Article

Metabolic myopathies encompass muscle glycogenoses (GSD) and disorders of muscle
fat oxidation (FAOD). FAODs and GSDs can be divided into two main clinical
phenotypes; those with static symptoms related to fixed muscle weakness and
atrophy, and those with dynamic, exercise-related symptoms that are brought about
by a deficient supply of ATP. Together with mitochondrial myopathies, metabolic
myopathies are unique among muscle diseases, as the limitation in exercise
performance is not solely caused by structural damage of muscle, but also or
exclusively related to energy deficiency. ATP consumption can increase
50-100-fold in contracting, healthy muscle from rest to exercise, and testing
patients with exercise is therefore an appropriate approach to disclose
limitations in work capacity and endurance in metabolic myopathies. Muscles rely
almost exclusively on muscle glycogen in the initial stages of exercise and at
high work intensities. Thus, patients with GSDs typically have symptoms early in
exercise, have low peak work capacities and develop painful contractures in
exercised muscles. Muscle relies on fat oxidation at rest and to a great extent
during prolonged exercise, and therefore, patients with FAODs typically develop
symptoms later in exercise than patients with GSDs. Due to the exercise-related
symptoms in metabolic myopathies, patients generally have been advised to shun
physical training. However, immobility is associated with multiple health issues,
and may even cause unwanted metabolic adaptations, such as increased dependence
on glycogen use and a reduced capacity for fatty acid oxidation, which is
detrimental in GSDs. Training has not been studied systematically in any FAODs
and in just a few GSDs. However, studies on single bouts of exercise in most
metabolic myopathies show that particularly moderate intensity aerobic exercise
is well tolerated in these conditions. Even low-intensity resistance training of
short duration is tolerated in McArdle disease. Training in patients with FAOD
potentially can also expand the metabolic bottleneck by increasing expression of
the defective, but partially functional enzyme. Exercise performance in metabolic
myopathies can be improved by different fuel supplementations and dietary
interventions and should be considered as adjunct therapy to exercise training.
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