After the 2006 birth of son Moses, her second child with hubby Chris Martin, Paltrow, 40, says she had an incredibly tough time shedding the baby weight.
"With my daughter [Apple, 8] it had been easier, but this time, no matter what I did, I felt stuck," Paltrow tells the January issue of Redbook (on newsstands December 18). "I couldn't shift the weight. But I met Tracy and she was this force from the second I met her."
During her initial evaluation with the fitness guru, "She pulled my pants off," the GOOP founder recalls. "I'll never forget it. She was like, 'Oh, my God, wow, I am just so surprised, because you look so good in clothes. I wasn't expecting this."
Despite the initial humiliation, the Oscar winner stuck with Anderson -- and says the expert's no-holds-barred approach paid off.
"[Working out with Tracy] did such wonders for my life, my confidence, my sex life, everything," Paltrow raves.