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The masses of some large Pleistocene species of the fossil family Glyptodontidae (Mammalia; Xenarthra) were estimated from the volumes of models. Their centres of mass were also estimated. Dimensions of limb bones and limb muscles were used to assess the athleticism of these species, using an approach previously applied to dinosaurs. The femora show higher athletic indicators (even when supporting the whole weight of the animal) than humeri in the quadrupedal stance. It is therefore proposed that performing strenuous locomotor activities bipedally was not only possible but even advantageous for minimizing risk of bone failure. The muscular dimensions analysed are consistent with this conclusion. The possible biological meaning of these mechanical results is considered. Since the smaller and older (early Miocene) glyptodont Propalaehoplophorus does not share this condition, it is suggested that it …