Respiratory failure due to diaphragm dysfunction is considered a main cause of death in nemaline myopathy (NM) and we studied both isometric force and isotonic shortening of diaphragm muscle in a mouse model of nebulin-based NM (Neb cKO). A large contractile deficit was found in nebulin-deficient intact muscle that is frequency dependent, with the largest deficits at low–intermediate stimulation frequencies (e.g., a deficit of 72% at a stimulation frequency of 20 Hz). The effect of the fast skeletal muscle troponin activator (FSTA) tirasemtiv on force was examined. Tirasemtiv had a negligible effect at maximal stimulation frequencies, but greatly reduced the force deficit of the diaphragm at sub-maximal stimulation levels with an effect that was largest in Neb cKO diaphragm. As a result, the force deficit of Neb cKO diaphragm fell (from 72% to 29% at 20 Hz). Similar effects were found in in vivo experiments on the nerve-stimulated gastrocnemius muscle complex. Load-clamp experiments on diaphragm muscle showed that tirasemtiv increased the shortening velocity, and reduced the deficit in mechanical power by 33%. Thus, tirasemtiv significantly improves muscle function in a mouse model of nebulin-based nemaline myopathy.