Passive stress of myofibril elements and ECM elements, and the average passive stress in the simulated fiber superimposed on the sarcomere length-tension diagram. Myofibril passive stress, ECM passive stress, and the sarcomere length-tension relationship were estimated from work by others (6,13,15, 25–27). The fiber average passive stress is calculated for a simulated fiber with 10 myofibrils laterally by summing the force contribution of each myofibril and the two ECM elements and dividing by the total cross-sectional area.