The word stretching misleads people.
No part of the human body becomes longer or more flexible because of so-called stretching.
Tendons and ligaments will not and CAN not stretch.
The muscles stretch only insomuch as rubber bands stretch and then return to their original shape and length.
What we perceive as flexibility and inflexibility flows out of residual tone in our muscles that results from unconscious and or forgotten commands from the cells that make up our brains.
Dead people, corpses, have remarkable flexibility (except for that time during which rigor mortis makes them stiff).
Even very old people who seemed inflexible and stiff in life have unimagineably flexible bodies in death, because they have no brain activity to give their muscles residual tone.
That which we refer to as stretching actually represents a non-verbal feedback learning process in which our body discovers muscle tone and either learns to relax the muscle (usually only temporarily) or to overcome the muscle.
Some forms of yoga offer good body feedback and others can cause injury.
One should "stretch" to the point of pleasant feedback and hold the so-called "stretch" for no more than two seconds (one Mississippi, two Mississippi).
One should alternate "stretches" between the left and right sides of the body, and between extension and flexion of the body, so that the body can teach itself to let go of unconsciously held muscle tone without self-injury.
The injuries that come from not-stretching come as the result of unconsciously-retained muscle tension or tone.
We learn much of this unconsciously-retained muscle tension from our culture and our family, and by age 15 we have locked it in as an almost permanent behavior.
Some forms of yoga, dance and physical therapy can reverse this and enable a person to discover many more options regarding movement and flexibility than one might think.
In the meantime, in terms of reduced injuries and arthritis later in life (past 35 years of age) it helps to "stretch" both ways, both sides, and for only two seconds per stretch in the earlier years.
Y'know what they say: "If I had known I would live this long I would've taken better care of myself."
No joke.