This discussion from another therapy list may also be of some interest here:

<<I have been looking for some good advise on stretching. A truly great bronc
rider ......recommends long periods of low load stretch which makes sense for
tendons/extensor hood and other tissue shortening situations in the
hand/foot. Others seem to suggest short duration high intensity stretching
for areas like the quads. What are your opinions on the best techniques for
stretching shortened tissue in large muscle groups, say
hamstrings/quads/illiopsoas/pecs? Low load long time or high load short time?
I appreciate this could very with the tissue involved, from say, an EPL
tendon repair to say, the IT Band, which some folk seem to say you can't
stretch and shouldn't even waste your time trying. I have professionally seen
gains in long duration low load stretching of large muscle tissue, but was
just wondering... >>

***Since you repeatedly have been asking for information on this topic, I
have collected some relevant extracts on this topic from one of my books.
Unfortunately, email does not yet allow one to paste diagrams directly into
the main body of the letter, so you will have to do your best with some crude
visualisations.

< The Biomechanics and Physiology of Flexibility

Siff M C   "Supertraining"   2000  (from Chs 1 and 3)

..... It is relatively meaningless to discuss muscle action without
considering the role played by the connective tissues associated with muscle.
  These tissues occur in the form of sheaths around muscle and its sub-units
at all levels, as linkages between myosin filaments,  as Z-discs at the ends
of muscle filaments, and as tendons at the ends of muscles.   Not only do
they protect, connect and enclose muscle tissue, but they play a vital role
in determining the range of joint movement (or flexibility), and improving
the efficiency of movement by storing and releasing elastic energy derived
from muscle contraction.....

All muscle comprises a contractile component, the actin-myosin system,  and a
non-contractile component, the connective tissue.   In mechanical terms,
muscle may be analysed  further (according to Levin & Wyman, 1927) in terms
of a contractile component in series with a series elastic component (SEC)
and in parallel with a parallel elastic component (PEC), as illustrated in
Figure 1.19.    Although the anatomical location of these elements has not
been precisely identified, the PEC probably comprises sarcolemma, rest-state
cross-bridging, and tissues such as the sheaths around the muscle and its
sub-units.  On the other hand, the SEC is considered to include tendon, the
cross-bridges, myofilaments, titin filaments and the Z-discs.  Of these
elements, the myofilaments apparently provide the greatest contribution to
the SEC (Suzuki & Sugi, 1983).

The PEC is responsible for the force exerted by a relaxed muscle when it is
stretched beyond its resting length, whereas the SEC is put under tension by
the force developed in actively contracted muscle.  The mechanical energy
stored by the PEC is small and contributes little to the energy balance of
exercise (Cavagna, 1977).  On the other hand, considerable storage of energy
occurs in the SEC, since an actively contracted muscle resists stretching
with great force, particularly if the stretching is imposed rapidly.  This
resistive force, exerted at the extremities of the muscle, and not the direct
lengthening of contracted muscle, is responsible for the storage of elastic
energy within the SEC.

Furthermore, it has been shown that mechanical strain imposed by stretching a
contracted muscle is smaller in a muscle with a preponderance of ST (slow
twitch) fibres, whereas the stored elastic energy is greater in FT fibres
(Komi, 1984). The same study has also suggested that the elasticity of the
SEC in a slow muscle is greater than that in a fast muscle.  These
differences are largely due to the fact that the concentration  of collagen
is higher in slow muscle than in fast muscle (Kovanen et al, 1984).  Such
findings agree with basic analysis of slow and fast movements.   The high
stiffness and low strain of a slow muscle clearly is most appropriate for
muscle function which is intended for continuous support of posture.
Conversely, the lower stiffness, greater compliance and lower elasticity of a
strongly contracting fast muscle is eminently suited to enhancing speed and
efficiency of movement.  Further research has indicated that the differences
in mechanical properties between fast and slow muscles in response to passive
stretching are to a large extent due to their content of collagen.

In addition to the differences regarding the collagenous component of muscle,
there are also differences in terms of the muscle fibres.  Apparently ST
fibres may be able to sustain cross-bridge attachments for longer periods
than FT fibres.  Therefore, prolonged muscle contraction would tend to be
more easily maintained in slow postural (tonic) than in fast (phasic)
muscles.  Consequently, stretching procedures would have to be applied for
longer periods on slow muscles to significantly enhance their flexibility.

The muscle fibres can also stretch passively and store elastic energy, like
tendons. In this respect, the myosin cross-bridges that are considered to
pull the actin filaments between the myosin filaments during muscle
contraction, are known to be compliant structures which may stretch
considerably before they detach from the activated sites on the actin
filaments. It is believed that this compliance may be caused by rotation of
the meromyosin heads of the cross-bridges and by elongation of its tail,
which appears to have a helical structure that would promote extensibility
(Huxley, 1974).  In other words, even a contracted muscle can stretch, not
only due to its collagenous component, but also due to its contractile tissue.

As stated earlier, if a relaxed muscle is stretched beyond its resting
length, it is the PEC which appears to be most exposed to the resulting
tension, whereas in active muscle it is the SEC which is subjected to most
tension.  This implies that static, relaxed ballistic and passive stretching
have the greatest effect on the PEC, while tense ballistic and active
stretching affects predominantly the SEC. The various PNF flexibility
techniques appear to be able to stretch and strengthen both the SEC and the
PEC, as well as the associated muscles.  Weight and other resistance training
routines based on the same PNF principles can achieve similar results.
.........

....  Since stretching is a particular type of mechanical loading,
application of stretching can be more effectively applied if the effects of
loading on collagen are studied carefully. In fact, physiological stretching
is possible because collagen is a viscoelastic material; that is, under rapid
loading it behaves elastically, while under gradual loading it is viscous and
can deform plastically.

Figure 1.21 illustrates the behaviour of collagenous tissue in response to
loading to failure.  The initial concave portion of the curve (Region I) has
been termed the 'toe region' and applies to the physiological range in which
the tissue normally functions. It probably represents a structural change
from the relaxed crimped state of the tissue to a straighter, more parallel
arrangement (Viidik, 1973). Little force is required to produce elongation in
the early part of this region, but continued force produces a stiffer tissue
in which the strain (i.e. elongation per unit length of tissue) is between
0.02 and 0.04 (Viidik, 1973). Cyclic loading up to this degree of strain
produces an elastic response, while unloading from this state restores the
original crimped (or planar zigzag) pattern and resting length of this
tissue. In other words, mild stretching of collagenous tissue within the
Region I will not produce long-term flexibility.

The next, almost linear, region (Region II) shows the response to increased
loading. Here the fibres have lost their crimping and are distinctly
parallel, a situation which is believed to be caused by reorganization of the
fibre bundles within the tissues. Small force decreases in the curve may
sometimes be observed just prior to the end of Region II, heralding the
early, sequential microfailure of some overstretched fibres. At this point,
the dangers of excessive stretching definitely become significant. Region III
corresponds to the force imposed on the tissues from the beginning of
microfailure to the sudden occurrence of complete failure (Region IV). Such a
situation will occur if the stretching in Region II continues to elongate the
tissues or if ballistic movements are applied in this state.

Since tendons and ligaments are viscoelastic, they also exhibit sensitivity
to loading rate, and undergo stress relaxation, creep and hysteresis. For
instance, Figure 1.22 represents the phenomenon of stress relaxation in an
anterior cruciate ligament while it is loaded at a finite strain rate and its
length is then held constant. The characteristic hysteresis curve (n = 1) in
Figure 1.22(a) corresponds to the case in which the ligament was loaded to
about one-third of its failure load and then immediately unloaded at a
constant rate.

If the curve fails to return to its starting point it indicates that the
material has become permanently deformed, a process which, if repeated
regularly, can lead to ligament laxity. Prolonged, excessive stretching of
this type encourages joint mobility at the expense of its stability so that
the joint then has to rely more on its muscles for stability. Despite the
widespread opinion that the muscles act as efficient synergistic stabilisers,
it should be remembered that the musculature cannot respond quickly enough to
protect a joint against injury if large impacts are applied rapidly,
particularly if they are torsional.  Since joint stability involves
three-dimensional actions over several degrees of freedom, the necessity for
appropriately conditioning all the interacting soft tissues becomes obvious.
Joint stabilisation and flexibility are discussed in greater detail later
(see 3.5.6).

Figure 1.22(b) refers to the case in which the ligament was subjected to the
same load F(0) and then the length was held constant, thereby revealing
asymptotic relaxation to a limiting value F(A). The hysteresis loop is
generally small for collagen and elastin, but large for muscle, while stress
relaxation is small for elastin, larger for collagen and very large for
smooth muscle.

Other loading phenomena also need to be noted. For example, if collagenous
tissue is tested by imposing a successive series of loading-unloading cycles
with a resting period of 10 minutes between each cycle, curves such as those
indicated by n = 2 and 3 in Figure 1.22 will be produced. Figure 1.22(a)
shows that the initial toe region increases in extent as the hysteresis
curves shift progressively to the right. At the same time, the stress
relaxation curves of Figure 1.22(b) shift upward. If the test is repeated
indefinitely, the difference between successive curves decreases and
eventually disappears. The tissue is then said to have been preconditioned, a
state which is achieved because the internal structure of the tissue alters
with cycling. This type of conditioning towards enhanced stability is the aim
of stretching exercises.

The hysteresis curve also offers a way of distinguishing between the relative
contributions of elasticity and viscosity to a tissue's behaviour. If the
vertical distance between the loading and unloading curves (e.g. in Fig 1.22)
is zero, the load-deformation graph becomes a straight line and the tissue is
purely elastic, obeying Hooke's Law (i.e. elongation x is directly
proportional to applied force F, or F = k.x). The larger the vertical
distance between the two curves, the more viscous is the material, the more
deformable it becomes and the more it dissipates imposed shocks. In addition,
the slope of the hysteresis curve gives a measure of the stiffness of the
tissue, with a steep slope being characteristic of a very stiff material that
does not extend much under loading.

The biomechanical performance of collagenous tissues depends largely on their
loading rate.  For instance, if a joint is subjected to constant low
intensity loading over an extended period, slow deformation of the tissues
occurs, a phenomenon known as creep and which is characteristic of
viscoelastic substances in general.  Furthermore, collagenous tissue
increases significantly in strength and stiffness with increased rate of
loading, thereby emphasizing the intelligent use of training with high
acceleration methods.  One study found an increase of almost 50% in load of
knee ligaments to failure when the loading rate was increased fourfold
(Kennedy et al, 1976).

Of further interest is the fact that, at slow loading rates, the bony
insertion of a ligament is the weakest component of the ligament-bone
complex, whereas the ligament is the weakest component at very fast loading
rates.  These results imply that, with an increase in loading rate, the
strength of bone (which also contains collagen) increases more than the
strength of the ligament (Frankel & Nordin, 1980).  Of added relevance is the
finding that the tensile strength of healthy tendon can be more than twice
the strength of its associated muscle, which explains why ruptures are more
common in muscle than in tendon (Elliott, 1967).

These facts are directly relevant to appreciating the difference between
static, passive and ballistic modes of stretching, with slow and rapid
loading rates having different effects on the each of the soft tissues of the
body (see Ch 3.5.8)........

...... At this point it is relevant to point out that stretching and
flexibility training are not necessarily synonymous.  Some flexibility
exercises are not stretching exercises although they increase range of
movement, because they may focus entirely on modifying neuromuscular
processes, in particular the stretch and tendon reflexes (see Fig 3.33) that
control the functional range of movement.  On the other hand, many stretching
exercises do not pay any deliberate attention to neuromuscular processes and
tend to concentrate on eliciting structural changes in the soft tissues.
Thus, static stretches may actually change the length of the muscle complex,
but have an inadequate effect on the dynamic range of movement required in a
given physical activity.  Therefore, it is vitally important to distinguish
between the different types stretching and flexibility exercises in order to
integrate the most appropriate and effective balance of static and dynamic
means of increasing functional ROM into an overall training programme.

For sports participants active flexibility is by far the more important, even
though passive flexibility provides a protective reserve if a joint is
unexpectedly stressed beyond its normal operational limits. The value of
active flexibility is emphasized by the fact that sporting prowess (rated in
terms of achievement standards in competition) correlates more strongly with
active rather than passive flexibility (a correlation coefficient of 0.81 vs
0.69) (Iashvili, 1982). This same Russian study of over 200 adult competitors
also concluded that traditional static and passive stretching exercises
develop mainly passive flexibility, whereas combined strength and stretching
exercises are considerably more effective in developing active flexibility,
particularly if strength conditioning is applied in the zone of active
muscular inadequacy. This finding will be appreciated more fully when the
biomechanics of the soft tissues is analysed later.

The current emphasis on flexibility neglects the equally important mechanical
qualities of the tissues comprising the joints, in particular their stiffness
and damping ratio. In other words, it is vital that these tissues offer each
joint an effective balance between mobility and stability under a wide range
of operating conditions. For instance, a joint whose tissues have low
stiffness (or high ability to be stretched easily), but a low damping ratio
(or poor ability to absorb tensile shocks) will be especially susceptible to
overload injuries (Siff, 1986).
Therefore, in analysing flexibility one has to consider the separate and the
interrelated effects of the ROM of the joints and the mechanical properties
of the tissues comprising them.....

----------------

Dr Mel C Siff
Denver, USA
http://groups.yahoo.com/group/Supertraining/