Described are GCF-8 inhibitors of the formula (I), and pharmaceutically acceptable salts thereof, wherein n, R1, R2, R5, R6, X and Z are defined herein. Also provided are pharmaceutical compositions comprising the same. Such compounds and compositions are useful in methods for inhibiting GDF-8 in a cell and methods for treating a patient suffering from a disease or disorder, wherein the patient would therapeutically benefit from an increase in mass or strength of muscle tissue.