Hydraulic circuit training machines were first developed for The Henley Corporation in the 1970s, and are now becoming an increasingly popular form of exercise.
[1] For circuit training applications, each piece of equipment is specifically designed for a given exercise.
The effort is applied through the range of motion of the exercise acts on a lever against a piston which moves linearly within a hydraulic cylinder.
Resistance to the motion is determined by the amount of effort applied and the adjustment size of the orifice.
This is a key feature of this design and allows the resistance to be adjusted to an individual's strength level.