The Speech Mechanism: A 4-part Series
Part 1: Lower Respiratory System

By Bryana Williams M.S. CCC-SLP

When it comes to communication, the vast majority of people turn to speech-it’s highly versatile, typically efficient, and allows us to layer multiple components like tone, intonation, and vocabulary choice to communicate a clear message. In fact, it’s such a ubiquitous skill that speaking people take it for granted-how often do you actually pay attention to the process of forming sounds and words? The answer for many people would be never!  But if you are a regular visitor to this site, you know speech isn’t accessible for everyone- a report from the NCBI indicates an estimated 1.3 percent of Americans (about 4 million people) cannot reliably meet their daily communication needs using natural speech (Beukelman and Mirenda, 2013). This is the reality for people with apraxia who consider themselves nonspeaking, minimally speaking, or unreliably speaking. Even when speech is available, it does not mean that it fully and accurately represents a person’s thoughts all the time-you can click here for a description of this from speller William Tziavaras 

Nonspeaking people, then, often turn to AAC options to replace or supplement their speech. For “natural” speakers, this can raise a few questions. Why can’t an apraxic person just learn use speech? Why is their rote speech different from prompted or purposeful speech? Why can they talk clearly one time and then sound unclear the next? Well in this four-part series, we are going to explore the major anatomical systems of our bodies that allow us to produce speech to help answer these questions. First up? Our lower respiratory system. 

Part One: The Powerhouse-The Respiratory System

Anatomically, the lower respiratory system is comprised of your lungs, rib cage, and several associated muscles. Together these components interact to complete “resting” or “tidal” breathing (i.e. intaking oxygen and removing carbon dioxide) as well as “speech breathing”: even though they rely on the same anatomy, they are two very different functions.

Let’s begin with resting breathing. This occurs the majority of the time since it is the primary function of the lungs. It includes a phase of active inhalation and a phase of passive exhalation, which together form one cycle of breathing. Each phase recruits the muscles in our chest and abdomen differently.

For the inhalation phase, we rely on two muscles/muscle groups. One you’ve probably heard of, the diaphragm is a large, domed muscle under the rib cage-when it contracts and flattens, it pulls on the bottom of the lungs and creates extra space for air to flow in. At the same time, another set of muscles called the external intercostals (sitting in between each rib)-contract to lift the whole rib cage up and out, similar to the motion of a hinged handle on a bucket. You can try a few breaths yourself to feel this motion.

The movement of the ribs and the flattening of the diaphragm then pulls on the lungs, which are encased within the rib cage and surrounded by a membrane layer called visceral or pulmonary pleura. The inside of the rib cage is also covered in a thin membrane (parietal or costal pleura), and sandwiched between the two is a small space filled with fluid called the pleural space. As the muscles expand the rib cage, it creates a vacuum of negative pressure in this space, drawing the lungs out and causing air to flow in an effort to equalize the pressure.

Once we’re ready to exhale, however, our muscles have a very easy job-all they have to do is relax! When they do so, the flexible tissues of our lungs and ribcage go through elastic recoil, rebounding into their resting positions and pushing on the lung tissue to create positive pressure. Much like an inflated balloon when you release your fingers from the neck, this sends the air rushing back out in our case, through the mouth or nose. In total, each resting breath cycle takes about 1-2 seconds (half for inhalation, half for exhalation) and typically don’t move the ribs out very far. 

It’s important to note here that tidal breathing is an unconscious process run by our medulla, a structure at the base of the brainstem-it is not something we have to think about. This is true whether you are a speaking or nonspeaking person.

When we switch to intentional speech breathing, though, we have to switch up the way we use our muscles. We still use the diaphragm and intercostals on inhalation, but the contraction is stronger and faster. This means our ribs and lungs move out to a greater degree and do so more quickly than in rest breathing. The result is more air in our lungs, and thus greater pressure-a necessity for producing long, fluent strings of speech! As a consequence, inhalation prior to speech is abrupt and takes up about 10% of the total cycle. Try this for yourself-place your hands on the sides of your rib and breath in as though you’re going to say a sentence. Feel the difference?

Now that we have plenty of air in our lungs, we need to control its escape-if it flows out too quickly, our ability to produce more than a few sounds or words is compromised. To ensure a steady, prolonged flow of air, the external intercostal muscles have to remain active and gradually release their contraction to allow the ribs to fall more gently.

In typical conversation, this is about all we need. But, we do sometimes require additional help from other muscles in our chest and abdomen if we need to force expiration. The first are our internal intercostals, which also sit between the ribs but behind the externals and running in the opposite direction. The second are our abdominals. 

Why on earth would we forcefully exhale? Well, there are a couple of reasons. First, if the pressure becomes too low, like when you are speaking and run out of air, contracting these muscles can keep the pressure up so we can finish our sentence. Secondly, controlling the pressure in our lungs is one way we manage the volume of our voice. When we yell or scream, we are using these accessory muscles to add more pressure. 

Whew, what a chore to just get some airflow! If you’ve made it this far with me, you’re probably seeing where the plans can go awry-remember that for “naturally” speaking people, years of practice have been available and this pattern of intentional breathing has become automatic-it doesn’t generally require us to pay attention in order for it to occur smoothly. But for an apraxic person, consciously controlling all of these factors is a LOT and can make intentional speech difficult to get out. For instance, they may struggle to initiate that quick, full inhale to start speech at all. They may have trouble holding the contraction to extend their exhale, which means it’s difficult to get more than a word (and sometimes even just a syllable) to come out. They may not be able to start their mouth moving until they get to the end of their breath. They may battle to control their volume-intentional speech may come out as a whisper or as a yell. Others may attempt a breath but end up recruiting less efficient muscles to lift the rib cage, such as the scalene muscles in the neck. Or, they may have a less common pattern like trying to speak as they inhale rather than on exhale. It is energy-intensive to break down the components of breath control and build them into reliable, purposeful movement. 

So if you are a nonspeaker with the goal of developing more purposeful speech (or a friend/family member/professional supporting them), remember you are doing big work -take it one breath at a time!

Keep an eye out for part two of this series, “The Voice Box”, in a few weeks!

Living in Atlanta and supporting non-speakers in Georgia and Nashville, Tennessee, Bryana Williams is a Speech-Language Pathologist and registered Spelling to Communicate Practitioner. She owns her own private practice and has, for the past three years, working as part of the I-ASC S2C Professional Training Course as a mentor, Cohort Captain, and continues to support I-ASC’s practitioner and CRP training initiatives through the development and improvement of training protocols.


The mission of I-ASC is to advance communication access for nonspeaking individuals globally through trainingeducationadvocacy, and research.  I-ASC supports all forms of augmentative and alternative communication (AAC) with a focus on methods of spelling and typing. I-ASC currently offers Practitioner training in Spelling to Communicate (S2C)with the hope that other methods of AAC using spelling or typing will join our association

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