Percentile reinforcement schedules have been used to systematically alter inter-response times, behavioral variability, breath carbon monoxide levels, duration of social behaviors, and various other properties of behavior. However, none of the previous studies have examined the effectiveness of percentile schedules in relation to the magnitude of muscular contractions. This control over magnitude of muscular responding has important implications relating to the strengthening of muscles and correct movements for patients receiving physical rehabilitation. There would be great utility in percentile schedules that can be implemented in rehabilitation situations by physical therapists and patients themselves to improve treatment outcomes – all of which could be possible without any behavioral training if the procedure is implemented via body sensors and smartphone applications. Using healthy adults and the aforementioned technology, this thesis focused on the design and testing of three percentile reinforcement schedule procedures to increase the strength of the vastus medialis muscle. Results indicate that the magnitude of muscular responses can be shaped using body sensors and contingent feedback, and the percentile schedule procedures have promising applications in the domain of physical therapy.