Its not invasive og difficult to research. All you need is ultrasound imaging and a stretching protocol.
Its a common assumption that stretching increases muscle length. A have not seen any studies proving it, but the research shows that increases in range of motion are due to increases in stretch tolerance, as in increased pain tolerance, not changes in the passive resistance of the muscle.
Weppler, C., H. & Magnusson, S., P. (2010). Increasing Muscle Extensibility: A Matter of Increasing Length or Modifying Sensation? Physical Therapy, 90
You would logical assume that more sarcomeres in series and greater length of each sarcomere would increase fascicle length and decrease passive resistance. If their is no changes in passive resistance i doubt that there are changes in fascicle length