There are many legends about Thaipusam. According to a popular legend, the kavadis symbolised the hills of burden that Idumban shouldered when Lord Murugan wanted to test his devotion to his master.
Are the devotees are empowered by the Spirit of Lord Murugan? They must be. That is the only way i think they can bear carrying the heavy kavadis with the piercings on their bodies. They are one with their Gods. It isn’t just a physical act but a spiritual one and an admirable display of devotion.