LOCKED KNEES

Symptom description
Knees are locked backward, giving a O shape to the leg, whereas a 3 to 5° of bending is physyological. 

Main suspect(s)
Absence of detailed proprioception of knee.
Weak gluteals. Wrong neuro motor control of hamstrings.

Consequence
Imbalance of higher segments & impact on breath.
​Possibly knee pain.

First aid exercise

Foreword : the whole Postural Transformation method takes seriously the view that a body is a whole and consequently that a symptom can find its source anywhere else in the body. In some rare cases, a one-fit-all prescripton can do magic and dissolve the symptom. This exercise might work on you and in all cases is one of the necessary ingredient to changing your posture for a better one.
And a longuer exercise video....

MORE reference information 
​On locked knees

From Jane Johnson, Expert therapist:
A short article on HumnaKinetics detailing:
  • Consequences of knee hyperextension
  • "It doesn’t stop at the knee"
  • What therapists can do to treat knee hyperextension
  • What clients can do to treat knee hyperextension

And in the do-it-yourself part, here is the short version of her recommandations:
. Become conscious of knee postures during everyday activities.
. Practice good knee alignment in static postures.
. Practice good knee alignment during dynamic functions.
. To improve proprioception, practice single-leg balancing with the knee in proper alignment.
. Perform exercises to improve the strength ratio between knee flexors and extensors.
. Avoid exercises and stretches that force the knee into extension. For example, take care with standing hamstring and calf stretches.

The last point, for example, should be a point of attention for yoga practitioners.