Balance and mobility are assessed by using the Timed Up and Go (TUG), a standard outcome measure for the elderly that is also increasingly being used in neuromuscular disorders. Participants are asked to rise from a chair, walk 3 meters, turn 180 degrees and return to a seated position in the chair.

Investigators will isolate DNA from blood cells in saliva - and will determine the methylation levels in the 4q region on chromosome 4 using this new technique. Investigators will compare methylation levels between participants and compare to other clinical information collected in this study.

A limited physical exam and strength testing will be used to derive two FSHD clinical severity scores. These severity scores both rank weakness in the face, shoulders, arms, distal, and proximal lower extremities on either a 10 or 15 point scale. The higher the severity score the more affected the individual.

The PROMIS57 is an instrument developed by the NIH which generates scores for physical function, and the impact of physical limitations on daily life. 57 questions are summed into a total score, which is transformed into a normalized t-score with 50 representing normal, and lower scores representing increasing disability.

This index measures upper extremity dysfunction. 20 questions are combined into a total score, the score is transformed into a normalized score with 80 representing normal, and lower scores representing increasing disability.

An exercise and pain assessment questionnaire was comprised specifically for this study and will be completed at every visit.

The FDI is a short 5 item questionnaire. The five questions are summed into total score which transformed onto a percentage scale, with 100 representing normal, and lower scores representing increasing disability.

The FSHD-RODS is a questionnaire about the relationship between daily activities and health. 32 questions are are constructed using a modern clinimetric technique, the Rasch analysis, which by ordering the items in increasing difficulty, allows for the calculation of the sum of item scores to achieve a total score. Answers provide information about how FSHD affects daily and social activities and to what degree participants are able to perform usual activities.

For sites who routinely obtain negative inspiratory or expiratory force (MIP/MEP) will be collected.

Time to ascend and descend 4 standard stairs.

For MMT a modified Medical Research Council 13-point scale will be used - with standardized positions for each muscle.

Whole body MRI will be performed for a sub-group of approximately 200 participants. MRI is recognized as a gold standard for body composition analysis, enabling a more complete description of a person's body composition profile from a single examination.In addition, we will include analysis to help make needle biopsy based on STIR and quantitative fat fraction more efficient with higher yield using standard fiducial markings and body landmarks.

Muscle biopsy will be performed for a sub-group of approximately 200 participants. Muscle biopsies will be performed at the baseline visit and for a subset of 20 participants at 4 months. We will use MRI analysis to help inform which lower extremity muscle will be chosen for biopsy. Muscle samples will be used to recapitulate RNA sequencing to confirm prior DUX4 target selection. In addition, other early or late genes of interest may be included which are FSHD-related but not specific to DUX4. We will also select 4 control genes for quality assurance, 2 to confirm muscle tissue, and 2 to rule out high fat content.

Reachable and Functional Workspace will be performed for a sub-group of approximately 200 participants. Participants are seated in front of a stereo-camera and perform a standardized upper extremity movement protocol under the supervision of a study clinical evaluator. Five-hundred-gram wrist weights will be added. The standardized simple set of movements consist of lifting the arm from the resting position to above the head while keeping the elbow extended, performing the same movement in vertical planes at around 0, 45, 90, 135 degrees. The second set of movements consists of horizontal sweeps at the level of the umbilicus and shoulder. Each set of movements is repeated three times for left and right arm.

The purpose of the wearable device is to accurately measure upper and lower limb movement during normal daily living. In FSHD, muscles are affected asymmetrically, therefore, measurements with the wearable device will be taken from the upper and lower limbs bilaterally.

32-item Motor Function Measure (MFM32) is a clinician-reported outcome measure used to assess the functional abilities of individuals with neuromuscular diseases.

We will collect the FSH-Composite outcome measure for children and adolescents. As with the FSHD-COM, the body regions assessed match areas of importance identified by patients and include leg function; shoulder and arm function; trunk function, hand function; and functional balance. The FSHD-COM Peds was modified by adding lighter weights to the shoulder abduction and flexion and elbow flexion items to accommodate motor development and lower strength levels seen in children. Most of the items are scored on a 5-point ordinal scale (0 = normal/not affected and 4= being severely affected or unable to complete the item) with the exception of the shoulder abduction/flexion and elbow flexion items which have been expanded and are scored on a 7-point scale using the additional weight categories. Thus, the total score for the FSHD-COM Peds is 84 (compared to 72 for the FSHD-COM).