Feasibility of blinding spinal manual therapy interventions among participants and outcome assessors: protocol for a blinding feasibility trial oleh Javier Muñoz Laguna, Astrid Kurmann, Léonie Hofstetter, Emanuela Nyantakyi, Lauren Clack, Heejung Bang, Nadine E. Foster, Julia Braun, Milo A. Puhan, Mazda Farshad, Cesar A. Hincapié