Résumé :
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Reliable investigation of exercising muscle within a superconducting magnet requires the construction of dedicated ergometers in order to perform standardised exercise protocols and to record mechanical variables. A few ergometers designed for quadriceps dynamic exercise have been described, but the corresponding mechanical data typically rely on the constancy of imposed work rates which is not satisfactory. Given that we have reached a point in the field of muscle energetics where absolute measurements are warranted to take the area forward, we designed an ergometer, including two force and two displacement transducers, allowing dynamic and isometric knee extension within a Magnetic Resonance (MR) system and accurate measurements of power output. On the basis of repeated measurements, the force and displacement transducers accuracy was 0.5% for values ranging from 0 to 394N and 3 % for values ranging from 0 to 20 cm. In addition, measurements were not affected by magnetic field and we were able to distinguish mechanical output during eccentric and concentric phases of exercise. MRS experiments in exercising muscle were conducted in 8 subjects. They performed two standardized dynamic alternate leg extension exercises (25 and 35 % of MVC) while the corresponding metabolic changes were measured using 31P-MRS. The mean power output produced during both exercises were 62 ± 17 and 79 ± 12 W. The corresponding metabolic changes were significant with a 20 to 40% PCr depletion and an end of exercise pH ranging from 7.0 to 6.3 pH units. Overall, the present ergometer is MR compatible. Dynamic and isometric leg extensions are possible while power output can be accurately quantified separately during the concentric and eccentric phases of exercise. Such an ergometer should be useful for future metabolic studies conducted in control subjects and patients for whom muscle energetics is impaired. Therapy follow-up is also possible
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