Biomechanical Principles: Internal and External Forces

Internal forces in the musculoskeletal system are produced by structures located outside the body.

Passive forces in the musculoskeletal system are typically generated by tension in stretched periarticular connective tissues.

External forces acting on the musculoskeletal system are mainly due to the force of attraction between two objects.

Sir Isaac Newton's observations in the 17th century laid the foundation for understanding human movement analysis.

Newton's laws of motion include the Law of inertia, Law of acceleration, and Law of resistance.

Active forces in the musculoskeletal system are generally produced involuntarily.

The outcome of all movement analysis is solely determined by external forces acting on the body being moved.

Newton's principles of mechanics are not relevant to human movement analysis.

Forces are scalar quantities.

Internal forces include gravity.

The center of gravity (CoG) of the body is always at the second sacral vertebra (S2).

To be stable, the line of gravity (LoG) must fall outside the base of support.

The larger the base of support, the lower the stability of an object.

The closer the center of gravity is to the base of support, the less stable the object is.

Internal forces include muscle.

The action line of gravity (LoG) is oriented vertically downward toward the moon.

The mass moment of inertia depends only on the mass of the body.

Linear acceleration is directly proportional to the mass of the body.

The formula for rotational movement is α = M/I.

If the sum of forces acting on a body is zero, acceleration is also zero.

A body will accelerate in the opposite direction of the resultant force.

Linear acceleration is directly proportional to the force causing it.

Angular acceleration is inversely proportional to mass moment of inertia.

Torque acceleration relationship formula is M = I * α.

The upper trapezius and serratus anterior are muscles that work together to produce a moment.

The situation where two muscles with different magnitudes and orientations work together is referred to as a force couple.

Kinetic friction is always greater than static friction.

Frictional forces can only resist the movement of an object when it is at rest, not when it is in motion.

Increasing the perpendicular forces pressing two objects together can decrease static friction.

To initiate dragging an object across a surface, it takes less force than to continue dragging it.

Friction resistance only occurs between two contacting surfaces.

A force couple refers to the situation where two muscles have identical magnitudes and orientations.

A second-class lever is common in the musculoskeletal system.

The calf muscles act as the external force in a second-class lever.

In a third-class lever, the muscle force always has greater leverage than the external weight.

Third-class levers provide more stability compared to second-class levers.

Vector resolution is about breaking multiple forces into a single force.

The axis of rotation in a third-class lever is located at one end of a bone.

The internal and external forces in a system always act in opposite linear directions.

Musculoskeletal levers are commonly used to rapidly accelerate objects in smaller ranges of motion.