Biomechanics of Vocal Tissues:
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 during oscillation.

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 break.

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:

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 contractions.

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.

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