kines Law of Motion

Third Law of Motion

Overview of Newton's Laws

  • First Law of Motion: An object remains at rest or continues in motion unless acted upon by an external force.
  • Second Law of Motion: This law describes acceleration in relation to force and mass.
    • Formula: F=mimesaF = m imes a
    • Where:
      • FF = Force
      • mm = Mass
      • aa = Acceleration
    • Explanation: The acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass.

Examples in Physical Therapy

  • Common Traumatic Event: Whiplash in car accidents.
    • Mechanism:
    • During a rear-end car collision, the body tends to keep moving forward due to inertia, while the head and neck remain stationary until a force acts upon them (e.g., the head hitting the chest, airbag deployment).
    • This sudden stop can strain neck muscles as they try to control the motion.

Biomechanics Concepts

  • Types of Biomechanics:
    • Static Biomechanics: Concerned with non-moving bodies (e.g., posture).
    • Dynamic Biomechanics: Focuses on moving bodies, which is often the focus in physical therapy.
Forces in the Body
  • Types of Forces:
    • Linear Forces: Movements in a straight line (e.g., rectilinear, curvilinear).
    • Angular Forces: Movement around an axis.
    • Linear Motion Examples:
      • Rectilinear Motion: Movement in a straight line, like walking.
      • Curvilinear Motion: Movement in a curved path, such as throwing a football or shooting a basketball.
        • Definition: All parts of the object move the same distance along a curve.
    • Angular Motion Example:
      • When the knee extends, the tibia moves less than the ankle, as the motion occurs around a joint axis.
Other Types of Motion
  • Curvilinear Motion: Defined as a motion that follows a curved path (e.g., long jump, javelin throw).
    • Examples include any thrown object that follows a parabolic trajectory.

Force Relationships

  • Parallel Forces: Forces acting in the same plane but opposite directions (e.g., back braces applying compressive forces).
  • Concurrent Forces: Combined forces acting on a common point, like the forces exerted by the deltoid muscle groups converging on the deltoid tuberosity.
  • Resultant Force: The vector sum of two or more forces acting at a point.
    • Example: The resultant force of the anterior and posterior deltoid actions will illustrate muscle imbalances if unequal.
Force Coupling
  • Definition: A force couple consists of two equal magnitude forces acting in opposite directions on either side of an axis.
    • Example: Scapular upward rotation involves three muscles (upper trapezius, lower trapezius, and serratus anterior) that produce rotary force without linear translation.
    • Importance: An imbalance amongst these muscles can lead to shoulder instability or dysfunction.
Additional Forces and Types
  • Compression: Reduction of joint space, often occurring during muscle contraction. Examples include carrying objects which approximate joint surfaces.
  • Distraction: Separation of joint surfaces, usually occurring in actions like pulling or carrying weight away from the body.
Shearing and Bending Forces
  • Bending: Involves compression on the concave side and tension on the convex side of a structure (e.g., spinal movements).
  • Shearing Forces: Involves gliding motions where surfaces move parallel to each other, such as in joint movements.

Torque and Joint Mechanics

  • Torque: The capacity to produce rotation around an axis due to force applied at a distance (lever arm).
    • Example: Carrying a box near the body to reduce torque on the lumbar spine.
    • Importance in lifting mechanics; lifting with a longer lever arm increases torque and mechanical stress on the back.
Moment Arm
  • Definition: The perpendicular distance between the force line and the joint axis, changing depending on joint position mainly at specific angles of movement.
    • Torque is maximized at a 90-degree angle during joint actions, requiring the greatest muscular effort.
Stability Concepts
  • Center of Gravity (COG): The balance point where torque is equal on all sides; varies with body shapes and movements of objects.
    • For adults, COG is located anterior to the S2 sacral vertebra.
    • COG becomes higher in children due to larger head-to-body ratio.
  • Base of Support: The area beneath an object that includes all points of contact. A wider base increases stability.
    • Example: Athletic positions encourage a lowered COG for better balance and increased force generation.
Lever Systems in the Body

  • Three Components of a Lever: Fulcrum (axis), force (muscle), and resistance (gravity/load).

  • Types of Levers:

    • Class I: Fulcrum in the middle; examples in the spine.
    • Example: Head balancing on the cervical spine.
    • Class II: Load in the middle; common in functional movements like calf raises (fulcrum at toes, resistance is body weight).
    • Class III: Most common in human joints; effort applied between load and fulcrum (e.g., bicep curls).
    • Efficient for motion but not for power.

  • Recommendation to familiarize yourself with the impact of lever classes on mechanics and injury prevention, especially in resistance exercises.