Yes there's definitely some muscles that need to be built up. Most noticeably for me are the ones on the sides of the foot that control rolling: in a conventional shoe there's a broad heel sole and a ridgid heel cup to give roll support. Without that, your foot wants to roll, even when walking flat. I needed to add some exercises to help supplement, and the ankles are still a little sore. But I'll take sore muscles over tendonitis.