Leg mechanism

A leg mechanism (walking mechanism) is a mechanical system designed to provide a propulsive force by intermittent frictional contact with the ground. This is in contrast with wheels or continuous tracks which are intended to maintain continuous frictional contact with the ground. Mechanical legs are linkages that can have one or more actuators, and can perform simple planar or complex motion. Compared to a wheel, a leg mechanism is potentially better fitted to uneven terrain, as it can step over obstacles. 
An early design for a leg mechanism called the "Plantigrade Machine" by Pafnuty Chebyshev was shown at the Exposition Universelle (1878). The original engravings for this leg mechanism are available. The design of the leg mechanism for the Ohio State Adaptive Suspension Vehicle (ASV) is presented in the 1988 book "Machines that Walk". In 1996, W-B. Shieh presented a design methodology for leg mechanisms.
The artwork of Theo Jansen, see Jansen's linkage, has been particularly inspiring for the design of leg mechanisms, as well as the Klann patent, which is the basis for the leg mechanism of the Mondo Spider.
Design goals.
Another design goal can be, that stride height and length etc. can be controlled by the operator. This can relatively easily be achieved with a hydraulic leg mechanism, but is not practicable with a crank-based leg mechanism.
The optimization has to be done for the whole vehicle – ideally the force/torque variation during a rotation should cancel each other out.
History.
Richard Lovell Edgeworth tried in 1770 to construct a machine he called a "Wooden Horse", but was not successful.
Patents.
Patents for leg mechanism designs range from rotating cranks to four-bar and six-bar linkages. See for example the following patents:
Complex mechanism.
Shown above are only planar mechanisms, but there are also more complex mechanisms: