QUICK SEARCH:   [advanced] Author: Keyword(s):
Home     Help     Feedback     Subscriptions     Archive     Search     Table of Contents

First published online August 17, 2006
Journal of Experimental Biology 209, 3370-3382 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.02392

This Article Summary Full Text Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Ahn, A. N. Articles by Full, R. J. Search for Related Content PubMed PubMed Citation Articles by Ahn, A. N. Articles by Full, R. J.

In situ muscle power differs without varying in vitro mechanical properties in two insect leg muscles innervated by the same motor neuron

A. N. Ahn^*, K. Meijer and R. J. Full

Department of Integrative Biology, University of California, Berkeley, CA 94720-3140, USA

View larger version (25K): [in a new window]   Fig. 1. Musculo-skeletal morphology of the hindlimb and muscles of interest. (A) Ventral view of the cockroach with the left hindleg in bold. (B) Dorsal, medial and ventral views (from left to right, respectively) of the left hindlimb. Muscle 178 (blue, shaded) inserts on the trochanter and originates on the dorsal side of the coxa closest to the body. The center image represents lateral view from the midline of the animal to clearly show the positions of the muscles. Muscle 179 (red, unshaded) inserts on the trochanter and originates on the ventral side of the coxa. Note that the moment arms are similar between the muscles, but muscle 178 is slightly shorter in length (see Table 1) and slightly broader with greater cross-sectional area. The circles in the dorsal and ventral views of the hindlimb indicate the axis of rotation of the joint.

View larger version (31K): [in a new window]   Fig. 2. Representative strain, total stress and power when muscles 178 and 179 operated under in vivo strain and stimulation conditions. The shaded area represents the shortening phase of the oscillatory cycle. The stimulation pattern, represented on the graphs by the yellow squares, was also determined during running at the animal's preferred speed (3 pulses at 100 Hz) (Full et al., 1998; Ahn and Full, 2002). (A) The strain pattern as determined during preferred speed running. Since muscle 178 is slightly shorter in length, 178 experiences slightly longer strain amplitudes (18.5% for muscle 178, 16.4% for muscle 179). (B) In situ muscle stress during imposed running conditions. The shaded areas represent the shortening (or stance) phase of the oscillatory cycle. (C) Instantaneous muscle power during running. Muscle 178 absorbs energy during lengthening, while generating power during the shortening phase. Muscle 179 also absorbs energy during lengthening, but did not generate power during the shortening phase. (D) Work loops for muscles 178 and 179 under in vivo conditions. Work loops show that muscle 178 generated no net mechanical power over the cycle, while muscle 179 absorbed net mechanical power over the cycle. The arrows indicate the directions of the loops. The dominant clockwise work loop for muscle 179 illustrates that this muscle generated higher forces during lengthening than during shortening, resulting in negative work or mechanical energy absorption when operating under the conditions experienced during preferred speed running. These data for muscle 179 were previously reported (Full et al., 1998; Ahn and Full, 2002), and are presented to allow direct comparison to the data of muscle 178.

View larger version (19K): [in a new window]   Fig. 3. Muscle power as a function of strain amplitude for muscles 178 and 179. Muscle power decreased with increasing strain for both muscles. The blue filled symbols and solid lines represent the data and regressions for muscle 178. The red broken lines represent the regression lines fitted to the data for muscle 179. Each line represents a different animal. Mechanical function for muscle 178 varied with the strain amplitude, while muscle 179 functioned to absorb energy at all strains. Data for muscle 179 have been previously published (Ahn and Full, 2002) and are presented for direct comparison with data for muscle 178.

View larger version (32K): [in a new window]   Fig. 4. Representative strain, total stress and power when muscles 178 and 179 operated under identical strain and stimulation conditions. The shaded area represents the shortening phase of the oscillatory cycle. The stimulation pattern, represented on the graphs by the yellow squares, was determined during running at the animal's preferred speed (3 pulses at 100 Hz) (Full et al., 1998; Ahn and Full, 2002). (A) The strain pattern was identical for both muscles (15% strain amplitude). (B) Muscle stress during imposed running conditions. The shaded areas represent the shortening (or stance) phase of the oscillatory cycle. (C) Work loops for muscles 178 and 179 under identical conditions. Work loops show muscle 178 generated net mechanical power during the cycle, while muscle 179 absorbed net mechanical power under identical conditions. Arrows indicate the direction of the loops.

View larger version (22K): [in a new window]   Fig. 5. Normalized force-velocity relationships for muscles 178 and 179. The normalized force-relative shortening velocity relationships did not differ between muscles 178 (filled, blue symbols) and 179 (red broken line). The solid blue line represents the curve fitted to the data for muscle 178 using the Hill equation. The arrow indicates the maximum shortening velocity experienced by both muscles when cyclically contracting at 15% strain. Different symbol shapes represent different individuals. The broken, red line representing the curve fit using the Hill equation for muscle 179 is presented for comparison (Ahn and Full, 2002).

View larger version (25K): [in a new window]   Fig. 6. Tetanic force-length relationships for muscles 178 and 179. Different symbol shapes represent different individuals. The solid blue arrow indicates optimal length for muscle 178 (filled, blue symbols), while the broken red arrow indicates optimal length for muscle 179 (open, red symbols). The region of grey shading represents the range of strains used by the muscles when cyclically oscillated at a strain amplitude of 15%. The filled and open horizontal bars span the ranges of strain experienced by the muscles under in vivo running conditions.

View larger version (19K): [in a new window]   Fig. 7. Force-length relationship using the in vivo stimulation pattern. Representative force-length relationships for muscles 178 (A) and 179 (B). The force-length relationships at all three different stimulation levels were obtained from some individuals. (C) Forces normalized to the 3s force at the optimal length determined from the tetanic force-length relationships of muscles 178 (solid blue symbols) and 179 (open red symbols). Forces were normalized to the force at the optimal lengths determined from the tetanic force-length relationships (8.5% strain for muscle 178 and 10.9% strain for muscle 179). Fourth-order polynomial curves were fit to the combined data for muscle 178 (bold, solid blue line; R2=0.95) and muscle 179 (red, broken line; R2=0.96). N=4 animals for each muscle. The bold, black lines represent the difference in isometric force generated by the two muscles at -8% strain, which both muscles experience in vivo (see Results).

View larger version (16K): [in a new window]   Fig. 8. Force depression due to active shortening in muscle 179. (A) Force plotted as a function of time. The solid black line represents the control contraction generated at the shorter length. The broken red line represents force with a shortening ramp generated during the contraction in muscle 179. `F' indicates the force depression or the percentage difference between the peak force of an isometric contraction held at the final, shortened length and the peak force of a contraction with active shortening. In this representative trial, active shortening (broken, red line) depressed the force generated by 16% as compared to the force generated during the control contraction (solid, black line). (B) Muscle length and stimulation (yellow squares) plotted as a function of time of the shortening contraction (broken, red line) compared with the control contraction (solid, black line).

View larger version (25K): [in a new window]   Fig. 9. Effect of strain on force depression due to active shortening in muscles 178 (filled, blue symbols) and 179 (open, red symbols). The solid blue line represents a regression of the data for muscle 178 (force depression=-0.01+4.90xstrain, R2=0.64; P<0.0001), whereas the broken red line represents a regression of the data for muscle 179 (force depression=-0.93+5.22xstrain, R2=0.56; P<0.0001). Each type of symbol represents a different initial length (circles, rest length; squares, RL+1 mm; diamonds, RL+2 mm; triangles, RL+3 mm). The muscles were shortened at 100 mm s-1 and stimulated with the 3s pattern. N=5 animals for each muscle.

View larger version (17K): [in a new window]   Fig. 10. Force enhancement due to passive pre-stretch in muscle 178. (A) Force plotted as a function of time. The thin black line represents the force measured in muscle 178 when stimulated isometrically at the longer length and serves as the control. The thick blue line represents the force measured in muscle 178 when stretched just prior to an isometric contraction. Force traces are aligned with respect to the stimulation to show the change in force with and without a passive pre-stretch. In this representative trial, the passive pre-stretch (thin, black line) enhanced the force generated by 6% as compared to the force generated during the control contraction (bold, blue line). (B) Muscle length and stimulation (yellow squares) plotted as a function of time of the contraction with the passive pre-stretch (thick, blue line) compared with the control contraction (thin, black line).

View larger version (19K): [in a new window]   Fig. 11. Change in force as a function of passive pre-stretch for both muscles. Positive values of force enhancement represent a larger isometric force following a passive stretch. A negative value of force enhancement represents a depression following a passive stretch. The solid, blue points and line represent the data and regression for muscle 178, respectively (force change=1.35+0.20xstrain; R2=0.28; P=0.35). The open red points and broken line represent force change for muscle 179 (force change=-2.62+0.25xstrain; R2=0.065; P=0.18). As stretch distance increased, the variation in force response increased. However, the changes in force in response to a passive stretch were independent of stretch distance in both muscles. For these data, the muscles were stretched at 15 mm s-1, and stimulated with the 3s pattern. N=4-7 animals for each muscle at each absolute distance of stretch.