I am in fairly good shape but feel like I am at a plateau. I started doing 6 100m sprints a day for 4 days. It felt great! I seriously thought I was seeing results already. After 4 days of hard sprints like this, my knees were in bad shape. The cartilage was swollen and even after a week of icing them multiliple times a day, my knees were weak. I've been advised to incorporate intervals of walking and jogging and not even thinking about sprints but I want to make sure this is worth my while. Intervals have helped me lose weight in the past but now that I'm at the weight that I want, they have not had the effect on me that sprints did. My question to you is, since my knees feel better now, is it safe to continue with sprints for my 'instant results' and continue doing the walking/jogging intervals for extra cardio? Also, how effective are sprints? Are they considered a good form of cardio? Thank you for your help!