For those confused by the ending including some of the other reviews:
The child adopted at the end of the film (the cosmonaut's son) is not the child seen in the orphanage throughout the film. Those scenes are flashbacks to Tanya when she was a child and thus the link between the running shoes in the box and the focus on the running shoes she unpacks when she arrives at the institute. The young Tanya's problems with walking presumably also linked to the scar seen on adults Tanya's back indicative of spinal surgery.
The child adopted at the end of the film (the cosmonaut's son) is not the child seen in the orphanage throughout the film. Those scenes are flashbacks to Tanya when she was a child and thus the link between the running shoes in the box and the focus on the running shoes she unpacks when she arrives at the institute. The young Tanya's problems with walking presumably also linked to the scar seen on adults Tanya's back indicative of spinal surgery.