This is an expansive topic, so let us see how simple we can make it for both of us? Let us firstly consider the simple process of ‘Ambulation … just walking along’. Ponder walking Up-hill, Level and Down-hill. If you have a sloped lawn, actually test these variables!
The ‘Two Patterns’ are simply ‘PUSH and PULL Basic’. We can ‘Push Forward’ with our ‘Brace or Back Foot’. We could arguably, ‘Pull Forward’ via the ‘Flex or Contraction Bending of our Front or Target Foot and Leg’. That is not very efficient. How could we walk or run by this ‘Target Leg and Hip Pull Basic Mechanical Pattern’? We also rightfully refer to these two methods of ambulatory propulsion as utilizing the ‘Lower Body Machine Push & Drag Links’.
A useful analogy is the ‘Two Wheel Drive Pick-Up Truck’. The front axle and wheels ‘Steer or Direct’ while the drive shaft and rear differential, axle and wheels ‘Push or Propel’. The Front Bumper can provide tow-truck drag. The Rear Bumper provides push source. (see ‘Lead & Lag’)