Figure 1.

Diagram for the function of the two components in active striated muscle. The thin filaments are the “contractile component”. The Z-filaments are the “series elastic component”. Torque produces rotations of the Z-filaments (arrows 1) and thin filaments (arrows 2 and 3). The opposed torque of arrow l and arrow 2 is balanced.

(a) to (b) Unloaded shortening. The distal parts of the thin filament helices rotate (arrows 3) and drill (= slide, arrows 4) into the myosin cross-bridges. No force arises, Z-filaments remain twisted, and the titin fibers shorten.

(a) to (c) Isometric force arises when a heavy load prevents the distal filament drilling (arrows 4). The Z-filaments are elastically stretched and untwisted by the now predominant tensile stress and proximal torque (arrows 2) of the thin filaments.

(c) to (b): A quick release of large amplitude results by thin filament drilling when the heavy load is reduced. The titin fibers shorten.

The Z-filaments are drawn with exaggereted largeness. A scheme of J. Spudich provided for Stryer’s “Biochemistry” (third edition, Figure 36–19) was used as the basis for this diagram.