There are two sets of muscles that stretch your fingers to reach wide chords. One set flares the fingers out so that your pinky and thumb will try to form a straight line. The other set widens the palm without bending the fingers. Therefore, when you are developing stretching muscles, you should work on these two separately -- it is more important to stretch the palm because bending the fingers outward causes stress.
To stretch the muscles for bending the fingers outward, place finger 234 on a table with thumb and pinky against the straight edge of a table, and push hand towards table so as to stretch the tendons between pinky and thumb.
To stretch the palm tendons, put your hands together, palm-to-palm, with the pinky of each hand interlocked with the thumb. Then push the hands towards each other so as to stretch the palm wider.
To exercise the stretching muscles, practice stretching wide and then relaxing instantaneously, very quickly, and try to do the same thing while playing. In this way, you can continually play wide chords without fatigue because you are resting between chords. When you get good at this, you can play many chords very rapidly and still relax between chords. Try to work mostly on the muscles that widen only the palms (and not the fingers), so that the fingers are free to play the keys without stress.