Proprioreseptive Input. Activation exercises

It is important to note that when performing concentric contractions alone, there may be a resultant unloading of the muscle spindle and decreased facilitation from the stretch reflexes as the muscle moves into the shortened range. Therefore, concentric contractions are ineffective and even detrimental when attempting to improve proprioceptive input to an inhibited muscle. [from MAT description]

What is Proorioreceotive Input(PI).
Why Spindels have less PI in shortened position?
What is Activation Exercise?
How to do Activation Exercise?

http://www.coachr.org/ankleproprio.htm
http://www.muscleactivation.com/FAQ.html