Nialda, it took til the first run of the second week that I couldn't handle running on them anymore. I tried to persevere, but they just wouldn't let me run! One was worse than the other. But the rest of me actually felt really good. Evn though I hate running afterwards I would feel great, sore, like muscle/effort sore, but it felt really good. I said to DH one day that now I know why people actually enjoy running, once you're fit and your body could handle the running, the high must really feel good, and be addictive!