6MWT 6-min walk test CC correlation coefficient DMD Duchenne muscular dystrophy MEP maximal expiratory pressure MGV mean grayscale value MGV-arm mean grayscale value of upper extremity muscles (average of the mean grayscale value of deltoid, biceps brachii, and flexor carpi radialis muscles) MGV-leg mean grayscale value of lower extremity muscles (average of the mean grayscale value of rectus femoris, tibialis anterior, and gastrocnemius muscles) MGV-neck mean grayscale value of head and neck...