They have two ports to allow air in, one for outstroke and one for instroke.

On instroke, the same relationship between force exerted, pressure and effective cross sectional area applies as discussed above for outstroke.

Her cheeks expanded obscenely on the instroke and hollowed on the outstroke, as she labored with mouth and hands.

For instroke, therefore, the relationship between force exerted, pressure, radius of the piston, and radius of the piston rod, is as follows:

The waiter was outside sweeping the street, and you could hear the scratchy hiss of his broom against the sidewalk, heavy on the outstroke, lifted clear on the instroke.

And, individual components of a natural motion (such as outstroke vs. instroke of a flapping appendage) can be programmed separately, which is certainly difficult to achieve when working with a live animal."

Due to the effective cross sectional area reduced by the area of the piston rod, the instroke force is less than the outstroke force when both are powered pneumatically and by same supply of compressed gas.