The biomechanical operations of the human body can be largely attributed to the classes of lever systems composing the human musculoskeletal system. The human body is predominantly composed of first class lever systems and third class lever systems with high distance advantages. A first class lever system consists of an exertion of effort on one side of a fulcrum and an acting force on the other side of the fulcrum. A third class lever system consists of an exertion of effort close to a fulcrum and an acting force further from the same fulcrum. A Wikipedia image of lever classes with first class at the top (load and effort reversed from most human muscle systems) and third class at the bottom can be found here: http://en.wikipedia.org/wiki/Lever#mediaviewer/File:Lever_%28PSF%29.png
By applying a large amount of effort, humans are able to move limbs in relatively wide arcs with appreciable speed, but humans are not relatively well equipped to exert large amounts of force in comparison to other animals. In other words, humans are more capable of greater feats of mobility than they are of feats of great power and generally can’t carry much more than their own body weight. Muscles operate by contracting and pulling, not by pushing. The biceps, for instance, attach to the forearm near the elbow and pull to lift the load of the arm all the way out to the hand at the same angle corresponding to a larger arc around the elbow joint. If a human were to extend their arm and pushed on an object with the back of their hand, the triceps attached to the forearm would contract and pull the hand in the opposite direction around the elbow joint.
Further clarification of human muscle lever system biomechanics can be found here: 
– Jared Martin
