I may have misunderstood this picture of new research related to the stretch-shortening cycle on the microscopic scale.
The Hill Muscle Model shows the functional components of a muscle on the smallest scale. It provides a way to visualize what is going on and to think about how active muscle components (actin & myosin) and passive muscle stretch component (recently also the Titin protein molecule) might function.
Recently, new research has emphasized the role played by passive Titin, the largest protein molecule, also located within the sacomere in parallel with the active Actin and Myosin structures. Recently it has been considered that Titin in each muscle cell
provides the main
stretch capability of the muscle. An older theory views stretch as an overall muscle stretch to include tendons.
Actin, Myosin Animation (no Titin) - Active Muscle Shortening
Sarcomere with Titin Illustrations
Actin, Myosin & Titin Illustrations
Report on Stretch Shortening Training with Biopsied Human Muscle Measurements. Added 2/5/2013
Powerpoint presentation Stretch Shortening Cycle including Titin. Added 2/5/2013
New research on Titin
This report in Figure 7 proposes a new way that Titin might be interacting with Actin to provide stretch functions.
I don't understand this last research but maybe it implies that a stretch can be deliberately activated at various lengths of the muscle.
This new research might be especially important along with other research that indicates muscle shortening might be faster
if 'passive' stretch is employed instead of active muscle shortening. (the Actin -Myosin animation above even looks slow).
In Biomechanics of Advanced Tennis
(2003). Elliott said
"10-20% of additional racket head speed is achieved following a stretch shortening cycle."
(This publication is now 10 years old so there may be different views in 2013.)
Is that a simple addition to racket head speed or is passive stretch derived muscle shortening the only
mode that can shorten that fast with control and reproducibility? Main principle of athletic movement?