Clinical efficacy will be assessed at regularly scheduled study visits, including functional tests such as the six-minute walk test (6MWT), time to stand (TTSTAND), time to run/walk 10 meters (TTRW), time to climb 4 stairs (TTCLIMB) and quantitative muscle testing (QMT). All patients will undergo a muscle biopsy of the bicep at baseline and a second muscle biopsy at Week 24.