▁extension ▁should ▁be ▁no ▁more ▁than ▁ 1 0 ▁degrees . ▁In ▁genu ▁rec ur vat um , ▁normal ▁extension ▁is ▁increased . ▁The ▁development ▁of ▁genu ▁rec ur vat um ▁may ▁lead ▁to ▁kne e ▁pain ▁and ▁kne e ▁o ste o arth rit is . ▁ ▁C aus es ▁The ▁following ▁factors ▁may ▁be ▁involved ▁in ▁causing ▁this ▁de form ity : ▁ ▁In her ent ▁la x ity ▁of ▁the ▁kne e ▁lig aments ▁ ▁We ak ness ▁of ▁b ice ps ▁fem oris ▁mus cle ▁ ▁Inst ability ▁of ▁the ▁kne e ▁joint ▁due ▁to ▁lig aments ▁and ▁joint ▁caps ule ▁inj uries ▁ ▁In app ropri ate ▁alignment ▁of ▁the ▁tib ia ▁and ▁fem ur ▁ ▁Mal union ▁of ▁the ▁b ones ▁around ▁the ▁kne e ▁ ▁We ak ness ▁in ▁the ▁hip ▁ext ensor ▁mus cles ▁ ▁Gast roc nem ius ▁mus cle ▁weak ness ▁( in ▁standing ▁position ) ▁ ▁Upper ▁motor ▁neur on ▁les ion ▁( for ▁example , ▁hem i ple gia ▁as ▁the ▁result ▁of ▁a ▁c ere bro vas cular ▁accident ) ▁ ▁Lower ▁motor ▁neur on ▁les ion ▁( for ▁example , ▁in ▁post - pol io ▁synd rome ) ▁ ▁Def ic it ▁in ▁joint ▁proprio ception ▁ ▁Lower ▁lim b ▁length ▁dis cre p ancy ▁ ▁Cong en ital ▁genu ▁rec ur vat um ▁ ▁C ere br al ▁p als y ▁ ▁Mus cular ▁d yst ro phy ▁ ▁Limited ▁d ors if lex ion ▁( plant ar