contexts for the human cognitive systems to understand and portray through actions and language. The motor system of the body is driven by muscles.

Muscles are comprised of cells that are able to contract when stimulated by the nervous system. There are three general types of muscle found in the human body: heart muscle, smooth muscle and striated muscle. Heart muscle is unique to the heart and smooth muscles form the various involuntary muscles of most organs. Striated muscles form the voluntary muscles that provide mobility to the skeleton. These muscles are linked to bones by tissues called tendons. Most of the movable portions of the skeletal arrangement have muscles arranged in opposition to each other across a joint; a configuration termed an antagonistic muscle pair. Consider for example that the arm can flex back and forth at the elbow joint. One muscle called the biceps connects the bones in the shoulder with the small bone in the forearm, the radius. A separate muscle, the triceps, located on what we typically view as the backside or underside of the arm connects the large bone in the upper arm, the humerus, with the large bone in the forearm, the ulna. Thus, the biceps and the triceps form an antagonistic pair. Muscles only exert force when they contract, so when the triceps contracts, the lower arm is extended away from the body. When the biceps muscle contracts it moves the forearm back toward the body. Similar muscle arrangements are found in virtually all moveable sections of the body.

It’s interesting to consider why expansion muscle tissue hasn’t developed through evolutionary processes. Perhaps it is for the same reason that it is hard to work with a rope by pushing on it. Highly directional extension processes typically require hydraulic operations which in turn tend to require rigid, yet extensible structures. Short of organic growth through cell division, it is difficult to identify any such processes in nature.

The control pathway for muscles consists of a direct motor neuron to muscle connection through a specialized variant of a synapse termed a neuromuscular junction. The motor neuron that comprises the pre-synaptic neuron of this junction, when its action potential is stimulated, emits the neurotransmitter acetylcholine. This neurotransmitter causes the muscle cells to contract. Following the contraction, the junction is reset when the enzyme acetylcholinesterase breaks down the acetylcholine. Without this resetting operation, the muscle would continue to contract. Repetitive activation of a stiated muscle tends to stimulate the growth of muscle mass and to enhance the efficiency of the contraction and reset mechanisms. Thus, an illustration of the concept of training or conditioning. Metaphorically similar is the concept of programming and teaching; an athlete programs her muscles in practice to move in a strict manner when swimming the butterfly stroke in competition during a swim meet. When the race starts, the body must function on learned behaviors.

The ability to walk erect on two legs is viewed as one of the major evolutionary changes that differentiate the human species from its more primitive primate ancestors. Using only two limbs for mobility leaves the arms and hands free for the creation and use of tools; particularly, ever more complex tools. In essence, by walking erect the human body is able to devote much more of the upper appendages to tool making and use.

The use of only two legs for mobility also offers the significant benefit of allowing the head, with its all important eyes, ears and brain to be positioned higher off the ground. The eyes and ears are obviously positioned so that they provide a superior longer-range sensory capability, and the brain is positioned in such a manner that it allows the rest of the body to protect it.

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