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In the human body’s musculoskeletal lever systems, muscles are usually at a force
disadvantage; that is to say that the forces produced by the muscles are greater than the
forces resisting them. This is because muscles’ insertion points tend to be closer to the
fulcrum than the resistance force. This can be seen in Figure 27 where the force
produced by the biceps femoris will need to be greater than the weight of the lower leg.
The force can be easily calculated using the principal of rotational equilibrium; the sum
of the moments acting about the knee joint must be zero. Therefore:
FdF - WdR = 0
Rearranging:
F=
Wd R 480  020
= 192 N

dF
005
Thus, the required biceps femoris force is four times that of the lower limb’s weight. It
is therefore at a considerable force disadvantage.
There are two reasons why muscles act mostly at a force disadvantage. They are range
of movement and speed of movement. The closer a muscle is inserted (attached) to a
joint then the smaller the change in muscle length required to produce a correspondingly
larger limb movement, similar the quicker the movement. Like with most things in life,
a compromise must be reached; in this case between the desire for strength and
mobility.
3.2.1 Lever system classes
Lever systems can be divided into three classes, which are dependent upon the positions
of the two forces relative to the fulcrum.
A first class lever has the fulcrum located between the effort and the resistance (Figure
28).
effort
FIRST CLASS LEVER
resistance
fulcrum
FIGURE 28. A FIRST CLASS LEVER.
Examples of first class levers are the crowbar, scissors and see-saw.
A second class lever has the resistance located between the effort and the fulcrum as
shown in Figure 29.
resistance
fulcrum
effort
FIGURE 29. A SECOND CLASS LEVER.
Unit 2 - Statics
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SECOND CLASS LEVER