The purpose of this study is to evaluate the effectiveness and safety of an investigational drug called reldesemtiv in amyotrophic lateral sclerosis (ALS). The study will evaluate patients' muscle strength and breathing, as well as conduct physical assessments and questionnaires. The questionnaires will address quality of life, mood, and ALS function. Blood and urine samples will also be collected. Additionally, researchers will evaluate the pharmacokinetics of reldesemtiv (how much reldesemtiv gets into the blood stream and how long it takes the body to get rid of it). Reldesemtiv is being studied as a potential new therapy for the improvement of skeletal muscle function in conditions associated with muscle weakness or fatigue, such as ALS. Participants will be randomly assigned to receive either reldesemtiv or placebo (inactive substance).