i am following your conversation and discussion and i am agree with siva ji about pt seems he had cocontraction of both knee flexors and extensors dyscoordination of the muscles actions..the patint is rigid and it looks like basal ganglia or thalamus injury would please tell me if he had it or not..??
mecconall have a taping approch for individuals waliking with hyperextension of the knees try it it may works to inhibit the quads in loading responce and helps the hamistring to have chance to initiat the movement..
let the patint stand on one leg and the affected leg on low step and start the tape from posterior superior femoral condylesand the tape will end in inferior to tibial tuberosity so it will cross anteriorly with each other.so the patint will walk with flexion pattern of affected leg.
try this exercise let the patient stand on 5 cm and the as progresion on 10cm hight step and let him step with the souned leg backward and down to the grauned and the affected leg will stay on the step and becarfull he will need you support.
good luck with your patient i hope he will improve.
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