Kinesiology and Biomechanics Introduction
Fundamentals of Kinesiology and Biomechanics
Kinesiology is the study of human movement, integrating anatomy, physiology, and basic biomechanics. It provides a foundation for understanding how the body normally functions, which is essential for identifying and treating abnormalities caused by injury.
The primary goal in therapy is to restore a patient's ability to perform activities they did prior to an injury or incident by returning them to "normal" functional movement.
For a thorough understanding of movement, one must consider both the musculoskeletal system (muscles, bones, joints, ligaments, and tendons) and eventually the nervous system.
Basic Mechanical Forces
Forces are described as a push or a pull exerted through a body segment. These forces are categorized based on their source.
Internal Forces: These originate within the body. The most common example is the pull of the muscles on bones.
External Forces: These originate from outside the body.
Gravity: The most persistent external force pulling everything toward the center of the earth.
Weight: Objects such as dumbbells or backpacks provide external resistance.
Ground Reaction Force: The force exerted by the ground back onto the body in response to the force the body exerts on the ground. A harder impact results in a larger ground reaction force.
Kinematics and Kinetics
Movement terminology is divided into how the limbs move and how the joint surfaces interact.
Osteokinematics: Describes the motion of bones or limbs through space. These are the visible movements typically associated with exercise, such as abduction, adduction, flexion, and extension.
Arthrokinematics: Refers to the movement that occurs specifically between the joint surfaces. Every joint has arthrokinematic motion.
The three basic types of arthrokinematic movement are roll, spin, and glide.
For example, in shoulder abduction, there is specific roll, spin, or glide occurring between the surface of the humerus and the scapula.
Muscle Direction and Line of Pull
The way a muscle is aligned—its line of pull—determines the movement it produces at a joint.
Deltoids Example: The deltoid muscle is composed of three parts with different lines of pull:
Anterior Deltoid: Primarily responsible for shoulder flexion.
Middle Deltoid: Responsible for shoulder abduction.
Posterior Deltoid: Primarily responsible for reaching back (horizontal abduction or extension).
When all three components fire together, the result is shoulder abduction.
Kinetic Chains
The body is functionally connected; an injury in the lower limb can impact the trunk, which can then impact the upper extremities.
Closed Kinetic Chain: The distal segment of the limb is fixed (stationary), while the proximal segment moves.
Examples: Squats (feet are fixed), push-ups (hands are fixed), standing up from a chair, or pistol squats.
Open Kinetic Chain: The distal segment is free to move while the proximal segment remains fixed or stable.
Examples: Bicep curls, riding a bike, or a "long arc quad" (extending the knee while sitting in a chair).
Hybrid Movements: Some activities like walking are a combination of both open and closed kinetic chains, transitioning through cycles of each.
Degrees of Freedom
Degrees of freedom refer to the number of planes in which a joint can move.
Uniaxial: Movement occurs in one plane around one axis.
Example: The elbow joint allows only flexion and extension in the sagittal plane.
Biaxial: Movement occurs in two different planes around two axes.
Example: The wrist allows for flexion/extension and radial/ulnar deviation.
Mnemonic for deviation: "Radial" is the thumb side because the thumb is "rad." "Ulnar" is the pinky side because it's "PU" (pinky-ulnar).
Triaxial: Movement occurs in three planes around three axes. This is the most mobile type of joint.
Example: The shoulder and hip. Both allow flexion/extension (sagittal), abduction/adduction (frontal), and internal/external rotation (transverse).
Measurement: Range of motion is measured using a tool called a goniometer, and measurement always begins from the anatomical position.
Scalar vs. Vector Quantities
In clinical documentation, it is important to distinguish between simple magnitude and directional movement.
Scalar: Includes only magnitude (e.g., "The patient walked 50 feet with a rolling walker").
Vector: Includes both magnitude and direction, providing a more thorough clinical picture (e.g., "The patient performed elbow flexion for 20 repetitions with a 10-pound weight").
Types of Loading and Joint Force
Compression (Approximation): Forces that push joint surfaces closer together. This happens naturally during weight-bearing, such as standing. Wearing a backpack on both shoulders causes compression in the spine.
Traction (Distraction): Forces that pull joint surfaces apart. This can be used in treatment to alleviate joint pain by creating space. Carrying a heavy bag at one's side causes traction at the shoulder joint.
Shear: Occurs when two surfaces slide across one another or in opposite directions. This is a common cause of injury, such as an ACL tear when an athlete stops and turns suddenly.
Bending: A combination of compression and traction. For example, in a spine with poor posture (slumping), the side tilted toward the movement is compressed (concave), while the opposite side is stretched via traction (convex).
Torsion: A twisting force involving a rotational component. An example is a spiral femur fracture, where a person's foot is planted and the body twists with enough force to spiral the bone.
Torque and Levers
Torque is the amount of force required to create rotation around a joint. In mechanics, distance matters:
The further away the resistance is from the axis (the joint), the more force is required to move the object.
Manual Muscle Testing (MMT): Clinicians can change the effort a patient must exert by moving their hand placement. Applying resistance distally (at the wrist for a shoulder test) is much harder for the patient to resist than applying it proximally (above the elbow).
The Three Classes of Levers
First-Class Lever: The axis is located between the force and the resistance.
Human Body Example: The head on the neck.
Mechanical Goal: Balance.
Second-Class Lever: The resistance is located between the axis and the force.
Human Body Example: A heel raise (standing on toes). The calf muscles (gastroc) allow the body to lift its entire weight easily.
Mechanical Goal: Strength and Power (like a wheelbarrow).
Third-Class Lever: The force is located between the axis and the resistance.
Human Body Example: A biceps curl.
Mechanical Goal: Speed and Distance. This is the most common lever type in the human body.
Equilibrium and Stability
Equilibrium is achieved when the sum of all internal and external forces acting on the body equals zero (). Stability and mobility exist in a balance termed homeostasis.
Center of Mass (COM): The point where all body mass is centered, typically located at the second sacral vertebra ().
Center of Gravity (COG): The point where gravity acts on the center of mass.
Base of Support (BOS): The area between the points of contact with the floor (e.g., the space between the feet or between a walker and the feet).
Increasing Stability:
Maintaining a lower center of gravity (e.g., a semi-squat).
Widening the base of support.
Keeping the line of gravity centered within the base of support.
Increasing the mass of the object.
Increasing friction between the object and the surface.
Questions & Discussion
Question: Does the shoulder have four degrees of freedom since it does horizontal abduction/adduction?
Response: No, it has three degrees of freedom. The rotations and horizontal abduction/adduction both occur in the transverse plane around a vertical axis. Degrees of freedom are counted by the number of planes a joint moves through.
Question: Can circumduction happen in the trunk or neck?
Response: Yes, although it is most commonly discussed regarding the arm at the shoulder. Neck circles involve a combination of flexion, extension, and lateral flexion. Trunk circumduction is possible but is generally considered less functional than arm circumduction.
Question: When studying levers for a test, how much detail is needed regarding the axis, resistance, and force arm?
Response: Focus on understanding the three different classes, their main goals (balance, power, or speed), and the body examples for each. Knowing that distal resistance is harder to overcome is more helpful than getting lost in the deep physics of lever arms.