Non-cross Bridge Viscoelastic Elements Contribute to Muscle Force and Work During Stretch-Shortening Cycles: Evidence From Whole Muscles and Permeabilized Fibers

Figures

• 

  Figure 1. Representative stretch-shortening cycles (SSCs) from whole wild type soleus muscles stimulated at two phases of activation (8.3 and 50%). Muscles were subjected to 3–4 SSCs at strain amplitude of ±5% L0 and a frequency of 5 Hz (200 ms/cycle). In this example, a muscle was stimulated at a phase of 8.3% (A,B) when the muscle was lengthening and at a phase of 50% (C,D) at the onset of muscle shortening. (A,C) illustrate three successive cycles. (B,D) show the third cycle from each trial, as well as the relationship between time (ms) and phase (0–100%). Dashed lines show length vs. time and solid lines show force vs. time. Muscle force (solid lines) is shown for SSCs with (blue lines) and without (black lines) a doublet stimulus added 10 ms after the first stimulus in each cycle. Red dots indicate individual stimuli, the arrow indicates the doublet.

• 

  Figure 2. Representative SSC experiments from permeabilized fiber bundles. Activation levels were controlled either by decreasing the concentration of calcium ions (pCa; A,C), or by 2,3-butanedione monoxime (BDM)-induced myosin inhibition (B,D). Length changes were performed using a “triangular” length trajectory between 2.4 and 3.0 μm SL at 2 Hz. (A,B) show all force trials collected for a representative fiber bundle for pCa-controlled and BDN-controlled fibers, respectively. Bundles were activated until force leveled off, and then subjected to multiple SSCs. (C,D) show the first four cycles at a higher time resolution for pCa and BDM trials, respectively.

• 

  Figure 3. Representative work loops from wild type and mdm soleus muscles stimulated with (blue = wild type; red = mdm) and without (black) a doublet. (A,D) Phase = 8.3% when the muscle is lengthening. (B,E) Phase = 50% when the muscle is shortening. (C,F) Phase = 83.3% when the muscle is shortening followed by lengthening. The effect of doublet stimulation on work per cycle (area within the work loop) changes with the phase of activation and is greatest when stimulation occurs during lengthening (phase < 50%). Black dots indicate individual stimuli. Arrows indicate the direction of a work loop.

• 

  Figure 4. Differences in peak stress with vs. without doublet stimulation across phase of stimulation in wild type (blue) and mdm (red) soleus. Doublet potentiation of peak stress is reduced in mdm muscles compared to wild type at all phases of activation (F = 52.3, p < 0.0001). Phase = 0% indicates a muscle at its shortest length at the onset of lengthening and phase = 50% indicates a muscle at its longest length just prior to shortening. Data represent means ± SE, N = 28 individuals, 112 trials.

• 

  Figure 5. Differences in net (A), negative (B), and positive (C) work per cycle with vs. without doublet stimulation across phase of activation in wild type (blue) and mdm (red) muscles. The greatest increase in work with doublet stimulation occurred when muscles were activated during lengthening and the smallest increase occurred when muscles were activated during shortening. Wild type muscles exhibited a significantly greater increase in net (F = 43.84, p < 0.0001), negative (F = 14.51, p = 0.007), and positive (F = 50.43, p < 0.0001) work than mdm muscles across all phases of activation. Phase = 0% indicates a muscle at its shortest length at the onset of lengthening and phase = 50% indicates a muscle at its longest length just prior to shortening. Data represent means ± SE, N = 28 individuals, 112 trials.

• 

  Figure 6. Peak force and work during SSCs vs. activation level for pCa- (blue) and BDM- (red) controlled fibers. All variables scaled linearly with activation level analysis of covariance (ANCOVA covariate effect < 0.001). Peak forces (A) were larger in BDM- vs. pCa- controlled trials across activation levels (p = 0.0014). Both negative (p < 0.001) and total work (p = 0.014) components were also larger in BDM vs. pCa controlled fibers. Compared to pCa-controlled trials, BDM-controlled trials had a modest but statistically significant steeper increase with activation level for positive work (B, ANCOVA interaction effect, p = 0.0072) and negative work (C, p = 0.031), but not for net work (D, p = 0.16). The 95% CIs are shown by the shaded regions around the lines. N = 15 pCa and 16 BDM experiments, n = 52 pCa and 76 BDM trials.