Should Your Voice Feel Tired When You Practice?
The strange thing about singing is that you already have all the strength needed in your throat to sing. This is different than say, guitar or piano where the finer muscles need to become stronger. Your tongue, jaw and other throat muscles are consistently used all day in speech and in swallowing, so not only are they very strong already, but they also tend to get overly involved in singing for most people, most of the time.
Learning to sing is not taxing for the muscles in your throat, but it is really taxing on your brain. Remember in kindergarten when you'd tighten your hold hand, arm, neck and shoulders while you struggled to form the cursive letter "G"? Was all that extra effort really needed? What about when you tighten your whole face, squint your eyes, and speak forcefully when you were trying to read words for the first time? Although it is natural to use more effort and tension than really needed while learning a new skill, developing new coordinations is very, very taxing on your brain and attention span.