What's everyone's suggestions to strengthen horses stifles. Mare gets injected of course but also want to help her get in shape 

I have always been told for strengthening stifles, lots of long trotting, walking up and down hills.

Backing up. Straight lines, circles, squares. Make sure they are good and warmed up though, and cool down appropriately. 

Long trotting, preferably in straight lines. Backing, lots of backing with their backs rounded an dnot hollowed out. W/T/L up hills, backing up hills. Transitions help. Circles, but their rear has to be up underneath and not swinging a hip out. Check out Joy Wargo's small circles concept, start slow and build up with that. Stepping over objects, ground poles/cavalettis etc. Or a log, shavings bag, feed bag. Anything about 12" high or less that they can step over repeatedly a couple days a week. Weighted bell boots on the back feet doing while light work can also help 

Riding ..... good 10-15 minute walking warmup .. extended walk ... before the real work begins. I see so many skip this step. And same with proper cool down 15 minutes of walking out. Backing up straight lines, backing uphill.. making your horse stop properly in a straight line and immediately backing up straight and then push off into a trot or lope. Once a week I log my horse. Put on my rope saddle and I have him pull a large heavy railroad tie I have in my pen. I have him pull it while backing up several steps and then I'll turn him around and have him pull it walking forward.. then to the left and to the right. Helps with topline as well.