How does a Muscle Work?

<1> Since a large muscle can exert more force through a longer distance than a small muscle it stands to reason, for comparative purposes, that we look at force per unit area of muscle. The force per area is called the stress. Note that the unit of stress is the same as that for pressure. The work to be done is caused by a force applied through a distance. In this case the distance a muscle is shortened. Since this distance is also dependent on the size of the muscle it is convenient to use the shortening per unit length also called the strain.

Stress = F/A
Strain = dL/L

Putting these together:

Stress x Strain = F x dL/A x L = Work/Volume.

The characteristic of muscle is the volume specific work. If the muscle has a certain density of mass per unit volume then we may get rid of the Volume in the Stress x Strain relation to obtain a mass specific work term.

W_V = W/V
W_M = \rho W/M

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