The key objective of this study has been to study how upper secondary school students develop knowledge about how tenseness affects their health and performing capacity. The way the students are offered to understand the object of learning, i.e. regulation of tenseness, is designed and analyzed by variation theory learning study. The design of the first lesson (A) was based on interviews. The remaining three lessons (B, C and D) were based on the analysis of the lessons and learning outcomes in the previous lessons (A, B and C). Students' understanding of regulation of tenseness depends on which critical aspect they are aware of. The students did not intentionally discern different responses of bodily tension, and had a limited number of ways to influence the levels of tenseness. The results show an increased learning outcome in all four groups. Results from the last lesson, D, show a very healthy increase (129%) in learning outcomes, and all students in that lesson improved their results. The effect of varying only the most important aspects appears in the last cycle, where the features (e.g. heart rate, respiration, muscle tension) contrasted more clearly, which developed the students' learning of the object of learning. The physical activities were kept invariant, and different responses of the sympathetic nervous system were contrasted, one at time, to establish knowledge of different bodily responses to tenseness. It appears sufficient to vary two critical aspects in one lesson to achieve the most powerful learning. Awareness about how the body responds to different kinds of pressure, physically or psychologically, is important for understanding the impact tenseness has on health and performing capacity. However, how the students are offered the aspects critical for developed understanding is crucial to increase their learning. The results show how the teachers, during the iterative process, discover what it takes to learn progressively during the learning study. The students´ results increase due to the more precise way the teachers pinpoint what is critical for the students´ understanding. The more familiar with how to design learning situations based on variations theory the teachers were, the more fruitful it was for the students` learning outcome.