Hill sprints are one of my favorite conditioning tools and I have written various articles about it. One tip which I learned from my Sambo Coach who was a world champion and olympian is to do hill sprints with a log on one of the shoulders. If you have not seen my articles on hill sprints and log training please see my articles liked as below:
When you run with one of the shoulders loaded, this activates to core since the core will have to work to balance the torso. When you do hill sprints with a log, you will be metabolically working your body while your core is being blasted at the same time. Every time you take a step there will be some bouncing and impact. The momentum of the log coming down on your shoulder will make you have to contract your core to ensure your torso does not collapse on one side. Due to the kinetic element of the workout the log's center of gravity will change during the sprints in reference to your shoulder (fulcrum point) and this will bring a whole different stimulus in to this workout which will lead more core muscles to be activated.
This workout can be done with a sand bag as well which is demonstrated in video below but if you want to gain the maximum core benefits, a log or a similar solid object will work better since it will have to balanced on your shoulder, unlike a sandbagThis workout is great for your arms and shoulders as well.