TORQUE
MOMENT
MOMENT OF FORCE

The three terms are synonymous

"The word 'strength' ... actually represents torque [or moment]. All real-world performance (eg, sprinting, lifting weights, getting out of bed, writing) represents manifestations of torque by the musculoskeletal system" (Lieber & Bodine-Fowler, 1993, p. 852).

TWO DEFINITIONS OF MOMENT:
  1. A moment is a turning effect, produced by a force at some distance from an axis of rotation:
    M=Fs

    Therefore, a moment's magnitude depends on:

    1. F, the force applied,
      • measured in Newtons (N) or pounds (lb)
      • depicted as a vector with a line of application

    2. s, the force's moment arm
      • measured in centimeters (cm) or inches (in)
      • the perpendicular distance FROM the force's line of application TO the axis of rotation

    Accordingly, a moment is measured in units like Newton*meters (or Newton*centimeters) or inch*pounds.

    We typically hyphenate these units; we use the * symbol here as a reminder that we derive these units as the product of two basic measurements.

  2. Just as a FORCE produces straight line (linear) acceleration in an object at rest.

    F=ma

    a MOMENT produces an angular acceleration in an object (like a body part) around an axis of rotation (like a joint axis).

    M=Ir

    where I is the moment of inertia and r is the angular acceleration.

Links to further study

Students in PHTH/OCTH 7143 begin the study of human movement by considering moments produced by gravity and by muscles. However, any force that acts at a distance from a joint produces a moment at that joint. In the future, we'll consider:
Reference: Lieber, R.L., & Bodine-Fowler, S.C. (1993). Skeletal muscle mechanics: Implications for rehabilitation. Physical Therapy, 73, 844-856.

Last updated 1-17-01 ©Dave Thompson PT
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