Exercises to strengthen lower leg?

[cwindle1]

Over the past few weeks my lower leg during flatwork has got much stronger but I've always struggled with week lower legs. Jumping is fine, my lower leg stays in place but during flatwork I struggle to keep my leg on the girth to help keep the horse from falling in and to help bend. What happens is that my leg sort of just goes back and forth, not loads but it can't stay in one place. My horse struggles to bend as it is but seeing as I can't keep my leg still and strong on his side this cant be helping the situation, also with keeping him forward to push him into the contact. Again, my heels do come up, not considerably loads but still, they do have the tenancy to just have a mind of there own. I've read lots of articles and most people suggest getting your core stronger however I feel that my core is quite strong. Does anyone know why this is happening? Any tips and exercises to strengthen lower leg?

 

[cwindle1]

I may add also, its worse on my own horse. I find when I ride others it keeps reasonably still but still not strong to keep the horse from occasionally falling in??

 

[Cowpony]

The best one I have found is to alternate between sitting and light seat in both trot and canter, gradually using each one for fewer and fewer strides, until you sit for 2, light seat for 2.