Methods and compositions for producing mature myotubes are provided herein. In some instances, the method involves contacting a myoblast in an in vitro culture with a compound, wherein the contacting the myoblast in the in vitro culture with the compound results in generation of mature myotubes or myotube-like cells. In some cases, methods of treatment are provided involving treating a subject with a compound such as a Chk1 inhibitor in order to treat muscle deficiency. The compound may be administered as a stand-alone therapy or in combination with a cell therapy, such as introduction of muscle precursor cells such as satellite cells or myoblasts. Methods for identifying compounds that induce formation of mature myotubes or myotube-like cells from myoblasts are also provided herein, as well as methods of using the identified compounds to treat subjects.