STRIDE Study - A Study in Subjects With LOPD Who Are Currently Being Treated With ERT

Inclusion Criteria: 1. Subject has a diagnosis of Pompe disease based on documented deficiency of GAA activity and a documented GAA mutation. 2. Male and female subjects between 18 years and 75 years, inclusive and ≥ 50 kg. 3. Subject must be currently receiving standard-of-care ERT (alglucosidase alfa) at a dose of 20 mg/kg dose every other week. 4. Subject must have been on ERT for the preceding 2 years or more. 5. Subject must have an upright forced vital capacity (FVC) within 35 to 90% of predicted normal (NHANES III reference values), based on the higher of the screening or baseline value, if their 6 minute walk distance (6MWD) is \> 200 m. Subject must have an upright FVC within 40 to 90% of predicted normal (NHANES III reference values), based on the higher of the screening or baseline value, if their 6MWD is ≤ 200 m. If FVC is between 80 and 90% of predicted normal, the subject may enter the study if the percent predicted FVC value drops by 10% predicted or more in supine position 6. Subject is able to walk at least 100 m in the 6MWT and the assessment is noted as valid. Exclusion Criteria: 1. Subject has received any investigational therapy or pharmacological treatment for Pompe disease, other than alglucosidase alfa within 30 days or 5 half lives, whichever is shorter, prior to the Baseline Visit or is anticipated to do so during the course of the study 2. Subject is on any of the following prohibited medications within 30 days of baseline: * miglitol (eg, Glyset) * miglustat (eg, Zavesca) * acarbose (eg, Precose, Glucobay) * voglibose (eg, Volix, Vocarb, Volibo) 3. Subject requires use of invasive or non-invasive ventilatory support for \> 6 hours a day while awake. 4. Subject has a medical or any other extenuating condition or circumstance that may, in the opinion of the investigator, pose an undue safety risk to the subject or compromise his/her ability to comply with protocol requirements. This includes clinical depression (as diagnosed by a psychiatrist or other mental health professional) with uncontrolled or poorly controlled symptoms. 5. Subject is breastfeeding, or is pregnant or planning to become pregnant within the next 2 years. 6. Other exclusion criteria according to the Lumizyme/Myozyme instructions for use.