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Characteristics of medial-lateral postural control while exposed to the external perturbation in step initiation

Controllability of posture in the medial-lateral direction is critical for balance maintenance, particularly in step initiation. The objective of the current study was to examine the effects of external perturbation and landing orientation on medial-lateral control stability in step initiation. Eleven young healthy participants stood on the force platform and waited for the instruction of taking a step while experiencing a pendulum perturbation applied at the lateral side of the right shoulder. Eight experimental conditions were conducted by two levels of step side (right or left), two levels of perturbation (with or without), and two levels of landing orientation (forward or diagonal). The center of pressure (COP), pelvic movements, and muscle activities were recorded and analyzed as the onset of COP and pelvic movement, the COP displacement, and cocontraction and reciprocal muscle activation pattern. The temporal events of COP and pelvic movement were not significantly different in all experimental conditions. However, COP and pelvic movement were significantly later in the diagonal condition. Most of the segments showed reciprocal muscle activation patterns in relation to the perturbation released time. Subsequently, all segments showed cocontraction muscle activation patterns, which was significantly affected by step side, perturbation, and orientation. The results suggest that how the CNS initiated a step was identical with the COP then pelvic movement. The outcome highlights the importance of external perturbation and foot landing orientation effects on postural adjustments, which may provide a different approach to help step initiation.

The lateral perturbation brought about by external impact causes side-specific postural control between the perturbed and contralateral side 13,14 . The correlated co-contraction and reciprocal activation of the trunk, thigh, and shank segments suggest that the CNS can modify postural muscle activation according to the perturbed and contralateral side of the body 13 , or a need to maintain body stability in the ML direction 14 . Corresponding postural control is also shown in the smaller COP displacements in the ML direction in response to the perturbation 14,15 . Together, step initiation or the lateral perturbations of the previous studies were all imposed as a single perturbation to balance maintenance. Effects of a single perturbation on postural adjustments have been well documented. Changes in muscle activities and COP are used to represent corresponding postural adjustments in response to the internal or external perturbations. When handling two perturbations simultaneously, instead of compromising the acceleration of arm flexion movement, the CNS recruits additional postural muscle activity to stabilize posture and reduce the COP displacement for the translational perturbation from the lower limb 16,17 . This indicates that strategies are adjusted based on the level of challenge to posture and balance, i.e. based on the context.
The external perturbation (pendulum impact) in the anterior-posterior direction during step initiation was shown to be a side-specific muscle coordination associated with both asymmetric perturbation and corresponding stance or swing leg side 18 . In spite of disturbed balance in the anterior-posterior direction, capacity of controlling ML postural stability has been associated with falls in older adults 19,20 . Step initiation 1,19 or the lateral perturbation 13,14 threatens postural stability in the ML direction and alters COP displacement. However, the external pulling system at the waist level could potentially improve step initiation in older adults 21 and Parkinson's patients 22 . This implies that the lateral perturbation might either assist or hinder movement depending on the impact of the swing or stance leg during step initiation. The objective of the current study was to investigate how medial-lateral perturbation (a pendulum would impact at the shoulder level on the right side of the body) affect postural muscles activity and COP in step initiation. We first hypothesized that muscle activity would be affected and modified accordingly, when taking a left or right step when a lateral perturbation applied at the right shoulder. Secondly, changes in foot landing orientation might affect the COP displacement and postural muscle activity of shank, thigh, and trunk segments.

Methods
participants. Eleven young volunteers (5 males, 6 females, age = 28.09 ± 4.35 years, height = 1.67 ± 0.07 m, mass = 71.09 ± 18.75 kg) with the right dominant legs participated in the experiment. All participants were free from any musculoskeletal disorder and neurologic disease that could affect performing the experimental tasks. The project was approved by the Institutional Review Board at the University of Illinois at Chicago I All participants provided written informed consent before taking part in the experimental procedures. The experiment was performed in accordance with relevant guidelines and regulations. procedure and instrumentation. The participants were instructed to stand still with their feet shoulder width apart and even body weight distribution between the left and right foot to the best of their effort on a force platform and waited for the experimenter's instruction. Feet position was marked on the force platform to reproduce across the trials. A pendulum with a 30 cm long wooden stick and a flag was used to induce perturbations at the shoulder level on the right side of the body and allowed the participants to see the approaching pendulum via peripheral vision without a need for head rotation shown in Fig. 1 18 . The length of the central rod of the pendulum was adjusted to each individual's shoulder height. A load (5% of the individual's body mass) was attached to the pendulum next to its distal end. The pendulum was positioned at an initial angle of 30° to the vertical (0.8 m from the body) and was released by an experimenter 1 to 2 seconds after the start of each trial data collection. Perturbation consisted of unidirectional force applied by the pendulum to the lateral side of the right shoulder. The experimenter would then give the instruction to move the right or left foot forward or diagonal (45 degree outward) to step on the white straight-line mark. The distance between the original standing position to the white straight-line mark was adjusted to each individual's step length. Two practice trials of initiating a step with the pendulum impact were performed prior to data collection. There were eight conditions (left or right foot, diagonal or forward step, and with or without perturbation). Three trials were collected in each condition. The order of the conditions was randomized across participants.
Two accelerometers were used in the experiment. The pendulum's accelerometer (model 208CO3, PCB Piezotronics Inc, USA) was attached to the pendulum and its signal was used to determine the timing of the pendulum released and impact. The accelerometer worn by the participant (model 1356a16, PCB Piezotronics Inc, USA) was attached to the dorsal surface of the participant at the level of L5S1 to detect the temporal events of the pelvic movement. Ground reaction forces and moments of forces were recorded using the force platform (Model OR-5, AMTI, USA). EMG of muscles were recorded using disposable surface electrodes (Red Dot, 3 M, USA). After cleaning the skin with alcohol prep pads, the electrodes were attached to the bilateral muscle bellies of tibialis anterior (TA), medial gastrocnemius (MG), rectus femoris (RF), biceps femoris (BF), gluteus medius (GM), external oblique (EO), rectus abdominis (RA), and erector spinae (ES). The selected muscles have been used in previous studies of step initiation 23,24 and anticipatory and compensatory postural adjustments involving externally induced body perturbations 15 . The placement of electrodes was based on recommendations reported in the literature 25 and the interelectrode distance was 2 cm. EMG signals were band-pass filtered (10-500 Hz) and amplified (gain 2000) using the EMG system (Myopac, RUN Technologies, USA). The accelerometer signals, forces, moments of forces, and EMGs were synchronized and digitized with a 16-bit resolution at 1,000 Hz by means of an analog-to-digital converter and customized LabVIEW 8.6.1 software (National Instruments, USA). Data processing. All data were processed offline using MATLAB software (MathWorks, Natick, MA, USA).
The signal from the pendulum's accelerometers was used to determine the time of pendulum release (T release ) following the verbal command and the time of impact (T impact ). The signal from the accelerometer worn by the participant was used to detect the timing of the movement onset that the pelvic started to move in the up-down (Pelvicy X-onset ), the left-right (Pelvic Y-onset ), and the forward-backward (Pelvic Z-onset ) directions. The onsets of the accelerometer signals were detected using the Teager-Kaiser onset time detection method 26 . The ground reaction forces and the moments of forces were filtered with a 20 Hz low-pass, 2 nd order, zero-lag Butterworth filter. Time-varying COP traces in the anterior-posterior and medial-lateral directions were calculated using the approximations described in the literature 27 . The onset of the COP moved away from the baseline was detected by the Teager-Kaiser method (COP AP-onset and COP ML-onset ). For magnitudes of the COP displacement, two posterolateral COP points were identified on the swing and stance legs, respectively. The first posterolateral COP on the swing leg was determined as COP ML1 in the ML direction and COP AP1 in the AP direction. The second posterolateral COP on the stance leg was determined as COP ML2 in the ML direction and COP AP2 in the AP direction. The first distance was compared to the baseline point (0, 0) and calculated as COP ML1-distance (COP ML1 -0) and COP AP1-distance (COP AP1 -0). The second distance between two posterolateral points was also calculated as COP ML2-distance (COP ML2 -COP ML1 ) and COP AP2-distance (COP AP2 -COP AP1 ). All variables were calculated for each trial and averaged over three trials.
All EMG data were high-pass filtered at 20 Hz, full-wave rectified, and low-pass filtered as linear envelope at 2 Hz (2 nd order Butterworth) 28 . Subsequently, the Teager-Kaiser method was used to detect the onset of muscle activity for individual muscle (EMG onset ). The integrals of EMG activity of all studied muscles (∫EMGs) were calculated during the four phases: 1) 50 ~ 250 ms, 2) 250 ~ 450 ms, 3) 450 ~ 650 ms, and 4) 650 ~ 850 ms in relation to T release (0 ms). These selected four phases were according to 50 ms of the postural response to the instruction cue 29 , a 200 ms window of postural adjustment evaluations 4,30 and COP shifting between the stance and swing legs during step initiation 3 . Moreover, ∫EMGs of the baseline activity during the 200 ms time window (0-200 ms) were obtained at the beginning of the trial.
Two approaches were used to evaluate changes in muscle activity during each phase. One approach was to eliminate effects of each muscle baseline activity by reduction of ∫ baseline; as a result, activation of muscles was described by values larger than zero (∫EMG > 0) and inhibition by values smaller than zero (∫EMG < 0). Thus, the ∫EMG were normalized by ∫EMGmax, which was the maximum value throughout all experimental trials for each muscle for each phase, separately as: Subsequently, the second approach was based on calculations of the sums and differences between normalized ∫EMG values of ventral and dorsal muscles for the shank, thigh and trunk in each phase, separately.
C indexes were calculated as the sum of the integrals of activity of the antagonist-agonist muscle pairs to represent co-activation and R indexes as the difference between the integrals of muscle activity in the muscle pairs to represent reciprocal activation 31 .
Thus, the C and R values for the shank segment in the 50~250 phase were calculated as:~∫ All variables were calculated for each trial and averaged over three trials.

Statistics.
A three-way repeated-measures ANOVA were performed with three factors: step (2 levels: right step and left step), perturbation (2 levels: with and without perturbation) and orientation (2 levels: forward and diagonal) on the onset and EMG integrals of four phases for individual muscles. A two-way repeated measures ANOVA was performed with two factors: step (2 levels: right step and left step) and orientation (2 levels: forward and diagonal) on a duration between T release and T impact . Three-way repeated measures ANOVA was performed with three factors: step (2 levels: right step and left step), orientation (2 levels: forward and diagonal), and perturbation (2 levels: with and without perturbations) on temporal variables (Pelvic X-onset , Pelvic Y-onset , Pelvic Z-onset , COP AP-onset , and COP ML-onset ), and on the COP distances (COP ML1-distance , COP AP1-distance , COP ML2-distance , and COP AP1-distance ).
In addition, identifications of either cocontraction (C) or reciprocal (R) activation pattern were employed in each phase for each segment. The EMG integrals of muscle coupling for the shank, thigh, and trunk segments were compared using C and R values. Paired sample t tests were used to determine that the significantly larger C values compared to R values indicated the presence of co-contraction in a segment. When ventral and dorsal muscles were activated (larger than the baseline), calculations from both positive values of ventral and dorsal muscles revealed higher C value than R value and vice versa. If R values were significantly larger as compared to C values, this would indicate a prevalence of reciprocal activation 13 . Subsequently, if a muscle coupling of the shank, thigh and trunk segments had C value larger than R value, three-way repeated measures ANOVAs were performed with three factors: step (2 levels: right step and left step), orientation (2 levels: forward and diagonal), and perturbation (2 levels: with and without perturbations) to evaluate the C rather than R, and vice versa.
Post hoc comparisons were performed using Tukey's Honest Significant Difference test for significant interactions. Statistical difference was set at p < 0.05. Means and standard errors are presented in the results and figures.

Results
The duration between T release and T impact was not significantly affected by the factors of step and orientation. The duration was not different across four conditions (2 level steps and 2 level orientation) and the mean duration was 552.11 ± 11.01 ms. Furthermore, the data averaged across eight conditions (2 level steps, 2 level orientation and 2 level perturbations) and presented in the temporal sequence was that COP ML-onset at 156.60 ± 11.57 ms and followed by COP AP-onset at 191.01 ± 13.52 ms, Pelvic X-onset at 233.17 ± 14.87 ms, Pelvic Y-onset at 233.57 ± 13.29 ms, and Pelvic Z-onset at 235.61 ± 13.08 ms in relation to T release . Repeated measures ANOVA revealed that the factor of orientation significantly affected the onset time of both COP and body movement (Table 1). Figure 2 illustrates that COP and pelvic movement initiated slower in the diagonal conditions than the forward conditions, except COP ML-onset .
The two representative points of COP, the first posterolateral COP on the swing leg (COP ML1 and COP AP1 ) and the second posterolateral COP on the stance leg (COP ML2 and COP AP2 ), are illustrated in Fig. 3 for all experimental conditions. In the left step conditions, COP moved towards to the left side (1 st point in Fig. 3A,C) then shifted towards to the right side (2 nd point in Fig. 3A,C). In the right step conditions, COP moved towards to the right side (1 st point in Fig. 3B,D) then shifted towards to the left side (2 nd point in Fig. 3B,D). Overall averaged cross the forward, diagonal, with, and without perturbation conditions, COP ML1 was 0.045 ± 0.009 m, COP AP1 was −0.031 ± 0.006 m, COP ML2 was −0.074 ± 0.005 m, and COP AP2 was −0.051 ± 0.006 m. COP showed the opposite (2019) 9:16817 | https://doi.org/10.1038/s41598-019-53379-9 www.nature.com/scientificreports www.nature.com/scientificreports/ direction in the right step conditions, COP ML1 was −0.040 ± 0.007 m, COP AP1 was −0.030 ± 0.004 m, COP ML2 was 0.089 ± 0.007 m, and COP AP2 was −0.049 ± 0.007 m.
In addition, Fig. 4 shows the C and R values in all the experimental conditions of each segment for each phase. First, the patterns of muscle co-contraction (C) and reciprocal (R) activation were identified for each segment. In the 50~250 ms phase, similar R and C values for the Trunk R and Thigh R segments and the Shank L segment were observed. While, significant larger R values in the Shank R and Trunk L segments with a significant larger C value in the Thigh L segment were observed. In the subsequent phases, significant larger C values were observed for all segments compared to R values. The large C values indicate co-contraction of muscle in all segments after 250 ms in relation to T release .
Further examinations of co-contraction and reciprocal activation of muscles were conducted by three-way repeated measures ANOVA and revealed significant main effects on all segments in the four phases ( Table 2). The factor of step significantly affected C values on the Shank L segment (50~250 ms phase and 250~450 ms phase), Shank R segment (250~450 ms phase and 650~850 ms phase), Thigh R segment (450~650 ms phases), Trunk L segment (650~850 ms phase), and Trunk R segments (250~450 ms phase and 450~650 ms phases). Figure 5 illustrates the effects of three factors and letters within the bars represent the corresponding experimental conditions for the significant main effect of the factor. The larger C values were observed in the left step conditions than the right step conditions, except the Shank R segment in the 250~450 ms phase. Additionally, the factor of perturbation   www.nature.com/scientificreports www.nature.com/scientificreports/ significantly affected C values of bilateral segments in the 250~450 ms, 450~650 ms, and 650~850 ms phases. All C values were significantly larger in the with perturbation conditions than the without perturbation conditions. The factor of orientation significantly affected C values on the Trunk L and Shank R segment in the 450~650 ms phase and showed the larger C values in the forward condition than the diagonal conditions.

Discussion
The current study is the first to investigate how external-lateral perturbations and landing orientation affect the COP displacement in left or right step initiation. The sequence of events observed were: COP ML-onset , then COP AP-onset , followed by Pelvic X-onset , Pelvic Y-onset , and Pelvic Z-onset with respect to T release . The factor of step affected the directional deviation of COP in the ML direction, not the AP direction. While the factor of orientation affected the onset time of COP in the AP direction and pelvic movement. The COP distances from the swing leg to the stance leg were affected by the factor of perturbation and orientation. The shorter COP distance was observed in the with perturbation condition and in the diagonal condition. Reciprocal activation patterns were observed on the left trunk and right shank segment in the 50~250 phase. After 250 ms in relation to T release , co-contraction patterns were observed and significantly affected by the factor of perturbation on bilateral shank, thigh, and trunk segments.  www.nature.com/scientificreports www.nature.com/scientificreports/ temporal events in step initiation. The synchronization timing was set at the time of the pendulum release (T release ) and the given instruction. After T release , the initiations of COP then pelvic movement were observed in all experimental conditions. The little variability in terms of duration indicated that the participants were aware of and impacted by the pendulum almost around the same time in the perturbation conditions. The sequence of temporal events was the same as in previous studies 9,10 and indicates that participants did not assume to make a left or right step before the instruction. In addition, the onset of COP in the ML then AP direction suggested the right or left step would be decided first and followed with body weight shifting on the swing leg in step initiation. Subsequently, the timing of the first and second posterolateral points were observed at 402.46 ± 28.40 ms and 686.86 ± 72.27 ms in relation to T release averaged over all conditions, respectively. It was confirmed that the external-lateral perturbation was imposed (at 552.11 ± 11.01 ms) in the period of body weight shifting between the swing and stance leg during step initiation 2,3 .

Effects of step.
The non-significant different onset of COP and pelvic movement between the right and left step conditions indicated that participants expected instructions to move to the right or left step first equally. The first and second posterolateral points were observed on the left and right sides in the left step condition, respectively and vice versa in the right step condition (Fig. 3). It confirmed that COP first shifted on the swing leg then toward the stance leg 2,3 . In addition, the first and second COP distances in both directions were not different between the left and right step conditions. These similar points and distances might support the idea that participants indeed stood with even body weight distribution between the left and right foot at the beginning of the trials.
Side-specific differences in muscle activation patterns between left and right segments was not observed in all experimental conditions. However, the C values were shown to be different for bilateral segments between the right and left step conditions (Fig. 5 & Table 2). The framework of C-and R-commands controlling a segment to effectively change muscle co-contraction if moving the endpoint of a segment while R-command will be more pronounced with no or small effects on the endpoint in response to the perturbations 32,33 . When the significant larger C value was observed, it indicated that coupling muscles were both activated and participants utilize co-contraction strategy to increase stability for the perturbations 13,34 . Taking the right step condition as an example, the right shank segment was the swing leg and increased TA muscle activity for the first posterolateral   www.nature.com/scientificreports www.nature.com/scientificreports/ COP distance 35,36 . This increased TA muscle activity was also used to prepare the forthcoming external lateral perturbation, which was anticipatory postural adjustment in relation to the perturbation 14 . Subsequently, in the perturbation impact phase, the larger C values were observed on the right thigh and shank segments in the left step condition. In the left step condition, right thigh and trunk segments were the stance side and increased muscle activities for loading weight 36 as well as for the perturbation 14 . On the contrary in the right step condition, right segments were the swing side and activated fewer muscle activities compared to acting as the stance leg in the left step condition.
Effects of the external-lateral perturbation. The external-lateral perturbation was imposed at the right shoulder level during the phase of weight shifting from the swing leg to the stance leg. The orientation of the COP trajectory and the perturbation were in the same direction in the right step conditions and showed the opposite direction in the left step conditions. When the left leg was the stance side (the right step condition), COP ML2-distance was smaller in the with perturbation condition (0.121 m) compared to the without perturbation condition (0.136 m). It might be comparable with the waist-pulling cable system for step initiation and showed improvements of decreasing weight transfer time in older adults 21 and Parkinson's patients 22 along with the pulling direction. Decreases in the horizontal distance has been observed in older adults with Tai Chi exercise and showed improvements of effective postural stability 2 . Similarly, the perturbation in the current study was like a push to help to shorten the ML COP distance in the right step condition. In the left step condition, COP ML2-distance was around 0.119~0.122 m regardless of the occurrence of the perturbations and smaller than the right step condition. When the right leg was the stance leg, the time of COP moved to the stance leg was close to the time of perturbation impact. Hence, participants attentively pulled themselves away from the side of the potential forthcoming perturbation and resulted in the same effects of the perturbation directly impacted the body.
In the current study, participants might expect the forthcoming pendulum due to it was released while the instruction but not actually impacted at the shoulder level in the without perturbation condition. Bilateral segments showed the same muscle activation patterns in the without and with perturbation conditions and acted as the early postural adjustment to the external-lateral perturbation 32,37 . Once participants were aware that the trial was without the perturbation, the C values of all segments were significantly smaller compared to the with perturbation condition. The muscle activities in the without perturbation condition represented their performance for step initiation and differences between two conditions indicated additional muscle activities for handling the external-lateral perturbation. The current outcomes supported the first hypothesis that the CNS increased corresponding muscle activities with cocontraction in the APA and CPA phase in relation to the predictable and asymmetrical perturbation 13,14 . Effects of landing orientation. There was a significant difference in onset of COP and pelvic movement between the forward and diagonal conditions, except COP in the ML direction. After initiating the COP displacement on the swing leg, COP shifted further lateral on the swing leg when intending to land the step diagonally. It was not only reflected by the position of COP ML1 (Fig. 3) but also the magnitude of COP ML1-distance . It was comparable with the ML displacement of the S1 period 1 with a shorter distance while COP reached to the www.nature.com/scientificreports www.nature.com/scientificreports/ first posterolateral point faster in the diagonal condition. Meanwhile, the shank and trunk segments showed less co-contraction in the diagonal condition. Together this may indicate that the CNS require more time to prepare COP movement and reduce muscle contractions for step landing diagonally. It was supported by the fact that young adults showed further ML COP displacement on the swing leg and finally making a quick step 38 as well as the larger step widths observed when balance is challenged by external perturbations 39 . Foot placement controllability in pathological gait 40 might also explain that difficulties of diagonal step initiation during the postural phase in Parkinson's disease 41,42 . conclusions In step initiation, the sequence of temporal events is from standing, COP shifts in ML direction to determine the swing and stance legs, and moves backward then following with pelvic movement. This sequence is consistent with the step side as well as landing orientation and not compromised by external perturbations. Instead, the COP distance and muscle co-contraction levels of segments were adjusted according to the appearance of perturbations and landing orientation to enhance postural stability. Control of step initiation through muscle coordination and foot placement might be benefits to individuals with gait stability difficulties and provide different approaches for developing rehabilitation programs.