Interesting. I saw a reference to retraining muscles on their site too. Not something I’ve heard of. If you are interested in the research behind stuttering therapies, there is a systematic review here by Bothe et al. (2006) here: https://www.researchgate.net/publication/6695770_Stuttering_Treatment_Research_1970-2005_I_Systematic_Review_Incorporating_Trial_Quality_Assessment_of_Behavioral_Cognitive_and_Related_Approaches The review shows that for adults prolonged speech programs tend to be the most effective.