The six-minute walk test (6MWT) measures the distance an individual is able to walk over a total of six minutes on a hard, flat surface. The goal is for the individual to walk as far as possible in six minutes. The individual is allowed to self-pace and rest as needed as they traverse back and forth along a marked walkway. The 6MWT has been used with a variety of other conditions than the chronic obstructive pulmonary disease (COPD) such as heart failure and stroke and is widely used in neuromuscular diseases. The six-minute-walk-test will be conducted as recommended by the American Thoracic Society. In this study, the six-minute-walk test will be performed once on visit 1 to detect the impact of muscle weakness, muscle stiffness and myotonia on muscular endurance.

Muscle ultrasound is an ideal imaging modality that allows for atraumatic, noninvasive, radiation-free point-of-care neuromuscular imaging. Muscular dystrophies are typically associated with an increase in the echogenicity from the muscle substance, distal attenuation of muscle echo and a corresponding loss of bone echo. Spinal muscular atrophies and neuropathies also showed an increase in muscle echo along with atrophy of the muscle and increase in depth of subcutaneous tissue, but a persisting bone echo. In several other myopathies, similar changes are seen. In semi-quantitative muscle ultrasound, muscle intensity will be documented using the 4-point Heckmatt score. In addition to that, cutis, subcutaneus fat and muscle will be measured in mm.

Muscle strength will be assessed by handheld dynamometry using the MicroFET2 myometer, produced by Hoggan Health Industries. This test is widely used in patients with neuromuscular diseases. To perform a test, the examiner holds the dynamometer stationary while the patient exerts a maximal force against the dynamometer. The patient makes a gradual increase in force and then completes an isometric hold for 4-5 seconds. The following muscle groups will be tested: Arm abduction, elbow flexion, elbow extension, knee extension, knee flexion, foot extension, foot flexion.

The secondary objective of this study is to compare stiffness, muscle tone, relaxation periods and elasticity with clinical muscle function tests, measured by clinical evaluation (MRC-scale).

The secondary objective of this study is to assess correlations between subcutaneous fat and muscle thickness and echogenicity, measured by muscle ultrasound and result values for stiffness, muscle tone, relaxation periods and elasticity.

The secondary objective of this study is to compare stiffness, muscle tone, relaxation periods and elasticity with clinical muscle endurance function tests, measured by the 6-minute walk test.