Torque and Moments of Force

Vocabulary practice flashcards focusing on torque, moment arms, static equilibrium equations, center of gravity determination, biomechanical stability, and gait mechanics.

Torque (Moment of Force)

The turning effect produced by a force that causes a change in angular motion.

Centric Force

A force whose line of action is directed through the center of gravity, causing changes in translation only.

Eccentric Force

A force whose line of action is not directed through the center of gravity, causing changes in both translation and rotation.

Force Couple

A pair of two eccentric, noncolinear forces of equal magnitude acting in opposite directions, causing changes in rotation only.

Moment Arm ($r$)

The perpendicular distance between the line of action of a force and a specified axis of rotation, representing the shortest line between the line of action and the axis.

Torque Equation

$T = F \times r$, where $T$ is torque measured in $\text{N}\,\text{m}$, $F$ is force in $\text{N}$, and $r$ is the moment arm in $\text{m}$.

Sense of Torque

The directional rotation of a torque about an axis, conventionally designated as positive ($+$) for counterclockwise and negative ($-$) for clockwise.

Muscular Torque

The turning effect created at a joint when a muscle produces a pulling force ($F_m$) acting across the joint at a specific moment arm distance from the joint axis.

Joint Angle Effect on Muscle Moment Arm

The variation in a muscle's moment arm length as a joint rotates; for example, the biceps brachii moment arm is longest when the elbow is flexed to $90^\circ$ and becomes shorter as the elbow extends.

Resistance Torque

The turning effect created by exercise weights or loads opposing muscle torque, which varies in magnitude across the joint range of motion (ROM) as its moment arm changes.

Static Equilibrium

A state of balance where all external forces sum to zero ($\Sigma F = 0$ in all directions) and all external torques sum to zero ($\Sigma T = 0$ about all axes).

Equations of Static Equilibrium

$\Sigma F = 0$ and $\Sigma T = 0$, representing the conditions where the sum of forces in all directions and the sum of torques around all axes must equal zero.

Net Torque

The sum of all torques acting about a common axis, calculated as $\Sigma T = T_{\text{clockwise}} + T_{\text{counterclockwise}}$.

Center of Gravity ($\text{cg}$)

An imaginary, non-fixed point in space within a body or system where the entire mass may be assumed to be concentrated and through which gravity acts downward.

Point of Balance

The center of gravity location where the sum of the torques of all individual body segments or elements equals zero ($\Sigma T_{\text{body parts}} = 0$).

Mathematical Determination of Center of Gravity

$\Sigma T = \Sigma (W \times r) = (\Sigma W) \times r_{\text{cg}}$, where $W$ is the weight of an element, $r$ is its moment arm, $\Sigma W$ is the total weight, and $r_{\text{cg}}$ is the distance to the center of gravity relative to the reference axis.

Standing Center of Gravity Height

The location of the center of gravity in anatomical position, situated at approximately $55\%$ of standing height in females and $57\%$ of standing height in males.

Parabolic Path of Center of Gravity

The fixed trajectory that a body's center of gravity follows while airborne as a projectile, around which body segments can rotate or move without altering the path.

Hang Time

The visual appearance of being suspended in air during a jump, created when an athlete raises their arms and legs upward relative to the center of gravity while the center of gravity continues its fixed parabolic trajectory.

Stability

The capacity of a body to return to equilibrium when displaced, which is enhanced by a lower center of gravity, a wider base of support, and greater object weight.

Base of Support

The area defined by the contact points between a body and the supporting surface, including the entire surface area enclosed between those contact points.

Stance Phase

The weight-bearing portion of the walking gait cycle comprising initial contact (heel strike), loading response (foot flat), midstance, terminal stance (heel off), and preswing (toe off), encompassing approximately $0\%$ to $60\%$ of the cycle.

Swing Phase

The non-weight-bearing portion of the walking gait cycle comprising initial swing, mid-swing, and terminal swing (acceleration and deceleration), encompassing approximately $60\%$ to $100\%$ of the gait cycle.