The contribution of muscular relaxation to systematic desensitization therapy was studied in four phobic patients. Removal of relaxation during a control phase of therapy made no difference to the patients' improving ability to perform in their phobic situation. In two subjects, progress through the hierarchy and therapeutic progress, as measured by self-rating, slowed on removal of relaxation; however, the overall effects were small. This suggests that variables other than relaxation are in large part responsible for the therapeutic effectiveness of desensitization. Relaxation paired with visualization of feared scenes helps some patients to approach their feared object or situation in imagination, and may motivate them to approach it in reality.
Agras WS, Leitenberg H, Barlow DH, Curtis N, Edwards J, Wright D. Relaxation in Systematic Desensitization. Arch Gen Psychiatry. 1971;25(6):511–514. doi:10.1001/archpsyc.1971.01750180031005