Firstly for all the people in the UK, who is the NSA? As the site states they are:

 The National Stuttering Association (NSA) is the largest self-help support organization in the United States for people who stutter. Our mission is to bring "hope, dignity, support, education, and empowerment to children and adults who stutter, and their families."

On the NSA website a Dr. Ben Watson is researching the effects of exercise on stammering using an online survey. The full description is:

We are conducting a survey to investigate the relation between regular exercise and stuttering frequency and severity.  Participation is voluntary and participants will be anonymous.  This study is approved by the Institutional Review Board of New York Medical College, Valhalla, NY.  Any questions should be addressed to Dr. Ben Watson (914) 594-4821 or .

If you are interested in participating please visit his site.

A more detailed description can be found on the BSA website the site reads:

Background
Stuttering is a complex disorder which interrupts the flow of speech. It is a condition in which the speaker repeats sounds and prolongs syllables in an irregular manner. People who stutter have several core characteristics that include, but are not limited to, blocking, which is characterized by a stutterer’s speech coming to a halt; repetition, in which sounds, syllables or words are repeated; and prolongations, in which a sound is extended beyond a normal duration. Facial expressions may alter due to tension in the lips, jaw, tongue, and upper chest. In moments of intense distress, the head may jerk uncontrollably. These secondary behaviors occur as a reaction to or in anticipation of the core stuttering behaviors described above. People who stutter avoid speaking situations such as placing phone calls and meeting new people......
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