Joint Structure and Function (5th Edition) PDF

Disclaimer: This eBook does not include ancillary media that was packaged with the printed version of the book. 2362_FM_i-xxii.qxd 2/3/11 4:24 PM Page i Joint Structure and Function FIF TH...

Disclaimer: This eBook does not include ancillary media that was packaged with the printed version of the book. 2362_FM_i-xxii.qxd 2/3/11 4:24 PM Page i Joint Structure and Function FIF TH EDITION A Comprehensive Analysis 2362_FM_i-xxii.qxd 2/3/11 4:24 PM Page iii Joint Structure and Function FIF TH EDITION A Comprehensive Analysis Pamela K. Levangie, PT, DSc, FAPTA Professor and Associate Chairperson Department of Physical Therapy MGH Institute of Health Professions Boston, Massachusetts Cynthia C. Norkin, PT, EdD Former Director and Associate Professor School of Physical Therapy Ohio University Athens, Ohio 2362_FM_i-xxii.qxd 2/3/11 4:24 PM Page iv F. A. Davis Company 1915 Arch Street Philadelphia, PA 19103 www.fadavis.com Copyright © 2011 by F. A. Davis Company Copyright © 2011 by F. A. Davis Company. All rights reserved. This product is protected by copyright. No part of it may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without written permission from the publisher. Printed in the United States of America Last digit indicates print number: 10 9 8 7 6 5 4 3 2 1 Acquisitions Editor: Melissa Duffield Manager of Content Development: George W. Lang Developmental Editor: Karen Carter Art and Design Manager: Carolyn O’Brien As new scientific information becomes available through basic and clinical research, recommended treatments and drug therapies undergo changes. The author(s) and publisher have done everything possible to make this book accurate, up to date, and in accord with accepted standards at the time of publication. The author(s), editors, and publisher are not responsible for errors or omissions or for consequences from application of the book, and make no warranty, expressed or implied, in regard to the contents of the book. Any practice described in this book should be applied by the reader in accordance with professional standards of care used in regard to the unique circumstances that may apply in each situation. The reader is advised always to check product information (package inserts) for changes and new information regarding dose and contraindications before administering any drug. Caution is especially urged when using new or infrequently ordered drugs. Library of Congress Cataloging-in-Publication Data Joint structure and function : a comprehensive analysis / [edited by ] Pamela K. Levangie, Cynthia C. Norkin.—5th ed. p. ; cm. Rev. ed. of : Joint structure and function / Pamela K. Levangie, Cynthia C. Norkin. 4th ed. c2005. Includes bibliographical references and index. ISBN-13: 978-0-8036-2362-0 ISBN-10: 0-8036-2362-3 1. Human mechanics. 2. Joints. I. Levangie, Pamela K. II. Norkin, Cynthia C. III. Levangie, Pamela K. Joint Structure and function. [DNLM: 1. Joints—anatomy & histology. 2. Joints—physiology. WE 300] QP303.N59 2011 612.7’5—dc22 2010033921 Authorization to photocopy items for internal or personal use, or the internal or personal use of specific clients, is granted by F. A. Davis Company for users registered with the Copyright Clearance Center (CCC) Transactional Reporting Service, provided that the fee of $.25 per copy is paid directly to CCC, 222 Rosewood Drive, Danvers, MA 01923. For those organizations that have been granted a photocopy license by CCC, a separate system of payment has been arranged. The fee code for users of the Transactional Reporting Service is: 8036–2362/11 0 + $.25. 2362_FM_i-xxii.qxd 2/3/11 4:24 PM Page v PREFACE TO THE FIFTH EDITION With the fifth edition of Joint Structure and Function, we that provides a strong foundation in the principles that un- maintain a tradition of excellence in education that began derlie an understanding of human structure and function more than 25 years ago. We continue to respond to the while also being readable and as concise as possible. We dynamic environment of publishing, as well as to changes hope that our years of experience in contributing to the taking place in media, research technology, and in the educa- education of health-care professionals allow us to strike a tion of individuals who assess human function. We include unique balance. We cannot fail to recognize the increased use of two- and four-color line drawings, enhanced instruc- educational demands placed on many entry-level health- tor’s tools, and new videos that all support and enhance the care professionals and hope that the updates to the fifth reader’s experience. edition help students meet that demand. However, Joint Our contributors are chosen for their expertise in the areas Structure and Function, while growing with its readers, con- of research, practice, and teaching—grounding their chapters tinues to recognize that the new reader requires elementary in best and current evidence and in clinical relevance. Patient and interlinked building blocks that lay a strong but flexible cases (in both “Patient Case” and “Patient Application” boxes) foundation to best support continued learning and growth facilitate an understanding of the continuum between normal in a complex and changing world. and impaired function, making use of emerging case-based We very much appreciate our opportunity to contribute and problem-based learning educational strategies. “Concept to health care by assisting in the professional development Cornerstones” and “Continuing Exploration” boxes provide of the students and practitioners who are our readers. the reader or the instructor additional flexibility in setting learning objectives. What remains unchanged in this edition of Joint Struc- PAMELA K. LEVANGIE ture and Function is our commitment to maintaining a text CYNTHIA C. NORKIN v 2362_FM_i-xxii.qxd 2/3/11 4:24 PM Page vi 2362_FM_i-xxii.qxd 2/3/11 4:24 PM Page vii ACKNOWLEDGMENTS The fifth edition of Joint Structure and Function is made We extend our continuing gratitude to F. A. Davis for possible only by the continued and combined efforts of their investment in the future of Joint Structure and Function many people and groups. We are, first and foremost, and its ancillary materials. Particular thanks go to Margaret grateful for the time, effort, and expertise of our esteemed Biblis (Publisher), Melissa Duffield (Acquisitions Editor), contributors with whom it has been a pleasure to work. Karen Carter (Developmental Editor), Yvonne Gillam Our thanks, therefore, to Drs. Sam Ward, Sandra Curwin, (Developmental Editor), George Lang (Manager of Content Gary Chleboun, Diane Dalton, Julie Starr, Pam Ritzline, Development), David Orzechowski (Managing Editor), Paula Ludewig, John Borstad, RobRoy Martin, Lynn Robert Butler (Production Manager), Carolyn O’Brien Snyder-Mackler, Michael Lewek, Erin Hartigan, Janice (Manager of Art and Design), Katherine Margeson (Illustra- Eng, and Sandra Olney, as well as to Ms. Noelle Austin tion Coordinator), and Stephanie Rukowicz (Assistant De- and Mr. Benjamin Kivlan. Additionally, we want to express velopmental Editor) who provided great support. As always our appreciation to the individuals who helped develop the we must thank the artists who, through the years, provided ancillary materials that support the fifth edition, including the images that are so valuable to the readers. These include the Instructor’s Resources developed by Ms. Christine artists of past editions, Joe Farnum, Timothy Malone, and Conroy and the videos developed by Dr. Lee Marinko and Anne Raines. New to the fifth edition is Dartmouth Publish- Center City Film & Video. We would also like to acknowl- ing, Inc., adding both new figures and enhanced color to edge and thank the individuals who contributed their the text. comments and suggestions as reviewers (listed on page xi), Finally, we acknowledge and thank our colleagues and as well as those who passed along their unsolicited sugges- families, without whose support this work could not have tions through the years, including our students. been done and to whom we are eternally indebted. vii 2362_FM_i-xxii.qxd 2/3/11 4:24 PM Page viii 2362_FM_i-xxii.qxd 2/3/11 4:24 PM Page ix CONTRIBUTORS Noelle M. Austin, PT, MS, CHT Michael Lewek, PT, PhD CJ Education and Consulting, LLC Assistant Professor Woodbridge, Connecticut Division of Physical Therapy www.cj-education.com University of North Carolina at Chapel Hill The Orthopaedic Group Chapel Hill, North Carolina Hamden, Connecticut Paula M. Ludewig, PT, PhD John D. Borstad, PT, PhD Associate Professor Assistant Professor Program in Physical Therapy Physical Therapy Division University of Minnesota Ohio State University Minneapolis, Minnesota Columbus, Ohio RobRoy L. Martin, PT, PhD, CSCS Gary Chleboun, PT, PhD Associate Professor Professor Duquesne University School of Physical Therapy Pittsburgh, Pennsylvania Ohio University Athens, Ohio Sandra J. Olney, PT, OT, PhD Professor Emeritus Sandra Curwin, PT, PhD School of Rehabilitation Therapy Associate Professor Queens University School of Physiotherapy Kingston, Ontario, Canada Dalhousie University Halifax, Nova Scotia, Canada Pamela Ritzline, PT, EdD Associate Professor Diane Dalton, PT, DPT, OCS Department of Physical Therapy Clinical Assistant Professor University of Tennessee Health Science Center Physical Therapy Program Memphis, Tennessee Boston University Boston, Massachusetts Lynn Snyder-Mackler, PT, ScD, SCS, ATC, FAPTA Janice J. Eng, PT, OT, PhD Alumni Distinguished Professor Professor Department of Physical Therapy Department of Physical Therapy University of Delaware University of British Columbia Newark, Delaware Vancouver, British Columbia, Canada Julie Ann Starr, PT, DPT, CCS Erin Hartigan, PT, PhD, DPT, OCS, ATC Clinical Associate Professor Assistant Professor Physical Therapy Program Physical Therapy Department Boston University University of New England Boston, Massachusetts Portland, Maine Sam Ward, PT, PhD Benjamin Kivlan, PT, SCS, OCS, CSCS Departments of Radiology, Orthopaedic Surgery, and Doctoral Student Bioengineering Duquesne University University of California San Diego Pittsburgh, Pennsylvania La Jolla, California ix 2362_FM_i-xxii.qxd 2/3/11 4:24 PM Page x 2362_FM_i-xxii.qxd 2/3/11 4:24 PM Page xi REVIEWERS John H. Hollman, PT, PhD Nancy R. Talbott, PhD, MS, PT Director and Assistant Professor, Program in Physical Associate Professor Therapy Rehabilitation Sciences Department of Physical Medicine and Rehabilitation University of Cincinnati Mayo Clinic College of Medicine Cincinnati, Ohio Rochester, Minnesota David P. Village, MS, PT, DHSc Chris Hughes, PT, PhD, OCS, CSCS Associate Professor Professor Department of Physical Therapy Graduate School of Physical Therapy Andrews University Slippery Rock University Berrien Springs, Michigan Slippery Rock, Pennsylvania Krista M. Wolfe, DPT, ATC Leigh K. Murray, PT, PhD Director, Physical Therapy Assistant Program Assistant Professor Allied Health Department Physical Therapy Department Central Pennsylvania College Walsh University Summerdale, Pennsylvania North Canton, Ohio Linda L. Wright, PhD, PT William K. Ogard, PT, PhD Professor, Department of Physical Therapy Assistant Professor Director, Educational Technology Physical Therapy Department College of Health Professions University of Alabama at Birmingham Armstrong Atlantic State University Birmingham, Alabama Savannah, Georgia Suzanne Reese, PT, MS Associate Professor Physical Therapist Assistant Program Tulsa Community College Tulsa, Oklahoma xi 2362_FM_i-xxii.qxd 2/3/11 4:24 PM Page xii 2362_FM_i-xxii.qxd 2/3/11 4:24 PM Page xiii CONTENTS IN BRIEF SECTION 1. Chapter 9. The Wrist and Hand Joint Structure and Function: Complex 305 Foundational Concepts 2 Noelle M. Austin, PT, MS, CHT Chapter 1. Biomechanical Applications SECTION 4. to Joint Structure and Hip Joint 354 Function 3 Chapter 10. The Hip Complex 355 Samuel R. Ward, PT, PhD RobRoy L. Martin, PT, PhD, Chapter 2. Joint Structure and Function 64 CSCS, and Benjamin Kivlan, Sandra Curwin, PT, PhD PT, SCS, OCS, CSCS Chapter 3. Muscle Structure and Chapter 11. The Knee 395 Function 108 Erin Hartigan, PT, PhD, Gary Chleboun, PT, PhD DPT, OCS, ATC; Michael Lewek, PT, PhD; and Lynn SECTION 2. Snyder-Mackler, PT, ScD, Axial Skeletal Joint Complexes 138 SCS, ATC, FAPTA Chapter 4. The Vertebral Column 139 Chapter 12. The Ankle and Foot Diane Dalton, PT, DPT, OCS Complex 440 RobRoy L. Martin, PT, PhD, Chapter 5. The Thorax and Chest Wall 192 CSCS Julie Starr, PT, MS, CCS, and Diane Dalton, PT, DPT, OCS SECTION 5. Chapter 6. The Temporomandibular Integrated Function 482 Joint 212 Pamela D. Ritzline, PT, EdD Chapter 13. Posture 483 SECTION 3. Cynthia C. Norkin, PT, EdD Upper Extremity Joint Chapter 14. Gait 524 Complexes 230 Sandra J. Olney, PT, OT, PhD, and Janice Eng, PT, OT, PhD Chapter 7. The Shoulder Complex 231 Index 569 Paula M. Ludewig, PT, PhD, and John D. Borstad, PT, PhD Chapter 8. The Elbow Complex 271 Cynthia C. Norkin, PT, EdD xiii 2362_FM_i-xxii.qxd 2/3/11 4:24 PM Page xiv 2362_FM_i-xxii.qxd 2/3/11 4:24 PM Page xv CONTENTS SECTION 1. ADDITIONAL LINEAR Joint Structure and Function: FORCE CONSIDERATIONS 23 Foundational Concepts 2 Tensile Forces 23 Chapter 1. Biomechanical Applications Joint Distraction 25 to Joint Structure and Revisiting Newton’s Law of Inertia 29 Function 3 Shear and Friction Forces 30 Samuel R. Ward, PT, PhD Considering Vertical and INTRODUCTION 4 Horizontal Linear Equilibrium 32 PART 1: KINEMATICS AND PART 2: KINETICS—CONSIDERING INTRODUCTION TO KINETICS 6 ROTARY AND TRANSLATORY DESCRIPTIONS OF MOTION 6 FORCES AND MOTIONS 33 Types of Displacement 6 TORQUE, OR MOMENT OF FORCE 33 Location of Displacement in Space 7 Angular Acceleration Direction of Displacement 9 and Angular Equilibrium 34 Magnitude of Displacement 9 Parallel Force Systems 35 Rate of Displacement 10 Meeting the Three Conditions INTRODUCTION TO FORCES 11 for Equilibrium 38 Definition of Forces 11 MUSCLE FORCES 39 Force Vectors 12 Total Muscle Force Vector 39 Force of Gravity 14 TORQUE REVISITED 41 INTRODUCTION TO STATICS Changes to Moment Arm AND DYNAMICS 18 of a Force 42 Newton’s Law of Inertia 18 Angular Acceleration With Changing Torques 43 Newton’s Law of Acceleration 18 Moment Arm and Angle TRANSLATORY MOTION of Application of a Force 44 IN LINEAR AND CONCURRENT FORCE SYSTEMS 19 LEVER SYSTEMS, OR CLASSES OF LEVERS 46 Linear Force Systems 19 Muscles in Third-Class Lever Systems 46 Concurrent Force Systems 21 Muscles in Second-Class Lever Systems 47 Newton’s Law of Reaction 22 Muscles in First-Class Lever Systems 48 xv 2362_FM_i-xxii.qxd 2/3/11 4:24 PM Page xvi xvi Contents Mechanical Advantage 48 GENERAL CHANGES WITH DISEASE, INJURY, Limitations of Analysis of Forces IMMOBILIZATION, by Lever Systems 50 EXERCISE, AND OVERUSE 98 FORCE COMPONENTS 50 Disease 98 Resolving Forces Into Perpendicular Injury 99 and Parallel Components 51 Immobilization (Stress Deprivation) 99 Perpendicular and Parallel Force Effects 51 Exercise 101 Translatory Effects of Force Overuse 102 Components 56 Summary 103 Rotary Effects of Force Components 57 Study Questions 104 References 104 MULTISEGMENT (CLOSED- CHAIN) FORCE ANALYSIS 58 Chapter 3. Muscle Structure Summary 61 and Function 108 Study Questions 61 Gary Chleboun, PT, PhD References 63 INTRODUCTION 109 Chapter 2. Joint Structure and Function 64 ELEMENTS OF MUSCLE Sandra Curwin, PT, PhD STRUCTURE 109 INTRODUCTION 65 Composition of a Muscle Fiber 109 Joint Design 65 The Contractile Unit 110 MATERIALS FOUND IN HUMAN The Motor Unit 112 JOINTS 67 Muscle Structure 114 Structure of Connective Tissue 67 Muscle Architecture: Size, Specific Connective Tissue Structures 73 Arrangement, and Length 115 GENERAL PROPERTIES OF Muscular Connective Tissue 117 CONNECTIVE TISSUE 80 MUSCLE FUNCTION 119 Mechanical Behavior 80 Muscle Tension 119 Viscoelasticity 84 Classification of Muscles 126 Time-Dependent and Factors Affecting Muscle Function 128 Rate-Dependent Properties 84 EFFECTS OF IMMOBILIZATION, Properties of Specific Tissues 85 INJURY, AND AGING 132 COMPLEXITY OF HUMAN Immobilization 132 JOINT DESIGN 87 Injury 133 Synarthroses 88 Aging 133 Diarthroses 89 Summary 133 JOINT FUNCTION 94 Study Questions 134 Kinematic Chains 94 Refereces 134 Joint Motion 95 2362_FM_i-xxii.qxd 2/3/11 4:24 PM Page xvii Contents xvii SECTION 2. Muscles Associated With the Axial Skeletal Joint Rib Cage 199 Complexes 138 Coordination and Integration of Ventilatory Motions 206 Chapter 4. The Vertebral Column 139 Diane Dalton, PT, DPT, OCS DEVELOPMENTAL ASPECTS OF STRUCTURE AND FUNCTION 207 INTRODUCTION 140 Differences Associated GENERAL STRUCTURE With the Neonate 207 AND FUNCTION 140 Differences Associated With Structure 140 the Elderly 207 Function 150 PATHOLOGICAL CHANGES IN REGIONAL STRUCTURE STRUCTURE AND FUNCTION 208 AND FUNCTION 154 Chronic Obstructive Pulmonary Structure of the Cervical Region 154 Disease (COPD) 208 Summary 210 Function of the Cervical Region 159 Study Questions 210 Structure of the Thoracic Region 162 References 210 Function of the Thoracic Region 163 Chapter 6. The Temporomandibular Structure of the Lumbar Region 164 Joint 212 Function of the Lumbar Region 169 Pamela D. Ritzline, PT, EdD Structure of the Sacral Region 171 INTRODUCTION 213 Function of the Sacral Region 174 JOINT STRUCTURE 214 MUSCLES OF THE VERTEBRAL Articular Structures 214 COLUMN 176 Accessory Joint Structures 215 The Craniocervical/Upper Capsule and Ligaments 216 Thoracic Regions 176 JOINT FUNCTION 217 Lower Thoracic/Lumbopelvic Regions 180 Joint Kinematics 217 Muscles of the Pelvic Floor 186 Muscles 220 EFFECTS OF AGING 187 Nerves 222 Age-Related Changes 187 Summary 188 Relationship to the Cervical Study Questions 188 Spine and Posture 222 References 188 Dentition 223 Chapter 5. The Thorax and Chest Wall 192 COMMON IMPAIRMENTS Julie Starr, PT, MS, CCS, and AND PATHOLOGIES 224 Diane Dalton, PT, DPT, OCS Age-Related Changes in the INTRODUCTION 193 TM Joint 224 GENERAL STRUCTURE Inflammatory Conditions 225 AND FUNCTION 193 Capsular Fibrosis 225 Rib Cage 193 2362_FM_i-xxii.qxd 2/3/11 4:24 PM Page xviii xviii Contents Osseous Mobility Conditions 225 Teres Major and Rhomboid Muscle Function 265 Articular Disc Displacement 226 Summary 266 Degenerative Conditions 226 Study Questions 267 Summary 227 References 267 Study Questions 227 References 228 Chapter 8. The Elbow Complex 271 Cynthia C. Norkin, PT, EdD SECTION 3. INTRODUCTION 272 Upper Extremity Joint STRUCTURE: ELBOW JOINT Complexes 230 (HUMEROULNAR AND HUMERORADIAL Chapter 7. The Shoulder Complex 231 ARTICULATIONS) 272 Paula M. Ludewig, PT, PhD, and John D. Borstad, PT, PhD Articulating Surfaces on the Humerus 272 INTRODUCTION 232 Articulating Surfaces on COMPONENTS OF THE the Radius and Ulna 272 SHOULDER COMPLEX 232 Articulation 273 Sternoclavicular Joint 232 Joint Capsule 276 Acromioclavicular Joint 236 Ligaments 277 Scapulothoracic Joint 240 Muscles 279 Glenohumeral Joint 245 FUNCTION: ELBOW JOINT INTEGRATED FUNCTION (HUMEROULNAR AND OF THE SHOULDER COMPLEX 257 HUMERORADIAL Scapulothoracic and Glenohumeral ARTICULATIONS) 281 Contributions 258 Axis of Motion 281 Sternoclavicular and Mobility and Stability 284 Acromioclavicular Contributions 259 Muscle Action 285 Structural Dysfunction 261 STRUCTURE: PROXIMAL MUSCLES OF ELEVATION 262 AND DISTAL ARTICULATIONS 288 Deltoid Muscle Function 262 Proximal (Superior) Radioulnar Joint 288 Supraspinatus Muscle Function 263 Distal (Inferior) Radioulnar Joint 288 Infraspinatus, Teres Minor, Articulations 290 and Subscapularis Muscle Function 263 Ligaments 290 Upper and Lower Trapezius and Serratus Anterior Muscle Muscles 291 Function 263 FUNCTION: RADIOULNAR Rhomboid Muscle Function 265 JOINTS 292 MUSCLES OF DEPRESSION 265 Axis of Motion 292 Latissimus Dorsi and Pectoral Range of Motion 292 Muscle Function 265 2362_FM_i-xxii.qxd 2/3/11 4:24 PM Page xix Contents xix Muscle Action 292 Summary 349 Stability 293 Study Questions 349 References 350 MOBILITY AND STABILITY: ELBOW COMPLEX 295 SECTION 4. Functional Activities 295 Hip Joint 354 Relationship to the Hand and Wrist 295 Chapter 10. The Hip Complex 355 EFFECTS OF AGE, GENDER, RobRoy L. Martin, PT, PhD, AND INJURY 297 CSCS, and Benjamin Kivlan, PT, SCS, OCS, CSCS Age and Gender 297 Injury 298 INTRODUCTION 356 Summary 301 STRUCTURE OF THE HIP JOINT 356 Study Questions 301 Proximal Articular Surface 356 References 301 Distal Articular Surface 358 Chapter 9. The Wrist and Hand Articular Congruence 362 Complex 305 Noelle M. Austin, PT, MS, CHT Hip Joint Capsule and Ligaments 363 INTRODUCTION 306 Structural Adaptations to Weight-Bearing 366 THE WRIST COMPLEX 306 FUNCTION OF THE HIP JOINT 368 Radiocarpal Joint Structure 307 Motion of the Femur on Midcarpal Joint Structure 310 the Acetabulum 368 Function of the Wrist Complex 312 Motion of the Pelvis on the Femur 369 THE HAND COMPLEX 319 Coordinated Motions of the Carpometacarpal Joints of the Fingers 319 Femur, Pelvis, and Lumbar Spine 372 Metacarpophalangeal Joints Hip Joint Musculature 374 of the Fingers 322 HIP JOINT FORCES AND Interphalangeal Joints of the Fingers 324 MUSCLE FUNCTION IN STANCE 379 Extrinsic Finger Flexors 326 Bilateral Stance 379 Extrinsic Finger Extensors 329 Unilateral Stance 380 Extensor Mechanism 330 HIP JOINT PATHOLOGY 385 Intrinsic Finger Musculature 335 Femoroacetabular Impingement 385 Structure of the Thumb 339 Labral Pathology 387 Thumb Musculature 341 Arthrosis 387 PREHENSION 343 Fracture 388 Power Grip 344 Summary 390 Precision Handling 346 Study Questions 390 FUNCTIONAL POSITION References 391 OF THE WRIST AND HAND 348 2362_FM_i-xxii.qxd 2/3/11 4:24 PM Page xx xx Contents Chapter 11. The Knee 395 Chapter 12. The Ankle and Foot Erin Hartigan, PT, PhD, DPT, OCS, Complex 440 ATC; Michael Lewek, PT, PhD; and RobRoy L. Martin PT, PhD, CSCS Lynn Snyder-Mackler, PT, ScD, SCS, INTRODUCTION 441 ATC, FAPTA DEFINITIONS OF MOTIONS 441 INTRODUCTION 396 ANKLE JOINT 443 TIBIOFEMORAL JOINT Ankle Joint Structure 443 STRUCTURE 396 Ankle Joint Function 447 Femur 396 THE SUBTALAR JOINT 448 Tibia 397 Subtalar Joint Structure 448 Tibiofemoral Alignment and Weight-Bearing Forces 398 Subtalar Joint Function 449 Menisci 399 TRANSVERSE TARSAL JOINT 455 Joint Capsule 401 Transverse Tarsal Joint Structure 455 Ligaments 404 Transverse Tarsal Joint Function 458 Iliotibial Band 411 TARSOMETATARSAL JOINTS 460 Bursae 411 Tarsometatarsal Joint Structure 460 TIBIOFEMORAL JOINT Tarsometatarsal Joint Function 462 FUNCTION 412 METATARSOPHALANGEAL Joint Kinematics 412 JOINTS 463 Muscles 417 Metatarsophalangeal Joint Structure 463 Stabilizers of the Knee 422 Metatarsophalangeal Joint PATELLOFEMORAL JOINT 424 Function 464 Patellofemoral Articular INTERPHALANGEAL JOINTS 467 Surfaces and Joint Congruence 425 PLANTAR ARCHES 467 Motions of the Patella 426 Structure of the Arches 467 Patellofemoral Joint Stress 427 Function of the Arches 468 Frontal Plane Patellofemoral Joint Stability 428 Muscular Contribution to the Arches 471 Weight-Bearing Versus MUSCLES OF THE ANKLE Non-Weightbearing Exercises AND FOOT 471 With Patellofemoral Pain 432 Extrinsic Musculature 471 EFFECTS OF INJURY Intrinsic Musculature 474 AND DISEASE 432 DEVIATIONS FROM NORMAL Tibiofemoral Joint Injury 433 STRUCTURE AND FUNCTION 475 Patellofemoral Joint Injury 434 Summary 476 Summary 435 Study Questions 477 Study Questions 435 Referneces 478 References 435 2362_FM_i-xxii.qxd 2/3/11 4:24 PM Page xxi Contents xxi SECTION 5. EFFECTS OF AGE, AGE Integrated Function 482 AND GENDER, PREGNANCY, OCCUPATION, AND Chapter 13. Posture 483 RECREATION ON POSTURE 515 Cynthia C. Norkin, PT, EdD Age 515 INTRODUCTION 484 Age and Gender 516 STATIC AND DYNAMIC Pregnancy 518 POSTURES 484 Occupation and Recreation 518 Postural Control 485 Summary 519 Major Goals and Basic Study Questions 519 Elements of Control 485 References 520 KINETICS AND KINEMATICS Chapter 14. Gait 524 OF POSTURE 489 Sandra J. Olney, PT, OT, PhD, Inertial and Gravitational Forces 489 and Janice Eng, PT, OT, PhD Ground Reaction Forces 489 INTRODUCTION 525 Coincident Action Lines 490 Gait Analysis 525 External and Internal Moments 490 Major Tasks of Gait 525 OPTIMAL POSTURE 491 Phases of the Gait Cycle 525 ANALYSIS OF STANDING GAIT TERMINOLOGY 527 POSTURE: VIEWED FROM Time and Distance Terms 527 THE SIDE 492 Kinematic Terms 528 Alignment and Analysis: Lateral View 492 Kinetic Terms 529 Deviations From Optimal Electromyography 531 Alignment Viewed From the Side 497 CHARACTERISTICS Optimal Alignment and Analysis: OF NORMAL GAIT 532 Anterior and Posterior Views 502 Time and Distance Characteristics 532 Deviations From Optimal Sagittal Plane Joint Angles 532 Alignment 503 Frontal Plane Joint Angles 534 ANALYSIS OF SITTING POSTURES 509 Ground Reaction Force and Center of Pressure 534 Muscle Activity 509 Sagittal Plane Moments 535 Interdiscal Pressures and Compressive Loads on the Spine 511 Frontal Plane Moments 537 Seat Interface Pressures 512 Sagittal Plane Powers 540 ANALYSIS OF LYING Frontal Plane Powers 542 POSTURES 514 Mechanical Energy of Walking 542 Interdiscal Pressures 514 Muscle Activity 545 Surface Interface Pressures 515 Gait Initiation and Termination 550 2362_FM_i-xxii.qxd 2/3/11 4:24 PM Page xxii xxii Contents TRUNK AND UPPER Assistive Devices 561 EXTREMITIES 552 Orthoses 561 Trunk 552 ABNORMAL GAIT 561 Upper Extremities 553 Structural Impairment 562 TREADMILL, STAIR, Functional Impairment 562 AND RUNNING GAITS 553 Summary 564 Treadmill Gait 553 Study Questions 564 Stair Gait 553 References 564 Running Gait 555 Index 569 Summary 558 EFFECTS OF AGE, GENDER, ASSISTIVE DEVICES, AND ORTHOSES 559 Age 559 Gender 560 2362_Ch01-001-063.qxd 1/29/11 4:40 PM Page 1 Joint Structure and Function FIF TH EDITION A Comprehensive Analysis 2362_Ch01-001-063.qxd 1/29/11 4:40 PM Page 2 Section 1 Joint Structure and Function: Foundational Concepts Chapter 1 Biomechanical Applications to Joint Structure and Function Chapter 2 Joint Structure and Function Chapter 3 Muscle Structure and Function 2 2362_Ch01-001-063.qxd 1/29/11 4:40 PM Page 3 Chapter 1 Biomechanical Applications to Joint Structure and Function Samuel R. Ward, PT, PhD “Humans have the capacity to produce a nearly infinite variety of postures and movements that require the tissues of the body to both generate and respond to forces that produce and control movement.” Introduction Introduction to Statics and Dynamics Newton’s Law of Inertia PART 1: KINEMATICS AND INTRODUCTION Newton’s Law of Acceleration TO KINETICS Translatory Motion in Linear and Concurrent Force Descriptions of Motion Systems Types of Displacement Linear Force Systems Translatory Motion Determining Resultant Forces in a Linear Force System Rotary Motion Concurrent Force Systems General Motion Determining Resultant Forces in a Concurrent Force System Location of Displacement in Space Newton’s Law of Reaction Direction of Displacement Gravitational and Contact Forces Magnitude of Displacement Rate of Displacement Additional Linear Force Considerations Tensile Forces Introduction to Forces Tensile Forces and Their Reaction Forces Definition of Forces Joint Distraction Force Vectors Distraction Forces Force of Gravity Joint Compression and Joint Reaction Forces Segmental Centers of Mass and Composition Revisiting Newton’s Law of Inertia of Gravitational Forces Vertical and Horizontal Linear Force Systems Center of Mass of the Human Body Shear and Friction Forces Center of Mass, Line of Gravity, and Stability Static Friction and Kinetic Friction Alterations in Mass of an Object or Segment Considering Vertical and Horizontal Linear Equilibrium Continued 3 2362_Ch01-001-063.qxd 1/29/11 4:40 PM Page 4 4 SECTION 1 Joint Structure and Function: Foundational Concepts PART 2: KINETICS—CONSIDERING ROTARY AND Lever Systems, or Classes of Levers TRANSLATORY FORCES AND MOTION Muscles in Third-Class Lever Systems Muscles in Second-Class Lever Systems Torque, or Moment of Force Muscles in First-Class Lever Systems Angular Acceleration and Angular Equilibrium Mechanical Advantage Parallel Force Systems Trade-Offs of Mechanical Advantage Determining Resultant Forces in a Parallel Force System Limitations of Analysis of Forces by Lever Systems Bending Moments and Torsional Moments Identifying the Joint Axis About Which Body Segments Force Components Rotate Resolving Forces Into Perpendicular and Parallel Meeting the Three Conditions for Equilibrium Components Perpendicular and Parallel Force Effects Muscle Forces Determining Magnitudes of Component Forces Total Muscle Force Vector Force Components and the Angle of Application of the Anatomic Pulleys Force Anatomic Pulleys, Action Lines, and Moment Arms Translatory Effects of Force Components Torque Revisited Rotary Effects of Force Components Changes to Moment Arm of a Force Rotation Produced by Perpendicular (Fy) Force Components Rotation Produced by Parallel (Fx) Force Components Angular Acceleration With Changing Torques Moment Arm and Angle of Application of a Force Multisegment (Closed-Chain) Force Analysis INTRODUCTION forces (Chapter 3). Subsequent chapters then examine the interactive nature of force, stress, tissue behaviors, and func- Humans have the capacity to produce a nearly infinite vari- tion through a regional exploration of the joint complexes of ety of postures and movements that require the structures the body. The final two chapters integrate the function of of the human body to both generate and respond to forces the joint complexes into the comprehensive tasks of posture that produce and control movement at the body’s joints. (Chapter 13) and gait (Chapter 14). Although it is impossible to capture all the kinesiologic In order to maintain our focus on clinically relevant elements that contribute to human musculoskeletal func- applications of the biomechanical principles presented in tion at a given point in time, knowledge of at least some of this chapter, the following case example will provide a the physical principles that govern the body’s response to framework within which to explore the relevant principles active and passive stresses is prerequisite to an understand- of biomechanics. ing of both human function and dysfunction. We will examine some of the complexities related to human musculoskeletal function by examining the roles of the bony segments, joint-related connective tissue struc- ture, and muscles, as well as the external forces applied to 1-1 Patient Case case John Alexander is 20 years old, is 5 feet 9 inches (1.75 m) those structures. We will develop a conceptual framework in height, and weighs 165 pounds (~75 kg or 734 N). that provides a basis for understanding the stresses on John is a member of the university’s lacrosse team. He the body’s major joint complexes and the responses to sustained an injury when another player fell onto the those stresses. Case examples and clinical scenarios will be posterior-lateral aspect of his right knee. Physical exami- used to ground the reader’s understanding in relevant nation and magnetic resonance imaging (MRI) resulted applications of the presented principles. The objective is in a diagnosis of a tear of the medial collateral ligament, to cover the key biomechanical principles necessary to a partial tear of the anterior cruciate ligament (ACL), understand individual joints and their interdependent and a partial tear of the medial meniscus. John agreed functions in posture and locomotion. Although we ac- with the orthopedist’s recommendation that a program knowledge the role of the neurological system in motor of knee muscle strengthening was in order before moving control, we leave it to others to develop an understanding to more aggressive options. The initial focus will be on of the theories that govern the roles of the controller and strengthening the quadriceps muscle. The fitness center feedback mechanisms. at the university has a leg-press machine (Fig. 1–1A) and This chapter will explore the biomechanical principles a free weight boot (see Fig. 1–1B) that John can use. that must be considered to examine the internal and exter- nal forces that produce or control movement. The focus will be largely on rigid body analysis; the next two chapters As we move through this chapter, we will consider explore how forces affect deformable connective tissues the biomechanics of each of these rehabilitative options in (Chapter 2) and how muscles create and are affected by relation to John’s injury and strengthening goals. 2362_Ch01-001-063.qxd 1/29/11 4:40 PM Page 5 CHAPTER 1 Biomechanical Applications to Joint Structure and Function 5 A Figure 1–1 A. Leg-press exer- cise apparatus for strengthening hip and knee extensor muscles. B. Free weight boot for strength- B ening knee extensor muscles. Side-bar: The case in this chapter provides a background for the presentation of biomechanical principles. The values and angles chosen for the forces in the various examples used in this case are representative but are not intended to correspond to values derived from sophisti- A cated instrumentation and mathematical modeling; dif- ferent experimental conditions, instrumentation, and modeling can provide substantially different and often contradictory findings. Human motion is inherently complex, involving multi- ple segments (bony levers) and forces that are most often applied to two or more segments simultaneously. In order to develop a conceptual model that can be understood and applied clinically, the common strategy is to focus on one segment at a time. For the purposes of analyzing John Alexander’s issues, the focus will be on the leg-foot segment, treated as if it were one rigid unit acting at the knee joint. Figure 1–2A and 1–2B is a schematic represen- B tation of the leg-foot segment in the leg-press and free weight boot situations. The leg-foot segment is the focus of the figure, although the contiguous components (distal femur, footplate of the leg-press machine, and weight boot) are maintained to give context. In some subsequent figures, the femur, footplate, and weight boot are omitted for clarity, although the forces produced by these seg- ments and objects will be shown. This limited visualiza- tion of a segment (or a selected few segments) is referred to as a free body diagram or a space diagram. If propor- tional representation of all forces is maintained as the forces are added to the segment under consideration, it is known as a “free body diagram.” If the forces are shown but a simplified understanding rather than graphic accu- racy is the goal, then the figure is referred to as a “space diagram.”1 We will use space diagrams in this chapter and text because the forces are generally not drawn in propor- tion to their magnitudes. As we begin to examine the leg-foot segment in either Figure 1–2 A. Schematic representation of the leg-foot seg- the weight boot or leg-press exercise situation, the first ment in the leg-press exercise, with the leg-foot segment high- step is to describe the motion of the segment that is or will lighted for emphasis. B. Schematic representation of the leg-foot be occurring. This involves the area of biomechanics known segment in the weight boot exercise, with the leg-foot segment as kinematics. highlighted for emphasis. 2362_Ch01-001-063.qxd 1/29/11 4:40 PM Page 6 6 SECTION 1 Joint Structure and Function: Foundational Concepts human movement, pure translatory movements are rare. Part 1: Kinematics and However, a clinical example of attempted translatory motion is joint mobilization, in which a clinician attempts Introduction to Kinetics to impose the linear motion of one bony segment on another, allowing joint surfaces to slide past one another. A specific example of such imposed motion is the anterior drawer test for anterior cruciate ligament (ACL) integrity DESCRIPTIONS OF MOTION at the knee (Fig. 1–3). This example of translatory motion Kinematics includes the set of concepts that allows us to assumes, however, that the leg segment is free and uncon- describe the displacement (the change in position over strained—that is, that the leg segment is not linked to the time) or motion of a segment without regard to the forces femur by soft tissues. Although it is best to describe pure that cause that movement. The human skeleton is, quite translatory motion by using an example of an isolated and literally, a system of segments or levers. Although bones unconstrained segment, segments of the body are neither are not truly rigid, we will assume that bones behave as isolated nor unconstrained. Every segment is linked to at rigid levers. There are five kinematic variables that fully least one other segment, and most human motion occurs describe the motion, or the displacement, of a segment: as movement of more than one segment at a time. The (1) the type of displacement (motion), (2) the location translation of the leg segment in Figure 1–3 is actually in space of the displacement, (3) the direction of the produced by the near-linear motion of the proximal tibia. displacement of the segment, (4) the magnitude of the In fact, translation of a body segment rarely occurs in displacement, and (5) the rate of change in displacement human motion without some concomitant rotation (rotary (velocity) or the rate of change of velocity (acceleration). motion) of that segment (even if the rotation is barely visible). Types of Displacement Rotary Motion Translatory and rotary motions are the two basic types Rotary motion (angular displacement) is movement of a of movement that can be attributed to any rigid segment. segment around a fixed axis (center of rotation [CoR]) in General motions are achieved by combining translatory and a curved path. In true rotary motion, each point on the seg- rotary motions. ment moves through the same angle, at the same time, at a constant distance from the center of rotation. True rotary Translatory Motion motion can occur only if the segment is prevented from Translatory motion (linear displacement) is the move- translating and is forced to rotate about a fixed axis. This ment of a segment in a straight line. In true translatory does not often happen in human movement. In the example motion, each point on the segment moves through the in Figure 1–4, all points on the leg-foot segment appear to same distance, at the same time, in parallel paths. In move through the same distance at the same time around A B Figure 1–3 An example of translatory motion is the anterior drawer test for ACL integrity. Ideally, the tibial plateau translates anteri- orly from the starting position (A) to the ending position (B) as the examiner exerts a linear load on the proximal tibia. Under ideal conditions, each point on the tibia moves through the same distance, at the same time, in parallel paths. 2362_Ch01-001-063.qxd 1/29/11 4:40 PM Page 7 CHAPTER 1 Biomechanical Applications to Joint Structure and Function 7 other bony forces acting on it to produce pure rotary motion. Instead, there is typically at least a small amount of translation (and often a secondary rotation) that accom- A panies the primary rotary motion of a segment at a joint. Most joint rotations, therefore, take place around a series of instantaneous center of rotations. The “axis” that is gen- erally ascribed to a given joint motion (e.g., knee flexion) is typically a midpoint among these instantaneous centers of rotation rather than the true center of rotation. Because most body segments actually follow a curvilinear path, the true center of rotation is the point around which true rotary motion of the segment would occur and is generally quite distant from the joint.3,4 Location of Displacement in Space The rotary or translatory displacement of a segment is com- monly located in space by using the three-dimensional Cartesian coordinate system, borrowed from mathematics, Figure 1–4 Rotary motion. Each point in the tibia segment as a useful frame of reference. The origin of the x-axis, moves through the same angle, at the same time, at a constant y-axis, and z-axis of the coordinate system is traditionally distance from the center of rotation or axis (A). located at the center of mass (CoM) of the human body, assuming that the body is in anatomic position (standing facing forward, with palms forward) (Fig. 1–5). According what appears to be a fixed axis. In actuality, none of the body to the common system described by Panjabi and White, segments move around truly fixed axes; all joint axes shift at the x-axis runs side-to-side in the body and is labeled in the least slightly during motion because segments are not suffi- body as the coronal axis; the y-axis runs up and down in ciently constrained to produce pure rotation. the body and is labeled in the body as the vertical axis; the z-axis runs front to back in the body and is labeled in the General Motion body as the anteroposterior (A-P) axis.3 Motion of a When nonsegmented objects are moved, combinations of segment can occur either around an axis (rotation) or along rotation and translation (general motion) are common. an axis (translation). An unconstrained segment can either If someone were to attempt to push a treatment table with rotate or translate around each of the three axes, which swivel casters across the room by using one hand, it would results in six potential options for motion of that segment. be difficult to get the table to go straight (translatory motion); it would be more likely to both translate and ro- y-axis tate. When rotary and translatory motions are combined, a number of terms can be used to describe the result. Curvilinear (plane or planar) motion designates a combination of translation and rotation of a segment in two dimensions (parallel to a plane with a maximum of three degrees of freedom).2–4 When this type of motion occurs, the axis about which the segment moves is not fixed but, rather, shifts in space as the object moves. The axis around which the segment appears to move in any part of its path is referred to as the instantaneous center x-axis of rotation (ICoR), or instantaneous axis of rotation (IaR). An object or segment that travels in a curvilinear z-axis path may be considered to be undergoing rotary motion around a fixed but quite distant CoR3,4; that is, the curvi- linear path can be considered a segment of a much larger circle with a distant axis. Three-dimensional motion is a general motion in which the segment moves across all three dimensions. Just as curvilinear motion can be considered to occur around a single distant center of rotation, three-dimensional motion can be considered to be occurring around a helical axis of motion (HaM), or screw axis of motion.3 Figure 1–5 Body in anatomic position showing the x-axis, As already noted, motion of a body segment is rarely y-axis, and z-axis of the Cartesian coordinate system (the coronal, sufficiently constrained by the ligamentous, muscular, or vertical, and anteroposterior axes, respectively). 2362_Ch01-001-063.qxd 1/29/11 4:40 PM Page 8 8 SECTION 1 Joint Structure and Function: Foundational Concepts The options for movement of a segment are also referred to as degrees of freedom. A completely unconstrained segment, therefore, always has six degrees of freedom. Segments of the body, of course, are not unconstrained. A segment may appear to be limited to only one degree of freedom (although, as already pointed out, this rarely is strictly true), or all six degrees of freedom may be avail- able to it. Rotation of a body segment is described not only as occurring around one of three possible axes but also as x moving in or parallel to one of three possible cardinal planes. As a segment rotates around a particular axis, the segment also moves in a plane that is both perpendicular to that axis of rotation and parallel to another axis. Rotation of a body segment around the x-axis or coronal axis occurs in the sagittal plane (Fig. 1–6). Sagittal plane motions are most easily visualized as front-to-back motions of a seg- ment (e.g., flexion/extension of the upper extremity at the glenohumeral joint). Rotation of a body segment around the y-axis or vertical axis occurs in the transverse plane (Fig. 1–7). Transverse plane motions are most easily visualized as Figure 1–7 The transverse plane. motions of a segment parallel to the ground (e.g., medial/lateral rotation of the lower extremity at the hip joint). Transverse plane motions often occur around axes Y that pass through the length of long bones that are not truly vertically oriented. Consequently, the term longitu- dinal (or long) axis is often used instead of “vertical axis.” Rotation of a body segment around the z-axis or A-P axis occurs in the frontal plane (Fig. 1–8). Frontal plane motions are most easily visualized as side-to-side motions of the segment (e.g., abduction/adduction of the upper extremity at the glenohumeral joint). Rotation and translation of body segments are not limited to motion along or around cardinal axes or within cardinal planes. In fact, cardinal plane motions are the Figure 1–8 The frontal plane. z exception rather than the rule and, although useful, are an oversimplification of human motion. If a motion (whether in or around a cardinal axis or plane) is limited to rotation around a single axis or translatory motion along a single axis, the motion is considered to have one degree of freedom. Much more commonly, a segment moves in three dimensions with two or more degrees of freedom. The following example demonstrates a way in which rotary and translatory motions along or around one or more axes can combine in human movement to produce Figure 1–6 The sagittal plane. two- and three-dimensional segmental motion. 2362_Ch01-001-063.qxd 1/29/11 4:40 PM Page 9 CHAPTER 1 Biomechanical Applications to Joint Structure and Function 9 Example 1-1 axes, we can describe three pairs of (or six different) anatomic rotations available to body segments. When the forearm-hand segment and a glass (all consid- Flexion and extension are motions of a segment occur- ered as one rigid segment) are brought to the mouth ring around the same axis and in the same plane (uniaxial or (Fig. 1–9), rotation of the segment around an axis and uniplanar) but in opposite directions. Flexion and extension translation of that segment through space occur simulta- generally occur in the sagittal plane around a coronal axis, neously. As the forearm-hand segment and glass rotate although exceptions exist (e.g., carpometacarpal flexion and around a coronal axis at the elbow joint (one degree of extension of the thumb). Anatomically, flexion is the direc- freedom), the shoulder joint also rotates to translate tion of segmental rotation that brings ventral surfaces of ad- the forearm-hand segment forward in space along the jacent segments closer together, whereas extension is the di- forearm-hand segment’s A-P axis (one degree of freedom). rection of segmental rotation that brings dorsal surfaces By combining the two degrees of freedom, the elbow joint closer together. axis (the instantaneous center of rotation for flexion of the Side-bar: Defining flexion and extension by ventral forearm-hand segment) does not remain fixed but moves and dorsal surfaces makes use of the true embryologic in space; the glass attached to the forearm-hand segment origin of the words ventral and dorsal, rather than using moves through a curvilinear path. these terms as synonymous with anterior and posterior, respectively. Abduction and adduction of a segment occur around the A-P axis and in the frontal plane but in opposite direc- tions (although carpometacarpal abduction and adduction of the thumb again serve as exceptions). Anatomically, ab- duction brings the segment away from the midline of the body, whereas adduction brings the segment toward the midline of the body. When the moving segment is part of the midline of the body (e.g., the trunk or the head), the ro- tary movement is commonly termed lateral flexion (to the right or to the left). Medial (or internal) rotation and lateral (or external) rotation are opposite motions of a segment that generally occur around a vertical (or longitudinal) axis in the trans- verse plane. Anatomically, medial rotation occurs as the segment moves parallel to the ground and toward the mid- line, whereas lateral rotation occurs opposite to that. When the segment is part of the midline (e.g., the head or Figure 1–9 The forearm-hand segment rotates around a coro- trunk), rotation in the transverse plane is simply called nal axis at the elbow joint and along A-P axis (through rotation at rotation to the right or rotation to the left. The exceptions the shoulder joint), using two degrees of freedom that result in a to the general rules for naming motions must be learned moving axis of rotation and produce curvilinear motion of the on a joint-by-joint basis. forearm-hand segment. As is true for rotary motions, translatory motions of a segment can occur in one of two directions along any of the three axes. Again by convention, linear displacement of a segment along the x-axis is considered positive when Direction of Displacement displacement is to the right and negative when it is to the Even if displacement of a segment is confined to a single left. Linear displacement of a segment up along the y-axis axis, the rotary or translatory motion of a segment around is considered positive, and such displacement down along or along that axis can occur in two different directions. For the y-axis is negative. Linear displacement of a segment rotary motions, the direction of movement of a segment forward (anterior) along the z-axis is positive, and such around an axis can be described as occurring in a clockwise displacement backward (posterior) is negative.1 or counterclockwise direction. Clockwise and counterclock- wise rotations are generally assigned negative and positive signs, respectively.5 However, these terms are dependent on Magnitude of Displacement the perspective of the viewer (viewed from the left side, flex- The magnitude of rotary motion (or angular displacement) ing the forearm is a clockwise movement; if the subject of a segment can be given either in degrees (United States turns around and faces the opposite direction, the same [US] units) or in radians (International System of Units movement is now seen by the viewer as a counterclockwise [SI units]). If an object rotates through a complete circle, it movement). Anatomic terms describing human movement has moved through 360°, or 6.28 radians. A radian is liter- are independent of viewer perspective and, therefore, more ally the ratio of an arc to the radius of its circle (Fig. 1–10). useful clinically. Because there are two directions of rotation One radian is equal to 57.3°; 1° is equal to 0.01745 radian. (positive and negative) around each of the three cardinal The magnitude of rotary motion that a body segment moves 2362_Ch01-001-063.qxd 1/29/11 4:40 PM Page 10 10 SECTION 1 Joint Structure and Function: Foundational Concepts squared (ft/sec2). Angular velocity (velocity of a rotating segment) is expressed as degrees per second (deg/sec), whereas angular acceleration is given as degrees per second squared (deg/sec2). An electrogoniometer or a three-dimensional motion analysis system allows documentation of the changes in displacement over time. The outputs of such systems are increasingly encountered when summaries of displace- ment information are presented. A computer-generated time-series plot, such as that in Figure 1–11, graphically portrays not only the angle between two bony segments (or the rotation of one segment in space) at each point