OTA 125: Kinesiology Chapter 2 Basic Biomechanics

I. Introduction

  • The musculoskeletal system is the internal system responsible for producing human movement.

  • The function of the musculoskeletal system is described by mechanics.

  • Mechanics applied to the human body is known as biomechanics.

A. Areas of Mechanics

  1. Statics

    • Focuses on factors associated with non-moving systems.

  2. Dynamics

    • Concerned with factors associated with moving systems.

    • Further divided into:

    • Kinetics: The study of forces producing stabilization or movement in a system.

    • Kinematics: The study of motion produced by forces, which incorporates factors of time, space, and mass.

B. Vectors

  • A vector describes both magnitude and direction.

  • Common vector measures include:

    • Force

    • Velocity

    • Acceleration

II. Force

  • Defined as the amount of push or pull applied to objects or body segments.

    • A push creates compression, while a pull creates traction.

A. Types of Forces

  1. Internal Forces:

    • Includes muscle contractions and ligamentous restraints.

  2. External Forces:

    • Includes gravity and any externally applied resistance.

B. Gravity

  • Described as the mutual attraction between the Earth and an object.

    • Gravitational Force: The force exerted on an object or person by gravity.

    • Weight: The result of a gravitational force multiplied by the mass of an object, always directed downward.

    • Ground Reaction Force: The upward force exerted by a supporting surface when an object pushes downward on it.

C. Friction

  • A force between two surfaces that increases resistance to the motion of one surface across the other.

    • Increases with compression and decreases with traction.

D. Types of Forces Acting on Objects

  1. Linear Forces:

    • Two or more forces acting along the same line.

  2. Parallel Forces:

    • Forces acting in the same plane and in the same or opposite directions.

  3. Force Couple:

    • A specific configuration of parallel forces where two or more forces act in different directions, producing clockwise or counterclockwise rotation.

  4. Concurrent Forces:

    • Two or more forces acting on an object with pushes or pulls in different directions.

    • Resultant Force Vector: Represents the sum of the magnitudes and directions of each force vector, indicating the overall magnitude and direction of movement resulting from all applied forces.

E. Effects of Force Application

  1. Traction:

    • Pulling apart of joint surfaces.

  2. Compression:

    • Pushing closer together of joint surfaces.

  3. Shear:

    • Gliding motion of joint surfaces parallel to one another.

  4. Bending:

    • Occurs when a force is not applied at the central axis of an object, causing it to bend:

      • Has a concave surface on one side and a convex surface on the other.

      • Results in compression on the concave side and traction on the convex side.

  5. Torsion:

    • Two opposing forces twisting within an object in opposite directions.

F. Motion Measurements

  1. Velocity:

    • Measured as change of distance over a given time, expressed in:

      • Feet/second

      • Miles/hour

  2. Acceleration:

    • Measured by change in velocity over time, expressed in:

      • Feet/second/second

      • Miles/second/second

III. Torque (T)

  • Defined as the tendency of a force to produce rotation about an axis.

  • The amount of torque generated depends on:

    • The amount of force applied (F).

    • The distance from the axis where the force is applied (Moment Arm, MA).

  • The formula for torque is given by: T = F imes MA

    • Torque increases with either an increase in applied force (F) or an increase in moment arm (MA).

A. Angle of Application of Force

  • Refers to the angle at which force is applied to a limb segment.

  • When force is applied perpendicularly to a limb segment, all force is used to generate torque.

  • When force is applied at an angle other than perpendicular:

    • Only the component of the force that is perpendicular to the limb segment generates torque.

    • The remaining component generates compression or traction through the axis of motion.

IV. Newton’s Laws of Motion

  1. First Law:

    • The law of inertia states that an object will stay at rest or move uniformly in a straight line unless acted upon by an external force.

  2. Second Law:

    • The law of acceleration states that the acceleration (a) of an object is inversely related to its mass (m) and directly proportional to the amount of force (F) applied.

    • This relationship can be expressed with the formula:
      F = ma

    • Where F is the force applied, m is the mass of the object, and a is the acceleration produced.

  3. Third Law:

    • The law of action-reaction states that for every action, there is an equal and opposite reaction.

V. Equilibrium and Stability

  • Equilibrium occurs when the sum of all forces acting on an object is equal to zero, resulting in no motion.

    • This equilibrium is dependent on the relationship between the center of mass, center of gravity, and base of support of the object.

A. Key Concepts

  1. Center of Mass (COM):

    • The point at which the sum of the mass of all body segments is located.

  2. Center of Gravity (COG):

    • The point at which gravitational force acts on the center of mass.

  3. Base of Support (BOS):

    • The area encompassed by the body's contact with the supporting surface.

  4. Line of Gravity (LOG):

    • An imaginary vertical line passing through the center of gravity toward the center of the Earth.

B. Factors Affecting Stability

  • More Stable:

    • Large base of support (BOS)

    • Wide BOS in the direction of disturbance

    • Lower center of gravity (COG) relative to BOS

    • Centered COG within the BOS

    • Greater mass

    • Increased friction between object and BOS

  • Less Stable:

    • Small base of support (BOS)

    • Narrow BOS in the direction of disturbance

    • Higher center of gravity (COG) relative to BOS

    • COG close to the margin of the BOS

    • Lesser mass

    • Decreased friction between object and BOS

VI. Types of Motion

  1. Linear Motion:

    • All parts of an object move the same distance at the same time.

  2. Curvilinear Motion:

    • Motion that occurs in a curved path, which is not circular.

  3. Angular Motion:

    • Movement of an object around a fixed point (axis).

    • All parts of the object move through the same angle, in the same direction, and at the same time, but do not move the same distance.

    • The joint acts as the axis of rotation.

VII. Simple Machines

  • Defined as tools that allow for a change of effort (F) or direction of applied force required to lift a load, or both.

  • There is an inverse relationship between the amount of force (F) and the distance over which the force must be exerted.

  • Consideration is given to both the magnitude and direction of the force vector.

A. Types of Simple Machines

  1. Levers

  2. Pulleys

  3. Inclined Planes

B. Mechanical Advantage

  • The relationship between the force expended and the load moved.

C. Levers

  • A lever is a rigid plank that can rotate about a fulcrum when forces are applied.

    • Fulcrum: The axis of the lever.

    • Levers have two moment arms:

      • Force Arm (FA): The distance from the axis to the point where the force is applied.

      • Resistance Arm (RA): The distance from the axis to the point where the resistance (load) is applied.

    • Representation of biological structures:

      • Joints act as axes; limb segments function as lever arms; muscles and weights act as forces.

D. Classes of Levers

  • First Class:

    • The axis is positioned between the force and the resistance.

    • Arrangement: F – A – R or R – A – F

  • Second Class:

    • The resistance is located between the axis and the force.

    • Arrangement: A – R – F or F – R – A

    • Mechanical advantage: FA is always longer than RA.

  • Third Class:

    • The force is located between the axis and the resistance.

    • Arrangement: A – F – R or R – F – A

    • Mechanical advantage: RA is always longer than FA.

E. Changing Class of Lever

  • A second-class lever can change to a third-class lever and vice versa, influenced by factors such as:

    • Increased load

    • Change of direction of movement

VIII. Pulleys

  • Defined as a grooved wheel that turns about an axis with a rope in the groove, providing a means to apply force and handle a load.

    • Utilized to change the direction of a force applied to lift a load or to change the magnitude of the force.

    • In the human body, the “wheel” is represented by a section of bone (e.g., malleolus), while the “rope” corresponds with a tendon or muscle.

A. Types of Pulleys

  1. Fixed Pulley:

    • A single pulley attached to a fixed point, functioning as a first-class lever.

  2. Movable Pulley:

    • Combines a fixed pulley to modify direction and a moving pulley to adjust the magnitude of force applied when lifting a load.

IX. Inclined Plane

  • Defined as a slanted surface that rises from one side to another, typically referred to as a ramp.

    • Requires less force over a greater distance to lift a load.

    • Federal Requirements for Wheelchair Ramp:

      • For every 1 inch of rise, there must be at least 1 foot of run.