I would even go a step further. In that, tensing the lips is not required. Yes but relaxing the lip muscles would also not be required, right? It's not that tensing my lips to its maximum extent prevents me from moving the lips in any way that would result in a speech block. For example, individuals with Motor Speech Disorders (MSD) or Spastic Dysarthria - tense the throat muscles and vocal cords with such intensity that it results in a strained, effortful voice. However, this almost never leads to speech blocks. In high school, if I found myself in a speech block, every time I re-tried to get out of the block was by forcing and tensing even harder, until I got the word out. It's more likely that, if we blame tension and expect (or require) less tension, that our defensive mechanism becomes activated and prevents us from saying the sound. Everyone has their own viewpoints, so why not share them: What does everyone (reading this post) think?