Comprehensive Study Notes on Kinesiology and Biomechanics

Fundamentals of Kinesiology and Biomechanics

Kinesiology is derived from the Greek word "Kineses," which translates to motion. It is defined as the scientific study of human body movement. In the academic sphere, kinesiology serves as an umbrella term for the scholarly study of human movement, encompassing various sub-disciplines including biomechanics, sport physiotherapy and medicine, motor development and learning, sport pedagogy, and sport and exercise psychology. Biomechanics specifically focuses on the study of motion and its causes in living things through the lens of physics and mechanics.

In the context of health care, clinical kinesiology is studied to understand the movement and forces acting on the human body. The primary objectives are to learn how to manipulate these forces to prevent injury, restore physical function, and achieve optimal human performance.

A distinction must be made between physiotherapy and kinesiology: a physiotherapist typically assesses and diagnoses sports injuries while working to reduce pain and swelling. In contrast, kinesiology focuses on the active component of rehabilitation. It is common for a physiotherapist to refer a patient to a kinesiologist after initial symptoms have lessened to begin this active phase of recovery.

The Mechanics of Human Movement

Biomechanics is defined as the study of the movement of living things using the science of mechanics (Hatze, 1974). Mechanics is a branch of physics concerned with the description of motion and how forces create that motion. For any movement to occur, a net force must be present. A wide variety of professions utilize biomechanics, including sports coaches, orthopedic surgeons, physical educators, physical therapists, prosthetists and orthotists, athletic trainers, strength and conditioning professionals, and occupational fitness consultants.

Mechanics is divided into three main areas relevant to biomechanics:

Rigid-body mechanics: This assumes that deformation of the body is so small that it can be ignored.
Deformable-body mechanics: This studies how forces stimulate growth or cause trauma and deformation.
Fluid mechanics: This involves the mechanics of gases and liquids.

Statics, Dynamics, and Kinematics

Rigid-body mechanics is further subdivided into statics and dynamics. Statics is the study of objects at rest or in uniform, constant motion. Dynamics is the study of objects being accelerated by the action of forces. Dynamics is divided into two branches: kinetics and kinematics.

Kinetics concentrates on the forces that produce or resist movement. Kinematics deals with the types of motion or movement without regard for the forces that produce them. Kinematics descriptors include the type, direction, and quantity of motion. It utilizes a three-dimensional coordinate system to describe the orientation of the body and segments in space.

Kinematics is divided into:

Osteokinematics: Concerns the movements of the bony partners or segments that make up a joint.
Arthrokinematics: Focuses specifically on the minute movements occurring within the joint and between joint surfaces.

Planes and Axes of Motion

The human body moves in three cardinal planes of motion, rotating around three corresponding axes (x, y, and z). This is based on the Cartesian coordinate system, where the origin is traditionally located at the center of mass ( $CoM$ ). As a segment rotates around an axis, it moves in a plane perpendicular to that axis and parallel to another.

Frontal Plane ( $XY$ Plane): Also known as the coronal plane. It is parallel to the frontal bone and coronal skull suture, dividing the body into front and back parts. It rotates around the anterior-posterior axis. Movements include abduction and adduction (hip, shoulder, digits), ulnar and radial deviation (wrist), and lateral flexion (neck, trunk).
Horizontal Plane ( $XZ$ Plane): Also known as the transverse plane. It is parallel to the horizon/floor and divides the body into upper and lower parts. It rotates around the longitudinal or $y$ -axis. Movements include medial and lateral rotation (hip, shoulder), pronation and supination (forearm), and eversion and inversion (foot).
Sagittal Plane ( $YZ$ Plane): Parallel to the sagittal suture of the skull, dividing the body into right and left sides. The axis of motion is the medial-lateral axis. Movements include flexion and extension (neck, trunk, elbow), and dorsiflexion and plantarflexion (ankle).

Dimensions and Types of Displacement

Motion can be classified into three types of displacement:

Translatory motion (linear displacement): Movement of a segment in a straight line.
Rotatory motion (angular displacement): Movement of a segment around a fixed axis (center of rotation [ $CoR$ ]) in a curved path.
Curvilinear (planar) motion: A combination of translation and rotation in two dimensions. The axis around which the segment move in any part of its path is the Instantaneous center of rotation ( $ICoR$ ) or Instantaneous axis of rotation ( $IaR$ ).

Range of Motion ( $ROM$ ) describes the magnitude of rotatory motion. It is clinically measured using goniometry in degrees ( $^{\circ}$ ). In the International System of Units ( $SI$ ), it is measured in radians. A radian is the ratio of an arc to the radius of its circle.

Conversion constants:

$1\text{ radian} = 57.3^{\circ}$
$1^{\circ} = 0.01745\text{ radians}$

Rate of displacement includes:

Speed: Displacement per unit time regardless of direction.
Velocity: Displacement per unit time in a given direction.
Acceleration: The change in velocity per unit time ( $m/sec^2$ ).
Linear velocity: Expressed as $m/sec$ ( $SI$ ) or $ft/sec$ ( $US$ ).
Angular velocity: Expressed as $deg/sec$ .

Kinematic Chains and Arthrokinematics

Kinematics refers to articulated segmental links, such as the pelvis, thigh, leg, and foot. An Open Kinematic Chain occurs when the distal segment rotates against a relatively fixed proximal segment. A Close Kinematic Chain occurs when the proximal segment rotates against a relatively fixed distal segment.

Arthrokinematics (accessory movements) describe how joint surfaces move relative to one another. While physiologic movement is voluntary, accessory movements normally accompany it. There are three types of surface motions:

Roll: Multiple points along one rotating surface contact multiple points on another (e.g., a tire rotating on a road).
Slide: A single point on one surface contacts multiple points on another (e.g., a tire skidding on ice).
Spin: A single point on one surface rotates on a single point on another (e.g., a toy top spinning on one spot).

Joint Position and Forces

Joint surface congruency determines stability. In a Close-packed position (or Closed-pack), joint surfaces have maximum contact, are tightly compressed, and ligaments/capsules are taut. Examples include full extension for the knee, wrist, and interphalangeal joints, and full dorsiflexion for the ankle. An Open-packed (or Loose-packed/Resting) position is any other position where congruency is minimized, reducing stability. Joint mobilization uses three main forces: Traction, Compression, and Shearing. Bending and torsional forces result from combinations of these.

Kinetics and Force Principles

Kinetics relates motion to its causes: forces and torques. The $SI$ unit of force is the Newton ( $N$ ), defined as $1\,kg \cdot m/s^2$ . Forces can stabilize the body but also have the potential to deform or injure it. Tissues are viscoelastic and exhibit "creep," which is the progressive strain of material under a constant load over time.

Injury depends on load characteristics (type, magnitude, rate, frequency) and tissue characteristics (material and structural properties). Internal forces are produced from structures within the body (active or passive). External forces come from outside (gravity, weights, physical contact, water, wind). Gravity is the most consistent external force.

Force vectors are represented by arrows: the base is the point of application, the shaft and arrowhead indicate direction/orientation, and the length indicates magnitude.

Gravity, Balance, and Stability

The center of gravity ( $COG$ ) is a hypothetical point where the mass of the body is concentrated. In the anatomic position, the $CoM$ of the human body lies approximately anterior to the second sacral vertebra ( $S2$ ). The Line of Gravity ( $LOG$ ) is the vertical projection of the $CoM$ to the ground.

Stability is determined by the Base of Support ( $BOS$ ):

A larger $BOS$ increases stability.
A lower $CoM$ (closer to the $BOS$ ) increases stability.
An object is only stable if its $LOG$ stays within its $BOS$ .

Equilibrium is a state where opposing forces or influences are balanced. Static equilibrium implies the body is at rest; dynamic equilibrium implies uniform motion. In both cases, the net force and net torque must be zero.

Newton's Laws and Mechanical Advantage

First Law (Law of Inertia/Equilibrium): An object remains at rest or in uniform motion unless acted upon by an unbalanced force. $\sum F = 0$ . Inertia is the resistance to change in motion and is proportional to mass.
Second Law (Law of Acceleration): Acceleration ( $a$ ) is proportional to the net force ( $F$ ) and inversely proportional to mass ( $m$ ). $a = \frac{F}{m}$ .
Third Law (Law of Reaction): For every action, there is an equal and opposite reaction.

Torque (or Moment of Force) is the strength of rotation produced by a force couple (two equal, opposite, non-collinear forces). It is the product of force magnitude and the moment arm ( $MA$ ), which is the shortest perpendicular distance between forces ( $T = F \times MA$ ).

Levers, Pulleys, and Inclined Planes

A lever is a rigid bar revolving around a fixed point called a fulcrum (axis).

First Class Lever: The axis is between the force and resistance (e.g., the atlanto-occipital joint in the neck).
Second Class Lever: The resistance is between the axis and the force.
Third Class Lever: The force is between the axis and the resistance.

Anatomic pulleys (bony prominences) alter the direction of muscle pull. For instance, the patella increases the moment arm of the quadriceps, allowing the same force to produce greater torque. Mechanical pulleys can be fixed or movable. A movable pulley requires half the force to move a weight but requires the rope to be pulled twice as far.

An inclined plane (e.g., a wheelchair ramp) demonstrates a trade-off: a longer ramp requires less force but covers a greater distance to reach the same height, whereas a shorter ramp requires more force over a shorter distance.