By Heller McAlpin Correspondent Cartoonist Alison Bechdel, who wrote about her parents in two celebrated graphic memoirs – “Fun Home” and “Are You My Mother?” – turns the focus on herself and her lifelong obsession with exercise in the ironically titled “The Secret to Superhuman Strength.” Just don’t mistake it for a self-help book filled with platitudes and exhortations. Bechdel, neither a cheerleader nor a boot-camp sergeant, is never shallow. In panels busy with expressive drawings, text, and commentary, Bechdel excavates her deepest thoughts and feelings. This latest memoir is a testimony to her determination to transcend her anxieties and find her way in life by dint of physical exertion and spiritual epiphanies. In the process, she channels her challenges into art.