Speech fluency consist of three variables: continuity, rate, and ease of speaking. Continuity refers to speech that flows without hesitation or stoppage. Rate refers the speed in which the words are spoken. For English-speaking adults, the mean overall speaking rate is 170 words per minute (w/m), substantially quicker than the approximately 120 w/m that stutterers produce. Ease of speaking refers to the amount of effort being expended to produce speech. Fluent speakers put very little muscular or physical effort into the act of speaking, while stutterers exert a relatively large amount of muscular effort to produce the same speech. In addition to the physical effort involved in producing speech, the mental effort is usually much greater in stutterers than non-stutterers.

Disfluency in speech, including repetitions and prolongations, is normal for all speakers, but stuttering is distinct from normal disfluency in that it occurs with greater frequency and severity—the disfluencies occur much more often and tend to last longer with more strain. The types of disfluencies are also markedly different: normal disfluencies tend to be a repetition of whole words or the interjection of syllables like "um" and "er," while stuttering tends to be repetition and prolongation of sounds and syllables. The various behaviors that can disrupt the smooth flow of speech include repetition, prolongations, and pauses:

Repetition is by far the most common behavior exhibited by stutterers. In speech, repetition occurs when a unit of speech, such as a phrase, word, or syllable, is superfluously repeated. (Examples of repetition for a phrase would be, "I want.. I want.. to go.. I want to go to the store," or, "I want to go to the - I want to go to the store." A word repetition would often resemble, "I want to-to-to go to the store," and a syllable or sound repetition being, "I wa-wa-want to go to the store," or, "I w-w-want to g-go to the store.") Repetition occurs in the speech of both stutterers and non-stutterers, but non-stutterers are less likely to repeat shorter units of speech, mainly repeating phrases and sometimes words but rarely syllables. Non-stutterers will also, in the majority of cases, repeat the unit once or twice as opposed to the 6 or so times common from stutterers.

Prolongations are one of the least typical behavior exhibited by stutterers. Prolongations normally happen with child stutterers and with the sounds /?/, /?/, /v/, or any other fricative consonant or vowel. With stutterers, prolonging a sound sometimes leads to a pitch and volume increase.

Pauses are also a common source of disfluency in both stutterers and non-stutterers. Most pauses can be divided into two categories: filled pauses and unfilled pauses.

* Unfilled pauses are extraneous portions of silence in the ongoing stream of speech. These pauses differ from the pauses that punctuate normal speech, where they reflect common sentence structure or are used to add a particular rhythm or cadence to speech. Unfilled pauses by stutterers are usually unintentional and may cause the larynx to close, restricting the flow of air necessary for speech. Stutterers refer to this as "blocking".

* Filled pauses are interjections typical in normal speech like "um", "uh", "er", and so on. In speech these serve as a kind of place-holder—a way a speaker lets listeners know that he or she still has the floor and is not finished speaking. In addition to being used as a way of preempting interruption, they are also used by stutterers as a way of easing into fluency or deflecting embarrassment when they cannot speak fluently. Each stutterer has different sounds that he or she personally finds difficult to speak, usually plosive consonants or close vowels, and by using filled pauses the stutterer can ease into continuous speech that otherwise would be more difficult to begin. This is a form of avoidance behavior. Another element of speech that is similar to filled pauses are parenthetical interjections—interjections like "so anyways", "like", or "you know". This pattern is quite common in teenagers and is also used heavily by some stutterers.

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