Images provided by:

(1) PhotoAlto / Odilon Dimier / Getty Images

(2-14) Brayden Knell / WebMD


Richard Cotton, PhD, spokesman, American Council on Exercise; chief exercise physiologist, MyExercisePlan web site.

Robert Gotlin, DO, director of orthopaedic and sports rehabilitation and coordinator of musculoskeletal and sports rehabilitation fellowship training program, Department of Orthopaedic Surgery, Beth Israel Medical Center, New York.

David Petersen, certified strength and conditioning specialist (CSCS); personal trainer; founder, BossFitness web site, Oldsmar, Fla.

Adam Rufa, physical therapist; certified strength and conditioning specialist (CSCS), Cicero, N.Y.