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A motor and a brake: two leg extensor muscles acting at the same joint manage energy differently in a running insect

A. N. Ahn^* and R. J. Full

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

View larger version (25K): [in a new window]   Fig. 1. Ventral view of the cockroach Blaberus discoidalis. The inset represents the left hind limb and muscles 177c and 179. Muscle 177c inserts on the medial tip of the trochanter and originates on the basalar plate (wing hinge). Muscle 179 inserts on the trochanter and originates on the ventral side of the coxa. The open circle indicates the axis of rotation of the coxa–trochanter–femur joint.

View larger version (22K): [in a new window]   Fig. 2. EMG onset and duration relative to the running cycle. (A) EMG onset phases are represented by the start of the solid boxes and the left-hand error bars. The duty cycle (or burst duration normalized to cycle duration) was 7.3±6.2 % (N=6) for muscle 177c (green box) and 17.0±9.1 % (N=6; means ± S.D.) for muscle 179 (red box) and is represented by the length of the box and the right-hand error bars. The shaded region is the phase during which the extensors were shortening during the running cycle. (B) Normalized muscle strain plotted versus phase. Muscle length for each muscle, where 177c is represented by the solid line and 179 is represented by the dashed line, was normalized relative to its maximum and minimum lengths.

View larger version (22K): [in a new window]   Fig. 3. The force–velocity relationships of maximally stimulated muscle (177c, 179) while shortening isotonically at different force levels. Both graphs represent the same data set, but are plotted with different abscissas. Each symbol represents a different animal (N=4 for each muscle). The filled symbols and solid line represent muscle 177c, whereas the open symbols and dashed line represent muscle 179. Force–velocity relationships were established for each individual (see text). The Hill coefficients were then averaged to obtain the overall force–velocity relationships for each muscle, represented by the lines. (A) Absolute force–velocity relationship. In absolute terms, the muscles shorten maximally at the same velocity during running (arrows; approximately 15 mm s–1). (B) Relative force–velocity relationship. Muscle 177c was more than twice as long as muscle 179. Therefore, in relative terms, maximum shortening velocity, Vmax, for muscle 177c was 5.7±0.4 L s–1 (N=4), where L is muscle length. The maximum in situ contraction velocity was approximately one-third Vmax or 1.7±0.2 L s–1 (N=7; solid arrow) for 177c. Vmax for muscle 179 was 4.9±0.4 L s–1 (N=4), while the maximum in situ relative contraction velocity of 179 was 3.7±0.1 L s–1 (N=6), which is near its Vmax (dashed arrow). The Hill constants, which represent the curvature of the force–velocity relationship are a=0.5±0.3 and b=2.7±1.3 (N=4) for muscle 177c and a=0.6±0.2 and b=2.5±0.9 (N=4) for muscle 179. Values are means ± S.D.

View larger version (28K): [in a new window]   Fig. 4. Function of muscles 177c and 179 during the preferred-speed running. (A,B) Simultaneous coxa–femur joint kinematics of the hindlimb and EMG recordings of muscles 177c and 179 during preferred-speed running. The coxa–femur joint angle increased with extension during stance (shaded areas) and decreased with flexion. Both muscles were activated during shortening. Note the overlap in muscle activity. (C,D) Individual muscle strain and stress during isolated muscle experiments. The shortening and lengthening patterns were determined from the joint kinematics during running. Muscle forces were measured as the muscles were stimulated to simulate preferred-speed running. (E) Work loops generated using in vivo conditions. The counterclockwise direction of the muscle 177c work loop illustrates that this muscle generated higher forces during shortening than during lengthening, resulting in positive work or power output (P). The clockwise direction of the work loop for muscle 179 illustrates that this muscle generated higher forces during lengthening than during shortening, resulting in negative work or energy absorption when operating under preferred-speed running conditions. The shaded areas indicate the stance phase of running when the joint angle increases and the muscles shorten. Squares represent the timing of muscle action potentials.

View larger version (13K): [in a new window]   Fig. 5. Muscle power as a function of strain. Each symbol represents a different animal. The closed green symbols represent muscle 177c. Muscle 177c always generated power when operating over a range of strain amplitudes near its in vivo strain (7 %). The open red symbols represent muscle 179. Muscle 179 always absorbed energy when operating over a range of strain amplitudes near its in vivo running strain (16.4 %). Muscle function was not sensitive to strain excursion (or joint angle amplitude) when operating under otherwise in vivo running conditions.

View larger version (20K): [in a new window]   Fig. 6. Muscle power output as a function of the phase of stimulation onset for muscles 177c and 179. (A) Muscle power for both muscles relative to the phase of stimulation onset. Muscle power varied as a function of stimulation onset phase for both muscles. The stimulation onset phase at which power peaks differed for the two muscles. The region within the vertical lines indicated by the double-headed arrow represents the range of stimulus onset phase within which muscle 177c performed positive work while muscle 179 absorbed mechanical energy. Within these lines, muscle 177c always functioned as a power generator when operating under preferred-speed running conditions (two muscle action potentials per cycle; 8 Hz cycle frequency). Muscle 179 always functioned as an energy absorber when operating under preferred-speed running conditions (three muscle action potentials per cycle; 8 Hz cycle frequency). The filled green (177c) and open red (179) symbols represent the measured mean onset phases during running, including standard deviations. (B) Normalized muscle strain plotted versus phase. Muscle length for each muscle, where 177c is represented by the green line and 179 is represented by the red line, was normalized relative to its maximum and minimum lengths.

View larger version (26K): [in a new window]   Fig. 7. Muscle strain, contraction velocity, contraction duration, and in situ force generation for muscles 177c and 179 as a function of time. (A) Muscle 179 (dashed lines) underwent larger strain amplitudes because it is shorter than 177c (solid lines). (B) Because both muscles cycled at (8 Hz) during running, 179 experienced faster shortening and lengthening velocities than the longer 177c. (C) Contraction duration is represented by the time to 50 % relaxation. During running, muscle 179 was stimulated for longer (three muscle action potentials at 100 pulses s–1) than 177c (two muscle action potentials at 100 pulses s–1) and thus 179 generated force for longer. (D) Muscle 177c generated force during muscle shortening and relaxed before the muscle began to lengthen. Muscle 179, however, generated lower levels of force during shortening, possibly because it shortened at very high velocities. Muscle 179 began generating force as the contraction velocity slowed and approached zero. Muscle 179 was still active as the muscle lengthened, when it generated its highest forces, resulting in net negative work during the running cycle. The shaded areas indicate the stance phase of running when the muscles shorten. Yellow squares represent the timing of muscle action potentials.

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