Biomechanics of Vocal Tissues:
Biomechanics is the study of how living tissue behaves when subjected
to physical forces. For instance, is the tissue relatively stiff, or
does it stretch or deform when forces are applied to it in various directions?
How much force can the tissue withstand before damage is done? Is the
tissue more resistant to some kinds of forces than others? Friction,
stretching and repeated impacts are some of the kinds of forces that
laryngeal tissue must be able to tolerate in everyday voice use.
Stretchiness, Squishy-ness and Stiffness
It is often difficult to measure the reactions of
live tissue in the human body (in vivo), due to the problem
of gaining access to the tissue in its natural environment. This
is especially difficult for the larynx, since it is located in the
highly sensitive airway. Scientists can get a picture of the vocal
folds vibrating from above, using a laryngoscope, but even this is
difficult, since it's rather hard to make natural sounds while you
have a camera apparatus stuck in your mouth or inserted into your
nose. Pictures of the larynx in action taken from below would present
even more difficulties, of course; a camera can't be inserted through
the vocal folds without triggering a very strong gag reflex, and
even if that problem could be overcome, there'd still be the difficulty
of having an object between the folds which would prevent them from
opening and closing as they normally do.
Given these difficulties, scientists are often forced
to remove tissue and study it in the laboratory (in vitro,
as in "in vitro fertilization"). Unfortunately, this can introduce
its own set of difficulties. Tissue which is taken out of its natural
environment in the body is deprived of its normal blood supply and
nerve stimulation, among other things, and thus cannot be relied
upon to behave the same way as it would in the body. Even with these
limitations, much has been learned about biomechanical properties
of vocal tissues through in vitro investigations.
Force, Stress & Strain
Stress measures how much force is spread over an area. We
are concerned with stresses on the various tissues in the larynx,
of course; these stresses can be applied in many directions. For
instance, a stress applied in a direction toward the surface of
the tissue (or any area) is called compressional stress,
and the amount of such a stress is known as pressure. Those
stresses which point away from the surface are called tensile
stresses, while those applied along a surface (tangentially)
are shear stresses.
The way that the medium (in our case, the laryngeal
tissue) responds to various stresses is called strain. These
responses include stretching (elongation) of the tissue, contraction
or shortening, thickening, or thinning of the tissue. Typically,
if tissue changes shape/size in one dimension, an opposite deformation
will take place in another dimension. For instance, shortening the
vocal folds causes them to become thicker, but stretching them to
make them longer results in them becoming thinner.
Some typical types of stress in the vocal folds are
compressional stress from the folds colliding tens or hundreds of
times per second, tensile stresses from the Bernoulli effect, and
shear stresses from the folds rubbing and sliding along each other
Force Elongation Curve
A common way to study tissue's response to stretching forces is to hook up
a small piece of vocal fold tissue to a stretching device, and measure
its response to stretching forces, thus generating a graph called a force-elongation
curve. The curve for human tissue indicates that only a small force is
needed to stretch the tissue initially, but then progressively larger forces
are needed to stretch the tissue further. Thus, the tissue becomes stiffer
as it is lengthened. Ultimately, enough stretching causes the tissue to
Muscles: Function & Composition
Muscle tissue allows us to move and assume various postures. Since humans have
so many different movement needs, the body also has many different types
of muscle, which are specialized to perform various tasks efficiently,
such as moving food through the digestive tract, pumping blood, running,
and so on.
Some muscles are called upon to execute rapid movements,
such as the eye muscles and the laryngeal muscles. In addition to
allowing us to speak and sing, the larynx also performs the vital
function of protecting the airway, and must therefore be extremely
quick to react. Other muscles need not react quickly, but need to
be able to work for long periods of time, or even constantly, without
becoming fatigued; heart muscle is a good example.
Different muscles get their unique properties from
the composition of muscle fiber type. Different fibers vary in their
individual (1) resistance to fatigue and (2) in their speed of response.
The larynx contains muscle fibers which are both very quick to react
and also high in endurance; these properties evolved because of the
constant need to protect the airway.
The anatomy of muscle tissue, from the largest to
the smallest units, is as follows:
|| A group of muscle fibers, enclosed by connective
|| One portion of the fascicle; each muscle fiber
is surrounded by blood vessels and nerves
|| A large number of these make up each fiber
|| The smallest unit; each myofilament is made
up of large protein molecules called actin and myosin
Within each myofilament, the two types of protein
molecules can slide past each other and 'grab' each other to produce
movement and force. Check out the external link to the Washington
University School of Medicine website. It presents a nice animation
of muscle fiber contraction.
Motor Units and Muscle Contraction
In order for muscles to contract, motor units of muscle fibers are recruited.
Each motor unit comprised of a number of muscle fibers, and is activated as
a whole by a nerve cell. Smaller units (less than 100 fibers) suffice for smaller
contractions, while larger units (1000 or more fibers) are recruited for larger
In order to generate stronger and stronger muscle
contractions, the number of nerve impulses sent to the motor units,
their firing rate, is increased. The more nerve impulses received
by a motor unit, the more often it will contract, thus producing
more active stress in the muscle.