Low impact intense cardio exercises - ideas?

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So my road running gave me shin splints something terrible. I'm taking a break from running to heal and instead I'm doing interval training. I alternate cardio and strength. I did mountain climbers and high knees in my livingroom last night and that's too high impact for me right now. I was hurting!

Does anyone have any ideas for low impact cardio exercises that will get my heart rate up quickly? For this routine I'm doing, I do 45 seconds training with 20 seconds of recovery. I need an exercise that will get my heart pumping fast within 45 seconds. I'm drawing a blank here...

Thanks!

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