This information is based on research studies by Per  A. Tesch, PhD., who is an internationally recognized expert in the field of strength training.  Tesch is a professor of physiology at the Karolinska Institute in Stockholm, Sweden. 

MRI (magnetic resonance imaging) was used to make these recomendations.  MRI were divided into three levels of gray.  Light gray meaning heavy muscle use, moderate gray meaning moderate muscle use and black, meaning the muscle was not used.  All MRI were done during weight training on experienced subjects all using correct movement techniques while performing 10-12 repetitions to failure.