Action potential occurs well before the contractile force attains its peak, and repolarization is completed well before the cell reaches its full resting value. The relaxation of the cardiac muscle takes place mainly during phase 4 of the action potential Action potential Contractile force Time FIGURE 16-2. Relationships between the developed force and the changes in transmembrane potential in a thin strip of ventricular muscle.(Redrawn from Kavaler F, Fisher VI Stuckey JH: Bull N Y Acad Med 41: 592, 1965Action potential occurs well before the contractile force attains its peak, and repolarization is completed well before the cell reaches its full resting value. The relaxation of the cardiac muscle takes place mainly during phase 4 of the action potential