Step, Swim, and Scratch Motor Patterns in the Turtle

Gammon M. Earhart and Paul S. G. Stein

Department of Biology and Program in Movement Science, Washington University, St. Louis, Missouri 63130


    ABSTRACT
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Earhart, Gammon M. and Paul S. G. Stein. Step, Swim, and Scratch Motor Patterns in the Turtle. J. Neurophysiol. 84: 2181-2190, 2000. The turtle generates a variety of coordinated hindlimb movements, including different forms of locomotion and scratching. The intact turtle produces forward step, forward swim, and backpaddle. Following spinal cord transection, rostral, pocket, and caudal scratches can be evoked by mechanical stimulation of the shell. Comparisons of the kinematics and motor patterns of these six behaviors provide insights regarding neuronal mechanisms underlying their production. All six behaviors were characterized by alternating hip flexion and extension and by an event during which force was exerted against a substrate. The portion of the cycle occupied by hip flexion or extension movement varied across behaviors. Hip extension occupied well over half the cycle period in the forward step and the caudal scratch. The cycle was split into approximately half hip flexion and half hip extension for the forward swim, the backpaddle, and the rostral scratch. Hip flexion occupied over half the cycle in the pocket scratch. The swim and scratch forms had curvilinear, crescent-shaped toe trajectories and a single burst of monoarticular knee extensor activity during each cycle. The forward step had a linear toe trajectory and two bursts of knee extensor activity during each cycle, one during swing and one during stance. Timing of monoarticular knee extensor onset was similar for: the forward swim, the rostral scratch, and the swing phase burst of forward step; the pocket scratch and the stance phase burst of forward step; and the backpaddle and the caudal scratch. Amplitudes of muscle activity varied among the six behaviors; high amplitudes of activity were associated with events during which force was exerted against a substrate. These times of force exertion were: stance phase in the forward step, powerstroke in the forward swim and the backpaddle, and rubs of the limb against the shell in the scratch forms. The six behaviors studied represent a range of parameter values, as evidenced by relative durations of hip flexion to hip extension, knee extensor phasing, and electromyogram (EMG) amplitudes. This range of behaviors could be produced by assembling different combinations of neurons from a common pool, with all six behaviors likely sharing some basic circuitry. The extent of shared circuitry may be greater between behaviors with similar timing, e.g., backpaddle and caudal scratch.


    INTRODUCTION
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Successful movement within a changing environment relies on the ability to select and coordinate different behaviors appropriate for different situations. Turtles, for example, must be able to locomote in both aquatic and terrestrial environments (Zug 1971). Within these different environments, turtles must be able to move in all directions as well as turn when a change in direction is desired. Thus successful movement requires recognition of salient environmental features, selection of a movement that will achieve the desired outcome, and production of the coordinated motor output for the selected movement. We find it useful to classify these movements according to the goals underlying each behavior. A movement in which an organism achieves a particular goal is a task; each different motor strategy used to perform a particular task is a form of that task (Stein et al. 1986b). In locomotion, for example, the goal of the organism is to move its center of mass from one point to another. Within the locomotor task, several forms can be used.

The turtle can produce hindlimb motor outputs for several locomotor forms. The forward step is a form of terrestrial locomotion (Walker 1971; Zug 1971; M. H. Schieber and P.S.G. Stein, unpublished observations). The forward swim and the backpaddle are forms of aquatic locomotion (Field and Stein 1997a,b; Lennard and Stein 1977; Stein 1978). Each of these three locomotor forms has a behavioral event during which force is exerted against a substrate. In the forward step, the event is stance; force is exerted against the ground during stance. In the forward swim and the backpaddle, the event is powerstroke; propulsive force is exerted against the water during powerstroke. The forward swim powerstroke is a hip extension movement with the foot held vertically and the toes and webbing spread (Davenport et al. 1984; Field and Stein 1997a; Lennard and Stein 1977; Stein 1978; Zug 1971). The forward swim powerstroke propels the turtle in the forward direction. The backpaddle powerstroke is a hip flexion movement with the foot held vertically and the toes and webbing spread (Field and Stein 1997a; Lennard and Stein 1977). The backpaddle powerstroke propels the turtle in the backward direction. By combining forward swim in one hindlimb with backpaddle in the other hindlimb, the turtle produces a turning behavior. The direction of the turn is away from the side of the forward swim and toward the side of the backpaddle.

The turtle therefore accomplishes the goal of locomotion using each of several forms to move in different environments and in different directions. The spinal turtle also produces motor outputs for scratch behaviors, during which the goal of the turtle is to rub a stimulated site on the shell surface with a nearby limb. The turtle produces three scratch forms: rostral, pocket, and caudal; each scratch form is used to rub a specific region of the shell (Mortin et al. 1985). Similar to the forms of locomotion, each scratch form has a behavioral event during which force is exerted against a substrate. The scratch event is a rub during which the hindlimb exerts force against the shell. The rub is performed with the dorsum of the foot in the rostral scratch, with the side of the knee in the pocket scratch, and with the heel of the foot in the caudal scratch (Field and Stein 1997a; Mortin et al. 1985).

In this study, we examined three locomotor and three scratch forms in the turtle. We compared characteristics of forward step, forward swim, backpaddle, rostral scratch, pocket scratch, and caudal scratch. This repertoire of coordinated hindlimb motor outputs represents a diverse group of movements. We focused on the kinematics and motor patterns for the six behaviors. We hypothesized that similarities and differences observed would provide insights regarding the possible sharing of circuitry among different behaviors. We report the first quantitative analyses of forward step kinematics and myography in the turtle, as well as the first quantitative phase analyses for backpaddle and scratch motor patterns. These data were also presented as a chapter of a doctoral thesis (Earhart 2000).


    METHODS
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Surgical preparation

Red-eared turtles (n = 34), Trachemys scripta elegans, were studied. All surgery was performed under hypothermic anesthesia (Melby and Altman 1974). Turtles were placed in crushed ice for at least 1 h before the start of surgery and kept in ice throughout the procedure.

We recorded EMGs using bipolar electrodes, each a pair of 100-µm silver wires with enamel coating that were glued together with Permabond 910 (National Starch and Chemical, Englewood, NJ). Electrodes were implanted in selected hindlimb muscles (Stein et al. 1982; Walker 1973): a monoarticular knee extensor (triceps femoris pars femorotibialis, FT-KE), two hip flexors (puboischiofemoralis internus pars anteroventralis, VP-HP, and anterior iliofemoralis, AI-HP), and a hip extensor (flexor cruris pars flexor tibialis internus, HR-KF). For implantation of VP-HP, a rectangular opening was drilled in the ventral plastron over the pelvic region, providing access to the muscle belly. For implantation of the other muscles, a triangular opening was drilled in the dorsal carapace. This opening exposed the AI-HP muscle belly. Knee and hip extensor muscle bellies were accessed through small incisions of the skin overlying each muscle. Electrodes for FT-KE and HR-KF were threaded under the skin and into the internal cavity exposed by the hole in the carapace. Electrode tips were implanted directly into the muscle bellies and glued in place. Following electrode placement in VP-HP, the electrode was glued to the plastron and covered with Op-Site transparent adhesive film (Smith and Nephew Medical, Hull, UK). The hole in the plastron was sealed with a wax plug (modern materials utility wax strips---large red; Heraeus Kulzer, South Bend, IN) that was flush with the plastral surface. Following electrode placement in AI-HP, FT-KE, and HR-KF, the electrodes were secured to the sides of the hole in the carapace, which was then sealed with wax. Skin incisions overlying the implanted knee extensor and hip extensor were closed with glue and covered with Op-Site. The turtle was removed from ice and warmed to room temperature.

For each electrode, the silver wires remaining external to the turtle were ensheathed in 750-µm-ID Tygon microbore tubing (Norton Performance Plastics, Akron, OH) to reduce movement artifact present when the wires touched one another directly. The tubes were then braided into a tether, allowing the turtle a 0.5-m radius within which it could freely move. Data for the three locomotor forms were then collected.

Thirteen turtles were studied only for intact behaviors. The other 21 turtles were studied for both intact and spinal behaviors. These 21 turtles were spinalized after completion of locomotor data collection. Each of these turtles was returned to crushed ice for at least 1 h and kept on ice throughout spinalization. For spinalization, a channel was drilled along the midline of the carapace over the second (D2, second postcervical) and third (D3) dorsal segments of the spinal cord (Zangerl 1969). The spinal cord was completely transected between the D2 and D3 segments, covered with Gelfoam absorbable gelatin sponges (Pharmacia and Upjohn, Kalamazoo, MI), and the midline channel sealed with a wax plug. The turtle was removed from the crushed ice and warmed to room temperature. Data for the three scratch forms were then collected.

Stimulation and recording

Reflective markers were placed over the hip, knee, ankle, and third toe of each turtle. Locomotor forms were recorded from freely moving, intact turtles. For forward step, turtles walked across a sheet of Plexiglas. Turtles were placed in a tank of water for forward swim and backpaddle. Scratch forms were recorded from spinal turtles resting on a sheet of Plexiglas and secured in place by a bandclamp encircling the midbody region of the shell. Scratching was evoked using the smooth glass tip of a probe to stimulate mechanically the shell bridge at the SP 2 site for rostral scratch, at the Fem 5 site for pocket scratch, and at the Anal 5 site for caudal scratch (Fig. 1A) (Mortin et al. 1985).



View larger version (15K):
[in this window]
[in a new window]
 
Fig. 1. Illustration of stimulation sites and hip angle (A). Mechanical stimulation at the SP 2 site evoked a rostral scratch, at the Fem 5 site evoked a pocket scratch, and at the Anal 5 site evoked a caudal scratch. A: dashed-line box is the region depicted in B-D. Toe trajectories for forward step (B), forward swim (C), and backpaddle (D) are shown. Thickened dashed lines denote stance phase in B and powerstrokes in C and D. Hindlimb position at mid-stance is shown in B and at mid-powerstroke in C and D.

Movement was recorded by use of a mirror placed below the turtle at an angle of 45°. Hindlimb movements were videotaped with a single camera at 60 Hz with a shutter speed of 1/250 s. Movements of the thigh were planar in all behaviors, but movements of the shank during stepping were not planar. Thus hip angle but not knee angle could be accurately measured using a single camera. EMGs from each muscle were amplified, filtered (100-1,000 Hz band-pass), and recorded on digital audio tape (DC-5 kHz band-pass). In all turtles, the monoarticular knee extensor, one hip flexor, and the hip extensor were recorded. In some turtles, both hip flexors were recorded. A synchronization signal, linked to opening of the camera shutter and generated by a Peak Event Synchronization Unit (Peak Performance Technologies, Englewood, CO), was recorded on the video and the digital audio tapes so that movement data and EMGs could be synchronized for analysis.

Data analysis

For inclusion in this study, a turtle had to produce forward swim and at least one of the other five behaviors of interest. In addition, artifact-free EMG recordings had to be obtained from at least three hindlimb muscles during these behaviors. Twenty-six of the 34 turtles studied qualified for inclusion. Of the 26 turtles included in this study, 5 were studied only in the intact condition and 21 were studied in both intact and spinal conditions. Of the 26 studied in the intact conditions, all produced forward swim, 15 produced forward stepping, and 14 produced backpaddle. Of the 21 turtles studied in the spinal conditions, 12 produced the scratch forms. Note that VP-HP was not recorded in all turtles. VP-HP recordings were obtained from 22 of the 26 turtles that produced forward swim, 12 of the 15 that performed forward stepping, 13 of the 14 that performed backpaddle, and all of the 12 that produced the scratch forms. Five of the 26 turtles performed all six behaviors studied.

Kinematic analyses were performed on a total of 300 cycles, 50 of each behavior. The 50 cycles of each behavior were composed of 10 cycles from each of the five turtles that performed all six behaviors. Episodes were selected for analysis if they contained at least four full cycles of a behavior. Analyzed cycles were taken from the middle portion of an ongoing response. Video tape records of selected episodes were replayed and the x and y coordinates for each marker manually digitized (30 Hz) using a Peak 5.2 system (Peak Performance Technologies). Hip angle was defined as the angle between the thigh and the ventral midline of the body (Fig. 1A). Hip angle values increased as the joint extended and decreased as the joint flexed. Toe trajectories were obtained by viewing videos frame by frame and tracing the path of the third toe on an acetate sheet placed over the video screen.

The timing of behavioral events was determined from video records. For the forward step, stance was defined as the time during which the foot was in contact with the ground. Stance began at the time of footfall and ended at the time of liftoff. For the forward swim, powerstroke was defined as the time during which the hip was extending and the foot was held in a vertical position with toes and webbing spread. For the backpaddle, powerstroke was defined as the time during which the hip was flexing and the foot was held in a vertical position with toes and webbing spread. For the scratches, minimum and maximum hip angles were used as reversal points for hip movements between flexion and extension.

Twenty cycles of EMGs from each turtle that produced each behavior were analyzed. EMGs selected for analysis were free from artifact. During the fastest stepping responses observed, the shell often struck the walking surface, creating large artifacts in the EMG records. Thus many of the fastest stepping responses observed were not analyzed.

In total, 520 forward swim cycles, 320 forward step cycles, 280 backpaddle cycles, and 240 cycles of each scratch form were analyzed. As for the kinematic analyses, selected cycles were taken from the middle of an ongoing response. Each episode of forward step, backpaddle, or scratch was matched with a forward swim episode from the same turtle. Each episode consisted of at least four cycles. EMGs were digitized at 2 kHz using Cambridge Electronic Design 1401 plus hardware and Spike2 software (Cambridge, UK). EMGs were rectified and averaged so that the mean of 20 successive rectified measurements was calculated, giving 100 data points per second.

Software custom written by Dr. Gavin Perry was used to merge kinematic and EMG files, aligning the two data sets using the synchronization signal recorded on each. EMG burst onset and offset times for each muscle were determined. Thresholds for selection of EMG burst onsets and offsets were set at 20% of the peak value for that burst. Two bursts of knee extensor activity per forward step cycle could be automatically recognized using this method, although there was not always a period of total quiescence between these two bursts. EMG amplitudes were obtained by averaging the activity within each burst. For each muscle, amplitudes were normalized to forward swim values, such that the amplitudes for a forward swim episode were 100% and amplitudes of the related forward step, backpaddle, and scratch episodes were expressed as a percentage of the forward swim values. Cycle period for a muscle was defined as the time from the onset of one burst to the onset of the next burst. The knee extensor had two bursts of activity per cycle in the forward step; its cycle period during forward step was defined as the time from onset of the swing phase burst to onset of the next swing phase burst. Duty cycle, defined as the percentage of cycle period during which a muscle was active, was calculated as burst duration multiplied by 100 and then divided by cycle period of that muscle.

Phase analyses were used to determine the timing of monoarticular knee extensor EMG onset and offset with respect to hip muscle activity. Dual-referent phase analyses were used to normalize the hip flexion and hip extension phases of each behavior (Berkowitz and Stein 1994b). Dual-referent analyses are preferred to single-referent techniques in situations where the duty cycle of the referent is variable. Dual-referent methods normalize for active and inactive periods of the referent; thus changes in referent duty cycle will not produce shifts in phase values obtained (see Fig. 2 of Berkowitz and Stein 1994b). Events occurring during the hip flexion portion of the cycle had phase values between 0.0 and 0.5; those occurring during hip extension had phase values between 0.5 and 1.0.

Kinematic phase analyses for timing of behavioral events in the forward step were performed using the method described by Field and Stein (1997a). For events occurring during hip flexion, phase was defined as the latency between hip flexion onset and event onset, divided by twice the duration of hip flexion. For events occurring during hip extension, phase was defined as the latency between hip extension onset and the event, divided by twice the duration of hip extension, plus 0.5.

Motor pattern phase analyses are described in Berkowitz and Stein (1994b). In the present study, we determined onset and offset phases of FT-KE using the hip flexor VP-HP as a referent. Hip flexor onset was given a phase value of 0.0 and hip flexor offset a phase value of 0.5. For events occurring during hip flexor activity, phase was defined as the latency between hip flexor onset and the event, divided by twice the duration of hip flexor activity. For events occurring during hip extensor activity, phase was defined as the latency between hip flexor offset and the event, divided by twice the duration of hip flexor quiescence, plus 0.5.

Phase data are cyclic; vector algebra techniques (Batschelet 1981) were used to determine average phase values. Each phase was converted to a two-dimensional unit vector with an angle of 2pi phi and a length of 1. Unit vectors were averaged using vector addition. The angle of the mean vector, divided by 2pi , was the mean phase; it had values between 0 and 1. Mean angular deviation, a measure of phase data dispersion, was also calculated (Batschelet 1981).

Statistical analyses

Kinematic and myographic parameters were compared across behaviors using nonparametric statistics. Mean EMG amplitudes and duty cycles were compared across locomotor forms using the Kruskal-Wallis ANOVA (Portney and Watkins 1993). Post hoc pairwise comparisons of each behavior to the others were conducted. Bonferroni corrections were used to account for the increase in type I error that resulted from doing multiple tests.

Mean phase of monoarticular knee extensor EMG onset and offset was compared across locomotor and scratch behaviors using the Watson U2 test for circular data (Batschelet 1981). Pairwise comparisons were used for all statistical analyses of phase values.


    RESULTS
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Kinematics

The forward step toe trajectory was a relatively linear path in the anterior-posterior direction (Fig. 1B). The toe trajectories for the forward swim and backpaddle were curved and similar in shape to one another (Fig. 1, C and D). Although the shapes of the forward swim and the backpaddle trajectories were similar, the direction of movement indicated by the arrows in Fig. 1, C and D, is opposite for the two behaviors. The posterior extent of the backpaddle toe trajectory was variable from cycle to cycle as shown in Fig. 1D. Toe trajectories for the three scratch forms, similar to those for the forward swim and backpaddle, are also curved and crescent-shaped (see Fig. 2 of Field and Stein 1997a). The forward step was distinct among the six behaviors: it was the only one of the six with a linear toe trajectory.

Despite differences in the shape of the forward step toe trajectory, the minimum and maximum hip angles for the forward step were not statistically different from those of the forward swim and the backpaddle (Fig. 2, Kruskal-Wallis ANOVA). There was a trend toward greater maximum hip angles in the forward step than in the swim forms. The minimum and maximum hip angles during forward step did not correspond exactly with the hip angles at footfall and liftoff: the hip angle at footfall was 30.9 ± 5.7°; the hip angle at liftoff was 118.8 ± 13.2° (mean ± SD). The phase of footfall within the hip movement cycle was 0.51 ± 0.04 and the phase of liftoff was 0.10 ± 0.05. For scratch minimum and maximum hip angles, see Fig. 4 of Field and Stein (1997a).



View larger version (20K):
[in this window]
[in a new window]
 
Fig. 2. Average minimum and maximum hip angles ± SDs for the 3 locomotor behaviors. Differences between the behaviors were not significant.

The six forms were also analyzed with respect to duration of movement phases as a function of cycle period (Fig. 3). Figure 3 shows durations of stance and swing, powerstroke and returnstroke, or flexion and extension for forward step, the two swim forms, and the three scratch forms, respectively, as a function of cycle period. Forward swim (Fig. 3B), backpaddle (Fig. 3C), and rostral scratch (Fig. 3D) displayed slopes near 0.50 for both phases of the cycle. Forward step (Fig. 3A) and caudal scratch (Fig. 3F) displayed slopes near 0.75 for the stance or extension phases. The slopes of the lines for the forward step agreed well with average percentages of the cycles spent in stance and swing, which were 76 and 24%, respectively. Pocket scratch (Fig. 3E) was the only behavior that showed longer flexion than extension durations, with a flexion slope of 0.65. 



View larger version (31K):
[in this window]
[in a new window]
 
Fig. 3. Durations of the different phases of movement versus cycle period for 50 cycles of each of the 6 behaviors. Durations of swing and stance, returnstroke and powerstroke, or flexion and extension are plotted vs. cycle period for forward step (A), the 2 swim forms (B and C), and the 3 scratch forms (D-F), respectively. Data for each phase were fitted with regression lines; slopes of these lines are given for each behavior.

Motor patterns

The forward step motor pattern was also distinct among the six behaviors studied. There were two bursts of monoarticular knee extensor (FT-KE) activity per cycle for the forward step, but only one FT-KE burst per cycle for the other five behaviors. Figure 4 shows hip angle and EMG records for two cycles of forward step. In the forward step, one FT-KE burst occurred during the swing phase, and a second FT-KE burst occurred during the stance phase (Fig. 4). The swing phase FT-KE burst began near the middle of the hip flexor burst and continued throughout hip flexor activity. The stance phase FT-KE burst began near the start of the hip extensor burst and continued throughout hip extensor activity. The amplitude of hip extensor activity was substantial during most of the stance phase when force was being exerted against the ground.



View larger version (23K):
[in this window]
[in a new window]
 
Fig. 4. Hip angle and EMG records for 2 cycles of forward step. The hip angle trace is thickened during the stance phase, the time of force generation against the substrate. Vertical lines divide the cycles into swing and stance. Note that there were 2 bursts of monoarticular knee extensor activity per cycle, 1 during swing and the other during stance.

Hip angle and EMGs are shown for two cycles each of forward swim (Fig. 5A) and backpaddle (Fig. 5B). In the forward swim and the backpaddle, a single FT-KE burst occurred during each cycle (Lennard and Stein 1977). Onset of the FT-KE burst occurred near the middle of the hip flexor burst for forward swim, but prior to onset of the hip flexor burst in the backpaddle. Powerstroke in the forward swim was associated with hip extension movement and high-amplitude hip extensor activity; powerstroke in the backpaddle was associated with hip flexion movement and high-amplitude hip flexor activity and knee extensor activity.



View larger version (24K):
[in this window]
[in a new window]
 
Fig. 5. Hip angle and EMG records for 2 cycles each of forward swim (A) and backpaddle (B), shown with the same set of vertical scales. The hip angle trace is thickened during the time of force generation against the substrate (i.e., powerstroke). Vertical lines divide the cycles into powerstroke and returnstroke phases. Note that forward swim powerstroke is associated with high-amplitude hip extensor activity and backpaddle powerstroke is associated with high-amplitude hip flexor activity.

Figure 6 shows motor patterns for the six behaviors. These data are all from the same turtle, and all panels are shown at the same set of gains. Comparisons of behavioral time scales, FT-KE timing with respect to hip flexor activity, EMG amplitudes, and EMG duty cycles can be made among all six behaviors using this illustration. The time scale for the forward step is much longer than that for the other five behaviors. The forward step was consistently slower than the swim or scratch behaviors, and its cycle period was more variable. Average cycle period for the forward step cycles that were analyzed was 1.79 ± 0.83 s. This average may be slightly high because motor patterns from the fastest stepping behaviors often contained artifact and thus were not selected for analysis. Average periods for the other five behaviors were: forward swim, 0.55 ± 0.18; backpaddle, 0.60 ± 0.24; rostral, 0.84 ± 0.30; pocket, 0.99 ± 0.32; and caudal, 0.79 ± 0.24 s.



View larger version (27K):
[in this window]
[in a new window]
 
Fig. 6. Motor patterns for forward step (A), forward swim (B), backpaddle (C), rostral scratch (D), pocket scratch (E), and caudal scratch (F). A longer time scale was used for forward step than for the other 5 behaviors. All EMG records are from the same turtle, and all are shown at the same set of gains so that amplitudes can be compared across behaviors.

Figure 6 also illustrates similarities and differences in FT-KE timing among the six behaviors. Onset of the swing phase FT-KE burst in forward step (Fig. 6A) occurred in the middle of the hip flexor burst, a timing similar to that in the forward swim (Fig. 6B) and rostral scratch (Fig. 6D). Onset of the stance phase FT-KE burst in forward step (Fig. 6A) occurred after hip flexor offset, a timing similar to that in the pocket scratch (Fig. 6E). FT-KE onset in the backpaddle (Fig. 6C) occurred late in hip flexor quiescence just prior to the onset of the hip flexor burst, a timing similar to that in the caudal scratch (Fig. 6F).

Average onset and offset phases for FT-KE during each of the six behaviors are shown in Fig. 7. Phase values of 0.0-0.5 indicate events that occurred during hip flexor (VP-HP) activity; phase values of 0.5-1.0 indicate events that occurred during hip flexor quiescence. There was no significant difference in FT-KE onset phase among the swing phase forward step burst, forward swim, and rostral scratch. There was also no significant difference in FT-KE onset phase between the stance phase forward step burst and the pocket scratch; in addition, there was no significant difference in FT-KE onset phase between backpaddle and caudal scratch. All other pairwise comparisons of FT-KE onset phase were significantly different (Watson U2, P < 0.05). There was no significant difference in FT-KE offset phase among forward swim, rostral scratch, and pocket scratch. There was also no significant difference in FT-KE offset phase between the swing phase forward step burst and backpaddle; in addition, there was no significant difference in FT-KE offset phase between the stance phase forward step burst and caudal scratch. All other pairwise comparisons of FT-KE offset phase were significantly different (Watson U2, P < 0.05).



View larger version (14K):
[in this window]
[in a new window]
 
Fig. 7. Mean triceps femoris pars femorotibialis (FT-KE) onset and offset phases ± angular deviations for each behavior. Phase values were determined using dual-referent analyses with the hip flexor puboischiofemoralis internus pars anteroventralis (VP-HP) as the referent. See text for description of statistical significance.

Additional phase analyses were conducted on the forward step motor patterns to determine interlimb phase. The phase of left VP-HP hip flexor activity using right VP-HP hip flexor as a referent was calculated. The onset phase of left VP-HP with respect to right VP-HP was 0.54 ± 0.11; the offset phase of left VP-HP with respect to right VP-HP was 0.90 ± 0.09. The forward step thus showed out-of-phase coordination between the hips, similar to the phasing noted previously for bilateral swimming and bilateral scratching (Field and Stein 1997b).

The six behaviors displayed a variety of different amplitudes of muscle activity. Amplitude variations across behaviors within a turtle are illustrated in Fig. 6, and average amplitudes across turtles are given in Table 1. High-amplitude knee extensor (FT-KE) activity in the backpaddle was associated with force generation during powerstroke (Fig. 6C). FT-KE amplitudes during the backpaddle were significantly higher than for the other five behaviors. FT-KE amplitudes for the three scratch forms and the backpaddle were significantly higher than during the swing or the stance bursts of the forward step (Table 1; Kruskal-Wallis ANOVA, P < 0.05).


                              
View this table:
[in this window]
[in a new window]
 
Table 1. EMG amplitudes

High-amplitude hip flexor activity was associated with force generation during the backpaddle powerstroke and the rostral scratch rub, which occurs near the end of hip flexion (Fig. 6, C and D) (Field and Stein 1997a). Average VP-HP amplitudes during rostral and pocket scratches were significantly higher than during the other four behaviors. Average AI-HP amplitudes during backpaddle, rostral scratch, and pocket scratch were significantly higher than during forward step, forward swim, or caudal scratch (Table 1; Kruskal-Wallis ANOVA, P < 0.05).

Forward step (Fig. 6A) and forward swim (Fig. 6B) had relatively high amplitudes of hip extensor (HR-KF) activity associated with generation of force against a substrate. High amplitudes of HR-KF activity were also noted in the pocket (Fig. 6E) and caudal (Fig. 6F) scratches. During pocket and caudal scratches force generation occurs during the rub, an event that occurs as the hip is extending during the pocket scratch and near the end of hip extension in the caudal scratch (Field and Stein 1997a). The backpaddle (Fig. 6C) and rostral scratch (Fig. 6D) had low-amplitude HR-KF activity. In some cycles of backpaddle, HR-KF activity was absent. These HR-KF omissions were noted in 7 of the 12 turtles that performed the backpaddle. Backpaddle omissions comprised 11.15% of all backpaddle cycles (see Robertson et al. 1985 for a definition of omissions). Overall, HR-KF amplitudes during forward step, forward swim, pocket scratch, and caudal scratch were significantly higher than during backpaddle or rostral scratch (Table 1; Kruskal-Wallis ANOVA, P < 0.05). All other amplitude comparisons were not statistically significant.

Motor pattern duty cycle was another means of distinguishing among the behaviors. Table 2 gives the average duty cycle percentages for each of the muscles studied for each behavior. FT-KE duty cycle for the stance phase burst of forward step was significantly greater than those for all other behaviors except backpaddle. FT-KE duty cycle for backpaddle was significantly greater than for the swing phase burst of forward step. VP-HP duty cycles for backpaddle and pocket scratch were significantly greater than those for forward step or caudal scratch. HR-KF duty cycles during forward step and forward swim were significantly greater than those of backpaddle and pocket scratch (Kruskal-Wallis ANOVA, P < 0.05). All other duty cycle comparisons were not statistically significant.


                              
View this table:
[in this window]
[in a new window]
 
Table 2. EMG duty cycles


    DISCUSSION
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

We have demonstrated kinematic and motor pattern differences and similarities between forms of a single task and between forms of two different tasks in the turtle. Our results indicate that: the motor pattern for forward stepping in the turtle shares features with stepping patterns of other limbed vertebrates; the six forms studied can be distinguished from one another on the basis of EMG phasing and EMG amplitude; and there may be sharing of circuitry among these six forms.

Forward stepping

The forward step motor pattern was distinct from those of the forward swim, the backpaddle, or the three scratch forms; only the forward step showed double bursting of the monoarticular knee extensor, FT-KE. In many cases, there was a clear quiescence between the two bursts; in other examples, there was no distinct pause in activity but rather a substantial decrease in EMG amplitude between the bursts. The swing phase FT-KE burst likely serves to extend the knee in preparation for footfall, while the stance phase FT-KE burst may serve to resist knee flexion. Jacobson and Hollyday (1982) proposed similar functions for the two bursts of FT-KE observed during chick walking. Double bursting of FT-KE is also present in lizard quadrupedal running (Reilly 1995). In intact, mesencephalic, and low spinal cats, double bursting of the monoarticular knee extensor vastus lateralis has been observed (Engberg and Lundberg 1969; Grillner 1973; Grillner and Zangger 1984). The presence of swing phase vastus lateralis activity is variable, as it is not seen in all cats or in all cycles produced by a given cat (Forssberg et al. 1980a,b; Smith et al. 1998; Wetzel 1981; Wetzel and Stuart 1976). In rats and dogs, vastus lateralis is active during both swing and stance (Goslow et al. 1981; Gruner and Altman 1980). Double bursting of vastus lateralis is also reported to occur in some humans as speed of progression is increased during running (Nilsson et al. 1985). In the forelimb, periods of activity during both swing and stance have been observed for monoarticular elbow extensors of the newt, lizard, rat, and cat (Cohen and Gans 1975; Delvolvé et al. 1997; English 1978; Jenkins and Goslow 1983). Thus double bursting of the monoarticular knee and/or elbow extensors during overground forward stepping appears to be a common feature among the motor patterns of many different limbed vertebrates.

The forward stepping reported here is also similar to that of other vertebrates in terms of percentage of the cycle spent in stance. Our average of 76% stance is similar to other reported values for turtles or tortoises: 74-86% stance for Chrysemys picta marginata (Walker 1971), 85% stance in Testudo graeca (Williams 1981), and 60-90% stance across several species (Zug 1971). Stance percentages of 70-75% have also been noted in alligator, salamander, chick, cat, and slow human locomotion (Ashley-Ross and Lauder 1997; Jacobson and Hollyday 1982; Nilsson et al. 1985; Reilly and Elias 1998; Trank et al. 1996).

Knee extension during swim powerstrokes: forward swim versus backpaddle

The knee remains extended throughout most of powerstroke during both the forward swim and the backpaddle (Field and Stein 1997a), but FT-KE activity during powerstroke was very different for the two behaviors. In forward swim, the force of the water against the anatomical ventral surface of the vertically oriented foot during the hip extension movement of powerstroke extended the knee; FT-KE activity was not necessary to maintain the extended position of the knee. In the backpaddle, the force of the water against the anatomical dorsal surface of the vertically oriented foot during the hip flexion movement of powerstroke would tend to flex the knee; high-amplitude FT-KE activity was necessary to maintain the extended position of the knee.

EMG phasing as a form discriminator

The timing of monoarticular knee extensor onset with respect to the hip motor pattern is a variable that has been used to distinguish among different forms. We demonstrate that differences in FT-KE onset can be used to discriminate among the three forms of locomotion. FT-KE onset phase can also be used to discriminate among rostral, pocket, and caudal scratches. Previous reports (Field and Stein 1997a), as well as our own data (unpublished observations), show that pocket and caudal scratches could not be distinguished from one another based on the phase of knee extension movement onset within the hip cycle despite differences in knee extensor onset phase. For rostral and pocket scratches, the phase of knee extension movement onset closely matches the phase of FT-KE onset. For caudal scratch, however, knee extension movement onset occurs prior to FT-KE onset. This difference may be related to the greater maximum hip angle in the caudal as compared with the rostral and pocket scratches. As the hip extends in the caudal scratch, biarticular muscles (e.g., triceps femoris pars iliotibialis) may be stretched across the hip, creating a passive pull at the knee that initiates knee extension. Knee extension onset in the caudal scratch may also be related to offset of the knee flexor, iliofibularis.

Based on observations of movement, Stein (1983) hypothesized that the following behaviors would have similarities: forward swim and rostral scratch; forward stepping and pocket scratch; and backpaddle and caudal scratch. Comparisons of FT-KE onset phase among the scratch and locomotor forms confirm and supplement these speculations, as FT-KE onsets are similar for: forward swim, rostral scratch, and the swing phase forward step burst; pocket scratch and the stance phase forward step burst; and caudal scratch and backpaddle. Thus FT-KE phase alone cannot be used to discriminate among all six forms studied. The similarities in FT-KE timing across forms support the possibility of shared circuitry among the different behaviors. Forms with similar timing of FT-KE onset may have greater overlap of circuitry than forms with differences in FT-KE timing.

EMG amplitudes as a form discriminator

Forms that cannot be distinguished from one another based on number of FT-KE bursts and/or FT-KE onset phase can be distinguished from one another based on EMG amplitudes. For example, forward swim and rostral scratch have similar FT-KE onsets, but forward swim has high-amplitude hip extensor activity and low-amplitude hip flexor activity compared with rostral scratch (Earhart and Stein 2000; Juranek and Currie 2000; Stein and Johnstone 1986). Backpaddle and caudal scratch have similar FT-KE onsets, but backpaddle has high-amplitude hip flexor activity and low-amplitude hip extensor activity compared with caudal scratch. High-amplitude EMG activity during each of the behaviors is associated with generation of force against the substrate.

Differences in amplitude between behaviors with similar FT-KE phasing may be produced by changes in interneuronal recruitment from classes active during both behaviors. A subset of neurons may be active during each behavior, and additional neurons may be recruited during the behavioral event of each to increase amplitudes during the time when force is exerted against a substrate. Such recruitment has been demonstrated in Xenopus, where additional interneurons not active in low-frequency swimming are recruited during struggling and during high-frequency swimming (Sillar and Roberts 1993; Soffe 1993).

Shared elements: presence and extent of overlap among circuits

Experimental evidence in recent years supports changes in our concepts of pattern generator organization (Stein et al. 1997). Pattern generators, once viewed as a static, unshared networks exclusive to single behaviors, are now viewed as dynamic, shared structures that can be reconfigured in multiple ways to produce different behaviors (Dickinson 1995; Getting 1989). In the turtle, studies support sharing between left and right side circuitry controlling the three scratch forms (Berkowitz 1999; Berkowitz and Stein 1994a,b; Currie and Gonsalves 1997, 1999; Field and Stein 1997b; Stein et al. 1986a, 1995, 1998a,b). There is also evidence for sharing of circuitry between rostral scratch and forward swim (Earhart and Stein 2000; Field and Stein 1997a; Juranek and Currie 2000; Stein and Johnstone 1986).

We provide evidence for similarities among the three forms of scratch and the three forms of locomotion, supporting the concept of shared circuitry among these six behaviors. We suggest that the behaviors represent a range of parameters, as evidenced by movement durations, monoarticular knee extensor phases, and EMG amplitudes. There may be some basic level of sharing among all six behaviors, with more extensive overlap of the networks controlling behaviors with motor pattern timing similarities (e.g., rostral scratch, forward swim, and forward step) and less overlap of the networks controlling behaviors with motor pattern differences (e.g., forward swim and caudal scratch). Future studies examining other naturally occurring behaviors could provide further support for sharing among networks. For example, bottom walking, an additional form of aquatic locomotion, appears to be a naturally occurring hybrid of forward step and forward swim behaviors (Zug 1971) and is worthy of further investigation. Additional studies using single-unit recording techniques will likely reveal neurons active during subsets of the six forms studied here, during all of these six forms, and perhaps during all six forms as well as other turtle hindlimb behaviors not described in this study.


    ACKNOWLEDGMENTS

We thank Dr. Gavin Perry for software development.

This work was supported by National Institutes of Health Grants R01-NS-30786 and T32-HD-07434 and by a Promotion of Doctoral Studies Award from the Foundation for Physical Therapy, Inc. to G. M. Earhart.

Present address of G. M. Earhart: Neurological Sciences Institute, Oregon Health Sciences University, 1120 NW 20th Ave., Portland, OR 97209.


    FOOTNOTES

Address for reprint requests: P.S.G. Stein, Dept. of Biology, Washington University, St. Louis, MO 63130 (E-mail: stein{at}biology.wustl.edu).

Received 1 May 2000; accepted in final form 28 July 2000.


    REFERENCES
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

0022-3077/00 $5.00 Copyright © 2000 The American Physiological Society