A strength measurement device called Accurate Test of Limb Isometric Strength (ATLIS) was developed to precisely and conveniently measure static limb strength in patients with ALS. The investigator will compare ATLIS data with data from the commonly used ALS outcomes measure, the ALS Functional Rating Scale-Revised (ALSFRS-R), as well as an exploratory measure, electrical impedance myography (EIM), in a prospective, longitudinal study. Both outcomes measures will be performed on 100 subjects collected preferably at bi-monthly clinic visits during the study period.

Cedars-Sinai Medical Center will enroll and collect data on 100 subjects. Subjects will ideally be tested every 1 to 4 months according to each patient's site visit schedule. Subjects will be tested at each visit using ATLIS, the ALSFRS-R questionnaire and EIM.

Inclusion criteria:

• Subjects 18 years or older diagnosed with clinically possible, laboratory supported probable, probable or clinically definite ALS according to the World Federation of Neurology Revised El Escorial Criteria.
• Capable of providing informed consent and complying with trial procedures.
• Active movement of at least two limbs at the time of study entry
• Intend to routinely attend clinic for neurological care at the clinical site

Exclusion criteria:

• Patients, judged by the site clinical investigator to have medical conditions that may limit their ability to safely exert maximal force using the muscles in their arms and legs will be excluded from this study (e.g. unstable hypertension, significant cardiac disease, significant musculoskeletal disorders, or other medical conditions).
• Patients judged by the site clinical investigator to have significant clinical evidence of unstable medical or psychiatric illness will be excluded.

Adults with ALS