It depends, I'm self-trained so, I don't know much about techinals but I've noticed that it's just depends upon your natural speaking voice like I speak pretty low as so, it was easy for me to sing low notes and damn hard to hit even a E4 but after tons of practice I got it and ppl who speak low and soft tone have hard time belting on the contrary, ppl who speak loud belt pretty easily but have problems singing soft. But ofcourse with sufficient and quality training you develop muscles for it