Isometric for "stiffness"

Im abt to enter a top speed (maintenance weights) phase with my gf, who Ive been training. She has very poor mechanics at top speed, so Im working on her form as well as her strength. I was thinking of using Isometric holds at the top portion of her squat to help develop “stiffness” that is needed for max velocity runs. She has fairly sensitive shins so any vertical plyos are out so Im hoping iso holds will help. Has anyone used them in this way?? What protocol has been used or does anyone suggest?? Chris T, your suggestions would be greatly appreciated.