Ebook The Olympic Textbook of Science in Sport: Part 2

Chapter 14 Biomechanics of Human Movement and Muscle-Tendon Function VASILIOS BALTZOPOULOS AND CONSTANTINOS N. MAGANARIS Biomechanics (derived from the Greek words b;oV/ veos for life-living and mhcanik:/mehaniki for mech-anics) is the scientific discipline for the study of the mechanics of the structure and function of living biological systems. In the human biological system the application of the principles and methods of mechanics, and in particular the study of forces and their effects, has led to a significant advancement in our knowledge and understanding of human move-ment in a whole spectrum of activities ranging from pathologic conditions to elite sport actions. The main aims of Biomechanics in the context of sports activities are: 1 To increase knowledge and understanding of the structure and function of the human muscu-loskeletal system; 2 To prevent injuries and improve rehabilitation techniques by examining the loading of specific structures in the human body during activity and their response; and 3 To enhance sports performance by analysing and optimising technique. This chapter will examine recent developments in the above areas. In particular, given that the generation of movement per se and investigations into technique improvement or reduction in loading and preven-tion of injuries depend primarily on the mechanics and control of muscles and joints, special emphasis will be placed on issues relating to muscle-tendon and joint function. The chapter will also consider Olympic Textbook of Science in Sport. Edited by Ronald J. Maughan © 2009 International Olympic Committee. ISBN: 978-1-405-15638-7 future developments in equipment and techniques necessary to overcome existing measurement and modeling limitations. These will allow easier develop-ment and more widespread application of subject-specific models in order to improve the contribution of biomechanics research and support services to performance enhancement and injury prevention. Biomechanical analysis of human movement Performance in all locomotory activities, including sports, depends on a number of factors related to the function and control of all the systems in the human body. Biomechanics is only one of the scientific disciplines, in addition to physiology, biochemistry, neuroscience, psychology etc., that contribute to the understanding and enhancement of performance and the prevention of overloading and injuries. Given the multi-factorial nature of human perform-ance, the contribution of biomechanics is crucial and is achieved using a combination of qualitative (Knudson & Morrison 1997) and quantitative (Payton & Bartlett 2008) experimental approaches, as well as theoretical approaches based on mathematical model-ing and computer simulation (Yeadon & King 2008). Qualitative approaches have been developed in recent years and the processes involved in conduct-ing an effective qualitative biomechanical analysis have been documented and described in great detail (Knudson & Morrison 1997). However, this approach essentially involves observation and sub-jective interpretation of the movement based on cer-tain principles before any intervention. This chapter 215 216 chapter 14 will concentrate on quantitative and theoretical approaches. It is generally agreed (e.g., Lees 1999, 2002; Bahr & Krosshaug 2005; Elliott 2006) that biomechanical research and scientific support services, whether for the prevention of injuries or the improvement of technique to enhance performance, should fol-low a sequence of important steps to ensure that any interventions are appropriate and that the out-come is evaluated and contributes to evidence-based practice: 1 Analysis of the specific problem to establish the relevant context (technique and wider performance factors or the extent and epidemiologic evidence of the injury); 2 Establishment of the key techniques, variables, desired characteristics, faults, coordination mech-anisms, or the mechanisms of injury and risk factors through observational, experimental, or theoretical approaches; 3 Design and implementation of an intervention; and 4 Evaluation of the intervention for improving per-formance or reducing injuries. The multi-factorial and multi-disciplinary nature of sports performance and sports injuries means that it is very difficult to control all of the implicated factors and to study only one or a few in isolation, given their complex interactions. This is also one of the main reasons for the lack of well-controlled intervention and prospective evaluation studies or randomized control trials, especially in quantitative approaches. Furthermore, the design and imple-mentation of an intervention necessitates collabora-tion with coaches or clinicians and other personnel. This highlights the need for effective communica-tion with other professionals involved in athlete training or rehabilitation, and is another reason for the lack of interventional and evaluation studies. Although biomechanics has had a tremendous impact in sports, the difficulties of outcome inter-vention and well-controlled evaluation studies has lead to there being only a small evidence base for biomechanical support and injury prevention interventions and some unfounded criticisms for the contribution and influence of biomechanics. It is important that future work addresses these shortcomings, especially with the advent of sophist-icated and versatile measurement, data collection, and analytical techniques. Experimental approaches Descriptive biomechanical analyses are usually based on the measurement of temporal (phase), kinematic, kinetic, or kinesiological/anatomical fea-tures of movement using the corresponding ex-perimental techniques (Bartlett 1999). Although a descriptive analysis of movement may provide a useful starting point, it is important to understand the underlying mechanisms of coordination and control of movement, or the mechanisms of injuries. The determination of key technique variables related to movement control and coordination mechan-isms, or the risk factors and the manner in which they are implicated in the mechanism of injury, is a very important step in any investigation, and in quantitative approaches these variables or factors are determined based on different methods that can be classified in general (e.g., Bartlett 1999; Lees 2002; Bahr & Krosshaug 2005) under the following headings: 1 Biomechanical principles of movement; 2 Hierarchical relationship (deterministic) diagrams; and 3 Statistical relationships. Biomechanical principles of movement are formu-lated by applying some of the fundamental mech-anical relations to the structural and functional characteristics of the neuromuscular system and to segmental motion and coordination. Although there is a general disagreement about the exact number, categorization, and even the definition and description of these principles (Bartlett 1999; Lees 2002), some of the more widely-accepted principles, such as the stretch-shortening cycle (SSC), the proximal to distal sequence of segmental action, and mechanical energy considerations have had a major impact on our understanding of the mech-anisms of control and coordination during move-ment and injuries. The SSC is explained in detail in Chapter 1, and it is important to emphasize that the main mechan-ism is based on the interaction between the muscle biomechanics of human movement 217 fascicles and the tendon in a muscle-tendon unit. During the preceding stretch or eccentric action phase, the muscle is activated so that elastic energy is stored in the tendon and is then released during the subsequent shortening phase, thus increasing the muscle force output (potentiation) above the level predicted by the isolated concentric force-velocity relationship alone, hence enhancing power production. In this way force production and timing in locomotory or throwing movements of short duration are optimized. However, the storage and utilization of elastic energy and the contribution of the stretch reflex to the potentiation of force depend on the muscles involved and their function (e.g., mono- or bi-articular), the intensity and the type of task or movement (e.g., the duration and optimal coupling between eccentric and concentric actions, or the contact phase) as they will influence fascicle-tendon interactions. It is therefore important to note that even universally accepted and well-defined biomechanical principles of movement require care-ful consideration when applied to different sports or activities. This is particularly relevant in jumping and throwing/hitting activities where the coupling (timing) between the stretch and shortening phases is crucial. In tennis, for example, the importance of a fast transition from the backswing to the forward swing of the racket or from knee flexion to extension during the serve is now clearly recognized (Elliott 2006). The proximal to distal sequence of segmental action has been widely accepted in throwing and ballistic activities in general where the maximiza-tion of the endpoint velocity is the main aim, but it was originally developed for movement constrained mainly in two dimensions. According to this prin-ciple, the movement of each distal segment starts when the velocity of its proximal segment is near maximum. However, more recent studies have shown that this sequence is not followed in many throwing or hitting activities of three-dimensional nature where significant internal or external rota-tions of segments around their longitudinal axis are involved and contribute significantly to the end point or implement velocity (e.g., Marshall & Elliott 2000). These important rotations for the potentiation of muscle forces not only play an important role in velocity generation, but also underline the import-ant contribution of the SSC potentiation in muscles, which contributes to segment longitudinal rotation and the interaction between the different biomech-anical principles. Movement control and coordination analysis based on nonlinear dynamics and dynamical sys-tems approaches and methodologies is one of the more recently emerging principles used to investig-ate the higher-order dynamics of movement (e.g., Hamill et al. 1999; Bartlett et al. 2007) and to establish the importance of variability for human movement and for the understanding of coordination and injury mechanisms. However, important questions, such as whether the complex variables used are the result or the cause of the injury, or whether they can be used for designing specific intervention measures to prevent the injury, are still unanswered; hence fur-ther work, including well-controlled prospective epi-demiological and intervention studies, is required. Mechanical energy and work principles are vital when examining the effects of not only the function of muscle-tendon units but also sports equipment in particular, because energy availability determines the ability to do work and increase performance, so the optimization of the energy transfer between athlete and equipment is crucial. This can be achieved by minimizing the energy lost, maximizing the energy returned, and optimizing the output of the musculo-skeletal system (Nigg et al. 2000). Although any use-ful energy return is controversial given that it relies on certain conditions about the amount (if any), timing, location, and frequency of the energy return (Stefanyshyn & Nigg 2000), the optimization of muscle force and power output by operating the muscle-tendon complexes at optimum length and velocity conditions is an important determinant of increased performance (e.g., Herzog 1996). Diagrammatic deterministic or conceptual models describe the hierarchical relationships of the various layers of factors that affect performance on the basis of temporal or mechanical principles (see Hay & Read 1982; Hay 1993). Assuming that certain criteria are satisfied when developing the model, these hier-archical relationships can then be useful in identify-ing important variables for biomechanical analysis, or they can form the basis of statistical models of 218 chapter 14 performance (Bartlett 1999). In injury prevention applications, the identification of risk factors and mechanisms of injury is based on similar diagram-matic models. These models describe the conceptual interaction of intrinsic and extrinsic risk factors in causing an injury and acting through a specific mechanism that is suggested to include information on aspects of the inciting event at different levels, which can be classified into one of four categories: playing situation, athlete/opponent behavior, whole body biomechanics, and joint tissue biomechanics (Bahr & Krosshaug 2005). The type and range of variables and factors resulting from the above approaches require instru-mentation and techniques that can accurately meas-ure a wide range of parameters. Such techniques include video and optoelectronic systems for kine-matic (position, velocity, acceleration etc) parameters, force plates and pressure sensors for kinetic infor-mation, electromyographic (EMG) systems for the assessment of muscle activity (see Payton & Bartlett 2008), and ultrasound systems for the imaging of muscle fascicles and tendon function (Maganaris 2003). The biomechanical study of sports injuries requires additional techniques that include clinical investigations based on medical imaging (compu-ted tomography (CT), magnetic resonance imaging (MRI), X-ray videofluoroscopy, arthroscopy etc.) and cadaveric studies (Krosshaug et al. 2005). Mathematical modeling, computer simulation, and optimization A theoretical approach is usually based on a sim-plified model of the essential aspects of the human body and can overcome some of the problems described above for experimental approaches. Math-ematical modeling in sports biomechanics, e.g., prediction of jump distance or height (Hatze 1981; Alexander 2003), is a powerful research tool because it can simulate effects that are impossible to study experimentally in a systematic way, thus allow-ing us to understand which parameters are more important for improving athletic performance. This enables appropriate strategies to be adopted for executing the sporting task, and guides the design of training programs. Modeling and computer simulation developments in human movement biomechanics have paralleled the technological development of computers and their processing power in the last few decades (Vaughan 1984), and there are now several dedicated computer software packages that allow mathematical model-ing and simulation of human movement. However, despite predictions of widespread use, the number of studies using computer simulation is still limited because of the difficulties in modeling the human body accurately, thus limiting realistic applications, except in certain types of activities such aerial move-ments and throwing events (see Yeadon & King 2008), and some clinical applications (e.g., Neptune 2000; McLean et al. 2003, 2004). The models used range in complexity from single-point mass models of the athlete or the throwing implement, to rigid body models of the whole body, a single segment, or a series of linked segments, to very detailed models of the musculoskeletal system including all the essential elements of its structure and function (Blemker et al. 2007; Delp et al. 2007). Given the complexity of the human body, all models are a simplification of the real structure and function of the modeled parts. The degree of simplification depends not only on the existing knowledge of the properties and function of the elements in the model, but also on the question to be answered. For example, in aerial sports movements rigid body models connected with pin joints are adequate for most questions, but in a model to study the loading in the knee joint during landings, a detailed model of the anatomical function of the patellar tendon is necessary as part of the knee joint kinematics modeling, including moment arms and geometrical data to allow accurate estimation of knee joint reaction forces and loading (e.g., Krosshaug et al. 2005). One of the other main problems in mod-eling and simulation is the development of models that are tailored to an individual athlete because of the difficulty in obtaining subject-specific data on the structure and properties of the modeled segments, joints, and muscle-tendon units (Yeadon & King 2008). These problems are further compounded by the difficulties of accurately measuring the joint mo-ment under different segment configuration and velo-city conditions (e.g., Baltzopoulos 2008). In inverse biomechanics of human movement 219 dynamics approaches specifically, the distribution of the calculated joint moment to the contributing muscles for the estimation of muscle-tendon forces and loading has been one of the fundamental prob-lems of biomechanics research. Various optimization techniques have been applied in the past (for a review see Tsirakos et al. 1997), and more recent techniques show particularly promising results (Erdemir et al. 2007). However, these all rely on accurate subject specific information about muscle properties and moment arms which are either difficult or not poss-ible to obtain in vivo. Activation criteria as opposed to optimization criteria for muscle force distribution have been proposed as the only way forward for this problem (Epstein & Herzog 2003). Human movement and the mechanics of muscle-tendon and joint function Human movement is the result of joint segment rotations generated by moments acting around the axes of rotations of joints. These moments result from muscle forces that are transmitted via tendons to the bones and in this way create rotation of the segment. Muscle force depends on the length, velocity, activation level, and previous activation state of the muscle (see Chapter 1 for further infor-mation). The function of the muscle in series with the tendon has important implications for their func-tion because the mechanical properties of the tendon, in particular its viscoelastic, time-dependent prop-erties, will affect the muscle length and velocity, and hence its force output. It is therefore clear that any attempt to optimize or change joint motion sequence (technique modification) will depend on the mech-anical properties of muscle and tendon and their interaction during the particular activity. For this reason it is important to consider the architectural and mechanical properties of muscles, the mech-anics of tendon function and force transmission and their interactions in order to understand the implications for human movement. Muscle architecture The term “muscle architecture” refers to the spatial arrangement of muscle fibers with respect to the Muscle fiber a Muscle belly b Distal tendon c Proximal end d Fig. 14.1 The main muscle architectures. (a) Longitudinal muscle. (b, c) Unipennate muscles of different pennation angles. (d) Bipennate muscle. axis of force generation in the muscle-tendon unit. Skeletal muscles may be categorized under two main types of architectural design – parallel-fibered and pennate. In parallel-fibered muscles, the muscle fibers run parallel to the action line of the muscle-tendon unit, spanning the entire length of the muscle belly (Fig. 14.1a). On the other hand, muscles with fibers arranged at an angle to the muscle-tendon action line are classified as pennate muscles. This specific angle is referred to as the pennation angle and necessitates that the fibers extend to only a part of the whole muscle belly length. If all of the fibers attach to the tendon plate at a given pennation angle, the muscle is termed unipennate (Figs. 14.1b,c). Multipennate structures arise when the muscle fibers run at several pennation angles within the muscle, or when there are several distinct intra-muscular parts with different pennation angles (Fig. 14.1d). Out of approximately 650 muscles in the human body, most have pennate architectures with resting pennation angles up to ∼30°(Wickiewicz et al. 1983; Friederich & Brand 1990). From the above definitions and illustrations it soon becomes apparent that pennation angle affects muscle fiber length; i.e., for a given muscle volume or area (if volume is simplified by projecting the muscle in the sagittal plane), the larger the penna-tion angle the shorter the muscle fiber length relat-ive to the whole muscle belly length. Since muscle fiber length is determined by the number of serial sarcomeres in the muscle fiber, the above rela-tionship means that increasing pennation angle penalizes the speed of muscle fiber shortening and the excursion range of fibers. However, pennation ... - tailieumienphi.vn