Overview of Basic Biomechanics

Biomechanics: application of principles and methods of mechanics to the structure and function of the human body to produce stability & movement
Statics: branch of mechanics involving factors associated with non-moving systems
Dynamics: branch of mechanics involving two factors associated with moving systems: kinetics & kinematics

Kinetics: describes forces producing stabilization or movement in a system
Kinematics: describes motion created by forces and incorporates the factors of time, space, and mass of a moving system

Magnitude: indicates the amount of a measurement
Direction: the direction in which magnitude is applied
Velocity: rate of change of position and direction of that change, measured by change of distance within a given time
- Measured in distance/time (ft/s or miles/hr)
- Dependent on musculoskeletal characteristics - limb segment movement & recruited muscles for movement
- Speed is scalar, not vector; does not account for direction in addition to mag
Acceleration: rate of change of velocity (magnitude) and direction of that change

Force: amount & direction of push or pull applied to objects or body segments
- Push - creates compression as two objects/segments are pushed together
- Pull - creates traction as two objects/segments are pulled away from each other
- Internal force: includes muscular contraction, ligamentous restraint, or bony support
- External force: includes gravity or any externally applied resistance
Gravity: mutual attraction between earth & an object
- - Gravitation force: force exerted on an object/person as a result of gravity
  - Weight: result of a gravitation force and mass of an object and always pushes directly downward
  - Ground reaction: the upward force a supporting surface exerts on an object when a person pushes down on the surface
Friction: force between two surfaces that increases resistance to motion of one surface across another; impediment to motion

Acts on Tissues & Joints

Traction: forces cause joint distraction, in which joint surfaces pull apart from one another, placing tension on tissue that holds the joints together
- Ex. Hanging from overhead bar
Compression: forces that cause joint approximation in which joint surfaces are pushed closer together
- Ex. Push ups - shoulder, elbow and wrist joints are approximated
- Prolonged compression = damage
Shear: forces causing a gliding motion in which joint surfaces move parallel to one another
- Ex. Knee extension - femur rotates on the tibia = shear
- Excessive forces = tissue strain/rupture
- Like scissors that when open, move in different directions
Bending: occurs when a force is not applied at the central axis of an elongated object and the object bends, creating a concave surface on one side and convex surface on the opposite side
- Compression at concave side and traction on the convex
- Ex. Spinal flexion --> bending at vertebrae --> decrease in height
Torsional: where two opposing forces creating twisting within an object
- Can lead to spiral fractures
- Torsion = Twist

Law of Inertia (Law 1): states that an object either stays at rest or remains in motion in a constant state, unless acted upon by an external force
Law of Acceleration (Law 2): defines the relationship between force (F), mass (m), and acceleration (a)
- (a) of an object is inversely related (as one increase, the other decreases) to the (m) of the object
- (a) is directly proportional to the (F) applied to the object
- F=m*a
- Force is necessary to overcome the inertia of an object
- The greater the mass, the more force is needed to act on the object
- The same force applied to objects of differing mass - object with greater mass will accelerate less than the object with less mass
Law of Action-Reaction (Law 3): states that for every action, there is an equal and opposite reaction
- The magnitude of the reaction is always equal to the magnitude of the action and occurs in the opposite direction
- Ex. Jumping on trampoline - action = force applied to the tramp by the person landing; reaction = force the tramp exerts back on the person --> rebound and person being pushed back in the air
- When the person lands on the tramp with a greater force, the rebound is greater because the reaction force is greater

Equilibrium: state exists when the sum of all forces acting on an object is equal to zero
Center of mass (COM): the point at which the sum of the mass of all body segments is located
- Depends on the position or configuration of the different body parts
Center of gravity (COG): point at which gravity acts on the COM
Base of support (BOS): area encompassed by a body's contact with a supporting surface
Line of gravity (LOG): imaginary vertical line passed through the COG toward the center of earth
- Degree of stability is determined by the interaction of BOS, COG, and mass of object/forces applied
- Since the COG is within the BOS, no forces act to destabilize the body

Angular motion (rotary motion): movement of an object around a fixed point (axis)
- All parts of the object move through the same angle in the same direction at the same time but not move the same distance
- Accounts for most body movements
- Movement outside the body tends to be linear

Walking is an example of simultaneous combined linear and angular motion - whole body exhibits linear motion when walking from point A to point B while hips, knees, and ankles exhibit angular motion

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