One-legged balance is a knee rehabilitation exercise.

With the eyes closed, and standing near a support, stand on one leg for 10 seconds. Open eyes, rest on two legs, and then repeat the exercisee.
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CLINICAL PUBLICATIONS
The effects of pain on quadriceps strength, joint proprioception and dynamic balance among women aged 65 to 75 years with knee osteoarthritis. Kim D, Park G, Kuo L-T and Park W. BMC Geriatr. 2018; 18: 245.