The length of the study, including screening, dosing, and follow-up, is approximately 20 weeks. After a one-week open-label phase during which all patients will receive CK-2017357 125 milligrams (mg) twice daily, patients will be randomized one to one (fifty-fifty) to receive double-blind CK-2017357 or matching placebo. The CK-2017357/placebo dose will be increased no faster than weekly to each patient's highest tolerated daily dose, with a maximum of 250 mg twice daily. The dose may be decreased based on tolerability. Patients will continue treatment at the highest tolerated dose to complete a total of 12 weeks of double-blind treatment. Patients may be on riluzole or not on riluzole at study entry. Patients not on riluzole must stay off riluzole. Patients on riluzole who are getting double-blind CK-2017357 will be given riluzole at half the labeled dosage (50 mg once a day instead of 50 mg twice a day). Blood tests for safety will be performed. Information about any side effects that may occur will also be collected.