Stretching- what, when and how.

Static stretching should only be performed after working out to areas that are excessively tight to reduce injury risk.
Dynamic stretching (swinging, bounding & bouncing) activate the neuromuscular system (GTOs: golgi tendon organs and muscle spindles) to produce the required tension and length in the motor unit (muscle fiber and neuron).
Accompanied prior with a warm-up to lubricate joints with synovial fluid to enhance performance (sprint, jump, throw and enduring activities).
In general, stiffness = stability, especially along the spine. Physical exertion in spinal flexion (sit-ups & crunches) place excessive strain on the lumbar spine. Planks, side planks and bird-dog poses are best (www.backfitpro.com). Dr. Stuart McGill is one of the world's most foremost spine biomechanists.
Stiffness in a joint increases stored energy in elasticity. The stretch-shorten cycle (SSC) is fully activated when performing plyometric exercises twice a week. See my training program for details.
The goal is to balance joint mobility along the sagital plane (left/right) to produce efficient coordinated primal movements. See my training program articles for details.