This article is from the Stretching FAQ, by Brad Appleton Brad_Appleton@ivhs.mot.com with numerous contributions by others.
Recall from our previous discussion (See "1.2.1 - How Muscles Contract")
that there is no such thing as a partially contracted muscle fiber: when a
muscle is contracted, some of the fibers contract and some remain at rest
(more fibers are recruited as the load on the muscle increases).
Similarly, when a muscle is stretched, some of the fibers are elongated and
some remain at rest (See "1.6 - What Happens When You Stretch"). During an
isometric contraction, some of the resting fibers are being pulled upon
from both ends by the muscles that are contracting. The result is that some
of those resting fibers stretch!
Normally, the handful of fibers that stretch during an isometric
contraction are not very significant. The true effectiveness of the
isometric contraction occurs when a muscle that is already in a stretched
position is subjected to an isometric contraction. In this case, some of
the muscle fibers are already stretched before the contraction, and, if
held long enough, the initial passive stretch overcomes the stretch reflex
(See "1.6.2 - The Stretch Reflex") and triggers the lengthening reaction
(See "1.6.3 - The Lengthening Reaction"), inhibiting the stretched fibers
from contracting. At this point, according to `SynerStretch':
When you isometrically contracted, some of the resting fibers would
contract, many of the resting fibers would stretch, and many of the
already stretched fibers, which are being prevented from contracting by
the inverse myotatic reflex [the lengthening reaction], would stretch
even more. When the isometric contraction was relaxed and the
contracting fibers returned to their resting length, the stretched
fibers would retain their ability to stretch beyond their normal limit.
... the whole muscle would be able to stretch beyond its initial
maximum, and you would have increased flexibility ...
The reason that the stretched fibers develop and retain the ability to
stretch beyond their normal limit during an isometric stretch has to do
with the muscle spindles (See "1.6.1 - Proprioceptors"): The signal which
tells the muscle to contract voluntarily, also tells the muscle spindle's
(intrafusal) muscle fibers to shorten, increasing sensitivity of the
stretch reflex. This mechanism normally maintains the sensitivity of the
muscle spindle as the muscle shortens during contraction. This allows the
muscle spindles to habituate (become accustomed) to an even