Being able to stand on one leg is one of those skills  that children, teachers and therapists really value. Standardized tests of motor function (such as the M-ABC) include standing on one leg as a measure of static balance. 

Standing on one leg requires good hip flexibility

If a child cannot stand on one leg it is generally assumed that the fault lies in poorly developed balance skills.

What is less often recognised is that standing on one leg has important biomechanical components.  It requires

• adequate ROM  of adduction/lateral rotation of the hip in full extension to allow the COM to be shifted over the narrow base of support (BOS) keeping the trunk upright. 
• adequate strength in the hip abductor and extensor muscles. 

Development of standing on one leg balance

The ability to shift the weight laterally with the pelvis level and the trunk upright develops with age in young children

At 4-5 years a child can  usually stand on one leg for a few seconds at a time. 

He/she does not keep the trunk steady but uses arm movements and sideways trunk movements to maintain balance.

In this clip Roan displays  typical 4-year-old control of standing on one leg. 

A 5-6 years a child should be  able to stand on one leg for up to 10s. 

At this age the child still uses arm and sideways trunk movements to maintain balance, but is starting to use small foot movements to balance. 

 At 6-7 years a  child should be able to stand steady on one leg for 10s.

 At 8-9 years children starts to be able to stand on one leg with the eyes closed for up to 10 seconds. 

(See Static balance norms in children.)

Roan (7 years) has difficulty standing on her left leg.

Roan (aged 7)  can stand on the left  leg for 10s but still uses some trunk action to maintain her balance. 

Notice that when she stands on the right leg her pelvis is tilted up on the left and the right hip is not adducted relative to the pelvis. This reflects some tightness she has in her left hip muscles and makes standing on the right leg tricky.

To compensate she uses trunk movements to try and maintain her balance. 

Typical  hip rotation in extension ROM = 45⁰ med and  lat rotation 

A child with good hip ROM has 45⁰ of medial and lateral rotation when the hip is in 0⁰ of extension and adduction. 

Roan's lateral hip rotation at the time this video clip was made was limited to 15⁰.

Active mobilization of hip lateral rotation improves right leg standing balance

A brief intervention using active mobilization aimed at increasing hip lateral rotation improves standing balance.

Note the improved ability to maintain the pelvis level and the trunk erect.