articular stability

What is a vector?

A quantity with both magnitude and direction, often represented by arrows. Force and acceleration are examples of vectors.

What is Newton’s 1st Law of Motion?

An object at rest stays at rest and an object in motion stays in motion unless acted on by an external force (Law of Inertia).

What is Newton’s 2nd Law of Motion?

F=ma; Force equals mass times acceleration.

What is Newton’s 3rd Law of Motion?

For every action, there is an equal and opposite reaction.

Define “moment” in biomechanics.

The tendency of a force to cause rotation about an axis; calculated as force × distance from the axis (lever arm).

What are the three types of levers in the human body?

• First-class: Effort—Fulcrum—Resistance (e.g., nodding the head).

• Second-class: Fulcrum—Resistance—Effort (e.g., standing on tiptoes).

• Third-class: Fulcrum—Effort—Resistance (e.g., biceps curl).

What defines a positive mechanical advantage?

When the effort arm (EA) is greater than the resistance arm (RA); magnifies force (e.g., second-class levers).

What defines a negative mechanical advantage?

When the resistance arm (RA) is greater than the effort arm (EA); magnifies speed (e.g., third-class levers).

What is skeletal stability in a static stance?

A near balance maintained by bone compression and ligament tension; minor adjustments occur through muscle contractions.

What is hypermobility?

Excessive joint motion outside normal planes, causing instability and possibly subluxation.

Define subluxation.

Partial joint dislocation where margins compress on one side and separate on the other, potentially leading to trauma and degeneration.

How does muscle activity contribute to joint stability?

By maintaining low angulation of forces across joints, enhancing compression and minimizing instability.

How do pronation and supination affect foot stability?

• Pronation increases angulation → decreased stability.

• Supination decreases angulation → increased stability.

What is the net force acting on a stationary body on a flat surface?

Zero; gravitational force is balanced by the ground reaction force.

What is a vector polygon and when is it formed?

A closed shape formed by summing forces in a single plane; occurs when net force is zero and the system is in equilibrium.

What is the difference between axial force and rotational force?

• Axial force: Linear, along one axis.

• Rotational force: Causes movement around an axis (moment/torque).

What happens to a spinning top when it slows down?

It wobbles due to angular acceleration and decreasing moment stability—an example of Newton's 1st Law (inertia and moment).

What anatomical structure acts as the lever in the musculoskeletal system?

Bones act as levers, muscles provide effort, joints act as fulcrums.

What contributes to osseous instability?

Large angulation of forces, excessive rotational forces, or weak muscle support.

What are the two main forces acting on weight-bearing bones?

• Compression force: Enhances stability.

• Rotational force: Increases instability.

Why is muscle tension important for stability during motion?

It maintains proper force angles at joints and counters destabilizing forces, preventing hypermobility or subluxation.

Why is standing still difficult to maintain for long periods?

Even static posture involves slight motions and requires periodic muscle contractions to maintain joint and osseous alignment.

What does the effort arm (EA) represent in a lever system?

The distance from the fulcrum to the point of muscle attachment where force is applied.

What does the resistance arm (RA) represent in a lever system?

The distance from the fulcrum to the resistance force vector (usually center of mass or load).

What is the clinical consequence of repeated subluxation?

Trauma to joint surfaces and potential development of degenerative joint disease (e.g., osteoarthritis).