Energy Cost of Force Production After a Stretch-Shortening Cycle in Skinned Muscle Fibers: Does Muscle Efficiency Increase?

Figures

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  FIGURE 1. A typical fiber response to an active shortening without a prior stretch (gray) and a stretch-shortening cycle (red). For the active shortening contraction, the fiber was passively stretched from a SL of 2.4 μm to a SL of 3.2 μm, activated and then actively shortened to a SL of 2.6 μm. After 15 s, the fiber was transferred to a fresh activating solution for 40 s and then deactivated. For the SSC contraction, the fiber was activated at a SL of 2.4 μm, actively stretched to a SL of 3.2 μm and immediately shortened to a SL of 2.6 μm. After 15 s the fiber was transferred to a bath of fresh activating solution for 40 s. The activating solutions in which the fibers were bathed for 40 s were used for ATP measurement. Note that the rate of force redevelopment and the steady-state force after shortening are smaller in the active shortening without prior stretch test compared to the SSC test. A fiber was activated by adding first a washing solution (free of EGTA and calcium) and then an activating solution (with high concentration of calcium). The noise on the graph indicates the time when the solution was changed. Active shortening lasted long enough to favor force depression.

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  FIGURE 2. Malachite green absorbance as a function of Pi (ATP) concentration. The standard curve was established by adding known amounts of Pi. When the concentration of Pi increased, the amount of malachite green increased and therefore the absorbance was increased too. Examples of the absorbance of malachite green in the diluted solutions of active shortening (CTL-2.6 and CTL-3.0) and SSCs (SSC-2.6 and SSC-3.0) contractions are also shown. The malachite green assay used in this study could detect differences in the concentration of Pi in the order of 0.1 μM.

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  FIGURE 3. Average total stress (A) and work performed during shortening (B) (±SEM) for the active shortening and SSC contractions at sarcomere lengths of 2.6 and 3.0 μm. Total stress and the work performed during shortening were greater after SSCs compared to the pure shortening contractions. ∗Indicates a significant difference from the corresponding pure active shortening contraction.

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  FIGURE 4. Average ATP consumption (A), average ATP consumption per unit of force (B) for the active shortening and the SSC contractions at sarcomere lengths of 2.6, 3.0, and 2.6 μm and 3.0 μm combined, and average ATP consumption per unit of force for the residual force enhancement (C) (±SEM). ATP consumption was greater after SSCs at a sarcomere length of 3.0 μm, and when 2.6 μm and 3.0 μm were combined, compared to the pure shortening contractions. ATP consumption per unit of force was not different after SSCs compared to the corresponding active shortening contractions but was reduced in the RFE compared to the isometric reference contractions. The residual force enhancement tests were performed twice, and results were combined for the first and second reference and active stretch contractions. ∗Indicates a significant difference from the corresponding pure shortening contraction (p < 0.05).

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  FIGURE 5. The change in stress (red) in response to the sinusoidal length changes (gray) performed to measure stiffness (A), and average stiffness (B) and stress relative to stiffness (C) (±SEM) for the active shortening and SSC contractions at sarcomere lengths of 2.6 and 3.0 μm. Stiffness was calculated as the average change in stress (peak-to-peak stress change) divided by the change in the initial fiber length (peak-to-peak length change expressed as strain) for 10 consecutive cycles. ∗Indicates a significant difference from the corresponding active shortening contraction (p < 0.05).