Effects of Force, Velocity, and Torque in Human Movement and Clinical Applications

Mechanical effects of forces

Traction, compression, shear, bending, and rotation (twisting).

Traction forces

cause joint distraction, in which joint surfaces pull apart, placing tension on tissues that hold joints together.

Compression forces

cause joint approximation, pushing joint surfaces closer together.

Shear

 cause a gliding motion where joint surfaces move parallel to each other.

Bending

occurs when a force is applied off the central axis of a long bone, creating compression on one side and traction on the other.

Torsion

are opposing forces creating twisting within an object, which can result in spiral fractures in bones like the tibia.

Velocity

The rate of change of position (magnitude) and its direction, measured as distance per time (e.g., ft/s).

Speed

only indicates magnitude, while velocity indicates both magnitude and direction.

Acceleration

The rate of change of velocity, measured as distance per time squared (e.g., ft/s²).

Gravity and acceleration

The pull of gravity produces acceleration, which, when multiplied by mass, creates a force termed weight (F = ma).

Torque

The tendency of a force to produce rotation about an axis, dependent on force and its distance from the axis.

Moment arm (MA)

The perpendicular distance between the line of pull of a muscle and the joint center of rotation.

Figure 2-15. Effect of moment arm on torque.

(A) Moment arm and angular force are greatest at 90 degrees.

(B) As joint moves toward 0 degrees, moment arm decreases and compression force increases.

(C) As joint moves beyond 90 degrees and toward 180 degrees, moment arm decreases and traction force increases.

Figure 2-16. Moment arm of quadriceps muscle

(A) with a patella and (B) without a patella.

Torque calculation

Torque = Force × Moment Arm (T = F × MA).

Angle of application of force

The angle at which force is applied; greatest rotary force occurs when applied at 90° to the limb segment.

Importance of traction forces

Traction forces are used in treatments such as joint mobilizations and spinal traction to relieve compression and pain.

Risks of prolonged compression forces

can damage articular cartilage, increasing the risk of degenerative joint changes.

Excessive shear forces

can cause tissue strain or rupture, especially in the knee joint between the femur and tibia.

Spinal flexion and bending forces

Prolonged spinal flexion imposes bending forces on vertebrae, reducing anterior vertebral height and contributing to conditions such as disc herniation.

Fracture type from torsional forces

A spiral fracture, commonly seen in the tibia when twisting forces exceed bone tolerance.

Velocity documentation importance

It allows clinicians to quantify and track patient progress, distinguishing between speed (magnitude) and velocity (magnitude + direction).

Patella efficiency

The patella increases the quadriceps' moment arm by directing the tendon outward, improving rotary motion and reducing joint compression.

Muscle efficiency for rotary motion

A muscle is most efficient at creating rotary motion when its line of pull is near 90° to the limb segment; efficiency decreases as the line of pull moves away from 90°.