Additional Study Information: The purpose of this study is to investigate the safety and effectiveness of tirasemtiv as a potential new therapy for the improvement of breathing, muscle weakness and muscle fatigue in patients with Amyotrophic Lateral Sclerosis (ALS). The effects of tirasemtiv on the performance of your elbow, wrist, knee, ankle, and hand muscles and how long you can maintain a task will be evaluated. How much tirasemtiv gets into the blood stream and how long it takes the body to get rid of it will also measured. The goal of this research study is to test the effectiveness and safety of tirasemtiv taken twice a day for 24 weeks and then for 48 weeks for the treatment of lung function, muscle fatigue and weakness in ALS.