Effects of different heights and task conditions on the biomechanical characteristics of children's lower limbs when descending steps

doi:10.21203/rs.3.rs-1981043/v1

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Effects of different heights and task conditions on the biomechanical characteristics of children's lower limbs when descending steps

https://doi.org/10.21203/rs.3.rs-1981043/v1

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Background: The ability of the human body to balance control is influenced by the cooperation of muscle strength and the nervous system. For steps of differing heights or different cognitive tasks, there are changes in neural control and sensory integration that change the balance control ability and movement control strategy of lower limbs. To explore the change rule of biomechanical characteristics of lower limbs of children with steps of differing heights and cognitive tasks, thereby revealing the characteristics and corresponding mechanisms of balance control when descending steps of differing heights and performing different cognitive tasks, thus providing much useful reference to aid the prevention and reduction of children's fall-related injuries from descending the steps.

Method: This study enrolled 20 volunteers from grades three to six who attend Southwest University Primary School (5 volunteers from each grade). Using kinematics (JVC9800 high-speed camera), dynamics (BIOFORCEN dynamic balance training evaluation system), electromyography (JE-TB0810 eight-channel electromyography tester), and other tools, the biomechanical parameters of step movements of subjects at differing step heights and performing different cognitive tasks were synchronously collected. The SPSS23.0 statistical analysis software was used to process the obtained parameters, and the significance level of all indexes was set as α = 0.05.

Result: 1. The angle of the knee and ankle joints at the first step under a single task were significantly less than those at the first step under a double task, and less than those at the third step while performing double tasks. 2. The changes of foot pressure (COP) root mean square of the left-right displacement under the first step and single-task step, root mean square of anterior-posterior displacement, total trajectory length of swing, and 95% confidence ellipse area were significantly < the COP root mean square of the left-right displacement under the third step and single-task step, the root mean square of the anterior-posterior displacement, the root mean square of swing trajectory length, 95% confidence ellipse area < the COP root mean square of the left-right displacement, the root mean square of anterior-posterior displacement, total swing trajectory length, 95% confidence ellipse area < the COP root mean square of the left-right displacement, the root mean square of anterior-posterior displacement, total swing trajectory length and 95% confidence ellipse area under the third step double-task step. 3. The mean EMG amplitude of biceps femoris, rectus femoris, tibialis anterior, and medial gastrocnemius from 200ms before touching the step to the moment the forefoot touched the step at the first step under a single task were significantly greater than those at the third step under a single task, which was also greater than those at the first step under double tasks, which was greater than those at the third step under double tasks.

Conclusion: Different step heights and cognitive tasks hindered the transmission of central nervous system information, reduced the expected posture adjustment of the body, and affected the pre-activation of lower limb muscles when descending steps, thereby affecting children's ability to balance control, and gait stability, thus increasing the risk of the child's fall.

step test

cognitive task

kinematics

balance control

electromyography

Falling is one of the most common accidents among children. It is reported(Bei et al., 2017; Wang, Dezheng, Ying, Wenlong, & Guohong, 2018)that the top three causes of children's accidental injuries are accidental falling, accidental tumbling, and traffic accidents, so the injuries caused by children's falling have become an important global public health problem. To reduce the risk of falling injury during children's growth and successfully complete various physical activities, it is very important to improve their balance control ability.

The ability to balance control means maintaining the vertical projection point of the center of mass (COM) in the base of support (BOS) or allowing the center of body mass to continuously track a new base of the support area. The ability to balance control is affected by the nervous system and muscle strength, and it needs the energy of the central and peripheral nervous system to control the muscles in various parts of the body to completely control the body (Kizilova & Karpinsky, 2007). In recent years, some scholars have pointed out that, in addition to the proprioceptive, vestibular, and sensory systems as important factors to maintain balance function, cognitive tasks also have an impact on balance control, and balance control when performing dual cognitive tasks will be worse than when performing single tasks(Huxham, Goldie, & Patla, 2001; Young, Williams, & Posture, 2015){Young, 2015 #19;Huxham, 2001 #20;Chengmin, 2018 #21}. When people perform dual tasks, they weaken their balance control over the body, thus reducing the proportion of proprioception, vestibular, and sensory system's ability to maintain balance, further increasing muscle contraction and tension, and reducing the joint action of stabilizing muscles, to increase the risk of falling(Ting et al., 2012). Compared with adults, children's heads are proportionately larger and they have a higher COM, immature neuromuscular development, movement strategies, and lack of perception, cognition, and experience, which makes children's balance control more important. However, children are also prone to go to high places in their lives, such as "going up and down stairs, taking things while standing on chairs, etc., so cognitive tasks will have an impact on children's balance control. Some scholars(B. Dingenen, Janssens, Claes, Bellemans, & Staes, 2016; Bart Dingenen, Janssens, Luyckx, Claes, & Staes, 2015)also believe thatwhen people walk while performing dual tasks, they become distracted from walking. Coupled with the decline of sensory function and central nervous system function, the walking task and cognitive task compete for attention resources, readjust the allocation, and allocate all the attention resources used for walking to complete the calculation task. As the effective attention resources allocated to the task of walking decrease, the balance control ability is reduced, and then the fall risk is increased. At present, most scholars have introduced performing dual-tasks as training in rehabilitation and athlete training through multiple disciplines, which has made some progress in training pathological old-age balance issues (such as stroke rehabilitation). Few studies have been conducted by using kinematics angle and muscle pre-activation angle, and few scholars have studied children's ability to balance during performing cognitive tasks. In addition, some scholars (Shen, Weijie, & Yu, 2016a; Shuyu, 2016)have agreed thatthe increase of ground height will have differing effects on the ability to control the balance of individuals when descending steps, the joint angle when landing, and muscle activation before touching the ground. So, will steps of differing heights have an impact on children’s ability to balance?

The purpose of this study is to explore children's balance control law with differing heights and cognitive tasks utilizing kinematics and EMG synchronous tests, to provide a useful reference for preventing and reducing children's fall injuries when going down steps, and to design appropriate training methods to improve children's ability to balance control.

1.1 Subjects

The subjects of this study were 20 healthy pupils from grades three to six in a primary school affiliated with Southwest University (5 from each grade), including 10 boys and 10 girls, with an average age of 10.50 ± 1.15, an average height of 145.80 ± 9.81 cm, an average weight of 43.80 ± 7.33 kg, an average BMI of 20.48 ± 1.23 kg/m², an average foot length of 21.07 ± 2.29 cm, and an average foot width of 8.14 ± 1.47 cm. The basic information of the subjects is not affected by gender factors (see Table 1).

Table 1 Basic gender information comparison
Gender	Age	Height	Body weight	BMI	Foot length	Foot width
Male	10.80±1.35	151.33±8.08	47.33±7.37	20.58±1.71	21.52±2.89	8.36±2.21
Female	10.17±1.02	137.50±4.95	38.50±3.54	20.34±0.41	20.47±1.59	7.91±1.21
P value	P>0.05	P>0.05	P>0.05	P>0.05	P>0.05	P>0.05

Inclusion criteria: Each subject must fill in the questionnaire about the

subjects’ physical health before the test, showing that they are in good health and willing to participate in this study. The subject's parents must fill in the notes and voluntary consent form. The subjects must have perfect limbs, no joint disease, be injury-free within the last year, and have no skeletal neuromuscular disease. Exclusion criteria: Those who suffered from sports injuries within the past week, had flat feet, and fell short of 83% accuracy with the cognitive tasks.

1.2 Test means and methods

1. Five steps made of plywood were 90cm in length, 28cm in width, and 18cm in height, and the highest step extended to a platform with a length of 28cm. 2. A high-speed camera (JVC9800, Victor Company, Japan) was set, the shooting frequency was set to 100 frames/second, and the camera was 3m away from the subject, with a camera height of 1.25m. The center of the lens was taken along the sagittal plane of the subject and covered the test action. A synchronous lamp was erected in front of the high-speed camera, and the camera lens faced the BIOFORCEN dynamic balance training evaluation system. Then, the video image analysis system developed by Beijing Sport University was used to analyze the motion images of the subjects from the first step to the second step to the third step, to obtain the related kinematic parameters. 3. A BIOFORCEN (BTA-200DP) dynamic balance training and evaluation system developed by Eli Intelligent Technology Co., Ltd. was used (for the test the instrument was embedded in the third step, which was 60cm long, 40cm wide, and 3.5cm high.) The interception frequency was set to 100Hz, and the data was transmitted to the computer through a data acquisition box and imported into the supporting software for interception and analysis of plantar pressure parameters. 4. Subjects wore JE-TB0810 surface electromyography instruments developed by Eli Intelligent Technology Co., Ltd. (during the test, the instrument was fixed at the hip joint, as shown in Figure 1), and the EMG signals from the four main muscles of the lower limbs during the test were collected, and the interception frequency was set to 1000Hz. The data was connected to the computer via a wireless receiver. To determine the reliability of receiving the test data, the uploaded data was saved in the EMG data analysis software for further interception and analysis after each test. 5. The high-speed camera and surface EMG were set synchronously. After subjects wore the EMG, the camera position and shooting range were fixed, and a wireless sensor was placed within the specified range, then they were controlled by triggering the remote-controlled device. Then a synchronizer was placed in front of the camera to trigger the synchronizer lamp after receiving the signal. Only when the synchronizer lamp was on, the information captured by the camera was effective, and the surface EMG began to collect data. 6. The lower limb joint angle, pressure center, and surface electromyography were collected synchronously under four conditions: descending the first step while performing a single task, descending the first step while performing a double task, descending the third step while performing a single task and descending the third step while performing a double task.

1.2.1 Walking down the stairs while performing a single task

Before the test, the subjects practiced several times to familiarize themselves with the movement process. The subjects stood barefoot on the second or fourth step. After hearing "start", the subjects completed one step down the stairs with natural movements and speed. This test was performed a total of three times.

1.2.2 Walking down the stairs while performing a double task

The process was the same as that for subjects performing a single task. The tester randomly reported a number between 50 and 10, completed the arithmetic of continuously subtracting three while at the same time they walked down the stairs, and stated the correct result, then the test was effective. The accuracy of cognitive tasks of the subjects was more than 83%.

1.2.3 Time phase division

1) Synchronization parameters: Therefore, in the sub-synchronous test, the kinematics sampling frequency was 100 frames/s, the dynamic balance training evaluation system sampling frequency was 100 frames/s, and the EMG sampling frequency was 1000 frames/s. Using the three parameters, standardized processing was conducted using the traditional method, and the whole action process was standardized in 0-100%, from the moment when the dominant leg left the step when the subject walked down the step to the moment the dominant leg touched the step again and remained standing, to realize the synchronization of complete actions. (See Figure 2)

2) EMG stage division: According to previous studies, the EMG status of muscles from 200ms before touching steps to the moment of touching steps was set as the comparative range of muscle pre-activation(Shen, Weijie, & Yu, 2016b; Weijie, Yu, Lingyan, & Rui, 2014). For the division of the buffer period, the tester set the EMG status of the lowest center of gravity muscle of the body from the moment of touching steps to touching the step as the comparison range of the buffer period (Shen et al., 2016b).

1.2.4 Data processing

Total excursion (TOTEX): The moving distance in unit time was obtained by subtracting the current coordinates from the coordinates of a time point after Anterior-posterior (AP) and Medial-lateral (ML), and the moving distance in unit time of AP and the square of the moving distance in unit time of ML were added before the root sign was taken, and all the distances were added up to equal the total moving distance.

N was the number of sample points, xi was instantaneous medial lateral-changes of foot pressure (ML-COP), and yi was instantaneous anterior-posterior-changes of foot pressure (AP-COP).

Root mean square distance (RDIST): The distance RD between AP and ML from the origin was calculated, then the RD was squared and averaged with the root sign as the root mean square distance.

Root mean square distance-ML\Root mean square distance-AP, (RDISTML\RDISTAP): it was obtained by averaging and taking the root sign after shaking the square of ML\AP in the left and right directions.

Mean distance (MDIST): The distance between AP and ML from the origin was calculated as RD, and then the average of RD was calculated as the distance, where N was the total length of data.

95% confidence circle area (95% CC AREA): Z0.95 was 1.645 by a normal distribution, SRD was obtained by subtracting the square of the mean square distance from the square of the mean square distance, and then the product of mean distance and Z0.95 and SRD was summed and squared, and finally 95% CC area was obtained by multiplying π.

4) Acquisition of myoelectricity (EMG) parameters: After full-wave rectification, the original EMG signal was smoothed with a low-pass filter (truncation frequency was 10 Hz), to obtain EMG signal values of four muscles of lower limbs when descending steps under different heights and cognitive tasks; To obtain more information about muscle activity, the EMG time phase burst and end and the relative percentage of each muscle discharge were determined by traditional methods. The main parameters included the normalized value of EMG amplitude and pre-activation time of four muscles, such as the tibialis anterior, biceps femoris, rectus femoris, and the medial gastrocnemius at 200ms before the dominant leg touches the step and the buffer stage of touching the step. Specific calculation principle:

Mean EMG amplitude: The conversion formula is as follows:

Standardized value of mean EMG amplitude: since subcutaneous tissue, fat, and electrode patches vary with people, as a result, it is impossible to directly compare EMG signals among individuals, so it is necessary to exclude individual differences from the data analysis. The method was to calculate the percentage of the signals in the test to the maximum potential of individual muscle groups based on the maximum potential of individual muscle groups compared with the signals collected in the test. The conversion formula is as follows:

1.3 Statistical analysis

The data analysis software SPSS 23.0 was used to analyze the kinematics and EMG parameters of descending steps captured by the one-way ANOVA, and the significance level of all parameters was set as α = 0.05.

2.1 Comparison of kinematic characteristics when descending steps of differing heights and performing different cognitive tasks

2.1.1 Comparison of angle characteristics of the landing leg, knee, and ankle at the moment of touching the step

Table 2 Statistical summary table of lower limb joint angle change points at the moment of touching the steps of differing heights while performing different cognitive tasks (N=20, unit: degree)
Parameter name	Work condition	Mean ± SD	t/F value	LSD multiple comparison
Knee angle (unit: degree)	The first step while performing a single task ^a	129.31±10.11	5.88***	a<c<b<d
	The first step while performing a double task ^b	161.66±16.05
	The third step while performing a single task ^c	158.81±13.99
	The third step while performing a double task ^d	165.79±11.96
Ankle angle (unit: degree)	The first step while performing a single task ^a	110.97±11.71	7.14***	a<c<b<d
	The first step while performing a double task ^b	128.55±15.66
	The third step while performing a single task ^c	123.25±12.76
	The third step while performing a double task ^d	134.11±10.13
Note: "" and " *" represent significant levels of 0.05 and 0.01, respectively. The four combinations of "the first step/single task", "the third step/single task", "the first step/double task" and "the third step/double task" are represented by a, b, c, and d respectively. The multiple comparisons meaning "a > b" means that the score of combination a is significantly higher than that of group b. If "a=b", it means that there is no significant difference between the two groups, and other groups have the same symbolic meaning, so it will not be stated herein.

Descending steps of differing heights and under different cognitive tasks significantly affected the knee and ankle angles at the moment of touching the steps (F=5. 88 p < 0.05). After LSD multiple comparisons found that the knee and ankle angles at the first step under a single task were significantly smaller than those at the third step while performing a single task, and the knee and ankle angles at the third step under a single task were smaller than those at the first step while performing a double task, and the knee and ankle angles at the first step under double task were smaller than those at the third step while performing a double task. The results showed that the knee and ankle angles at the moment of touching steps increased with the increase of cognitive task difficulty and height. (See Table 2)

2.1.2 Landing leg joint angle comparison at the moment the center of gravity of the human body reaches the lowest point during the buffer process of touching the step

Table 3 Statistical summary table of lower limb joint angle changes of the lowest center of gravity of the body at the moment of touching the step when descending steps of differing heights and under different cognitive tasks (N=20, unit: degree)
Parameter name	Work condition	Mean ± SD	t/F value	LSD multiple comparison
Knee angle (unit: degree)	The first step while performing a single task ^a	157.12±7.95	2.08
	The first step while performing a double task ^b	159.06±8.81
	The third step while performing a single task ^c	162.62±9.90
	The third step while performing a double task ^d	166.64±8.06
Ankle angle (unit: degree)	The first step while performing a single task ^a	107.11±9.03	1.23
	The first step while performing a double task ^b	115.11±10.19
	The third step while performing a single task ^c	112.11±13.03
	The third step while performing a double task ^d	120.90±11.16
Note: "" and " *" represent significant levels of 0.05 and 0.01, respectively. The four combinations of "the first step/single task", "the third step/single task", "the first step/double task" and "the third step/double task" are represented by a, b, c, and d respectively. The multiple comparisons meaning "a > b" means that the score of combination a is significantly higher than that of group b. If "a=b", it means that there is no significant difference between the two groups, and other groups have the same symbolic meaning, so it will not be stated herein.

Descending steps of differing heights and under different cognitive tasks doesn’t affect the knee and ankle angles with the lowest center of gravity at the moment of touching the steps (P>0.05). The results showed that there was no significant difference in the knee and ankle angles at the moment of the lowest center of gravity with the increase in cognitive task difficulty and height. (See Table 3)

2.2 Pressure center deviation comparison of landing leg during support period at differing heights and under cognitive tasks

Table 4 Statistical table of pressure center movement of landing leg during supporting period under different heights and different cognitive tasks (N=20, unit: mm)
Parameter name	Work condition	Mean ± SD	t/F value	LSD multiple comparison
Root mean square of the left-right displacement	The first step while performing a single task ^a	22.74±11.11	4.11^***	a<c<b<d
	The first step while performing a double task ^b	32.19±12.34
	The third step while performing a single task ^c	28.74±14.13
	The third step while performing a double task ^d	35.51±20.15
Root mean square of the forward and backward displacement	The first step while performing a single task ^a	39.01±15.19	8.45^***	a<c<b<d
	The first step while performing a double task ^b	66.99±21.07
	The third step while performing a single task ^c	47.69±18.22
	The third step while performing a double task ^d	84.92±32.55
Total trajectory swing length	The first step while performing a single task ^a	285.71±88.58	14.20^***	a<c<b<d
	The first step while performing a double task ^b	387.23±100.09
	The third step while performing a single task ^c	345.41±108.50
	The third step while performing a double task ^d	410.55±94.03
95% confidence elliptical area	The first step while performing a single task ^a	105.34±33.31	10.67^***	a<c<b<d
	The first step while performing a double task ^b	198.77±66.17
	The third step while performing a single task ^c	135.34±46.72
	The third step while performing a double task ^d	226.24±85.85
Note: "" and " *" represent significant levels of 0.05 and 0.01, respectively. The four combinations of "the first step/single task", "the third step/single task", "the first step/double task" and "the third step/double task" are represented by a, b, c, and d respectively. The multiple comparisons meaning "a > b" means that the score of combination a is significantly higher than that of group b. If "a=b", it means that there is no significant difference between the two groups, and other groups have the same symbolic meaning, so it will not be stated herein.

Steps of differing heights and under different cognitive tasks significantly affected the root mean square of left-right displacement, root mean square of anterior-posterior displacement, total trajectory length, and 95% confidence ellipse area of COP during a support period (F=4. 11, 8.45, 14.20, 10.67, all P < 0.001). Further LSD multiple comparisons found that the root mean square of the left-right displacement of COP at the first step while performing a single task, the root mean square of forward and backward displacement, total trajectory length of swing, 95% confidence ellipse area were significantly less than those at the third step while performing a single task, which was less than those at the first step under double task, which was less than those at the third step while performing a double task. The results showed that the root mean square of the left-right displacement, root mean square of forward and backward displacement, the total trajectory length of swing, and 95% confidence ellipse area of COP increase with the increase of cognitive task difficulty and height. (See Table 4)

2.2 Stability comparison of the human body during descending steps of differing different and while performing different cognitive tasks

2.2.2 Comparison of EMG characteristics of the main muscle groups in the landing leg from 200ms before touching the board to the moment when the forefoot touches the step

Table 5 Statistical summary table of the mean EMG amplitude change parameters of the main muscle groups of the lower limbs from 200ms before touching the board to the moment of touching the step under different visual input and different cognitive tasks (N=20, unit: %)
Parameter name	Work condition	Mean ± SD	t/F value	LSD multiple comparison
Biceps femoris muscle	The first step while performing a single task ^a	13.14±4.10	7.98***	a<c=b<d
	The first step while performing a double task ^b	8.01±3.89
	The third step while performing a single task ^c	10.12±3.39
	The third step while performing a double task ^d	6.35±1.67
Rectus femoris	The first step while performing a single task ^a	9.12±4.17	6.21***	a<c=b<d
	The first step while performing a double task ^b	5.70±2.45
	The third step while performing a single task ^c	6.54±2.64
	The third step while performing a double task ^d	4.92±2.33
Tibialis anterior muscle	The first step while performing a single task ^a	12.72±5.27	9.54***	a<c=b<d
	The first step while performing a double task ^b	8.09±3.33
	The third step while performing a single task ^c	9.14±4.76
	The third step while performing a double task ^d	6.94±2.25
Medial gastrocnemius	The first step while performing a single task ^a	10.97±4.49	10.17***	a<c=b<d
	The first step while performing a double task ^b	6.38±3.65
	The third step while performing a single task ^c	7.64±3.15
	The third step while performing a double task ^d	5.18±2.52
Note: "" and " *" represent significant levels of 0.05 and 0.01, respectively. The four combinations of "the first step/single task", "the third step/single task", "the first step/double task" and "the third step/double task" are represented by a, b, c, and d respectively. The multiple comparisons meaning "a > b" means that the score of combination a is significantly higher than that of group b. If "a=b", it means that there is no significant difference between the two groups, and other groups have the same symbolic meaning, so it will not be stated herein.

Descending steps of differing heights and while performing different cognitive tasks significantly affected the mean EMG amplitude of biceps femoris, rectus femoris, tibialis anterior, and medial gastrocnemius from 200ms before touching the board to the moment of touching the board of the forefoot (F=7. 98, 6.21, 9.54, 10.17, P < 0.001). Further LSD multiple comparisons found that the mean myoelectricity amplitude of biceps femoris, rectus femoris, tibialis anterior, and medial gastrocnemius from 200ms before touching the board to the moment when the forefoot touches the plate at the first step while performing a single task was significantly greater than the mean myoelectricity amplitude of the biceps femoris, rectus femoris, tibialis anterior and medial gastrocnemius from 200ms before contacting the step to the moment when the forefoot touches the step at the third step while performing a single task; the mean myoelectricity amplitude of the biceps femoris, rectus femoris, tibialis anterior and medial gastrocnemius from 200ms before contacting the step to the moment when the forefoot touches the step at the first step while performing a double task was greater than those at the third step under double task. The results showed that the mean EMG amplitude of the biceps femoris, rectus femoris, tibialis anterior, and medial gastrocnemius increased with the increase of cognitive task difficulty or height from 200ms before touching the step to the moment of touching the step with the forefoot. (See Table 5)

2.2.3 EMG Comparison of the main muscle groups of the landing leg from the moment when the forefoot touches the step to the entire foot touching the step

Table 6 Statistical summary table of mean EMG amplitude change parameters of main muscle groups from the lower limbs during the buffer period under steps at differing heights and different cognitive tasks (N=20, unit: %)
Parameter name	Work condition	Mean ± SD	t/F value	LSD multiple comparison
Biceps femoris muscle	The first step while performing a single task ^a	16.12±5.12	1.11
	The first step while performing a double task ^b	13.37±4.48
	The third step while performing a single task ^c	15.37±3.67
	The third step while performing a double task ^d	14.78±4.31
Rectus femoris	The first step while performing a single task ^a	21.01±6.54	2.01
	The first step while performing a double task ^b	17.94±7.11
	The third step while performing a single task ^c	19.29±3.14
	The third step while performing a double task ^d	16.04±2.35
Tibialis anterior muscle	The first step while performing a single task ^a	46.91±4.13	3.10
	The first step while performing a double task ^b	37.66±2.88
	The third step while performing a single task ^c	45.73±5.95
	The third step while performing a double task ^d	31.60±4.72
Medial gastrocnemius	The first step while performing a single task ^a	34.23±5.20	2.89
	The first step while performing a double task ^b	25.81±6.17
	The third step while performing a single task ^c	33.33±4.60
	The third step while performing a double task ^d	21.80±3.77
Note: "" and " *" represent significant levels of 0.05 and 0.01, respectively. The four combinations of "the first step/single task", "the third step/single task", "the first step/double task" and "the third step/double task" are represented by a, b, c, and d respectively. The multiple comparisons meaning "a > b" means that the score of combination a is significantly higher than that of group b. If "a=b", it means that there is no significant difference between the two groups, and other groups have the same symbolic meaning, so it will not be stated herein.

Descending steps at differing heights and under different cognitive tasks did not affect the mean EMG amplitude of biceps femoris, rectus femoris, tibialis anterior, and medial gastrocnemius in a buffer period (P > 0.05). The results showed that the mean EMG amplitude of biceps femoris, rectus femoris, tibialis anterior, and medial gastrocnemius had no significant difference in the buffer period with the increase in cognitive task difficulty and height. (See Table 6)

3.1 Characteristic analysis of the lower limb joint angle at different heights and while performing different cognitive tasks

The study found that the lower limb joint angle while performing a single task was lower than that while performing a double task at the moment of touching the step. In daily functional movements, extra cognitive tasks cause distraction, and at the same time, it also affects the ability of functional movement control and sensory system information integration (Horak & Ageing, 2006). Some scholars (Fantong, 2016; Wei, 2019)believe that, double tasks distract the subject’s attention, which leads to a decrease in the flexion of the knee joint and ankle joint when descending steps, resulting in a larger angle of knee and ankle joints than a single task, thus increasing the risk of falling.

The study also found that descending steps of differing heights had an impact on the knee and ankle angles at the moment of touching steps. This result was consistent with previous studies (Shen et al., 2016b; Shuyu, 2016). The lower limb joint angle at the moment of touching the third step was lower than that at the moment of touching the first step. This study also found that the knee and ankle angles at the first step while performing a single task were significantly smaller than those at the third step while performing a single task, which was smaller than those at the first step while performing double tasks, which was smaller than those at the third step while performing double tasks. This also confirmed that with the difficulty of performing cognitive tasks, the influence of height from the ground on the joint angle of lower limbs was higher than that of height from the ground, and the knee and ankle angles also increased at the moment of touching the step (Horak & Ageing, 2006; Shen et al., 2016a).

3.2 Characteristic analysis of plantar pressure at different heights and while performing different cognitive tasks

Li, Bherer, Mirelman, Maidan, and Hausdorff (2018)found that children's balance control while performing a single task was better than that while performing double tasks. According to the principle of overlapping neural circuits, when cognitive and motor tasks occupy the same neural circuits, serious interference between two single tasks in double tasks also occurred. Attention distribution is an important factor affecting balance control. When people perform double tasks, part of their attention is allocated to cognitive tasks, and the attention allocated to completing actions will decrease, and the ability to balance control will also weaken (Fujita, Kasubuchi, Wakata, Hiyamizu, & Morioka, 2016; Xiuen, Jiejiao, Haitao, & Yong, 2016). As a result, the ability of the human body to maintain balance when completing the dual cognitive tasks is reduced, and the COP root mean square displacement in front and back direction, root mean square displacement in left and right direction, the total trajectory length of swing and 95% confidence ellipse area will also increase.

This study also found that descending steps of differing heights affect the root mean square of forward-backward displacement, root mean square of left-right displacement, total trajectory length of swing, and 95% confidence ellipse area of COP during the supporting period. Descending steps of differing heights from the ground affect the ability to control the balance of the subjects, with the increase of height, the root mean square displacement of COP in forward and backward directions, root mean square displacement in left and right directions, the total trajectory length of swing and 95% confidence ellipse area will also increase (Miaomiao, Jingxian, & Lin, 2018; Shen et al., 2016b).

The study also found that the root mean square of the left-right displacement of COP at the first step while performing a single task, the root mean square of forward and backward displacement, total trajectory length of swing, and 95% confidence ellipse area were significantly less than those at the third step under single task, which was less than those at the first step while performing double tasks, which was less than those at the third step while performing double tasks. Different heights from the ground have an impact on the balance of the body, and people also have certain interference on the balance control ability of the body when performing dual tasks (Qi, Zhong, Lei, Minhua, & Qing, 2010; Ziwen et al., 2020). In addition, the impact of performing cognitive tasks on balance control is greater than that of height from the ground(Li et al., 2018; Miaomiao et al., 2018). Therefore, a person’s ability to control their balance when descending from the third step while performing double tasks is weaker than that from the third step while performing a single task, the first step while performing double tasks, and the first step while performing a single task.

3.3 EMG analysis at different heights and under different cognitive tasks

3.3.1 Characteristics analysis of surface EMG from 200ms before touching the step at different heights and under different cognitive tasks

This study showed that muscle pre-activation while performing a single task was better than that under double tasks. Some scholars(Chmielewski et al., 2021; Yiou, Caderby, Delafontaine, Fourcade, & Honeine, 2017)believe that, the related muscles of the human body will be pre-activated in the process of expected posture adjustment, and the body’s stability is related to the adjustment of the corresponding muscle pre-activation, and higher muscle pre-activation can improve the body’s stability. If a person's expected posture adjustment will decline when completing dual tasks, and the external information can’t be well provided to the central nervous system, it will cause the phenomenon of muscle pre-activation decline, which will lead to the decline of the body's ability to balance control (Plate, Klein, Pelykh, Singh, & B?Tzel, 2016; Uemura et al., 2012). This study found that under the same task, the muscle pre-activation in the first step was better than in the third step. Landing at different heights from the ground had different effects on the muscle activation before touching the ground, the higher the height, the weaker the muscle activation of descending steps (Shen et al., 2016b; Shuyu, 2016). In addition, the effect of performing cognitive tasks on individual muscle pre-activation is greater than that of height from the ground (Feng & Jie, 2017; Plate et al., 2016). Therefore, height from the ground and cognitive tasks are important factors affecting muscle pre-activation, and muscle pre-activation play a decisive role in joint stability and body stability.

3.4 Brain reflex function when descending steps while performing different visual and cognitive tasks and the corresponding balance control mechanism in children

Good muscle pre-activation can increase muscle activation during the buffer stage of touching steps, thus improving the stability of knee and ankle joints and reducing body shaking during the supporting stage (Bencke, Zebis, & Kinesiology, 2011; Klyne, Keays, Bullock-Saxton, Newcombe, & Kinesiology, 2012). Under normal conditions, descending steps have a good feed-forward regulation and good lower limb muscle activation before touching steps, so the human body can better maintain knee and ankle joint stability by increasing the angle of knee flexion and dorsiflexion. In addition, ankle joint strategy is the key to reducing body shaking, and the ankle joint is the first joint part of touching steps. The higher the activation of calf-related muscles, the better the ground reaction force can be buffered, which is helpful for the stability of the ankle joint at the stage of touching steps, thus improving the balance control ability of the body and reducing body shaking (Kmr, Psa, & Ckacd, 2021; Nyland et al., 2013). On the contrary, the pre-activation of muscles related to lower limbs when descending steps decreases at different heights and under cognitive tasks, which reduces the joint regulation ability and muscle activation level, thus causing the musculoskeletal system to bear greater impact force (Shen et al., 2016b; Shuyu, 2016), and further increasing the risk of instability and falling of knee and ankle joints (Sun & Wang, 2019; Weijie et al., 2014). Therefore, it can be concluded that muscle pre-activation can positively predict the flexion of knee and ankle joints and the ability to control the balance of the body, and muscle pre-activation can also reversely predict the strength of external interference.

3.5 Conclusion

1. Different step heights and cognitive tasks have negative effects on the stability of balance in normal children when descending steps. However, because cognitive tasks directly affect the transmission of information to the central nervous system, which makes the expected posture adjustment of the body decline, the influence of neural control on children's balance control is significantly higher than that of muscle ability. 2. Muscle pre-activation has positive benefits to stability when touching steps. Therefore, observing the muscle pre-activation status of subjects at different heights under double tasks can serve as a potential index of children's ability to balance development and also as a potential index of children's falling risk. 3. The change of balance control ability caused by different step heights indicates that the information stimulation of vision to the central nervous system has a significant positive influence on children's cognitive work, and visual deprivation may promote children to focus more on gait movements and weaken cognitive work when descending steps.

Changes of foot pressure (COP)
The center of mass (COM)
The base of support (BOS)
Total excursion (TOTEX)
Anterior-posterior (AP)
Medial-lateral (ML)
Medial lateral-changes of foot pressure (ML-COP)
Anterior-posterior-changes of foot pressure (AP-COP)
Root mean square distance (RDIST)
Root mean square distance-ML(RDISTML)
Root mean square distance-AP(RDISTAP)
Mean distance (MDIST)
Confidence circle area (CC AREA)
Myoelectricity (EMG)

Ethics approval and consent to participate

The study was approved by Research Ethics Committee of Chongqing University of Posts and Telecommunications. The approval No. is NSFCLL202205. The research procedures and methods were in accordance with the ethical standards as laid down in the Declaration of Helsinki and its later amendments or comparable ethical standards. Written informed consent was obtained from all volunteers in the study.

Consent for publication

All participants of case reports had consent for publication.

Availability of data and material

The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author.

Competing interests

All authors declare no conflflict of interest.

Funding
Chongqing Municipal Sports Bureau

This study was supported by the Sports Scientific Research Project in Chongqing Municipal (D202002) and the Social Science Planning Project in Chongqing Municipal (2021WT19), and The Humanities and Social Sciences Research Base Project of Chongqing Municipal Education Commission in 2022 (22KJD074, 22SKJD077).

Authors' contributions

JT and HY developed the experimental protocol survey. JT and HY recruited the students enrolled in Southwest of China. HY and XL undertook the statistical analysis and graphical representation of the database. JT and XL revised the draft. All authors designed this study, contributed to the article, and approved the final manuscript.

Acknowledgements

We are grateful to the School of Physical Education of Southwest University for providing the relevant equipment and Teacher Ouyang Yiyi for their joint efforts.

Authors' information

Jinsheng Teng, China, 1974-, Associate Professor, PhD; Research Direction: Scientific Fitness and Guidance, Physical Education Theory.

Huijun Yang, China, 1988-, lecturer, Master; Research Direction: Health Evaluation and Health Promotion, Smart Sports.

Xu Li, China, 1991-, lecturer, Master; Research Direction: Smart Sports, Physical Education.

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Effects of different heights and task conditions on the biomechanical characteristics of children's lower limbs when descending steps

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