qxd 9/11/08 16:04 Page 215 Chapter 14 Biomechanics of Human Movement and Muscle-Tendon Function VASILIOS BALTZOPOULOS AND CONSTANTINOS N. MAGANARIS Biomechanics (derived from the Greek words β;ος/ future developments in equipment and techniques veos for life-living and μηχανικ:/mehaniki for mech- necessary to overcome existing measurement and anics) is the scientific discipline for the study of the modeling limitations. These will allow easier develop- mechanics of the structure and function of living ment and more widespread application of subject- biological systems. In the human biological system specific models in order to improve the contribution the application of the principles and methods of of biomechanics research and support services to mechanics, and in particular the study of forces and performance enhancement and injury prevention. their effects, has led to a significant advancement in our knowledge and understanding of human move- Biomechanical analysis of human ment in a whole spectrum of activities ranging from movement pathologic conditions to elite sport actions. The main aims of Biomechanics in the context of Performance in all locomotory activities, including sports activities are: sports, depends on a number of factors related to the 1 To increase knowledge and understanding of function and control of all the systems in the human the structure and function of the human muscu- body. Biomechanics is only one of the scientific loskeletal system; disciplines, in addition to physiology, biochemistry, 2 To prevent injuries and improve rehabilitation neuroscience, psychology etc., that contribute to the techniques by examining the loading of specific understanding and enhancement of performance structures in the human body during activity and and the prevention of overloading and injuries. their response; and Given the multi-factorial nature of human perform- 3 To enhance sports performance by analysing and ance, the contribution of biomechanics is crucial optimising technique. and is achieved using a combination of qualitative This chapter will examine recent developments in the (Knudson & Morrison 1997) and quantitative (Payton above areas. In particular, given that the generation & Bartlett 2008) experimental approaches, as well as of movement per se and investigations into technique theoretical approaches based on mathematical model- improvement or reduction in loading and preven- ing and computer simulation (Yeadon & King 2008). tion of injuries depend primarily on the mechanics Qualitative approaches have been developed in and control of muscles and joints, special emphasis recent years and the processes involved in conduct- will be placed on issues relating to muscle-tendon ing an effective qualitative biomechanical analysis and joint function. The chapter will also consider have been documented and described in great detail (Knudson & Morrison 1997). However, this Olympic Textbook of Science in Sport. Edited by Ronald approach essentially involves observation and sub- J. Maughan © 2009 International Olympic Committee. jective interpretation of the movement based on cer- ISBN: 978-1-405-15638-7 tain principles before any intervention.qxd 9/11/08 16:04 Page 216 216 c h a p ter 14 will concentrate on quantitative and theoretical shortcomings, especially with the advent of sophist- approaches. icated and versatile measurement, data collection, It is generally agreed (e., Lees 1999, 2002; Bahr & and analytical techniques. Krosshaug 2005; Elliott 2006) that biomechanical research and scientific support services, whether Experimental approaches for the prevention of injuries or the improvement of technique to enhance performance, should fol- Descriptive biomechanical analyses are usually low a sequence of important steps to ensure that based on the measurement of temporal (phase), any interventions are appropriate and that the out- kinematic, kinetic, or kinesiological/anatomical fea- come is evaluated and contributes to evidence-based tures of movement using the corresponding ex- practice: perimental techniques (Bartlett 1999). Although a 1 Analysis of the specific problem to establish the descriptive analysis of movement may provide a relevant context (technique and wider performance useful starting point, it is important to understand factors or the extent and epidemiologic evidence of the underlying mechanisms of coordination and the injury); control of movement, or the mechanisms of injuries. 2 Establishment of the key techniques, variables, The determination of key technique variables related desired characteristics, faults, coordination mech- to movement control and coordination mechan- anisms, or the mechanisms of injury and risk factors isms, or the risk factors and the manner in which through observational, experimental, or theoretical they are implicated in the mechanism of injury, is a approaches; very important step in any investigation, and in 3 Design and implementation of an intervention; quantitative approaches these variables or factors and are determined based on different methods that 4 Evaluation of the intervention for improving per- can be classified in general (e., Bartlett 1999; Lees formance or reducing injuries. 2002; Bahr & Krosshaug 2005) under the following The multi-factorial and multi-disciplinary nature headings: of sports performance and sports injuries means 1 Biomechanical principles of movement; that it is very difficult to control all of the implicated 2 Hierarchical relationship (deterministic) diagrams; factors and to study only one or a few in isolation, and given their complex interactions. This is also one 3 Statistical relationships. of the main reasons for the lack of well-controlled Biomechanical principles of movement are formu- intervention and prospective evaluation studies or lated by applying some of the fundamental mech- randomized control trials, especially in quantitative anical relations to the structural and functional approaches. Furthermore, the design and imple- characteristics of the neuromuscular system and mentation of an intervention necessitates collabora- to segmental motion and coordination. Although tion with coaches or clinicians and other personnel. there is a general disagreement about the exact This highlights the need for effective communica- number, categorization, and even the definition and tion with other professionals involved in athlete description of these principles (Bartlett 1999; Lees training or rehabilitation, and is another reason for 2002), some of the more widely-accepted principles, the lack of interventional and evaluation studies. such as the stretch-shortening cycle (SSC), the Although biomechanics has had a tremendous proximal to distal sequence of segmental action, impact in sports, the difficulties of outcome inter- and mechanical energy considerations have had a vention and well-controlled evaluation studies has major impact on our understanding of the mech- lead to there being only a small evidence base anisms of control and coordination during move- for biomechanical support and injury prevention ment and injuries. interventions and some unfounded criticisms for The SSC is explained in detail in Chapter 1, and the contribution and influence of biomechanics. it is important to emphasize that the main mechan- It is important that future work addresses these ism is based on the interaction between the muscle 9781405156387_4_014.qxd 9/11/08 16:04 Page 217 biomechanics of human movement 217 fascicles and the tendon in a muscle-tendon unit. velocity generation, but also underline the import- During the preceding stretch or eccentric action ant contribution of the SSC potentiation in muscles, phase, the muscle is activated so that elastic energy which contributes to segment longitudinal rotation is stored in the tendon and is then released during and the interaction between the different biomech- the subsequent shortening phase, thus increasing anical principles. the muscle force output (potentiation) above the Movement control and coordination analysis level predicted by the isolated concentric force- based on nonlinear dynamics and dynamical sys- velocity relationship alone, hence enhancing power tems approaches and methodologies is one of the production. In this way force production and timing more recently emerging principles used to investig- in locomotory or throwing movements of short ate the higher-order dynamics of movement (e., duration are optimized. However, the storage and Hamill et al. 1999; Bartlett et al. 2007) and to establish utilization of elastic energy and the contribution of the importance of variability for human movement the stretch reflex to the potentiation of force depend and for the understanding of coordination and injury on the muscles involved and their function (e. However, important questions, such mono- or bi-articular), the intensity and the type as whether the complex variables used are the result of task or movement (e., the duration and optimal or the cause of the injury, or whether they can be coupling between eccentric and concentric actions, used for designing specific intervention measures to or the contact phase) as they will influence fascicle- prevent the injury, are still unanswered; hence fur- tendon interactions. It is therefore important to note ther work, including well-controlled prospective epi- that even universally accepted and well-defined demiological and intervention studies, is required. biomechanical principles of movement require care- Mechanical energy and work principles are vital ful consideration when applied to different sports when examining the effects of not only the function or activities. This is particularly relevant in jumping of muscle-tendon units but also sports equipment in and throwing/hitting activities where the coupling particular, because energy availability determines (timing) between the stretch and shortening phases the ability to do work and increase performance, is crucial. In tennis, for example, the importance of a so the optimization of the energy transfer between fast transition from the backswing to the forward athlete and equipment is crucial. This can be achieved swing of the racket or from knee flexion to extension by minimizing the energy lost, maximizing the energy during the serve is now clearly recognized (Elliott returned, and optimizing the output of the musculo- 2006). skeletal system (Nigg et al. Although any use- The proximal to distal sequence of segmental ful energy return is controversial given that it relies action has been widely accepted in throwing and on certain conditions about the amount (if any), ballistic activities in general where the maximiza- timing, location, and frequency of the energy return tion of the endpoint velocity is the main aim, but it (Stefanyshyn & Nigg 2000), the optimization of was originally developed for movement constrained muscle force and power output by operating the mainly in two dimensions. According to this prin- muscle-tendon complexes at optimum length and ciple, the movement of each distal segment starts velocity conditions is an important determinant of when the velocity of its proximal segment is near increased performance (e. However, more recent studies have Diagrammatic deterministic or conceptual models shown that this sequence is not followed in many describe the hierarchical relationships of the various throwing or hitting activities of three-dimensional layers of factors that affect performance on the basis nature where significant internal or external rota- of temporal or mechanical principles (see Hay & tions of segments around their longitudinal axis are Read 1982; Hay 1993). Assuming that certain criteria involved and contribute significantly to the end are satisfied when developing the model, these hier- point or implement velocity (e., Marshall & Elliott archical relationships can then be useful in identify- 2000). These important rotations for the potentiation ing important variables for biomechanical analysis, of muscle forces not only play an important role in or they can form the basis of statistical models of 9781405156387_4_014.qxd 9/11/08 16:04 Page 218 218 c h a p ter 14 performance (Bartlett 1999). In injury prevention Modeling and computer simulation developments applications, the identification of risk factors and in human movement biomechanics have paralleled mechanisms of injury is based on similar diagram- the technological development of computers and their matic models. These models describe the conceptual processing power in the last few decades (Vaughan interaction of intrinsic and extrinsic risk factors 1984), and there are now several dedicated computer in causing an injury and acting through a specific software packages that allow mathematical model- mechanism that is suggested to include information ing and simulation of human movement. However, on aspects of the inciting event at different levels, despite predictions of widespread use, the number which can be classified into one of four categories: of studies using computer simulation is still limited playing situation, athlete/opponent behavior, whole because of the difficulties in modeling the human body biomechanics, and joint tissue biomechanics body accurately, thus limiting realistic applications, (Bahr & Krosshaug 2005). except in certain types of activities such aerial move- The type and range of variables and factors ments and throwing events (see Yeadon & King resulting from the above approaches require instru- 2008), and some clinical applications (e.
