Summary The basic field equations for viscoelastic soft tissues are employed for exploring the relationship between time varying mechanical stresses and dynamic deformations of human fasciae in manual therapy. The predicted stress range for plastic deformation of plantar fascia, based on the viscoelastic model is close to experimental data. Fascia lata (in vitro) and plantar fascia (in vitro) are found to exhibit similar behavior, irrespective of how the fasciae are deformed. Manual therapists should avoid increasing deformation with time. It is desirable to produce deformation which should become constant after some time, and be applied for the duration up to 60 s. The therapists may apply almost the same load to produce the same plastic deformation for plantar fascia and fascia lata. Greater loads are needed to produce the same strain with higher rate of deformation. The data presented in this paper may also be useful for surgeons in planning the orientation of fascia in knee and hip replacement surgeries.