Primates that move by quadrupedal locomotion have arms (forelimbs) and legs (hindlimbs) of equal length, while bipedal humans have longer legs than arms. In comparison, Lucy maintains relatively longer arms to her legs.

Lucy’s intermembral index (i.e., radius length plus humerus length, divided by femur length plus tibia length) is 84.6. The mean of this measurement in humans is 71.8, indicating humans have shorter arms relative to their legs. The common Chimpanzee measurement is 106. In other words, Lucy appears to have intermediate measurements between modern humans and chimpanzees.