FIG. 4.

(a) Common principles underlying most measurements of contraction; an axial force can be measured directly using an isometric or strain gauge force transducer. Conversely, during an isotonic contraction, perpendicular cell thickening can be used as a correlative measurement due to the conservation of volume within a contracting cell. The titin structure within the CM offers a baseline of passive resistance ($\stackrel{\rightharpoonup }{T}$_passive), to which the applied force vector from active contraction ($\stackrel{\rightharpoonup }{F}$_a) is added. (b) Principles of contraction within a sarcomere of stable A-band dimensions that allow for sarcomeric shortening measurements of isotonic (above) and isometric contraction (below) such as seen in TEM studies.²⁶³ The scale bar on the left is 1 μm. (c) The heart makes use of both near-isotonic (auxotonic) and isometric contractions with each cycle, as illustrated by a standard pressure-volume loop. The cycle occurs in a counter-clockwise direction, with systole and diastole ending before isometric phases at the top-left and bottom-right corners of the loop, respectively. Passive diastolic filling at near-constant pressure results in eccentric contraction in the myocardium, which corresponds to the degree of preload or end-diastolic pressure.