Force-frequency relation or Bowditch treppe. Developed force of contraction in the isolated papillary muscle at increasing stimulation rates. The stimulus rate is shown as the action potential duration on an analog analyzer. The tension developed by papillary muscle contraction is shown as developed force. An increased stimulation rate increases the force of contraction. On cessation of rapid stimulation, the contraction force gradually declines. Heart rate is a leading determinant of cytosol calcium concentration, and strictly linked to contractility. In the healthy heart, a frequency increase up to 180 beats per minute provides for faster systolic calcium SR release (increased contractility or developed force) and for faster diastolic SR calcium reuptake (positive lusitropic effect). (Modified from Opie LH. Normal and abnormal cardiac function. Chapter 14, page 443. In Braunwald Zipes Libby, Heart disease, 6th edition, W. B Saunders Company, 2001).