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The Journal of Neurophysiology Vol. 79 No. 5 May 1998, pp. 2329-2337
Copyright ©1998 by the American Physiological Society
1 Department of Physical Therapy and 2 Division of Neuroscience, University of Alberta, Edmonton T6H 2G4, Canada
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ABSTRACT |
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 Abstract
 Introduction
Methods
Results
Discussion
References
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Yang, Jaynie F., Marilee J. Stephens, and Rosie Vishram. Transient disturbances to one limb produce coordinated, bilateral responses during infant stepping. J. Neurophysiol. 79: 2329-2337, 1998. Transient disturbances were applied to the lower limbs of infants (3-10 mo of age) while they were supported to stepped on a treadmill. The aim was to determine how stepping infants respond to novel disturbances that would disrupt equilibrium during independent walking. Their responses were also compared with those from lower mammals and adult humans. In the first series of experiments, the motion of the limb in the swing phase was transiently stopped by the experimenter grasping the limb for a short time (0.1-1.7 s). During such disturbances, the stance phase was prolonged in the contralateral limb, and the onset of the swing phase was delayed. The degree to which the stepping was modified in the contralateral limb depended on the amount of load experienced by that limb. If the contralateral limb was bearing very little weight at the time of the disturbance, its rhythm did not change appreciably. In the second series of experiments, load was added to the infant by pushing down on the pelvis during the stance phase. This greatly prolonged the stance phase and delayed the swing phase. It did not increase the amplitude of the extensor electromyogram (EMG) of the loaded limb. In conclusion, the neural circuitry controlling stepping in the infants responds to disturbances in an organized fashion that is conducive to maintaining equilibrium and forward progression.
Infant stepping exhibits many of the characteristics of adult walking (Yang et al. 1998 Subjects
Twenty-one infants were studied. None could walk independently. Twelve were studied during disturbances applied in the swing phase and 12 during loading of the stance phase. Three were studied during both types of disturbances. The infants ranged in age from 3 to 10 mo (7.2 ± 1.9 mo, mean ± SD). Infants were recruited through the maternity wards of hospitals, and the public health division of Capital Health in Edmonton. Ethical approval was obtained from the appropriate facilities. A parent provided informed, written consent for the infant to participate in the study. Only healthy babies, born at or after 32-wk gestation were included.
Recording procedures
All subjects were weighed on an infant scale or on the force platform, depending on where the experiment was performed (i.e., laboratory with or without Gaitway treadmill system). Beckman type surface electromyographic (EMG) electrodes were placed over four muscle groups in the lower limbs in either of the following combinations: quadriceps (quads) and tibialis anterior (TA) of both limbs, gastrocnemius-soleus and TA of both limbs, or quads, hamstrings, gastrocnemius-soleus, and TA of the left limb. Generally, miniature electrodes (2 mm recording diameter) were used for the lower leg, and regular electrodes (7 mm recording diameter) were used for the thigh muscles, except for the very young infants, in which miniature electrodes were used on all muscles. The skin was cleaned with rubbing alcohol before application of electrodes. The electrode pairs were separated by ~1 cm.
SWING PHASE DISTURBANCES.
Once good sustained stepping was obtained, disturbances were randomly applied by hand to the right lower extremity. The right limb was stopped momentarily during its flight, either in the first or second half of the swing phase. The duration of this disturbance was varied between 0.1 and 1.7 s. The duration was estimated from the video record, with an accuracy of 33 ms.
STANCE PHASE DISTURBANCES.
Trials were also performed in which the limb was not disturbed during the swing phase, but additional load was applied instead during the stance phase, to examine the effect of load alone. Sudden loading was applied by having the infant bear more of her/his own weight, as controlled by the experimenter supporting the infant, or by another experimenter pushing down transiently on the pelvis. Loads were always applied transiently during the stance phase of one step. The amount of load added could not be controlled exactly. Because the FSRs only provided qualitative information about loading, five subjects were studied in another laboratory, where a Gaitway treadmill system (Kistler Instruments, Amherst, NY) with a force platform embedded under the treadmill belt, was available (courtesy of Dr. Brian Andrews). In these experiments, the exact amount of force could be estimated. FSRs were not used in these experiments, because the force platform provided information on foot contact.
Data analysis
A hard copy of the raw data were printed on a chart recorder, and the sequences of good data corresponding to the video record were identified. The EMG data were full-wave rectified and low-pass filtered at 30 Hz, then A/D converted together with the FSR (or force platform), goniometer, and synchronization pulse data at 250 Hz (Axotape, Axon Instruments, Foster City, CA).
SWING PHASE DISTURBANCES.
In the initial analysis, all swing phase disturbances were included in the analysis, so long as they 1) did not cause the infant to slip, 2) were preceded and followed by good steps, 3) did not contain disturbances that extended over more than one step cycle, and 4) did not cause both limbs to leave the treadmill (i.e., not airstepping). The number of successful right limb disturbances ranged from 1 to 14 per subject (mean, 4.4; median, 4). A total of 58 disturbances fit the criteria set out above.
LOADING DISTURBANCES.
Trials in which the infant was transiently loaded were analyzed in a similar way. The duration of the load was estimated from video. Response to loading was quantified by comparing the duration of the disturbed step cycle to that preceding and following it. In some trials, the stepping rhythm stopped after load was applied. These trials were analyzed separately. For the experiments carried out on the Gaitway treadmill, the force platform was calibrated with known weights. Four force-sensitive cells, one at each corner of the force platform, measured vertical force. The total force was the sum of the four signals. The amount of load added in each loading disturbance was estimated as follows. The average force over the stance phase was estimated for the disturbed step and the preceding undisturbed step. The difference in force between these two stance phases represented the amount of load added during the disturbance. The amplitude of the extensor EMG (either gastroc-soleus or quads) and their burst duration during loading was compared with that in undisturbed walking. Only subjects who showed extensor EMGs free of artifact and clear FSR signals were used. The average EMG amplitude during the stance phase was calculated by dividing the area under the rectified and smoothed EMG signal during the stance phase by the duration of the stance phase. The stance phase was defined by the FSR or force platform signal indicating foot contact with the ground. The burst duration of the extensor EMGs was estimated visually. Both the amplitude and duration estimates from the EMG signal were analyzed using custom-written programs (MATLAB, MathWorks, Natick, MA).
Statistical analysis
A repeated measures analysis of variance (ANOVA) was used to compare the durations of the pre-, during, and postdisturbance steps at a significance level of 0.05. Bonferroni t-test was used to compare the data post hoc. The level of significance for the post hoc tests were adjusted to 0.017 to guard against an increase in type I errors with multiple comparisons (Myers 1979 Disturbing the motion of the swing limb
When the right limb was suddenly stopped during the middle of the swing phase, the rhythm in the contralateral limb was altered. Figure 1 shows stick figures of the contralateral (left) limb during the step preceding the disturbance, and the disturbed step. The most extended position achieved in each step is also shown. The right limb was held fixed for 500 ms during the swing phase in this disturbance. Note that the left limb reaches a much more extended position during the disturbed step. Extension occurs mostly at the metatarsophalangeal joint, but also at the hip and ankle (9 and 16°, respectively, in this case). EMG activity from the same trial is shown in Fig. 2. There was a prolongation of extensor activity (L Quad), and a delay in the onset of the flexor activity (L TA).
Transient loading of the stance limb
Transient loading during the stance phase also resulted in a prolongation of the stance phase and a delay in the onset of the swing phase (Fig. 6). Note the prolongation of the gastrocnemius-soleus (GS) EMG, and the delay in onset of the TA EMG (Fig. 6A). The FSR signals from both feet (Fig. 6B) further show that while the left stance phase was prolonged, the right swing phase was also prolonged, so that the two limbs remained 50% out-of-phase with each other. The duration of the step cycle and the stance phase are shown for 11 of the 12 subjects in Fig. 7, A and B. In the remaining subject, loading always stopped the stepping rhythm, so that the duration of the step with added load was very long, and there were no post disturbance steps. Both the stance phase and the cycle duration of the disturbed step were significantly prolonged compared with both the pre- and postdisturbance steps. The pre- and postdisturbance steps were not significantly different from each other (Bonferroni t-test, post hoc).
The data show that infant stepping is highly responsive to sensory input from the periphery. Transient disturbances applied to one limb in the swing phase affect the rhythm in the contralateral limb. Load added during the stance phase prolongs the duration of the step cycle. The responses to these external disturbances appear organized in a way that would facilitate equilibrium and forward progression during walking.
Characteristics of the response to disturbances
Halting the motion of the swing limb causes the contralateral leg to prolong ground contact. In bipedal walking, this could prevent falling. Similar disturbances in adults produce comparable responses (Dietz et al. 1986 Lower centers in the central nervous system contain details for how to respond to disturbances
Bernstein (1967) Comparison between the responses in adults and infants
The maturity and sophistication of the response in these infants can be estimated by comparing them to adults faced with similar disturbances. When the right limb was unexpectedly stopped during its swing phase and prevented from progressing into stance in these infants, the contralateral limb remained in the stance phase. For bipedal walking, this is an important strategy to maintain balance. Dietz et al. (1986) Summary
In summary, infants respond to unexpected disturbances during stepping in an organized fashion that appears to be suited for maintaining equilibrium and forward progression. All this occurs well before independent walking is possible. Some of the responses are similar to those seen in reduced cat preparations, whereas others show some similarity with those seen in adults.
INTRODUCTION
Abstract
Introduction
Methods
Results
Discussion
References
). For example, the muscle activation patterns show clear alternation between flexor and extensor muscles of the lower limb. Moreover, the infant is capable of adapting her/his walking to the speed of the treadmill belt in a way very similar to that seen in adults (Grillner et al. 1979), suggesting that sensory input from the periphery is used to modify the stepping rhythm in a rather mature way. Others have shown that coordination of the lower limbs remains unaltered even when each limb is driven by treadmill belts running at different speeds (Thelen et al. 1987), similar to that seen in adults (Dietz et al. 1994).
with Fouad and Pearson 1997). Can infants respond to transient disturbances during stepping in a coordinated manner, with appropriate changes in the muscle activity of both limbs? If so, are the responses conducive to maintaining equilibrium and forward progression?
; Hirschfeld and Forssberg 1994; Sveistrup and Woollacott 1996). A fully developed, adultlike pattern of postural control, however, emerges considerably later, in the second decade of life (Forssberg and Nashner 1982; Hirschfeld and Forssberg 1992).
; Nashner 1980). Moreover, the responses are especially pronounced in the stance limb, regardless of whether the disturbance is applied to the stance or swing limb (Dietz et al. 1986). Such well-coordinated responses are also seen in intact and reduced quadrupedal animals during walking. For example, electrical stimuli applied to one limb generates responses in both limbs in the cat (Duysens and Loeb 1980; Gauthier and Rossignol 1981). Moreover, mechanical disturbances, such as unexpectedly stepping in a hole, generate coordinated responses from both limbs to prevent the animal from falling (Gorassini et al. 1994; Hiebert et al. 1994).
), and an extensor plantar response remains (Peiper 1961). In the first series of experiments, the right limb was suddenly halted during the swing phase, and the response in the contralateral limb was examined. During the disturbance, the stance phase of the contralateral limb was prolonged. The magnitude of the prolongation, however, was dependent on the amount of weight bearing at the time of the disturbance. Thus, in another series of experiments, transient loads were applied during stepping, to determine how adding more load affected the overall step cycle. The results indicated that increased load during the stance phase prolonged the stance phase and delayed the swing phase. This pattern is similar to that seen in decerebrate or spinal cats (Duysens and Pearson 1980; Pearson et al. 1992). Such responses could assist the maintenance of equilibrium during walking. They are present before the infant can stand or walk independently.
METHODS
Abstract
Introduction
Methods
Results
Discussion
References
). They were asked to have the infant practice stepping for 1 or 2 min, twice a day (see Yang et al. 1998).
). Student's t-test was used to compare stepping in 1) practiced versus unpracticed subjects and 2) early versus late disturbances in the swing phase. Differences in the EMG amplitude and burst duration during the stance phase were compared for loaded versus normal steps using a paired t-test. All t-tests were performed at a level of 0.05.
RESULTS
Abstract
Introduction
Methods
Results
Discussion
References
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FIG. 1.
Stick figures from a step preceding (A) and during a disturbance (B) for the limb contralateral to the disturbance. The step starts from the stick figure at the far right when the foot 1st makes contact with the ground. Each sequential stick figure is shifted in position to the left to allow better visualization of the movement (interframe interval is 33 ms). The sequence ends at the far left with foot-floor contact. A sketch of the same limb showing the most extended position achieved in the late stance phase (corresponding stick figures indicated by arrows) is shown to the right. Note that the limb achieves greater extension during a disturbed step.
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FIG. 2.
Muscle activity and foot contact pattern from the same subject as that shown in Fig. 1, during the same disturbance of the swing limb. The rectified and smoothed electromyogram (EMG) from a knee flexor and an ankle extensor muscle of each limb are shown for the step preceding, during, and after a disturbance to the right limb. The force-sensitive resistor (FSR) data (in arbitrary units) for each limb is shown below the corresponding EMG traces. The motion of the right limb was halted for 500 ms during the swing phase (solid line at the top of the graph). The EMG activity of the right limb is considerably altered by the disturbance. Note that the contralateral limb (left) prolonged its extensor activity, and delayed the onset of flexor activity. Note that the FSRs do not indicate the step following the disturbance clearly, so the timing of foot-contact ( 
) and lift-off (
) has been added. The timing was derived from the video record.
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FIG. 3.
Muscle activity, foot-contact patterns, and goniometer signal from the right hip for one subject during disturbance of the swing limb (right) in stepping. Steps preceding, during, and after the disturbance are shown. The signal from the force-sensitive resistor (FSR) indicates foot contact with the ground when the signal is high (arbitrary units). The goniometer signal is represented in degrees, with zero degrees representing the neutral position (i.e., trunk and thigh aligned), and positive values representing flexion. Note that the hip angle never achieves the neutral position during stepping in this subject, reflecting the more flexed posture of infants. Note that during the disturbance to the right limb (duration shown by solid line at the top of the graph), the goniometer signal on the right hip shows a prolongation of the flexed position as a result of the disturbance. As in Fig. 2, the stance phase is prolonged on the contralateral (left) side while the flexor activity is delayed. Note also that when the right limb resumes stepping, it does so in coordination with the left side.
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FIG. 4.
Duration of the step cycles on the left side preceding, during, and after a disturbance to the right limb. The disturbances were applied during the swing phase on the right side. Averages across 12 subjects are shown with SE. The disturbed step was significantly different from the pre- and postdisturbance steps. The pre- and postdisturbance steps were not significantly different from each other.
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FIG. 5.
Examples from 2 subjects during disturbances when the weight borne on the contralateral side was very low. A: the disturbance was applied to the right limb, and started toward the end of the swing phase. At that time, weight support on the left was already very low (see L FSR). The disturbance did not delay the onset of the swing phase on the left side [left tibialis anterior (L TA)]. Note also that when the right limb resumed stepping, it did so in coordination with the left side. B: in this subject, a prolonged disturbance to the right swing limb over 2.8 s did not prevent the left limb from continued stepping. The foot-contact and lift-off times are indicated with solid downward and open upward triangles, respectively. The FSR signal is in arbitrary units. The EMG from the gastrocnemius-soleus (GS) is not as clear when weight support is low.
).
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FIG. 6.
Data from 2 subjects, showing the response to transient loading during the stance phase. Note that during the application of load (solid line at the top of the graph), the FSR signal is larger. Moreover, the extensor activity on the loaded side is prolonged and the flexor activity is delayed. Note also that in B, the swing phase was prolonged on the contralateral side during the disturbance, so that the 2 limbs remained 50% out-of-phase with each other. The FSR signals are in arbitrary units.
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FIG. 7.
Effect of load on the duration of the step cycle (A), the duration of the stance phase (B), the amplitude (C), and the duration (D) of the extensor EMG burst. A: the duration of the step cycle is shown for steps preceding, during, and after the application of additional load, averaged across 11 subjects (mean ± SE). The step with the added load is significantly longer (28%) than the steps preceding and after it. The pre- and postloading steps are not significantly different from each other. B: most of the change in the duration of the step cycle resulted from a prolongation of the stance phase. C: the EMG amplitude for steps with and without extra load were averaged separately across the stance phase for each subject. Pooled data across subjects are shown here (n = 4 for quadriceps and n = 6 for gastrocnemius-soleus). EMGs were not significantly different between normal steps and steps with added load. D: there was a trend for the EMG burst durations to be longer during loaded steps, but the difference was not significant.
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FIG. 8.
Data from a single subject during a loading disturbance applied to the right limb during the stance phase. These data were collected on the treadmill with a force platform. The EMG data are shown together with the data from the force platform. Foot-contact ( ) and lift-off () times are shown for the right foot at the bottom of the graph. Note the prolongation of extensor activity, and the delay of the onset of flexor activity. This was a trial with a particularly high loading force (48% of the subject's body weight). Note the increase in force during a disturbed step, and the clear demarkation of foot-contact in the force signal for all steps.
DISCUSSION
Abstract
Introduction
Methods
Results
Discussion
References
). An intriguing aspect of our results is the importance of afferent input from the ipsilateral limb in controlling that limb during walking. For example, when the movement of the swing limb was temporarily halted, the response in the contralateral stance limb was very dependent on the conditions in the stance limb. If the weight support was high, the response was clear. If the weight support was low, the effects were muted.
). Together, these results are consistent with those reported for spinal and decerebrate cats (e.g., Conway et al. 1987; Duysens and Pearson 1980; Gossard et al. 1994; Giuliani and Smith 1985; Pearson and Collins 1993; Whelan et al. 1995). In some of these studies, loading was simulated by electrical stimulation of the muscle nerve (Conway et al. 1987; Gossard et al. 1994; Guertin et al. 1995; McCrea et al. 1995; Pearson and Collins 1993; Pearson et al. 1992; Whelan et al. 1995) or the ventral root (Duysens and Pearson 1980; Pearson et al. 1992). In other studies, muscle force was increased by direct stretch of the extensor muscles (Duysens and Pearson 1980) or transient unweighting or weighting of the hindlimbs (Hiebert 1997). In all cases, increase in load prolonged the stance phase, whereas decrease in load shortened the stance phase.
,b). The results suggest that the circuitry in the spinal cord/brain stem for controlling how to respond to loads during walking is more potent in the infant. With maturation, changes occur that modify how adult humans respond to load. More modest effects from activating load-sensitive afferents were also reported for the intact cat (Whelan and Pearson 1997). Presumably, the influence of the cerebrum modifies the behavior of the spinal/brain stem circuitry. Indeed, recordings from cells in the motor cortex in intact cats suggest that a large proportion of these cells fire at the transition from the stance to swing phase, and may play a role in this transition (Armstrong and Drew 1984; Drew 1991).
). Detailed examination of the data suggests that the effect of hip position may have been quite weak, however. As seen in Fig. 1, under load, the hip was extended beyond the extension angle achieved in undisturbed steps, yet the flexor burst was still inhibited. Distinguishing the importance of hip position versus load on the limb will require further experiments.
suggested that the details of movement are organized at lower levels of the nervous system, to free the higher centers of such laborious tasks. With respect to walking, considerable evidence from animal work supports his idea (e.g., reviewed in Grillner 1981; Rossignol 1996). Some data from humans also suggest that the human spinal cord may be capable of generating the rhythmic movements of walking, although the evidence is less direct (e.g., Bussel et al. 1989; Calancie et al. 1994; Dietz et al. 1996; Holmes 1915; Kuhn and Macht 1948). The present study provides further evidence that the circuitry for stepping is operational in infants. It is probably controlled by the lower centers in the CNS (Forssberg 1985). Not only do these lower centers control the details for generating the rhythmic movements, but they also contain the circuitry for responding to unexpected disturbances in an organized, useful way, like that seen in spinal cats (Forssberg et al. 1975, 1977).
; Hadders-Algra et al. 1996; Hirschfeld and Forssberg 1994; Sveistrup and Woollacott 1996). Presumably, the circuitry for such behavior is later modified and refined by experience, as shown by others for other motor tasks such as postural control (Hadders-Algra et al. 1996; Sveistrup and Woollacott 1997) and precision grip (Eliasson et al. 1995; Forssberg et al. 1991).
provided a similar but much shorter disturbance (20-160 ms) to adults by holding the swing limb momentarily at different points during the swing phase with a cord attached to the lower leg. When the disturbance was applied early in the swing phase, the contralateral stance phase was prolonged, just as in infants.
) or weights applied to the body (Stephens and Yang 1996b), has little effect on the duration of the step cycle. Transiently adding loads equivalent to 30% of the adult's body weight causes an increase in the amplitude of the extensor EMGs, but prolongs the duration of the stance phase only slightly (3%). The same load has an even smaller effect on the cycle duration (0.5%) (Stephens and Yang 1996b). In contrast, load experienced by the lower limbs during the stance phase of infant stepping has a very powerful effect on the duration of the step cycle (28%, see Fig. 7), but no effect on the EMG amplitude. Based on the results obtained from the treadmill instrumented with a force platform, the loads applied to the infant were in the same range as those applied in the adult (22% of body weight for the infants and 30% body weight for the adults).
). In contrast, when load was applied during the stance phase in the adult in a very controlled and consistent way, there was no real danger to the individual or to their forward progression. Thus higher centers in the nervous system could diminish the influence of the spinal/brain stem system, increasing extensor EMG amplitude to avoid disruption of the stepping rhythm. In contrast, halting the swing limb threatens equilibrium and continued forward progression. In this case, it is functionally important that the adult also modify its stepping, to ensure stability. We do not know whether the response in the adult represents the behavior of similar spinal/brain stem circuits as in the infants, or whether supraspinal systems take over the control, and happen to generate a similar response.
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ACKNOWLEDGEMENTS |
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We thank the University of Alberta Hospitals, Royal Alexandra Hospital, Caritas Health Group, Public Health Division of Capital Health, and S. Gorgichuk and N. Doucette for assistance with recruitment of subjects. We thank Dr. B. Andrews for use of the Gaitway treadmill/forceplate systemand Dr. M. Piper for use of the infant weight scale. We thank Drs. J. Bobet, M. Gorassini, and R. B. Stein for helpful comments on earlier versions of this manuscript.
This work was supported by grants from the Medical Research Council of Canada and the Natural Science and Engineering Research Council of Canada to J. F. Yang. M. J. Stephens was supported by a studentship from the Alberta Heritage Foundation for Medical Research.
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FOOTNOTES |
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Address for reprint requests: J. F. Yang, Dept. of Physical Therapy, 250 Corbett Hall, University of Alberta Edmonton, Alberta T6G 2G4, Canada.
Received 3 October 1997; accepted in final form 21 January 1998.
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REFERENCES |
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Abstract
Introduction
Methods
Results
Discussion
References
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