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Comparison of the mechanical advantage of different types of lever arms (A–D). A. The relationship of the vector of the force produced by muscle
contraction versus resistance of externally applied force and the fulcrum for rotation or angular displacement in a type 2 lever arm. B. The mechanical
advantage (shaded area) of this system, a type 2 lever arm system, due to the larger moment arm (distance of arrow from fulcrum) of the contracting
muscle’s long lever arm compared to the lever arm of the external force. This type of system has a mechanical advantage ratio > 1. This is produced by
the moment arm of the muscle force (F × df)/(R × dr). Thus, with a force equal to resistance, there is a mechanical advantage > 1 due to the longer
moment arm from the muscle contraction based on the orientation of its insertion. During a single-leg stance, in which the external load that must be
Source: Kinesiology, CURRENT Diagnosis & Treatment: Physical Medicine & Rehabilitation
overcome by the active muscles is usually approximately 1.5–2 times body weight, this muscle group is capable of producing forces of contraction likely
IB, Cruz
CURRENT
Diagnosis
& Treatment:smaller,
Physical
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Rehabilitation;
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greater than Citation:
200–400Maitin
kg. Clearly,
anE.
externally
applied
force magnitudes
produced
force of one2015
upper
limb during
muscle testing, will not
Accessed:
May
11,
2017
be able to overcome this muscle unless significant weakness is present. C. The relationship of the vector of the force produced by muscle contraction
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versus resistance
of externally
applied forceEducation.
and the fulcrum
forreserved
rotation or angular displacement in a type 3 lever arm. D. The mechanical advantage of
this system, a type 3 lever arm system, is due to the larger moment arm of the external resistance’s long lever arm compared to the lever arm of the