To establish whether different implant-supported prosthetic reconstructions influence jaw muscle resistance to clenching efforts, sustained submaximal (50%) clenching was performed in two groups of patients who were edentulous in one or both jaws. One group consisted of patients who were rehabilitated by means of an overdenture on two to four implants connected by a bar; the other group consisted of patients who were rehabilitated with an implant-supported fixed prosthesis on four to seven implants in either the maxilla or the mandible. Results indicated that after rehabilitation with implant-supported fixed reconstructions, the myoelectric output level increased over time, which was parallel to a higher bite force level. For the other group, such a time-dependent change in electromyographic amplitude did not occur. On the other hand, power spectrum analysis revealed a significant downward shift of the mean power frequency during sustained clenching after rehabilitation with implant-supported overdentures but not with implant-supported fixed prostheses. The absence of a spectral shift in the latter group probably expressed a fear of biting too hard and fracturing the prosthesis.