The most basic principle of muscle activity is that muscle can only function by contracting, or tightening. A muscle crosses over a joint, (sometimes more then one joint), and moves the joint by pulling on it. Contrary to what some people may believe the muscle cannot push on anything. When a muscle has contracted completely and the associated joint has reached the  end of it's range of motion, it then needs the contraction of an opposing muscle to return. An example of this would be as such;

  • The biceps cross the elbow joint on the inside of the joint, thus when the biceps contracts, the arm is pulled into a bent position from an original straight position.

  • The triceps cross the elbow joint on the outside of the joint, thus when the triceps contracts, the arm is once again returned to a straight position.

  • All muscles in the body work upon this principle of pulling on a joint, and for every muscular action there is an equal and opposite reaction from an opposing muscle, or muscles.

Of course all of this information is presented in a somewhat simplified manner. Nothing that your body does is actually that simple. For instance when a bench press is performed, it is easy to say that the chest muscles contract to pull the upper arms together and forward . However, the triceps are also involved in extending the arm, the front and medial deltoid are involved, and due to the fact that it crosses the shoulder joint, the biceps are even somewhat involved. So the point is, there is no way to absolutely isolate a muscle, there will always be other muscles involved in assistance and stabilization.

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