Lean body mass is a measure of body weight without any body fat, calculated by subtracting fat mass from total body mass. A lower lean body mass is associated with health issues of the skeletal muscle such as osteoporosis and impaired muscle balance.
A person's lean body mass is strongly influenced by genetics: two genotypes have now been shown to be associated with lower lean body mass. Learn more here: http://www.ncbi.nlm.nih.gov/pubmed/19268274
Take a look at your genetic tendency for lean body mass!