As always, I liked your post - a great summary. Let me throw something out at you This has always confused me. As you stated, the muscle spindle lies within the muscle fibers (Intrafusal fibers) and act as a protective mechanism, activating the muscle to prevent it from being overstretched and causing damage. Like you said they record the changes in length and rate of change in length.
Then you have the GTO which also acts as a protective mechanism, measuring tension and rate of change in tension and when the tension becomes too great - inhibits the muscle.
So here we have 2 protective mechanisms, one protects the muscle by activating it while the other protects it by inhibiting it. Here's my problem: it has never been explained to me why the muscle spindles become overactive due to poor posture. Furthermore, is it really the GTO overriding the muscle spindle? I've never found that in any physiology book. Lastly, what about muscular adhesions? What is mechanically/physiologically happening with adhesions and how does the process of SMR (or massage) release them?
Interesting questions! Let's take a closer look.
1. So here we have 2 protective mechanisms, one protects the muscle by activating it while the other protects it by inhibiting it. Here's my problem: it has never been explained to me why the muscle spindles become overactive due to poor posture.
Poor posture cause muscle imbalances, and puts pressure and tensions where they were not intended to be. In fact, in part due to poor posture, most people have major muscle imbalances, which usually cause an entire muscle group to get very tight, or over activated. In other words, imbalances cause certain muscles to over-compensate as way to protect areas that might be harmed by this imbalance. This often causes cause the muscle spindles to become overactive.
When poor posture is continual, the imbalance remains. Activation of the muscle spindle will send sensory impulses to the spinal cord via type 1a afferent nerves, informing the CNS that the muscle is being stretched. Impulses returning to the muscles via α motor neurons cause the muscle to contract, thereby resisting the stretch. This is why ballistic activities have fallen from favor for some (not all) clinicians because it is believed that the facilitation of the muscle spindle during ballistic may cause microtrauma in the muscle because of the tension created when the muscle is stretched (Bandy & Sanders, 2007, http://books.google.com/books?id=Zm3XAqi5OTYC&dq=is+it+really+the+gto+overriding+the+muscle+spindle). However, ballistic activities are still recommended for dynamic athletes (see attached notes as an extra resource).
There are both agonist and antagonist muscles that work ...
This solution explains why the muscle spindles become overactive due to poor posture. In relation to the muscle spindles becoming overactive due to poor posture, it discusses the idea that it is due to the GTO overriding the muscle spindle. In relation to muscular adhesions, it explains the mechanically/physiologically process that is happening with adhesion and how the process of SMR (or massage) releases them. Supplemented with an article on restoring range of motion and improving flexibility.