Stretching is one of the most neglected areas of most peoples workout.
Just think for a moment about a Lion as he wakes from his sleep, he immediately stretches his whole body.
The Lion knows instinctively that stretching is good for his muscles and his strength, you should do the same before each and every workout.
Ligament, tendon, muscle and joints are all flexible.
They can stiffen, limiting your range of motion, or if correctly warmed up and stretched, will give a longer range of motion bring more muscle fibres into play.
Stretching also makes your workout safer.
As you extend your muscles when training with weights, they can easily be pulled to far if your range of motion is limited, this can cause overextension of a ligament or tendon and result in a sprain or strain.
But if you take the time and extra effort to stretch and warm up the areas you are about to train, the muscles will be able to adjust and ready themselves for the workout ahead.
A muscle can contract, but it cannot stretch itself.
It has to be stretched by the pull of an opposing muscle.
As you already know you must train using a full range of motion to bring the most muscle fibres into play, you will find that the muscle that is contracting will stretch the opposite muscle.
Muscles contracted against heavy resistance tend to shorten with the effort.
This is the reason stretching before your workout is so important, allowing you to train longer, harder and more safely.
You can prepare for your workout by doing any number of standard stretching exercises, or even better would be to take part in a yoga or stretching class at your gym, this would be a great way to defend yourself from injuries.