When bending down to reach for a toy on the floor, infants who are new to standing with support tend to reach down by tipping the trunk forwards and keeping the knees in extension.
As their ability to control knee extension and balance improves, infants start to flex their knees when reaching down to the floor.
If an infant tends to lock the knees into extension they may needs some extra training to encourage them to flex the knees as they reach down.
Practice sitting down and standing up
Another way to provide an infant with opportunities to gain more control over knee flexion and extension in standing is to practice standing up from sitting on a step.
Let the infant stand on a knee high step facing a chest high support surface. Initiate games that encourage the infant to stand up and sit down.
Posting games are often good for encouraging repeated actions: put the posting box on the support surface and the toys to be posted on the floor.
Practice lifting "heavy" bottles
The tendency to lock the knees in extension when standing is often a non-variable, fixed strategy to provide some stability. One way to introduce some variability is to introduce a task that "pushes" the system to adapt its responses.
Lifting and moving lifting partially filled plastic bottles is one way to encourage a change of strategy.
Let the infant stand facing a chest high support. Position one or more partially filled water bottles on the floor next to the infant and encourage her to lift the bottles up onto the support surface.
The effort required to maintain balance while lifting the bottles will often lead to some adaptive and more variable flexion in the knees.