Physiological Significance of the Force-Velocity Relation in Skeletal Muscle and Muscle Fibers

Figure 1. Experimental setup to record the force–velocity relation of skeletal muscle. A frog sartorius muscle (M) is held vertically between the long arm (A1) of the lever and a force transducer (F), while a weight W is put to the short arm (A2) of the lever to apply constant afterload (P) to the muscle. Muscle length is initially adjusted by stops (S1 and S2) at a slack length (Lo) where resting force is just barely perceptible. Then, the muscle is stimulated by a single electrical current pulse through a pair of electrodes (E1 and E2) to produce a twitch (at 0 °C). Force and length changes in the muscle during a twitch are recorded by a force transducer (F) and a photoelectric device (PE), respectively.

Figure 2. Force (upper traces) and length (lower traces) changes of a muscle during twitches under three different afterloads. The muscle first develops isometric force, and when the force reaches the value equal to the afterload, it starts shortening under the constant afterload with a constant velocity.

Figure 3. The force–velocity (P–V) relation of a whole frog sartorius muscle obtained during an afterloaded twitches at 0 °C. The P–V curve fits part of rectangular hyperbola, so that the relation between P and V are expressed as (P + a) V = b (Po − P). From Ref. [1].

Figure 4. Double-hyperbolic P–V relation obtained from single tetanized intact frog muscle fibers. Inset shows semi-logarithmic plot of the same P–V data points. From [2].

Figure 5. Huxley contraction model, constructed to account for the P–V relation of Hill in terms of distribution of the A–M link between myosin head (M) extending from a myosin filament and corresponding myosin-binding site (A) in an actin filament. (A) Diagram showing a myosin head (M) connected to a myosin filament via springs S1 and S2, and a myosin-binding site (A) on an actin filament. Arrows indicate directions of relative sliding between actin and myosin filaments. (B) Diagram showing the rate constants (f and g) as functions of distance (x) of M from its equilibrium position (0). (C) Diagrams showing distribution of the A–M link along the x axis at V = 0 (isometric condition), at V= 0.25Vmax, and V = Vmax. For further explanation, see text. From Ref. [9].

Figure 6. Transient oscillatory changes in shortening velocity (isotonic velocity transient) in tetanized single intact frog muscle fibers following various magnitudes of quick changes in load from Po to P < Po. Changes in fiber length and force are shown in upper and lower records, respectively. From Ref. [10].

Figure 7. Isotonic velocity transients following quick changes in load from (A) Po to P < Po and (B) Po to P > Po in tetanized intact single muscle fibers. The values of P are expressed relative to Po on the left of each record. From Ref. [12].

Figure 8. Records of fiber length (upper traces) and force (lower traces) of experiments in which the tetanized intact muscle fiber was first made to shorten isotonically under a load 0.8Po and, after 50 ms, subjected to quick increases in load from 0.8 Po to 0.85Po in (A), to 0.9Po in (B), to 1.1Po in (C), and to 1.3Po in (D). Note marked changes in fiber length with alternate lengthening and shortening in (A–C). From Ref. [12].

Figure 9. Records of isotonic shortening of an intact single muscle fiber during afterloaded twitches under two different afterloads. Note that a single fiber starts shortening linearly after preceding isometric force development equal to the afterload without any transient changes. From Ref. [13].

Figure 10. Enhancement of mechanical performance in tetanized intact muscle fibers when they are first lengthened by quick changes in load from Po to P > Po and then subjected to quick changes in load from P > Po to P < Po. Note that the fiber shortens under a load = Po (B), as well as under loads P < Po (C, D), while it is lengthened under a load > Po (A); and also note that the fiber starts shortening without any velocity transients. From Ref. [15].

Figure 11. Shift of the P–V relation to the right along the force (load) axis as the result of enhancement of mechanical performance in isotonically lengthening muscle fibers. Open circles and the solid line show the P–V relation, obtained in the control condition, while filled and dotted circles show data points, obtained during isotonic lengthening under a load of 1.32~1.38Po and 1.61~1.67Po, respectively. Note that the shift of the P–V curve occurs at all levels of shortening velocity. From Ref. [15].

Figure 12. Two different modes of ATP-induced myosin head power strokes. (A) Structure of the myosin head, which consists of a catalytic domain (CAD), a converter domain (CD), and a lever arm domain (LD). Approximate attachment regions of antibodies 1 and 2 are indicated by numbers 1 and 2, respectively. (B) The mode of myosin head power stroke at standard ionic strength. The amplitude of movement is larger at the distal CAD than at the proximal CAD so that the myosin head is oblique to actin and myosin filaments at the end of a power stroke. (C) The mode of a myosin head power stroke at low ionic strength. The amplitude of movement is the same at both the distal and the proximal CAD so that CAD is perpendicular to actin and myosin filaments at the end of power stroke. From Ref. [19].