Clinical Kinesiology: Essential Topics and Musculoskeletal Principles

Learning Objectives of Clinical Kinesiology

• Kinematics and Kinetics: Develop a foundational understanding of basic concepts regarding the motion of bodies (kinematics) and the forces that cause or result from that motion (kinetics).

• Arthrokinematics and Osteokinematics: Describe the movement of joint surfaces (arthrokinematics) and the movement of bones (osteokinematics) and apply these to identify normal versus abnormal movement patterns.

• Kinesiological Application: Apply principles of kinematics and kinetics to the human body, specifically regarding functional movement and the mechanisms of injury.

• Levers and Mechanical Advantage: Apply knowledge regarding the classifications of levers and the concept of mechanical advantage to the human body and movement.

Musculoskeletal Forces and the Stress-Strain Curve

• Common Musculoskeletal Forces: The forces most frequently applied to the musculoskeletal system include:     * Tension     * Compression     * Bending     * Shear     * Torsion     * Combined loading (combo loading)

• Experimental Observation: Tolerance of loads is observed experimentally by plotting the amount of force (stress) required to deform an excised tissue (strain).

• Stress-Strain Curve Components:     * A = Small Tension: Initial loading where tissue begins to tighten.     * B = Elastic Zone: The area where tissue returns to its original length after the load is removed.     * C = Plasticity (Plastic Zone): The area where the tissue undergoes permanent deformation; it will not return to its original length after the load is removed.     * D = Initial Failure: The point where individual fibers begin to break.     * E = Mechanical Failure: The point where the tissue is completely ruptured.

• Viscoelasticity: A factor of time that causes the Stress-Strain curve to change its properties.

• Creep: A phenomenon occurring when a tissue is subjected to a long, constant stress, leading to progressive deformation over time.

Internal and External Forces

• Internal Forces: These are produced from structures located within the body. They are divided into two categories:     * Active Forces: Produced by stimulated muscle, generally under volitional control.     * Passive Forces: Generated by non-muscular sources such as gravity or tension in stretched connective tissues (ligaments, capsules).

• External Forces: Produced by sources outside the body, such as gravity pulling on a limb or a weight being lifted.

Musculoskeletal Torques

• Translation vs. Rotation: Forces applied directly to a body segment can potentially translate it. However, forces acting at a distance from the axis of rotation at a joint produce a potential rotation.

• Torque (Moment): The quantification of the rotational potential of a force. It is the product of a force and its moment arm.

• Moment Arm: The perpendicular distance between the axis of rotation and the line of action of the force.

• Moment Arm Variations and Force Requirements:     * A longer moment arm results in greater torque for the same amount of force.     * If the internal moment arm is shorter than the external moment arm, more internal (muscle) force is necessary to overcome the external force.     * If the internal moment arm is longer, less internal force is necessary to produce the same torque.

• Static Rotary Equilibrium (Isometric Status): Occurs when the Internal Torque matches the External Torque ($\text{Internal Torque} = \text{External Torque}$). In this state:     * The internal moment equals the external moment.     * There is no resultant motion.     * There is no rotation.

Musculoskeletal Levers and Mechanical Advantage

• Lever Classes:     * First Class: The axis (fulcrum) is located between the internal and external forces (e.g., the head resting on the first cervical vertebra).     * Second Class: The external force is located between the axis and the internal force (e.g., calf raise; internal force has a longer moment arm).     * Third Class: The internal force is located between the axis and the external force. This is the most common lever type in the human body.

• Mechanical Advantage (MA): This is the ratio of the internal moment arm (IMA) to the external moment arm (EMA). It is calculated as:     * $\text{MA} = \frac{\text{IMA}}{\text{EMA}}$

• Interpretation of MA:     * MA > 1: The system is able to balance the torque equilibrium with an internal (muscle) force that is less than the external force.     * MA < 1: Requires greater internal force to produce movement or maintain equilibrium. This is characteristic of third-class levers.

• The Force-Distance "Trade-off":     * Since most extremities function as third-class levers (\text{MA} < 1), they require large internal forces to create movement.     * This is particularly significant in high-velocity activities (e.g., throwing), where high forces are required both to initiate motion and to decelerate the extremity.     * The combination of a "bad" system (low mechanical advantage) requiring high force to move and high force to slow down contributes to injury risk.

Arthrology: Joint Structure and Classification

• Classification by Structure and Movement Potential:     * Synovial (Diarthrodial) Joints: These joints are specialized for movement. They always possess seven essential elements:         1. Synovial fluid         2. Articular cartilage         3. Articular capsule         4. Synovial membrane/bursa         5. Capsular ligaments         6. Blood vessels         7. Sensory nerves     * Additional Synovial Elements: To accommodate varied functional demands, some joints may include a labrum, fat pads, plica, menisci, or discs.

• Mechanical Classification of Synovial Joints:     * Hinge Joint: Primary movement is flexion and extension (e.g., humeroulnar joint).     * Pivot Joint: Rotation around a single axis (e.g., atlantoaxial joint).     * Ellipsoid Joint: Bi-axial movement with one surface being oval/convex and the other concave (e.g., radiocarpal joint).     * Ball-and-Socket Joint: Multi-axial movement allowing for the greatest range of motion (e.g., glenohumeral joint).     * Plane Joint: Gliding or sliding between flat surfaces (e.g., intercarpal joints).     * Saddle Joint: Each surface has both a convex and concave curvature (e.g., carpometacarpal joint of the thumb).     * Condyloid Joint: Similar to ball-and-socket but with more limited rotation (e.g., tibiofemoral joint).

Biologic Materials and Connective Tissues

• Primary Biological Materials: Tissue in the body includes muscle, nerve, epithelium, and connective tissue.

• Relevant Connective Tissues in Kinesiology:     * Dense Connective Tissue: Found in tendons, fascia, and the fibrous layer of joint capsules.     * Articular Cartilage: Covers the ends of bones.     * Fibrocartilage: Found in intervertebral discs and menisci.     * Bone: Provides rigid support.

• Composition of Connective Tissue:     * Fibers:         * Type I Collagen: Stiff and thick; ideal for tendons and ligaments; transmits force to bone.         * Type II Collagen: Thinner; maintains shape after deformation; primary component of hyaline cartilage.         * Elastin: Provides elastic quality; allows "give" when elongated.     * Ground Substance: Consists of Glycosaminoglycans (GAGs), water, and solutes. Plays a critical role in shock absorption.     * Cells: Responsible for maintenance and repair.

Specific Tissue Characteristics

• Dense Connective Tissue:     * High proportion of Type I collagen.     * Resists natural stresses and transmits force.     * Most effective when stretched parallel to its long axis.     * Dense "Irregular" Connective Tissue: Similar to regular but more disorganized to resist forces from multiple directions.

• Articular Cartilage:     * Specialized hyaline cartilage composed of Type II collagen.     * Forms load-bearing surfaces; is avascular and aneural.     * Chondrocytes: Nourished by synovial fluid via the "milking action" of joint loading and unloading.     * Zonal Arrangement:         * Superficial Zone: Fibers parallel to the surface (gliding surface).         * Transitional Zone: Randomly oriented fibers.         * Deep Zone: Fibers perpendicular to the surface.         * Tidemark: The boundary between the deep zone and subchondral bone.     * Repair: Adult articular cartilage has poor repair capacity if damaged significantly.

• Fibrocartilage:     * Combines properties of dense connective tissue and articular cartilage.     * Acts as an ideal shock absorber (e.g., intervertebral discs, menisci, labrum).     * Nourishment is dependent on the diffusion of nutrients from synovial fluid, assisted by intermittent weight bearing.     * Vascularity: Only the outer rim (e.g., outer $\frac{1}{3}$ of the knee meniscus) has a direct blood supply; some repair is possible only in this vascularized periphery.

• Bone:     * Structure: Cortical (compact) bone forms the outer shaft; cancellous bone forms the interconnecting network at the ends.     * Osteon (Haversian System): The structural subunit of cortical bone.     * Matrix: Contains calcium phosphate crystals to accept high compressive forces.     * Sensitivity: Periosteum and endosteum are richly vascularized and innervated with sensory receptors for pressure and pain.     * Remodeling: Follows the SAID principle (Specific Adaptation to Imposed Demands); bone adapts its strength based on physical activity.

Effects of Aging and Immobilization on Tissue

• Aging:     * Decreased GAG replacement and repair leads to decreased water binding.     * This results in decreased tolerance to compressive forces and decreased force distribution.     * Fluidity decreases, and adhesions may form.     * Altered bone metabolism leads to osteoporosis and slower healing rates.

• Immobilization:     * Greatly decreases the mechanical strength of tissue due to lack of external force (SAID principle).     * Decreased tensile strength in ligaments can be observed within weeks.     * Recovery of strength takes much longer than the period of immobilization.     * Clinical judgment is required to balance the need for immobilization (healing) with its negative effects.

Muscle Structure and Morphology

• Structural Organization:     * Epimysium: Surrounds the entire muscle belly (tight collagen).     * Perimysium: Surrounds fascicles; acts as a conduit for vessels and nerves.     * Endomysium: Surrounds individual muscle fibers; meshwork of collagen connected to the fiber.

• Muscle Morphology:     * Fusiform: Fibers run parallel to the tendon (e.g., biceps brachii).     * Pennate: Feather-like arrangement (unipennate, bipennate, multipennate).

• Muscle Architecture Concepts:     * Physiologic Cross-Sectional Area (PCSA): The amount of contractile protein available. A thicker muscle has greater force potential.     * Pennation Angle: The angle between fibers and the central tendon.         * $0^{\circ}$ angle: $100\%$ of force is transferred to the tendon.         * $30^{\circ}$ angle: $\cos(30) \approx 86\%$ of force transferred.         * Pennate muscles allow more fibers to be packed into a given volume, increasing PCSA.

Muscle Length-Tension and Force-Velocity Relationships

• Passive Length-Tension Curve (PLTC): Connective tissues within the muscle generate resistive tension when elongated (like a spring). As a muscle is stretched toward its maximum, it stiffens to transfer force and provide stability.

• Active Length-Tension Curve: Describes the force produced by the muscle fiber's active contraction.     * Sarcomere: The fundamental force generator (Z-disc to Z-disc).     * Sliding Filament Hypothesis: Actin filaments slide over myosin; cross-bridges form. More cross-bridges lead to more force.     * Ideal Resting Length: The length where the greatest number of cross-bridge attachments can form, resulting in maximum potential active force.

• Total Length-Tension Curve: The summation of active and passive forces. At lengthened positions, passive tension dominates.

• Insufficiencies (Two-Joint Muscles):     * Active Insufficiency: A muscle is so short that it cannot generate significant tension to complete a range of motion (e.g., gripping with a flexed wrist).     * Passive Insufficiency: A muscle is so elongated that passive tension restricts the range of motion (e.g., hamstring tightness limiting hip flexion while the knee is straight).

• Force-Velocity Relationship:     * Concentric Activation: Force and velocity are inversely related. As load increases, maximal contraction velocity decreases. At maximal load, velocity is zero (isometric).     * Eccentric Activation: Force and velocity are directly related. A greater load causes a faster lengthening velocity. Eccentric contractions develop higher force than concentric ones due to passive tension and rapid cross-bridge reattachment.     * Clinical Note: Eccentric work has a higher metabolic cost and higher onset of Delayed Onset Muscle Soreness (DOMS).

Nervous System Activation of Muscle

• Motor Unit: Consists of an alpha motor neuron and all the muscle fibers it innervates.

• Recruitment: The nervous system activates more motor neurons to increase force.     * The Size Principle: Smaller motor neurons (fatigue-resistant, low amplitude) are recruited before larger ones (easily fatigued, high amplitude) for a smooth gradation of force.

• Rate Coding: Increasing the rate of excitation (action potentials) to regulate force.     * Twitch: Response to a single action potential.     * Unfused Tetanus: A series of twitches that produce increasing force.     * Fused Tetanus: The maximum stable level of muscle force where twitches are no longer distinguishable.

• Muscle Fatigue: A decline in muscle force under stable activation. The nervous system compensates by increasing rate coding or recruitment of additional motor units.

Electromyography (EMG)

• Definition: The recording and amplification of Motor Unit Action Potentials (MUAP).

• Signal Meaning: EMG indicates the relative timing and level of "neural drive" to a muscle. Higher MUAP summation generally indicates higher muscle force.

• Types of Electrodes:     * Surface Electrodes: Non-invasive; detect signals from a large area.     * Fine Wire (Insertional) Electrodes: Specific to deep muscles or specific muscle regions.

Newton’s Laws of Motion in Movement Analysis

• Law of Inertia (First Law): A body remains at rest or constant velocity unless acted upon by an external force.     * Mass Moment of Inertia: Distribution of mass relative to the axis of rotation. Athletes alter body positions (e.g., tucking in a dive) to change this.

• Law of Acceleration (Second Law): Force equals mass times acceleration ($F = m \times a$).     * Angular Counterpart: Torque equals the mass moment of inertia times angular acceleration ($\tau = I \times \alpha$).     * Impulse-Momentum: Impulse is the average force multiplied by time. This is used in the design of helmets and shoes to decrease peak force by increasing impact time.     * Work-Energy: Work ($W$) is the force ($F$) times distance ($d$). Power ($P$) is the rate of work ($\frac{W}{t}$ or $F \times v$).

• Law of Action-Reaction (Third Law): For every action, there is an equal and opposite reaction.     * Ground Reaction Force (GRF): The force exerted by the ground back onto the foot during gait. It changes in magnitude and direction throughout the gait cycle.

Advanced Movement Analysis Concepts

• Anthropometry: Measurement of physical features like length, mass, volume, and center of mass. This is essential for calculating inertial properties.

• Free Body Diagram: A simplified sketch representing all relevant forces (muscle, gravity, friction, etc.) acting on a system.

• Joint Reaction Force: The force produced by one joint surface pushing back against another, primarily caused by muscle activation and gravity.

• Reference Frames:     * Relative: One segment relative to an adjacent segment (e.g., knee flexion angle).     * Global: Motion relative to a fixed point in space (e.g., position in a room).

• Vector Resolution: Breaking a resultant force into components:     * Parallel Component: Can cause compression or shear (translation).     * Perpendicular Component: Creates the moment arm for rotation.

• Angle of Insertion: The angle at which a muscle attaches to a bone changes through the range of motion, altering the muscle's leverage and internal torque potential.

• Mechanical Advantage Case Study (Hip OA): Severe hip osteoarthritis can shorten the femoral neck, decreasing the internal moment arm ($\text{IMA}$) of the hip abductors. This requires the muscles to produce much higher forces. Relocating the greater trochanter laterally can surgically increase the $\text{IMA}$ to reduce muscle force requirements.

Measurement Systems in Kinesiology

• Kinematic Systems (Measure Motion):     * Electrogoniometer     * Accelerometer     * Imaging: Photography, Cinematography, Videography, Optoelectronic systems     * Electromagnetic tracking devices

• Kinetic Systems (Measure Force):     * Mechanical: Hand-held dynamometer     * Transducers: Force plate     * Electromechanical: Isokinetic dynamometer