New methods of functional evaluation of patients with metabolic myopathies : the effects of exercise training

INTRODUCTION Mitochondrial myopathies (MM) and myophosphorylase deficiency (McArdle’s disease, McA) are genetic disorders characterized by impairments of energy metabolism which translate into reduced exercise tolerance. At present the therapeutic interventions available for these patients are very limited. Evidence has been provided that aerobic training should be considered as a treatment for these conditions. Aim of the present study was to utilize non−invasive methods of functional evaluation, specifically aimed at oxidative metabolism at the skeletal muscle level, in order to evaluate the effects of an aerobic exercise training. METHODS 7 MM and 8 McA patients underwent 12 weeks of exercise training (4v/weeks) at home at an heart rate (HR) corresponding to about 65-70% of the maximal HR. Oxygen uptake (V’O2) and skeletal muscle (vastus lateralis) fractional O2 extraction (by NIRS) were assessed during incremental and moderate and high-intensity constant-load exercises: All tests were carried out on a cycle ergometer before (BEFORE) and at the end (AFTER) of the training period. Before and about 2 months after the termination of the training period average daily energy expenditure (EE) of the patients were determinated (SenseWear Armband). RESULTS V’O2peak increased significantly with training both in MM (from 14.7 ± 3.0 [x ± SD] mL/kg/min BEFORE to 17.6 ± 3.3 AFTER) and in McA (from 17.4 ± 4.0 mL/kg/min to 20.4 ± 4.4 mL/kg/min). Peak skeletal muscle (vastus lateralis) fractional O2 extraction increased with training both in MM (from 22.0 ± 16.5 % to 32.6 ± 14.5) and in McA (from 17.8 ± 17.9 % to 43.4 ± 17.4). During the high-intensity constant-load exercise, MM and McA patients showed, after training, clear signs of increased exercise tolerance, such as lower HR (from 134.3 ± 26.6 b/min to 121.2 ± 21.9) and lower scores at the Borg’s scale of perceived exertion (from 14.5 ± 1.2 to 12.3 ± 1.5). EE was not different BEFORE (36.6 ± 9.2 kcal/day/kg) vs. AFTER (35.7 ± 13.4). CONCLUSION In MM and McA patients a 12 weeks aerobic training program significantly increased exercise tolerance. Our findings confirm that near infrared spectroscopy can effectively detect the functional improvements obtained by training, yielding insights also on the mechanisms of the improvements at the pathophysiological level. Surprisingly, the improvements in exercise tolerance obtained by the training program did not determine an increase in average daily energy expenditure.