Henry Gray (18251861). Anatomy of the Human Body. 1918.
cross-section than are the fine-fibered muscles. Fick estimates the average strength as about 10 kg. per square cm. This is known as the absolute muscle strength. The total strength of a muscle would be equal to the number of square centimeters in its physiological cross-section x 10 kg.
FIG. 365 A, fusiform; B, unipinnate; C, bipinnate; P.C.S., physiological cross-section. (See enlarged image)
The work Accomplished by Muscles.For practical uses this should be expressed in kilogrammeters. In order to reckon the amount of work which a muscle can perform under the most favorable conditions it is necessary to know (1) its physiological cross-section (2) the maximum shortening, and (3) the position of the joint when the latter is obtained.
Work = lifted weight x height through which the weight is lifted; or
Work = tension x distance; tension = physiological cross-section x absolute muscle strength.
If a muscle has a physiological cross-section of 5 sq. cm. its tension strength = 5 x 10 or 50 kg. If it shortens 5 cm. the work = 50 x .05 = 2.5 kilogrammeters. If one determines then the physiological cross-section and multiplies the absolute muscle strength, 10 kg. by this, the amount of tension is easily obtained. Then one must determine only the amount of shortening of the muscle for any particular position of the joint in order to determine the amount of work the muscle can do, since work = tension x distance.
The tension of a muscle is, however, not constant during the course of contraction but is continually decreasing during contraction. It is at a maximum at the beginning and gradually decreases.
This can be illustrated by the work diagram Fig. 366.
A M D (ordinate) = tension.
A V X (abscissa) = shortening.
A D = tension of muscle in extended or antagonistic position.
A V = amount of actual shortening.
A M = tension in midposition = absolute muscle strength.
D V = shows how the tension sinks from maximum (in the extended position of the muscle) where it is about double that in the midposition (M) to nothing on complete contraction.