Quantitative testing of handgrip strength, myotonia, and fatigue in myotonic dystrophy

An electrically calibrated, isometric hand dynamometer was used to quantitate maximum voluntary contraction (MVC(, relaxation time and fatigue time (at 50% MVC) in 18 normal and 10 myotonic dystrophy subjects. Precise measurements of MVC and fatigue time were obtained in normals and myotonic dystrophy patients. Relaxation times were markedly prolonged and were quite variable in all patients with myotonic dystrophy in contrast to the highly reproducible times in normals. Neither "warm-up" activity nor acetazolamide treatment altered relaxation time.