Movement Science Week 3 - Kinetics Review

Study Notes

Kinetics studies forces acting on material bodies, including the human body, body segments (e.g., finger, ankle, joint), or an entire body.
Forces produce, arrest, or stop motion in the whole body or individual segments.

Internal forces originate within the body; muscles are a primary example.
External forces act on the body from outside sources. Gravity is the most common; others include pushes, pulls, and shoves.

The lecture notes mention a prior kinematics lecture; the difference between kinetics and kinematics should be known from previous learning.

Forces have magnitude (quantity of motion, measured in units like feet, pounds, inches, etc.) and direction (positive and negative components).
Human motion is three-dimensional (x, y, and z axes), with x representing left/right, y representing up/down (superior/inferior), and z representing anterior/posterior.

Osteokinematic motion, movement produced by forces, is demonstrated using axes:
- Medial-lateral (ML) axis (x-axis): allows for flexion and extension.
- Anterior-posterior (AP) axis (z-axis): allows for abduction and adduction.
- Vertical axis: allows for rotation.
Joints have varying degrees of freedom and orientations of axes of rotation determining the allowed motions.

Tug-of-war illustrates internal and external forces, their magnitudes, and directions.
The outcome depends on whether forces are equal or unequal.

Gravity: the weight of an object, the most prevalent force.
Muscle force: internal force; active (contraction) and passive (stretching) components due to viscoelastic properties.
Externally applied resistance: pulleys, dumbbells, manual resistance (e.g., manual muscle testing).
Friction: force between contacting objects in motion.

Weight: force of gravity on an object; varies with gravitational pull (e.g., less on the moon or at higher altitudes).
Mass: amount of matter in an object; remains constant regardless of location.

Mass: typically measured in kilograms (SI unit); slugs are also used in British and US systems, but the lecture does not typically use slugs.
Weight: usually measured in pounds or Newtons.

Moment: product of force and linear distance; quantifies force size.
Moment arm: perpendicular distance between the axis of rotation and the line of force.
Torque: product of force and moment arm in rotary motion; similar to a moment but specifically for rotational movement.

Reducing the moment arm by flexing the knee (shortening the distance between the weight and the axis of rotation) decreases torque on the hip joint.
This is useful in rehabilitation to reduce stress after surgery or injury.
Internal moment arm: perpendicular distance between muscle and axis of rotation.
External moment arm: perpendicular distance between an external force and the axis of rotation.
Reducing the moment arm (from 4.5 feet to 4 feet) reduced torque from 450 foot-pounds to 400 foot-pounds while keeping the force constant (10 pounds).