Oxygen Cost of Dynamic or Isometric Exercise

Abstract

1898 The oxygen cost per kg of active skeletal muscle during dynamic knee extensions has been estimated. However, assumptions of active skeletal muscle mass may have underestimated relative VO2. The use of electromyostimulation (EMS) and magnetic resonance imaging (MRI) could aid in the more accurate quantification of active muscle mass and thus a more accurate estimation of the oxygen cost per kg of active muscle. PURPOSE: To quantify the oxygen cost per kg of active skeletal muscle mass during one-legged and two-legged dynamic knee extensions, as well as two-legged isometric exercise. METHODS: Seven recreationally active males and one female volunteered to participate (28 ± 3 years, 176 ± 7 cm, and 79 ± 13 kg (± SD)). EMS was used to evoke 2 bouts of dynamic knee extension on a Krogh ergometer and one bout of isometric exercise in a specially designed chair. Contractions were evoked at 30 Hz and amplitude adjusted to allow three minutes of exercise at 30 contractions per minute. Stimulation amplitude was identical for two-legged dynamic and isometric exercise. Right leg amplitude was replicated for one-legged exercise. Whole body VO2 was measured by indirect calorimetry and active muscle mass determined using MRI. The oxygen cost per kg of active skeletal muscle was calculated as the increase in VO2 divided by the active muscle. RESULTS: VO2 per kg of active muscle was 724 ± 50, 618 ± 60, and 320 ± 34 ml • kg−1 • min−1 for one- or two-legged dynamic extensions and two legged isometric actions respectively. No significant difference in oxygen cost was observed between one- and two- legged dynamic exercise. However, the oxygen cost for isometric exercise was significantly lower than that of dynamic exercise. CONCLUSIONS: This study indicates that oxygen cost per kg of active skeletal muscle is higher than previous estimates. These findings also strengthen evidence for higher oxygen cost of dynamic versus isometric contractions. Supported by NIH grant HD 39676(GAD)