On the Cavalry Commander (1.5-1.6)

The bones of the shanks should be thick,[*] since these are the pillars of the body; but not thick with veins nor with flesh, else when the horse is ridden over hard ground, these parts are bound to become charged with blood and varicose; the legs will swell, and the skin will fall away, and when this gets loose the pin,[*] too, is apt to give way and lame the horse.

If the colt’s knees are supple when bending as he walks, you may guess that his legs will be supple when he is ridden too, for all horses acquire greater suppleness at the knee as time goes on. Supple knees are rightly approved, since they render the horse less likely to stumble and tire than stiff legs.