Purpose. This study explored the effects of unstable shoe design on oxygen consumption. Methods. Oxygen consumption (VO 2 ) and heart rate (Hr) were measured in 16 individuals while barefoot, wearing unstable shoes (Masai Barefoot Technology) and wearing conventional sport shoes while standing and walking on a treadmill and for 5 individuals while walking around a 400 m track. Results. When wearing the MBT shoes, a significant (p < 0.01) increase of 9.3 ± 5.2% in VO 2 was measured while

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... quietly for 6 min. No differences in VO 2 and Hr were observed between the MBT shoes or weight-adjusted conventional shoes (to match the weight of the MBT shoes) while walking on a treadmill. However, significant increases (p < 0.01) in VO 2 (4.4 ± 8.2%) and Hr (3.6 ± 7.3%) were observed for the MBT shoes compared with being barefoot. No significant differences in VO 2 and Hr were recorded while walking around a 400 m track either with MBT shoes, weight-adjusted conventional shoes or barefoot. Nonetheless, a comparison of the MBT shoes with barefoot revealed a tendency for VO 2 to be higher when wearing the MBT shoes (7.1 ± 6.5%, p < 0.1) although Hr was not significantly affected. Conclusions. The unstable shoe design predominantly effects oxygen consumption while standing, most likely due to increased muscle activity of the lower extremities.