Comprehensive University Study Notes on Kinesiology and Human Biomechanics

General Structure of Movement: Passive and Active Sectors

Movement is divided into two primary sectors: the passive sector and the active sector. The passive sector includes elements that do not independently generate force but are moved by it, such as articular capsules, ligaments, articular facets, and specialized structures like glenoid labrums, menisci, and cartilage. It also includes bones, which act as mechanical levers, as well as synovial fluid, serous bursae, fasciae, and subcutaneous cellular tissue. The active sector is primarily composed of muscles. A muscle is considered active when it is contracted, possessing a force vector that dictates movement. When a muscle is relaxed, it functions as part of the passive sector. Additionally, specialized tissue structures like keloids (inelastic scars caused by excess tissue) can limit movement.

The Mechanics of Levers in the Human Body

A lever is a rigid bar that rotates around a fixed axis, known as a fulcrum or support point ( $A$ ), when a force ( $F$ ) is applied to overcome a resistance ( $R$ ). Levers are designed to make force application more efficient, often allowing a relatively small force to overcome a significantly larger resistance. The effectiveness of a lever is determined by the Moment of Force, calculated as:

$\text{Moment of Force} = \text{Force} \times \text{Lever Arm}$

The lever arm is the perpendicular distance from the fulcrum to the point where the force is applied. In anatomical terms, the rigid bar is the bone, the force is provided by the muscle, and the resistance is usually the center of gravity or an external load. The center of gravity is the point where the weight of an object is concentrated, typically located distal to the insertion of flexor muscles in body segments. Greater distance from the fulcrum requires greater force application.

Classification of Levers

Levers are categorized into three classes or "genres" based on the relative positions of the fulcrum, force, and resistance:

First Class (Balance/See-saw Type): The fulcrum is positioned between the force and the resistance ( $F - A - R$ ). An anatomical example is the head-neck junction, where the atlanto-occipital joint is the fulcrum, the neck extensors provide the force, and the weight of the head is the resistance.
Second Class (Nutcracker Type): The resistance is located between the fulcrum and the force ( $F - R - A$ ). This is exemplified by the foot during plantar flexion (standing on tiptoes). The contact point with the ground is the fulcrum, the body weight is the resistance, and the calf muscles provide the force. These levers are designed for power.
Third Class (Tweezer/Hinge Type): The force is applied between the fulcrum and the resistance ( $R - F - A$ ). Most levers in the human body are of this type because muscle insertions are typically very close to the joint (the fulcrum). An example is the elbow joint during a bicep curl, where the joint is the fulcrum, the biceps provides the force at its insertion, and the weight in the hand is the resistance.

Articular Classification and Properties

Joints (articulations) are classified based on their range of movement and structural characteristics:

Synarthrosis: Joints with minimal to no movement, such as the sutures of the skull.
Amphiartrosis: Joints allowing limited movement in specific situations, such as the pubic symphysis, intervertebral discs, and the sacroiliac joint.
Diarthrosis (Synovial): Joints with significant range of joint play and movement. Examples include the elbow, hip, knee, and shoulder.

Factors affecting articular stability include the specific joint configuration (congruence), surrounding musculature (muscles crossing the joint), skin, adipose tissue, and intra-articular atmospheric pressure.

Structural Anatomy of Diarthrodial Joints

Diarthrodial joints feature specialized structures to manage friction and load:

Articular Cartilage: Transmits forces while increasing the area of load distribution and reducing friction. It is avascular and non-innervated, receiving nutrients via imbibition from synovial fluid (acting like a sponge). Wear of this tissue leads to arthrosis.
Articular Capsule: A fibrous membrane composed of dense connective tissue ( $90\%$ collagen and elastin) that surrounds the bone ends. It contains receptors providing conscious proprioception of joint position.
Ligaments: Provide stability and guide movement by preventing excessive range. Their mechanical properties depend on fiber orientation; elastic fibers are less resistant to traction but more deformable, whereas collagen fibers are highly resistant to traction.
Synovial Membrane and Fluid: The membrane produces synovial fluid, which lubricates the joint and facilitates sliding by reducing the friction coefficient. The membrane contains folds (plicae) to allow for full range of motion.
Anatomical Congruence: Refers to the matching of curvature axes between articular surfaces. Incongruent joints utilize structures like menisci (knee) or glenoid labrums (shoulder/hip) to increase surface contact and improve concordance.

Osteokinematics and Articular Positions

Osteokinematics is the study of bone movement in space across different planes. Basic terminology includes:

Zero Position (Anatomical Position): The internationally accepted neutral starting point for measuring joint movement. Most joints begin at zero, though the forearm's neutral position is defined when the thumb points upward.
Resting Position (Loose-Packed): The position where periarticular structures are most relaxed, joint contact is minimal, and instability/joint play is maximal. It is the easiest position for manual mobilization.
Locking Position (Close-Packed): The position of maximal tension in the capsule and ligaments, requiring maximal contact between surfaces. Joint sliding is highly restricted.
Articular Hardness (End-feel): The structure that limits movement can be hard (bone hitting bone, e.g., elbow extension), capsulo-ligamentous (firm, e.g., knee extension), or soft (tissue approximation, e.g., elbow flexion).

Arthrocinematics and the Concave-Convex Rule

Arthrocinematics focuses on the relationship between articular surfaces during movement. Two primary movements occur: rolling and sliding.

Rolling: New equidistant points on one surface contact new equidistant points on another. Rolling occurs in the same direction as the bony movement, regardless of the surface shape. Less congruent surfaces exhibit more rolling.
Sliding: The same point on one surface contacts new points on another. More congruent or flat surfaces exhibit more sliding.

The Concave-Convex Rule:

Concave Surface Move: If a concave surface moves on a fixed convex surface, the sliding occurs in the same direction as the bony movement (e.g., knee extension where the tibia moves on the femur).
Convex Surface Move: If a convex surface moves on a fixed concave surface, the sliding occurs in the opposite direction of the bony movement (e.g., shoulder abduction or plantar flexion of the ankle).

If rolling occurs without sliding, it can lead to a "wedge effect," compressing and pinching intra-articular structures, resulting in injury.

Muscle Physiology and the Motor Unit

The motor unit is the functional unit of skeletal muscle, consisting of a single motor neuron and all the muscle fibers it innervates. Small muscles for fine movements may have a dozen fibers per unit, while large muscles for gross movements can have up to $1000$ to $2000$ fibers. Muscles follow the "all-or-nothing" principle of stimulation.

Muscle Components:

Contractile Component: Actin and myosin filaments.
Non-Contractile Component: Fascia and tendons.
Passive Tension: Developed when a muscle is stretched beyond its resting length ( $L_0$ ). It involves elastic elements like the endomysium (surrounding individual fibers) and epimysium (surrounding the entire muscle).
Active Tension: Generated by contractile elements. Maximal active force is obtained when a fiber is at its intermediate resting length.

Types of Muscle Contractions and Functions

Contractions are defined by the relationship between motor moment ( $M_m$ ) and resistance moment ( $M_r$ ):

Isometric: $M_m = M_r$ . No change in muscle length.
Concentric: M_m > M_r. Muscle shortens.
Eccentric: M_m < M_r. Muscle lengthens; generates the highest tension levels.
Isokinetic: Muscle contracts at a constant angular velocity, requiring specialized equipment.

Tension levels vary such that: \text{Eccentric} > \text{Isometric} > \text{Concentric}. Muscle strength is also influenced by temperature; higher temperatures increase nerve conduction velocity, enzymatic activity, and collagen elasticity.

Functional Classifications:

Agonists: Primary movers.
Antagonists: Oppose the movement.
Stabilizers (Fixators): Provide a fixed point for other muscles to work.
Synergists (Neutralizers): Eliminate unwanted muscle components during a movement.

Muscle Insufficiency and Functional Ranges

Active Insufficiency: Occurs when an agonist muscle, crossing multiple joints, reaches a state of maximal shortening and cannot generate more force to complete the full range of all joints involved.
Passive Insufficiency: Occurs when an antagonist muscle is stretched to its limit across multiple joints, preventing the agonist from completing the movement (e.g., being unable to fully extend the knee when the hip is fully flexed due to hamstring tension).

Movement Analysis and Measurement

Movement evaluation can be qualitative (assessing end-feel, fluidity, pain, and noise) or quantitative (using scales and instruments to measure the Maximum Range of Movement, or ROM).

Instruments for ROM measurement:

Manual Goniometer: The standard tool. The axis must be over the joint center, and arms must follow the longitudinal axes of the segments. Measurement is compared against the healthy side from a zero position.
Plumb-line Goniometer: Used for vertical planes.
Magnetic Goniometer: Used for horizontal planes.
Inclinometer: Placed on the bone segment to measure vertical inclination.
Electromyography (EMG): Records electrical signals of muscle contraction to analyze chronology and approximate force.
Dynamometry: Measures mechanical force. Isokinetic dynamometry maintains constant speed throughout the range.
Ground Reaction Plates: Measure forces between the patient and the support surface.

Kinesiology of the Spine (Raquis)

The spine serves three primary functions: loading, protection (of the spinal cord/vessels), and movement. It possesses contradictory traits of rigidity and flexibility.

Spinal Curves: In the sagittal plane, the spine has four curves: cervical lordosis, dorsal kyphosis, lumbar lordosis, and sacral kyphosis. These curves increase the spine's resistance to axial compression according to the formula:

$\text{Resistant} = (N^2) + 1$

where $N$ is the number of curves. A spine with three curves is $10$ times more resistant than a straight spine ( $R = 1$ ).

Vertebral Structure: Vertebrae consist of a body (for weight-bearing) and a posterior arch (for protection and muscle insertion). Vertebral bodies feature trabecular patterns (vertical, horizontal, and oblique) that follow lines of stress to maximize strength. Areas with fewer trabeculae are prone to fractures, such as "vertebral wedging" in the anterior portion during sudden flexion.

Intervertebral Discs and Movement

The disc consists of the annulus fibrosus (concentric layers of fibrocartilage) and the nucleus pulposus (a gelatinous core that is $88\%$ water). The disc functions like a hydraulic shock absorber.

Flexion: The nucleus moves posteriorly.
Extension: The nucleus moves anteriorly.
Lateral Inclination: The nucleus moves to the side of convexity (the opposite side of the tilt).
Rotation: The annulus undergoes shearing stress, and the height of the disc decreases.

The Cervical Spine

The cervical spine is divided into the upper cervical complex ( $C1$ /Atlas, $C2$ /Axis, and the Occipital) and the lower cervical spine ( $C2$ to $T1$ ).

Atlas ( $C1$ ): Lacks a vertebral body; has two lateral masses that articulate with the occipital condyles (atlanto-occipital joint), allowing for "yes" nodding (flexion/extension).
Axis ( $C2$ ): Features the odontoid process (dens), which acts as a pivot for the rotation of the Atlas (atlanto-axial joint).
Lower Cervicals ( $C3-C7$ ): Feature uncinate processes and uncovertebral joints, which are unique to the cervical region and contribute to lateral flexion and rotation coordination (Fryette’s Second Law: side-bending and rotation occurring to the same side).

The Thoracic (Dorsal) Spine and Rib Cage

The thoracic region (12 vertebrae) is the least mobile due to its attachment to the ribs. It protects the vital visceral organs.

Articulations: Includes costovertebral (rib head with vertebral body) and costotransverse (rib tubercle with transverse process) joints.
Rib Mechanics: During inspiration, the upper ribs demonstrate "pump handle" movement (increasing anteroposterior diameter), while the lower ribs demonstrate "bucket handle" movement (increasing transverse diameter).
Musculature: The diaphragm is the primary inspiratory muscle. Intercostal muscles facilitate expansion (external) and contraction (internal) of the rib cage.

The Lumbar Spine and Pelvis

The lumbar vertebrae are the largest, designed for weight-bearing. $L3$ is often the most mobile and horizontal vertebra, acting as a functional relay. $L5$ is a transition vertebra, often featuring a wedge-shaped body.

Sacroiliac Joint (ASI): Connects the sacrum to the iliac bones. Movement here is defined as nutation (sacral base moves anteriorly and inferiorly, while the coccyx moves posteriorly) and contranutation (the reverse).
Hip Joint (Coxofemoral): A highly stable enarthrosis (ball and socket). Stability is maintained by deep acetabular depth, the labrum, a thick capsule, and powerful ligaments like the iliofemoral (Bigelow's ligament). The "Triangle of Ward" is a zone of relative weakness in the femoral neck trabeculae.

The Knee Joint Complex

The knee is composed of the femorotibial and femoropatellar joints.

Menisci: Increase congruence and load distribution. The medial meniscus is "C" shaped and less mobile; the lateral meniscus is "O" shaped.
Patella: A sesamoid bone that increases the lever arm of the quadriceps, making it more efficient and protecting the tendon from friction.
Stability: Provided by the Cruciate Ligaments ( $LCA$ preventing anterior tibial displacement, $LCP$ preventing posterior) and Collateral Ligaments (limiting varus/valgus stress).
Alignment: Normal physiology includes a valgus angle of $170-175$ degrees. The "Q Angle" (quadricipital angle) is higher in women due to a wider pelvis.

The Ankle and Foot

Talocrural Joint: Composed of the tibia, fibula, and talus (astragalus). It is most stable in dorsal flexion because the talus is wider anteriorly, wedging into the malleolar mortise.
Subtalar Joint: Responsible for inversion (adduction, plantar flexion, and supination) and eversion (abduction, dorsal flexion, and pronation).
Midtarsal (Chopart) and Tarsometatarsal (Lisfranc) Joints: Critical for footprint dynamics and adapting to terrain.
Plantar Vault: The weight of the body is distributed from the talus to the calcaneus and the first and fifth metatarsal heads.

Temporomandibular Joint (ATM)

The TMJ is a bicondylar joint that acts as a functional enarthrosis. It is divided by an intra-articular disc into a superior compartment (for sliding/protrusion) and an inferior compartment (for hinge/rotation). The joint opens and closes $1500$ to $2000$ times per day. Opening involves two stages: an initial rotation and a subsequent anterior translation of the condyle.

The Shoulder Complex

The shoulder comprises five joints: three anatomical (glenohumeral, acromioclavicular, sternocostoclavicular) and two functional (subdeltoid, scapulothoracic).

Glenohumeral Joint: Extremely mobile but unstable. Stability depends on the glenoid labrum (ventosa effect), negative intra-articular pressure, and the "rotator cuff" (supraspinatus, infraspinatus, teres minor, subscapularis) which coapt the humeral head.
Scapulohumeral Rhythm: For every $2$ degrees of glenohumeral abduction, the scapula rotates $1$ degree.

Elbow and Wrist Mechanics

Elbow: A hinge joint (humero-ulnar) and a pivot joint (radio-ulnar). Flexion is limited by soft tissue approximation or the coronoid process; extension is limited by the olecranon process. Pronosupination involves the radius rotating around the ulna.
Wrist: Composed of the radiocarpal and midcarpal joints. Extension is the most stable position for the radiocarpal joint. The carpal tunnel, formed by the flexor retinaculum, houses the median nerve. The scaphoid bone is particularly important as it tends to flex during radial deviation.