The purpose of the study is to use a new research imaging technique, a kind of magnetic resonance imaging (MRI), to measure important metabolic features of muscle, including mitochondrial function, in people with mitochondrial disease and in healthy individuals. (Mitochondria are tiny organelles that generate energy for the body.)

It is hoped that this new strategy will help physicians to understand better the health problems of people with mitochondrial disease. Eventually, this could lead to better diagnostic and treatment approaches.

There are two study-related visits.

The first visit is a "screening" visit to ensure eligibility. This includes fasting blood tests. The second visit is an MRI scanning session. This also takes around 2 - 3 hours, with no more than 1.5 hours spent in the actual MRI machine. It may be possible to complete these two visits on the same day. Otherwise, the MRI session should occur within 3 months of the screening visit. In addition, an optional physical capacity visit will occur on the same day as the MRI session.

Group 1) Mitochondrial Disease:

Inclusion Criteria:

• Genetic diagnosis of mitochondrial disease
• Be able to perform sub-maximal leg exercise for several minutes
• Be able to provide written, informed consent
• Cognitively and medically stable and able to comply with study procedures
• Able to fast for 4 hours prior to blood draw and/or MRI scanning
• Be willing to stop taking any over-the-counter vitamins and supplements that are neither prescribed nor recommend by their physician for 2 weeks prior to MRI scanning

Exclusion Criteria:

• Diabetes
• Alcohol/substance abuse
• Smoking
• Use of any investigational agents within 4 weeks of enrollment
• Any contraindication to MRI scanning

Group 2) Healthy Individuals:

Inclusion Criteria:

• Be able to perform sub-maximal leg exercise for several minutes
• Be able to provide written, informed consent
• Cognitively and medically stable and able to comply with study procedures
• Able to fast for 4 hours prior to blood draw and/or MRI scanning
• Be willing to stop taking any over-the-counter vitamins and supplements that are neither prescribed nor recommend by their physician for 2 weeks prior to MRI scanning

Exclusion Criteria:

• Carry a diagnosis of mitochondrial disease
• Have a first-degree relative with a diagnosis of mitochondrial disease
• Diabetes
• Alcohol/substance abuse
• Smoking
• Use of any investigational agents within 4 weeks of enrollment
• Any contraindication to MRI scanning

Two groups of subjects: 1) mitochondrial disease and 2) healthy individuals