Cornell University Ergonomics Web

1. Force and Torque Relationships in the Body - When we do work, muscles contract, causing bones such that bones act as a series of levers. For example, when raising a coffee mug the mug acts as a load (or resistance) on the forearm (lever). Two sets of forces are generated:

a. Force - a function of the strength of muscle contraction which is translated into torque.

b. Torque (Moment) - tendency for forces developed by muscle-tendon complexes to cause rotation at joints. When a perpendicular force is applied on a lever arm at some distance from its axis of rotation (fulcrum) this causes a rotational tendency (torque or moment). Torques generated by the body translate muscle contraction into mechanical work. Different types of work require different oxygen supply and needs.

i) Positive Work - Whenever the product of muscle force and the force arm is greater than the product of the force required to overcome the resistance and the resistance arm this causes movement (dynamic torque) or positive work.

ii) Negative Work - If the load requires a greater force than can be generated then extension of muscles occurs and this is called negative work. This type of work occurs in situation such as lowering a heavy box.

iii) Static Torque - occurs when the product of muscle force and the force arm equals the product of the force required to overcome resistance and the resistance arm, the opposing forces are in balance.

2. Lever Systems in the Body - Three classes of lever systems are found in the body:

a. First class lever - Fulcrum (center of rotation) is the joint, and this is located between the load (resistance) and force (muscle).

When Force Arm length is greater than Resistance Arm length there is a mechanical advantage. For example, when a person is looking down into a microscope the fulcrum is at the lanto-occipatal joint connecting the head and spinal column. The mass of head is the resistance and the force is the contractions of muscles in the back of the neck. Fatigue of neck muscles from static contraction can lead to neck pain.

b. Second class lever - The Force Arm is always larger than the Resistance Arm. There are only a few examples in the body, e.g. opening mouth against a resistance (gooey fudge/toffee).

c. Third class lever - Here, lever systems are always at a mechanical disadvantage because Resistance Arm always exceeds the Force Arm, so greater force is needed to move the load. There are many examples in the body such as raising a coffee cup.


Calculating simple torque?

T= F x D (Distance of the load from the fulcrum)

straight arm lift
(counter clockwise rotation sugar (load) 1 kg.
______________________ r
s D=0.5 meter
shoulder (fulcrum)

Lift load 1' off counter

SI Unit - Newton (1 = 0.22 lb. feet)

- force required to raise .22 lb. through distance of 1 foot (2.2 lb. = 1 kg)
2.2 lbs = 1 kg.
In = .1 kg foot
Force required to raise 1 kg. through 1 foot = 10 Newtons

T = F X D
= 10 x .5
= 0.5 Newtons meter