or friction from muscles and joints. External forces come from the ground or external loads, from active bodies (such as the force exerted by a faller in football) or passive sources (such as wind resistance). A wide variety of kinetic analyzes can be performed. The moments produced by muscles across joints, the flow of mechanical power to or from the same muscles, and the changes in body energy caused by this power flow are all considered part of the dynamics. It is in dynamics that we can really uncover the cause of movement, thus gaining some insight into the mechanisms involved, movement strategies and neural compensation. The future of biomechanics depends heavily on kinetic analysis, as the information provided allows us to make very clear assessments and interpretations.
As with the kinematics, all basic kinetic concepts will be covered in detail in 2D analyses. Three-dimensional analysis adds an additional force vector in the global reference system (GRS), but, because of the two additional planes, there are two additional moment vectors. The 3D analysis techniques are considerably more complex; however, within any of these three planes, the interpretation is the same as in 2D analyses.
Similar to kinematics, all basic dynamic concepts will be covered in detail in 2D analysis. A 3D analysis adds an additional force vector in the global reference system, but because of the addition of two planes, there are two additional moment vectors. Three-dimensional analysis techniques are relatively complex, but in any of these three planes, the interpretation is the same as in two-dimensional analysis.
1.3.4 Anthropometry
Many of the earlier anatomical studies involving body and limb measurements were not considered to be of interest of biomechanics. However, it is impossible to evolve a biomechanical model without data regarding masses of limb segments, location of mass centers, segment lengths, centers of rotation, angles of pull of muscles, mass and cross-sectional area of muscles, moments of inertia, and so on. The accuracy of any analysis depends as much on the quality and completeness of the anthropometric measures as on the kinematics and kinetics.
Much of the early anatomical research involved body and extremity measurements and was not considered to be of biomechanical interest. However, no biomechanical model can be built without data on the mass of the limb segment, center of mass position, segment length, center of rotation, angle of tension of the muscle, mass and cross-sectional area of the muscle, moment of inertia, etc. The accuracy of any analysis is as important as the quality and completeness of anthropometric measurements, as are kinematics and dynamics.
1.3.5 Muscle and Joint Biomechanics
One body of knowledge that is not included in any of the preceding categories is the mechanical characteristics of the muscle itself. How does its tension vary with length and with velocity? What are the passive characteristics of the muscle—mass, elasticity, and viscosity? What are the various characteristics of the joints? What are the advantages of double-joint muscles? What are the differences in muscle activity during lengthening versus shortening? How does the neural recruitment affect the muscle tension? What kind of mathematical models best fit a muscle? How can we calculate the center of rotation of a joint? The final assessment of the many movements cannot ignore the influence of active and passive characteristics of the muscle, nor can it disregard the passive role of the articulating surfaces in stabilizing joints and limiting ranges of movement.
One piece of knowledge not included in the previous classifications is the mechanical properties of the muscles themselves. How does muscle tension vary with length and speed? What are the passive properties of muscle—mass, elasticity, and viscosity? What are the different properties of joints? What are the advantages of bijoint muscles? What is the difference in muscle activity during lengthening and shortening? How does neural recruitment affect muscle tone? What mathematical model is best for describing muscles? How do we calculate the center of rotation of a joint? The final assessment for many sports cannot ignore the influence of the active and passive properties of the muscles, nor the passive action of the articular surfaces in stabilizing the joint and limiting the range of motion.
1.3.6 Electromyography
The neural control of movement cannot be separated from the movement itself, and in the electromyogram (EMG) we have information regarding the final control signal of each muscle. The EMG is the primary signal to describe the input to the muscular system. It gives information regarding which muscle or muscles are responsible for a muscle moment or whether antagonistic activity is taking place. Because of the relationship between a muscle’s EMG and its tension, a number of biomechanical models have evolved. The EMG also has information regarding the recruitment of different types of muscle fibers and the fatigue state of the muscle.
The neural control of movement is inextricably linked to movement itself, and in electromyography (EMG) we can obtain information about the ultimate control signals of each muscle. EMG is the main signal describing the input to the muscular system. It provides information about which muscle or muscles are responsible for generating muscle torque, or whether there is antagonistic activity. Due to the relationship between the EMG of a muscle and its tension, many biomechanical models have emerged. EMG also provides information about the recruitment of different types of muscle fibers and the fatigue state of the muscle.
1.3.7 Synthesis of Human Movement
Most biomechanical modeling involves the use of inverse solutions to predict variables such as reaction forces, moments of force, mechanical energy, and power, none of which is directly measurable in humans. The reverse of this analysis is called synthesis, which assumes a similar biomechanical model, and using assumed moments of force (or muscle forces) as forcing functions, the kinematics are predicted. The ultimate goal, once a valid model has been developed, is to ask the question, “What would happen if?” Only through such modeling are we able to make predictions that are impossible to create in vivo in a human experiment. The influence of abnormal motor patterns can be predicted, and the door is now open to determine optimal motor patterns. Although synthesis has a great potential payoff, the usefulness of such models to date has been very poor and has been limited to very simple movements. The major problem is that the models that have been proposed are not very valid; they lack the correct anthropometrics and degrees of freedom to make their predictions very useful. However, because of its potential payoff, it is important that students have an introduction to the process, in the hope that useful models will evolve as a result of what we learn from our minor successes and major mistakes.
Most biomechanical modeling involves the use of inverse solutions to predict variables such as reaction forces, moments, mechanical energy, and power that cannot be measured directly. The inverse of this analysis, called synthesis, assumes a similar biomechanical model and uses the assumed moments (or muscle forces) as forcing functions to predict kinematics. Once a valid model is built, the ultimate goal is to ask the question: "What would happen if...?" Only through this modeling approach will we be able to make predictions that are impossible to create in human experiments. It is possible to predict the effects of abnormal movement patterns and begin to determine the best movement patterns. Despite the huge potential rewards of synthesis, the practicality of such models has so far been very poor, limited to very simple motions. The main problem lies in the lack of validity of the proposed models, lacking correct anthropometric data and degrees of freedom, making their predictions not very useful. However, because of the importance of its potential rewards, it is important to give students some insight into the process, and hopefully what we learn from both small successes and major mistakes will lead to the development of useful models.
1.3.8 Biomechanical Motor Synergies
With the increased technology, biomechanics has made great strides in analyzing more complex total body movements and, because of the considerable interactions between adjacent muscle groups, it is becoming necessary to identify motor synergies. In a new chapter, we use several techniques to identify two or more muscle groups acting synergistically toward a common goal.
Biomechanics has made significant advances in the analysis of complex whole-body motion as technology has advanced. Because of the complex interactions between adjacent muscle groups, it is necessary to identify motor synergy. In the new chapter, we use a variety of techniques to identify situations where two or more muscle groups work together to achieve a common goal. This chapter focuses on understanding and describing the coordinated action of multiple muscle groups in complex movements.