Biomedical subjects

G B Gillis

Publications and source records attributed to G B Gillis.

5 recordsLinked to original sources

PubMed2001

How muscles accommodate movement in different physical environments: aquatic vs. terrestrial locomotion in vertebrates.

Representatives of nearly all vertebrate classes are capable of coordinated movement through aquatic and terrestrial environments. Though there are good data from a variety of species on basic patterns of muscle recruitment during locomotion in a single environment, we know much less about how vertebrates use the same musculoskeletal structures to accommodate locomotion in physically distinct environments. To address this issue, we have gathered data from a broad range of vertebrates that move successfully through water and across land, including eels, toads, turtles and rats. Using high-speed video in combination with electromyography and sonomicrometry, we have quantified and compared the activity and strain of individual muscles and the movements they generate during aquatic vs. terrestrial locomotion. In each focal species, transitions in environment consistently elicit alterations in motor output by major locomotor muscles, including changes in the intensity and duration of muscle activity and shifts in the timing of activity with respect to muscle length change. In many cases, these alterations likely change the functional roles played by muscles between aquatic and terrestrial locomotion. Thus, a variety of forms of motor plasticity appear to underlie the ability of many species to move successfully through different physical environments and produce diverse behaviors in nature.

Animals↗

PubMed2001

Hindlimb muscle function in relation to speed and gait: in vivo patterns of strain and activation in a hip and knee extensor of the rat (Rattus norvegicus).

Understanding how animals actually use their muscles during locomotion is an important goal in the fields of locomotor physiology and biomechanics. Active muscles in vivo can shorten, lengthen or remain isometric, and their mechanical performance depends on the relative magnitude and timing of these patterns of fascicle strain and activation. It has recently been suggested that terrestrial animals may conserve metabolic energy during locomotion by minimizing limb extensor muscle strain during stance, when the muscle is active, facilitating more economical force generation and elastic energy recovery from limb muscle-tendon units. However, whereas the ankle extensors of running turkeys and hopping wallabies have been shown to generate force with little length change (<6% strain), similar muscles in cats appear to change length more substantially while active. Because previous work has tended to focus on the mechanical behavior of ankle extensors during animal movements, the actions of more proximal limb muscles are less well understood. To explore further the hypothesis of force economy and isometric behavior of limb muscles during terrestrial locomotion, we measured patterns of electromyographic (EMG) activity and fascicle strain (using sonomicrometry) in two of the largest muscles of the rat hindlimb, the biceps femoris (a hip extensor) and vastus lateralis (a knee extensor) during walking, trotting and galloping. Our results show that the biceps and vastus exhibit largely overlapping bursts of electrical activity during the stance phase of each step cycle in all gaits. During walking and trotting, this activity typically commences shortly before the hindlimb touches the ground, but during galloping the onset of activity depends on whether the limb is trailing (first limb down) or leading (second limb down), particularly in the vastus. In the trailing limb, the timing of the onset of vastus activity is slightly earlier than that observed during walking and trotting, but in the leading limb, this activity begins much later, well after the foot makes ground contact (mean 7% of the step cycle). In both muscles, EMG activity typically ceases approximately two-thirds of the way through the stance phase. While electrically active during stance, biceps fascicles shorten, although the extent of shortening differs significantly among gaits (P<0.01). Total average fascicle shortening strain in the biceps is greater during walking (23+/-3%) and trotting (27+/-5%) than during galloping (12+/-5% and 19+/-6% in the trailing and leading limbs, respectively). In contrast, vastus fascicles typically lengthen (by 8-16%, depending on gait) over the first half of stance, when the muscle is electrically active, before shortening slightly or remaining nearly isometric over much of the second half of stance. Interestingly, in the leading limb during galloping, vastus fascicles lengthen prior to muscle activation and exhibit substantial shortening (10+/-2%) during the period when EMG activity is recorded. Thus, patterns of muscle activation and/or muscle strain differ among gaits, between muscles and even within the same muscle of contralateral hindlimbs (as during galloping). In contrast to the minimal strain predicted by the force economy hypothesis, our results suggest that proximal limb muscles in rats operate over substantial length ranges during stance over various speeds and gaits and exhibit complex and changing activation and strain regimes, exemplifying the variable mechanical roles that muscles can play, even during level, steady-speed locomotion.

Animals↗

PubMed2000

Hindlimb extensor muscle function during jumping and swimming in the toad (Bufo marinus).

Many anurans use their hindlimbs to generate propulsive forces during both jumping and swimming. To investigate the musculoskeletal dynamics and motor output underlying locomotion in such physically different environments, we examined patterns of muscle strain and activity using sonomicrometry and electromyography, respectively, during jumping and swimming in the toad Bufo marinus. We measured strain and electromyographic (EMG) activity in four hindlimb muscles: the semimembranosus, a hip extensor; the plantaris, an ankle extensor; and the gluteus and cruralis, two knee extensors. During jumping, these four muscles are activated approximately simultaneously; however, joint extension appears to be temporally staggered, with the hip beginning to extend prior to or initially faster than the more distal knee and ankle joints. Mirroring this pattern, the gluteus and plantaris shorten quite slowly and over a small distance during the first half of limb extension during take-off, before beginning to shorten rapidly. The hip and knee extensors finish shortening near the point of take-off (when the feet leave the ground), while the ankle-extending plantaris, which exhibits the longest-duration EMG burst, on average, always completes its shortening after take-off (mean 26 ms). During swimming, activation of the four muscles is also nearly synchronous at the start of a propulsive stroke. The onset of fascicle shortening is temporally staggered, with the knee extensors beginning to shorten first, prior to the hip and ankle extensors. In addition, the knee extensors also often exhibit some degree of slow passive shortening prior to the onset of EMG activity. The offset of muscle shortening during swimming is also staggered, and to a much greater extent than during jumping. During swimming, the cruralis and gluteus finish shortening first, the semimembranosus finishes 30-60 ms later, and the plantaris, which again exhibits the longest EMG burst, finishes shortening last (mean 150 ms after the cruralis). Interestingly, much of this extended shortening in the plantaris occurs at a relatively slow velocity and may reflect passive ankle extension caused by fluid forces, associated with previously generated unsteady (accelerative) limb movements, acting on the foot. Average EMG burst intensity tends to be greater during jumping than during swimming in all muscles but the gluteus. However, EMG burst duration only changes between jumping and swimming in the cruralis (duration during jumping is nearly twice as long as during swimming). The cruralis is also the only muscle to exhibit substantially greater fractional shortening during jumping (mean 0.28) than during swimming (mean 0.20 active strain, 0.22 total strain). On the basis of these results, it appears that toad hindlimb function is altered between jumping and swimming. Moreover, these functional differences are influenced by passive effects associated with physical differences between the external environments, but are also actively mediated by shifts in the motor output and mechanical behavior of several muscles.

Animals↗

PubMed2000

Patterns of white muscle activity during terrestrial locomotion in the American eel (Anguilla rostrata).

Eels (Anguilla rostrata) are known to make occasional transitory excursions into the terrestrial environment. While on land, their locomotor kinematics deviate drastically from that observed during swimming. In this study, electromyographic (EMG) recordings were made from white muscle at various longitudinal positions in eels performing undulatory locomotion on land to determine the muscle activity patterns underlying these terrestrial movements. As during swimming, eels propagate a wave of muscle activity from anterior to posterior during terrestrial locomotion. However, the intensity of EMG bursts is much greater on land (on average approximately five times greater than in water). In addition, anteriorly located musculature has higher-intensity EMG bursts than posteriorly located muscle during locomotion on land. EMG duty cycle (burst duration relative to undulatory cycle time) is significantly affected by longitudinal position during terrestrial locomotion, and duty cycles are significantly greater on land (0.4-0.5 cycles) than in water (0. 2-0.3 cycles). Finally, as in swimming, a phase shift in the timing of muscle activity exists such that posteriorly located muscle fibers become activated earlier in their strain cycle than do more anteriorly located fibers. However, fibers become activated much later in their muscle strain cycle on land than in water. Therefore, it is clear that, while eels propagate a wave of muscle activity posteriorly to generate backward-traveling waves that generate propulsive thrust both in water and on land, the specific patterns of timing and the intensity of muscle activity are substantially altered depending upon the environment. This suggests that physical differences in an animal's external environment can play a substantial role in affecting the motor control of locomotion, even when similar structures are used to generate the propulsive forces.

Anguilla↗

PubMed1999

Dynamics of muscle function during locomotion: accommodating variable conditions.

Much of what we know about animal locomotion is derived from studies examining animals moving within a single, homogeneous environment, at a steady speed and along a flat grade. As a result, the issue of how musculoskeletal function might shift to accommodate variability within the external environment has remained relatively unexplored. One possibility is that locomotor muscles are differentially recruited depending upon the environment in which the animal is moving. A second possibility is that the same muscles are recruited, but that they are activated in a different manner so that their contractile function differs according to environment. Finally, it is also possible that, in some cases, animals may not need to alter their musculoskeletal function to move under different external conditions. In this case, however, the mechanical behavior appropriate for one environmental condition may constrain locomotor performance in another. To begin to explore the means by which animals accommodate variable conditions in their environment, we present three case studies examining how musculoskeletal systems function to allow locomotion under variable conditions: (1) eels undulating through water and across land, (2) turkeys running on level and inclined surfaces, and (3) ducks using their limbs to walk and to paddle. In all three of these examples, the mechanical behavior of some muscle(s) involved in locomotion are altered, although to different degrees and in different ways. In the running turkeys, the mechanical function of a major ankle extensor muscle shifts from contracting isometrically on a flat surface (producing little work and power), to shortening actively during contraction on an uphill gradient (increasing the amount of work and power generated). In the ducks, the major ankle extensor undergoes the same general pattern of activation and shortening in water and on land, except that the absolute levels of muscle stress and strain and work output are greater during terrestrial locomotion. In eels, a transition to land elicits changes in electromyographic duty cycles and the relative timing of muscle activation, suggesting some alteration in the functional mechanics of the underlying musculature. Only by studying muscle function in animals moving under more variable conditions can we begin to characterize the functional breadth of the vertebrate musculoskeletal system and understand more fully its evolutionary design.

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