Part 1 (dose escalation, open-label) Part 1 will consist of up to 6 cohorts (A to F) of patients and will evaluate multiple ascending dose levels of ACE-083 in either the tibialis anterior (TA) or biceps brachii (BB) muscle. Patients in each cohort will be enrolled in a 4-week screening period before beginning treatment. A Safety Review Team (SRT) will meet to review data for each cohort when at least 4 patients within a cohort have completed their Day 43 visit prior to dose escalation.