Kinesiology levers/motion

The Third Law of Motion

Overview of Newton's Laws of Motion

1. First Law of Motion: An object at rest will remain at rest, and an object in motion will continue in motion with the same speed and in the same direction unless acted upon by a net external force. This property is known as inertia.

2. Second Law of Motion: This law states that the acceleration of an object is directly proportional to the net force acting upon it and inversely proportional to its mass. Mathematically, this is expressed as:
   $F=m\cdot a$
   where:

   • F is the net force applied to the object,

   • m is the mass of the object,

   • a is the acceleration produced.
     This means that the greater the force applied to an object, the greater the acceleration it will experience; conversely, a larger mass will result in less acceleration from the same force.

3. Third Law of Motion: For every action, there is an equal and opposite reaction. This principle means that forces always occur in pairs; if one object exerts a force on a second object, the second object exerts an equal and opposite force on the first.

Application in Physical Therapy

1. Example of Inertia:

   • A common example seen in physical therapy regarding the law of inertia is whiplash resulting from a rear-end car accident. In this scenario, the body moves forward due to inertia when the car is suddenly stopped, potentially causing injuries to the neck and spine.

   • In whiplash, the head and neck continue to move forward while the body stops, resulting in strain to the neck muscles as they attempt to stop this motion, which may lead to muscle strain if they are unable to counteract the force.

Biomechanics

1. Static and Dynamic Biomechanics:

   • Static biomechanics deals with forces acting on a body at rest while dynamic biomechanics focuses on bodies in motion. Clinical applications often emphasize dynamic biomechanics in understanding movements and injuries.

2. Forces in the Body:

   • Forces can be categorized into several types:

     • Linear forces: movement along a straight line.

     • Angular forces: movement around an axis. For example, knee extension is an angular motion as different body parts move different distances relative to the axis (the knee joint).

     • Curvilinear motion: movement that follows a curved path, such as the trajectory of a thrown football or a basketball.

3. Types of Motion:

   • Rectilinear motion: straight line movement.

   • Curvilinear motion: path that follows a curved trajectory, relevant in many sports like basketball, football, and javelin throw.

   • Angular motion: movement of an object around a fixed point, exemplified in joint movements where different segments travel different distances.

Force Applications

1. Force Couples:

   • A force couple consists of two forces that are equal in magnitude but act in opposite directions on either side of an axis, producing rotation. An example includes the upward rotation of the scapula via muscles like the upper and lower trapezius and serratus anterior. Issues arise when one muscle in a force couple is stronger or tighter, leading to improper scapular motion.

2. Types of Forces:

   • Concurrent Forces: Two or more forces acting at a common point but originating from different directions, resulting in a resultant force that defines directional movement. E.g., the deltoid muscle forces acting on the deltoid tuberosity.

3. Linear vs. Angular: Different types of motions imply specific force applications, critical in therapy and rehabilitation settings where understanding muscle dynamics is important for injury rehabilitation.

4. Compression and Distraction:

   • Compression: The act of pushing forces together (e.g., muscle contractions creating joint stability).

   • Distraction: The pulling apart of forces, typically applied in manual therapy practices.

Lever Mechanics in the Body

1. Basic Components of a Lever:

   • Fulcrum: The pivot point.

   • Effort: The applied force.

   • Resistance: The weight being moved.

2. Classes of Levers:

   • Class I Lever: Fulcrum is between load and effort (e.g., neck extension around the cervical vertebra).

   • Class II Lever: Load is between fulcrum and effort (e.g., calf raise where the pivot is at the toes, the load is the body weight through the tibia).

   • Class III Lever: Effort is between resistance and fulcrum (e.g., a bicep curl).

Stability and Center of Gravity

1. Center of Gravity (CoG):

   • Located anterior to the S2 sacral vertebra in adults, it significantly affects balance and stability.

   • In children, the CoG is higher due to proportionally larger heads, which contributes to an unsteady gait.

2. Base of Support:

   • The area beneath an object that ensures its stability. A wider base allows for better stability while a narrower base increases the difficulty of maintaining balance.

   • Stability diminishes when the CoG shifts outside the base of support, increasing the likelihood of falling.

3. Friction and Mass:

   • Greater friction between surfaces enhances stability.

   • An increase in mass results in increased gravitational force, which also contributes to improved stability.

Practical Applications in Rehabilitation

1. Assessments for Balance:

   • Functional Reach Test: Measures how far a person can reach without losing balance, indicating fall risk.

   • Tragus Wall Test: Measures the distance between the tragus of the ear and the wall; significant forward tilt may indicate an increased risk of falls.

2. Therapeutic Techniques:

   • Utilizing progressive challenges to balance, such as throwing a ball or standing on unstable surfaces, encourages improvements in proprioception and stability in patients.

The notes capture the essential elements of physics and biomechanics as they pertain to physical therapy, ensuring a comprehensive understanding of the concepts presented in the transcript.