Functional diversification within and between muscle synergists: intrinsic muscle dynamics redistribution of energy via

6. Recording in vivo neuromuscular function(guinea fowl LG and DF-4) This slide illustrates the anatomy and methods in a lateral view of a guinea fowl hindlimb. Click-On the left the Lateral head of the Gastrocnemius is shown. For the rest of the talk I will use the abbreviation LG and represent this muscle in orange. The Gastrocnemius originates on the femur and crosses the knee. The tendon converges to a common Gastroc. tendon that crosses the ankle joint. Click-The Digital Flexor to the lateral toe is deep to the gastrocnemius and can be seen on the right. For the rest of this talk this muscle will be abbreviated DF, and represented in green. The digital flexor group is comprised of multiple muscles which originate on the femur and tibiotarsus. The long thin tendons cross behind the ankle and TarsoMetatarsoPhalangeal joints continuing to the end of each digit. Thus this muscle-tendon system crosses all of the joints in the hindlimb except the hip. Click- In each muscle we implanted sonomicrometry crystals to measure muscle fascicle length change (point out crystals), and EMG electrodes to measure muscle activity, which are not shown. Click- We also attached E type tendon buckles to the associated tendons (point out tendon buckle) in order to measure muscle-tendon force. This slide illustrates the anatomy and methods in a lateral view of a guinea fowl hindlimb. Click-On the left the Lateral head of the Gastrocnemius is shown. For the rest of the talk I will use the abbreviation LG and represent this muscle in orange. The Gastrocnemius originates on the femur and crosses the knee. The tendon converges to a common Gastroc. tendon that crosses the ankle joint. Click-The Digital Flexor to the lateral toe is deep to the gastrocnemius and can be seen on the right. For the rest of this talk this muscle will be abbreviated DF, and represented in green. The digital flexor group is comprised of multiple muscles which originate on the femur and tibiotarsus. The long thin tendons cross behind the ankle and TarsoMetatarsoPhalangeal joints continuing to the end of each digit. Thus this muscle-tendon system crosses all of the joints in the hindlimb except the hip. Click- In each muscle we implanted sonomicrometry crystals to measure muscle fascicle length change (point out crystals), and EMG electrodes to measure muscle activity, which are not shown. Click- We also attached E type tendon buckles to the associated tendons (point out tendon buckle) in order to measure muscle-tendon force.

11. The first goal in this study was to address how body mechanics respond to a drop perturbation. The drop in substrate is 8cm, and if the body fell this entire distance this would lead to a 1.5 J decrease in PE of the CoM. The mechanics are likely to differ depending on the control mechanisms involved. If the bird managed to fully compensate for the step by adjusting limb parameters appropriately it could prevent any decrease in CoM height, and maintain the original trajectory. If the body is allowed to fall some of the distance, two additional possibilities arise, the PE can be converted to KE as illustrated in Case 2, or the muscles can absorb energy, preventing an increase in KE and resulting in a new trajectory at the lower height. Of course the behavior can be a combination of these possibilities. The first goal in this study was to address how body mechanics respond to a drop perturbation. The drop in substrate is 8cm, and if the body fell this entire distance this would lead to a 1.5 J decrease in PE of the CoM. The mechanics are likely to differ depending on the control mechanisms involved. If the bird managed to fully compensate for the step by adjusting limb parameters appropriately it could prevent any decrease in CoM height, and maintain the original trajectory. If the body is allowed to fall some of the distance, two additional possibilities arise, the PE can be converted to KE as illustrated in Case 2, or the muscles can absorb energy, preventing an increase in KE and resulting in a new trajectory at the lower height. Of course the behavior can be a combination of these possibilities.

13. In the drop perturbation trials, both of these distal muscles exhibit rapid changes in force development. As the foot breaks through the tissue, the force rapidly declines, and then when the limb is loaded again, force recovers. The fall and rise in force is more rapid and prominent in the DF than the LG. In both muscles, there is also extended EMG activity associated with the stance phase following the perturbation.In the drop perturbation trials, both of these distal muscles exhibit rapid changes in force development. As the foot breaks through the tissue, the force rapidly declines, and then when the limb is loaded again, force recovers. The fall and rise in force is more rapid and prominent in the DF than the LG. In both muscles, there is also extended EMG activity associated with the stance phase following the perturbation.

14. LG in vivo length and force during breakthrough perturbation:velocity- and shortening-deactivation of force (KEh mode),followed by force- and length-dependent feedback?

15. To address these we measured the latency of the reflex response resulting from a tendon tap on the achilles tendon while recording muscle length and activity. By measuring the time between the muscle stretch from the tap and the first EMG spike that follows, we can estimate the reflex latency. In the case illustrated here, the latency is only about 5ms. To address these we measured the latency of the reflex response resulting from a tendon tap on the achilles tendon while recording muscle length and activity. By measuring the time between the muscle stretch from the tap and the first EMG spike that follows, we can estimate the reflex latency. In the case illustrated here, the latency is only about 5ms.

16. Next I will compare this to muscle performance during locomotion of obstacles. I created an obstacle treadmill, in which the birds encountered 5 and 7cm obstacles every 10 to 12 steps. From the bird’s perspective, these obstacles matched the surrounding surfaces to minimize visual cues. However, they are white on the side so that they can be seen in the lateral view videos. Next I will compare this to muscle performance during locomotion of obstacles. I created an obstacle treadmill, in which the birds encountered 5 and 7cm obstacles every 10 to 12 steps. From the bird’s perspective, these obstacles matched the surrounding surfaces to minimize visual cues. However, they are white on the side so that they can be seen in the lateral view videos.

19. LG length and force during 5 cm obstacle step<length- & velocity-dependent effects on force>hypothesis: intrinsic effects of muscle properties reinforced by subsequent Force & Length neural feedback Here a video will be played in slow motion, synchronized to muscle recordings from the lateral gastrocnemius. Muscle fascicle length is shown on the top trace, and muscle force on the bottom trace with the EMG activity in the background. The dashed lines are representative traces from level running. The trace for the obstacle step in the video will be overlaid in solid lines. You can see that as soon as the foot contacts the obstacle, the muscle remains at a longer length, and the force rapidly rises. The mechanical performance of the muscle changes dramatically.Here a video will be played in slow motion, synchronized to muscle recordings from the lateral gastrocnemius. Muscle fascicle length is shown on the top trace, and muscle force on the bottom trace with the EMG activity in the background. The dashed lines are representative traces from level running. The trace for the obstacle step in the video will be overlaid in solid lines. You can see that as soon as the foot contacts the obstacle, the muscle remains at a longer length, and the force rapidly rises. The mechanical performance of the muscle changes dramatically.

20. Neural feedback alters EMG intensity (LG) & time course (DF) during obstacle steps If we look at the total EMG intensity over the course of the stride, we can see when the muscle recruitment pattern deviates significantly from control levels. In these graphs the white lines are the mean and 95% confidence intervals for the muscle force, length and total EMG intensity during level running. Overlaid in colored lines are 3 obstacle steps for the same bird. The onset times of the EMG do not significanlty differ from control, but the intensity of the LG is significantly higher than control shortly after the perturbation. Although the DF intensity is much more variable it also significantly deviates from control intensity levels at various points in the stride. If we look at the total EMG intensity over the course of the stride, we can see when the muscle recruitment pattern deviates significantly from control levels. In these graphs the white lines are the mean and 95% confidence intervals for the muscle force, length and total EMG intensity during level running. Overlaid in colored lines are 3 obstacle steps for the same bird. The onset times of the EMG do not significanlty differ from control, but the intensity of the LG is significantly higher than control shortly after the perturbation. Although the DF intensity is much more variable it also significantly deviates from control intensity levels at various points in the stride.

22. Acknowledgements