The five leading causes of workplace injuries drive nearly 72% of the nation’s direct workers compensation costs, according to research released Tuesday by Liberty Mutual Group Inc. Overexertion—or injuries caused by lifting, pushing, pulling, holding and carrying—costs businesses $12.5 billon in direct annual expenses and accounts for more than 25% of the national burden, according to Liberty Mutual’s Workplace Safety Index. The index, compiled by the Liberty Mutual Research Institute for Safety, relies on data that researchers collected on injuries causing at least six missed days of work.