I'm currently training each musclegroup once per week. I have heard from several people that there are a few muscle groups which can safely be trained twice a week due to their ability to recover more quickly than other groups. I was told these include:
Traps
Biceps
Calves
Abs
When I say train safely, I mean train without the concern of overtraining. Does anybody agree with this? Does anybody disagree? Please provide evidence/examples in either case, thank you!