Hm...I would ditch the lower body during track season. Your sprint workouts are going to hit those really hard. Work on growth in the off season. Definitely work for speed, and work your shoulders and upper back...the upper body and core in general. Just my opinion. I was a sprinter, so I rarely worked my legs in season (with weights). Building strong shoulders took a few seconds off my 200m though...smartest thing I ever did, weight wise, for track.