BACKGROUND: Oxygen cost of different muscle actions may be influenced by different recruitment and rate coding strategies. The purpose of this study was to account for these strategies by comparing the oxygen cost of dynamic and isometric muscle actions relative to the muscle mass recruited via surface electrical stimulation of the knee extensors. METHODS: Comparisons of whole body pulmonary delta VO2 were made in seven young healthy adults (1 female) during 3 minutes of dynamic or isometric knee extensions, both induced by surface electrical stimulation. Recruited mass was quantified in T2 weighted spin echo magnetic resonance images. RESULTS: The delta VO2 for dynamic muscle actions, 242 +/- 128 ml x min(-1) (mean +/- SD) was greater (p = 0.003) than that for isometric actions, 143 +/- 99 ml x min(-1). Recruited muscle mass was also greater (p = 0.004) for dynamic exercise, 0.716 +/- 282 versus 0.483 +/- 0.139 kg. The rate of oxygen consumption per unit of recruited muscle (VO2(RM)) was similar in dynamic and isometric exercise (346 +/- 162 versus 307 +/- 198 ml x kg(-1) x min(-1); p = 0.352), but the VO2(RM) calculated relative to initial knee extensor torque was significantly greater during dynamic exercise 5.1 +/- 1.5 versus 3.6 +/- 1.6 ml x kg(-1) x Nm(-1) x min(-1) (p = 0.019). CONCLUSION: These results are consistent with the view that oxygen cost of dynamic and isometric actions is determined by different circumstances of mechanical interaction between actin and myosin in the sarcomere, and that muscle recruitment has only a minor role.