Effects of eccentric contraction on force enhancement in rat fast‐twitch muscle

Figures

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  FIGURE 1. Force record in measurements of passive force enhancement (PFE) and residual force enhancement (RFE). Maximal Ca2+‐activated force (max Ca2+ force or M) and passive force (P) were evaluated using two different procedures. In the first procedure, the force (M1) was measured by immersing the skinned fiber to the maximal Ca2+‐activated solution after stretching the sarcomere length (SL) from 2.4 to 3.2 μm in the relaxation solution. P1 is the passive force at 3.2‐μm SL after M1 measurement. In the second procedure, the SL was stretched from 2.4 to 3.2 μm while the fiber was immersed into the maximal Ca2+‐activated solution (the max Ca2+ force measured is referred to as M2). P2 is the passive force at 3.2‐μm SL after M2 measurement. RFE is a difference between M1 and M2, and PFE is a difference in between P1 and P2.

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  FIGURE 2. Effects of eccentric contraction (ECC) on isometric torque in whole muscle. Intact gastrocnemius muscle was electrically stimulated in situ at 80 Hz for 1 s every 4 s for 200 cycles. During the stimulation, the foot was forcibly dorsiflexed by a servomotor from 110° to 50° at 60°/s. (A) Isometric torque at pre‐ and post‐ECC. (B) Ratio of force at 20 Hz to that at 100 Hz. Values are means + SD; N denotes number of muscles. Individual data are displayed as gray or black circles. a p <0.05, vs. pre‐ECC (two‐way repeated ANOVA and paired t‐test for (A and B), respectively). Symbols for significant differences between stimulation frequencies are not shown. Post, post‐ECC; Pre, pre‐ECC.

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  FIGURE 3. Effects of eccentric contraction on titin degradation. For the protocols of eccentric contraction (ECC), see legend of Figure 2. (A) Electrophoretic analysis of titin. Proteins were separated by agarose‐strengthened 2% SDS‐PAGE and were visualized with Coomassie Blue R staining. T1 and T2 are full‐length and degraded titin, respectively. (B) Ratio of degraded titin to total titin (T1 + T2). Values are means + SD; N denotes number of muscles. Individual data are displayed as gray or black circles.

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  FIGURE 4. Effects of eccentric contraction on passive torque in whole muscle (A) and passive force in skinned fiber (B). For the protocols of eccentric contraction (ECC), see legend of Figure 2. Values are means ± SD; N and n denote number of muscles and skinned fibers, respectively. Individual data are displayed as gray or black circles. a p <0.05, vs. rested fibers in matching condition (two‐way ANOVA). A main effect of ECC was observed for whole muscle passive force (Pre < Post; two‐way repeated ANOVA). Symbols for significant differences between ankle angles and sarcomere lengths (SLs) are not shown in (A and B), respectively. ECC, fibers subjected to ECC; post, post‐ECC; Pre, pre‐ECC; Rest, rested fibers.

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  FIGURE 5. Effects of eccentric contraction and active stretch on maximal Ca2+‐activated force (max Ca2+ force) in skinned fiber. For the protocols of max Ca2+ force measurement and eccentric contraction (ECC), see legend of Figures 1 and 2, respectively. (A) max Ca2+ force (M) 1 and 2. (B) Difference between M1 and M2. (C) Ratio of M2 to M1. Values are means + SD; n denotes number of fibers. Individual data are displayed as gray or black circles. Main effects of ECC and active stretch were observed (Rest > ECC and M2 > M1; two‐way repeated ANOVA). Rest, rested fibers; ECC, fibers subjected to ECC.

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  FIGURE 6. Effects of eccentric contraction and active stretch on passive force in skinned fiber. For the protocols of passive force measurement and eccentric contraction (ECC), see legend of Figures 1 and 2, respectively. (A) Passive force (P) 1 and 2. (B) Difference between P1 and P2. (C) Ratio of P2 to P1. Values are means + SD; n denotes number of fibers. Individual data are displayed as gray or black circles. a p <0.05, vs. rested fibers in matching condition (two‐way repeated ANOVA); b p <0.05, vs. P1 within fibers (two‐way repeated ANOVA); c p <0.05, vs. rested fibers (t‐test). Rest, rested fibers; ECC, fibers subjected to ECC.

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  FIGURE 7. Effects of eccentric contraction and active stretch on ratio of passive force (P) to maximal Ca2+‐activated force (M) in skinned fiber. For the protocols of P and M measurement and eccentric contraction (ECC), see legend of Figures 1 and 2, respectively. Values are means + SD; n denotes number of fibers. Individual data are displayed as gray or black circles. a p <0.05, vs. rested fibers in matching condition (two‐way repeated ANOVA); b p <0.05, vs. P1/M1 within fibers (two‐way repeated ANOVA). Rest, rested fibers; ECC, fibers subjected to ECC.