Dependence of maximum performance time on work intensity in patients with a hereditary myopathy with succinate dehydrogenase deficiency.

Patients with a hereditary mitochondrial myopathy with succinate dehydrogenase (SDH) deficiency and abnormal lactacidosis during physical exercise have a low work capacity when exercising for about 10-15 min. Their maximum voluntary muscular strength is fairly normal. The relationship between the time (t) and a constant workload (N) that a healthy subject can maximally sustain can be expressed as: log t = beta + alpha log N. For normal subjects the constant alpha is approximately -5 and the constant beta has a large interindividual variation. Of four myopathy patients alpha was determined from two or three maximum bicycle exercise tests of different duration (including ramp- and steady-state tests using a new application of the method of adding submaximal loads to the final maximum workload). The value of alpha varied between -1.0 and -1.81 and beta had low values, both significantly different from those of healthy subjects. The alpha values explain the divergent results that may be obtained with different types of exercise tests in some of these patients, i.e. a normal or moderately reduced capacity in exercise tests of short duration (for example a short Tornvall or a ramp type of test) and a very low exercise capacity in tests of longer duration (for example a steady state type of test with workloads chosen to allow at least two loads). The low absolute value of alpha may be related to the abnormally increased anaerobic metabolism of these patients during exercise, caused by the SDH deficiency.