Effects of exercise-induced gradual versus instant avatar transformation on muscle strength and sense of embodiment in patients with chronic low back pain and healthy adults

Introduction The appearance of a virtual body (avatar) can influence a user’s attitudes and behavior, a phenomenon known as the Proteus effect. The present study examined whether linking an avatar’s transformation to the user’s own physical effort can strengthen this effect. Methods A screen-based system was developed that displayed a body-tracked avatar whose apparent muscle mass increased gradually and in real time in response to the user’s squatting exercise (the Gradual method). This was compared with a conventional approach in which the avatar switched to a muscular physique instantaneously at the end of the exercise (the Instant method). Sixteen patients with chronic low back pain and sixteen healthy adults experienced both methods in counterbalanced order. Grip, trunk extensor, and knee extensor strength, sense of embodiment (body ownership and sense of agency), and usability were assessed, with pain and fear of movement added for the low back pain group. Results The Gradual method produced a significantly greater acute increase in grip and trunk extensor strength and a higher sense of agency than the Instant method. The advantages in trunk extensor strength and sense of agency showed no significant order dependence in the present sample, whereas order effects on the other measures indicate that the overall comparison should take experience sequence into account. Pain and fear of movement did not differ between methods. Discussion These preliminary findings indicate that effort-linked, real-time avatar feedback was associated with acute increases in maximal voluntary strength and sense of agency. For exercise therapy, making exercise effort immediately visible through feedback on a body-tracked avatar may represent a feasible and well-tolerated support tool during exercise, although its effects on engagement and clinical outcomes require longitudinal evaluation.