Effect of initial foot angle (IFA) on kinematics and dynamics of body during sit-to-stand transfer

Abstract

Sit-to-stand (STS) is considered the most common functional activities in daily life and the basis for other activities. The elderly and patients with lower limb disorders could not complete the STS motion very well due to limb pain and muscle weakness. Physiotherapist find that specific STS transfer strategies can make patients more easily to complete this task. However, few researchers pay attention to the effect of initial foot angle (IFA) on STS motion. Twenty-six healthy subjects were randomly selected to perform STS transfer experiment. The motion characteristic parameters of subjects under 4 different IFAs (nature, 0°, 15°, and 30°) were obtained, including the percentage of duration in each phase, the velocity of joints, rotation angle and angular velocity of joints (shoulder, hip and knee), center of gravity (COG) trajectory. the change of plantar pressure parameters, and dynamic margin of stability. By comparing the motion characteristic parameters obtained under different IFAs and carrying out statistical analysis, the influence of different IFAs on body kinematics and dynamics during STS task was further explored. The kinematic parameters obtained under different IFAs are significantly different. The percentage of duration in each phase of the STS transfer was different with different IFA, the main differences were in phase I and phase II. The phase I of U15 took 24.5% T, while phase I of N, U0 and U30 took about 20% T, and the maximum difference was (U15-U0) 5.4%. The phase II of U15 took the least time, about 30.8% T. When the IFA is nature (N) and 15°(U15), the trajectories of COG are basically in coincidence; when the IFA is 0°(U0) and 30°(U30), the displacement of COG in anterior-posterior direction is larger. The larger the IFA, the smaller the plantar pressure parameter. When the IFA is 15°, the COG is close to the center of limits of stability, which can provide a better stability. This paper summarizes the influence under 4 different experimental conditions of IFAs on STS transfer, so as to provide a starting point and bases for clinicians to develop rehabilitation training protocols and STS motion strategies for patient.

What is known.

• According to the data of STS motion experiment of 26 subjects, the human motion characteristics under 4 different experimental conditions of IFAs are summarized, including kinematics parameters, plantar pressure parameters and COG trajectory.

What is new.

• This paper summarizes the influence under 4 different experimental conditions of IFAs on STS transfer, so as to provide a starting point and bases for clinicians to develop rehabilitation training protocols and STS motion strategies for patient.

1. Introduction

Sit-to-stand (STS) is a very important daily motion, we need to repeat at least 60 ± 22 STS activities in a day.^[1–3] Although this motion seems simple, it needs the complex coordination between the central nervous system and neuromuscular system.^[4] During the STS transfer, we should ensure the balance of the center of gravity (COG) on the narrow base of support.^[5–8] For the elderly and patients with lower limb disorders, it is difficult to complete the STS transfer activities in daily life due to the weakness of lower limb muscle strength and joint damage.^[9] Therefore, many researchers have conducted in-depth study on STS motion, including the law of motion of healthy people during STS and the influence of different strategies on STS motion.^[10–12]

Some studies have shown that the STS motion is easily affected by the initial sitting posture,^[13,14] including adjusting the height of the chair, increasing the trunk flexion and changing the initial foot position.^[15,16] Shoichi Kawagoe et al^[17] studied the influence of foot fore-and-aft positioning on dynamics during STS transfer. In their study, they found that it will directly affect the horizontal displacement of the COG and reduce the stability of the body when the foot is in the front. Therefore, they suggested that sufficient space should be reserved under the seat to allow the foot to be placed backward and make patient as easy as possible to stand up. The research results of Patrick et al^[18] showed that the placement of foot and arm in sitting posture had a significant effect on the STS time of patients with chronic stroke. Spherd et al^[19] explored the effect of different foot positions on biomechanics of young women during STS transfer, set 3 different foot positions in the experiment (feet back, preferred position and feet forward). They found that as the feet were placed further forward, the overall time taken to perform the STS transfer increased. Medeiros et al^[20] studied the influence of seat height and foot position on the posture control of children with cerebral palsy during the STS task. They found that different seat height and foot position would produce different results on the linear displacement (x and y) of shoulder and knee joint in the sagittal plane, but had no effect on the posture control. Hyeon-Je et al^[21] studied the effects of foot symmetry and asymmetry on body dynamics of stroke patients during STS training. Thirty-six stroke subjects participated in their experiment, the results showed that the peak value of ground reaction force under foot asymmetry condition increased significantly compared with foot symmetry. Farqalit et al^[22] evaluated the effect of foot position on the balance and STS ability of patients with chronic stroke during STS training. They found that foot position asymmetry improved the balance and STS ability of patients with stroke compared with foot position symmetry. Jeon et al^[23] studied the effects of foot fore-and-aft positioning and symmetry on muscle activity and balance of healthy adults during STS task. They completed 6 groups of experiments. They mainly explored the change of COG and ground reaction force under various conditions. The results showed that with the forward of foot initial position, the forward displacement of body COG and trunk flexion increased. Gillette et al^[24] studied the influence of foot symmetry and asymmetry on the torque of each joint during STS task, found that the torque of hip joint increased significantly when the foot was placed symmetrically. Many researchers have studied the fore-and-aft position and symmetry of the foot.^[25–27] They found that the fore-and-aft position and the symmetry of the foot have a great impact on the motion characteristics of STS transfer.

Understanding and quantifying these effects provides clinicians with a new STS motion strategy to help the elderly and patients with lower limb disorders. However, in the current studies, the initial foot angle (IFA) of the subjects was not limited, but was chosen by the subjects themselves. There are few literatures about the influence of IFA on STS motion. We believe that different IFAs will have a significant impact on STS motion. Understanding these effects can make clinicians have a new understanding of STS motion, so as to develop a reasonable STS motion strategy, and better help the elderly and patients with lower limb disorders to complete STS transfer. Therefore, it has great significance to study the IFA.

The purpose of this paper was to explore the influence of different IFAs (nature, 0°, 15°, and 30°) on body kinematics and dynamics during STS transfer. For this purpose, we obtained the STS kinematics and dynamics parameters under 4 different experimental conditions of IFAs, including percentage of duration in each phase, the peak value of each joint velocity, the maximum tilt angle of trunk and shank, and the peak value of joint angular velocity. We also calculated the position of the COG, the dynamic margin of stability and measured the plantar pressure parameters. By comparing the motion characteristic parameters under different experimental conditions, this paper analyzes how different IFAs affect body kinematics and dynamics. The results showed that a larger or smaller IFA would reduce the stability, the larger IFA, and the smaller the plantar pressure parameter.

2. Methods

2.1. Participant

Twenty six healthy adult males were randomly selected to participate in this study. The inclusion criteria were no cognitive impairment, the STS transfer experiment could be completed in cooperation, no medical history or sequelae affecting body movement, and no balance dysfunction. The inclusion time was from April 10 to April 25, 2020 at Tianjin University of science and technology, and all the subjects were right hand dominant. Their average age, height, weight and BMI were 27 (SD 6) years, 174 (SD 6.5) cm, 69 (SD 8.9) kg and 22.7 (SD 2.7) kg/ m² respectively. The study size is 26 adult healthy men, and the study sample of the experiment covers 5 to 99 percentile human, the selection of sample is of statistical significance. This study was approved by Academic Ethics and Scientific Ethics Special Committee of Academic Committee of Tianjin University of Science and Technology. All subjects signed an informed consent statement prior to participating in this study.

2.2. Equipment

We used a high-definition camera (EOS 200D II, Canon) to record the joint trajectory. In the experiment, we placed the camera on the left side of the subjects and collected the video at a rate of 60 fps/s. In order to obtain plantar pressure, we used a flexible film pressure sensor (MD30-60, Leanstar, Suzhou, China) with a range of 10kg, thickness < 0.6mm, response point < 200g, response time < 1ms, and measurement diameter of 23mm. Flexible film pressure sensor is a kind of resistance sensor. The output resistance decreases with the increase of the pressure applied to the surface of the sensor. The pressure can be measured by a specific pressure-resistance relationship. In order to ensure the accuracy of the data, pressure sensors were set on the soles and heels of the subjects.

2.3. Procedure

During the experiment, 1 researcher was responsible for recording videos and the other was responsible for collecting plantar pressure. In order to prevent the shake of clothes from affecting the accuracy of experimental data, subjects were asked to wear black tights. At the same time, in order to ensure the accuracy of plantar pressure measurement, and the subjects completed the STS movement without wearing the shoes. To obtain kinematic data, red markers were attached to the following anatomical landmarks on the left side of the subject’s body: shoulder, waist, knee, hip, and ankle joints. The waist point is located at 60% of the line between the shoulder joint and the hip joint. Subjects were seated on the seat of an armless, backless chair, which was adjusted to 100% of each subject’s knee height. Subjects were instructed to fold their arms across the chest and to rise without bringing their arms forward. Subjects began to perform STS transfer at the word “start,” at the same time, the researchers turned on video recording and begin to measure plantar pressure. The movement ended with the subject’s self-report “stop,” at the moment, and researchers finished the data collection and checked whether there were any incorrect data. Subjects performed the STS task at natural (self-selected) speed. Four different experimental conditions of IFAs were set: Nature (N), 0°(U0), 15°(U15), 30°(U30). Data were collected for 2 trials for each subject. Subjects were given adequate rest between trials to avoid fatigue. We defined the time to complete the STS as T under each condition of IFAs.

In the process of experiment, the bias mainly came from the subject, the researcher who carried out the experiment and the measurement process, and so we paid special attention to control the possible bias factors in the experimental process to ensure the accuracy and reliability of the measurement results.

2.4. Data analysis

In order to simplify the movement model, it is assumed that the STS transfer is completed in the sagittal plane. We established a link segment model and a cartesian coordinate system with the ankle joint as the origin in the sagittal plane, as shown in Figure 1a. STS motion is a complex and continuous motion process. In order to analyze the mechanics of STS transition in more detail, the principle of biomechanics was adopted to simplify it into a motion chain. In the conversion process, the planar motion chain model of ankle joint, knee joint, hip joint and shoulder joint played the role of hinge, and the change of joint angle is also the most significant.^[28]

Figure 1.

Human models and experimental conditions. (a) Link segment model of human body. (b) Four different experimental conditions of IFAs. (θ₁: Knee angle; θ₂: Hip angle; θ₃: Trunk angle; θ₄: Tilt angle of trunk; θ₅:Tilt angle of shank). IFAs = initial foot angles.

Through the experiment, we got the position coordinates of each joint of the subjects. A high-definition camera was used to sample the sagittal plane images in STS motion, and then Adobe Photoshop 2018 was used to extract each frame. With the ankle marker as the origin of coordinates, a rectangular coordinate system was established, as shown in Figure 1b. Pixel position coordinates of knee, hip, shoulder were obtained, and actual position coordinates of each joint marker were obtained through calibration.

In order to obtain the rotation law of each joint, we used the position coordinates of adjacent joint points to calculate the rotation angle of each joint. We used SPSS 26 (IBM Corporation) to analyze and process the data, and 1-way analysis of variance method was used to analyze the significant differences among different motion characteristic parameters. We used finite difference method to calculate the velocity and angular velocity of the marker.^[29]

The central difference method was used to calculate the velocity:

Where vⁱ are the velocity of the marker at time i, ∆t is the sampling interval of the data, and S is the linear position.

The forward and backward difference method were used to obtain a velocity at time 1 or n:

$${\displaystyle \begin{array}{l}{\displaystyle v_1=\frac{s_2-s_1}{\mathrm{\Delta }t}}\end{array}}$$

$${\displaystyle \begin{array}{l}{\displaystyle v_n=\frac{s_n-s_{n-1}}{\mathrm{\Delta }t}}\end{array}}$$

By using the body segment parameters,^[30] as shown in Table 1, we obtained the trajectory of COG.

Table 1.

The body segment parameters.

Body segment	Segment definitions		Segmental mass/total body mass (%)	CM/segment length (%)
	Proximal end	Distal end		Proximal	Distal
HAT	Head	Spine-base	61.31	54.9	45.1
Thighs	Hip	Knee	14.19	45.3	54.7
Legs	Knee	Ankle	3.67	39.3	60.7
Feet	Ankle	Foot	1.48	48.6	51.4

CM = center of mass, HAT = head and trunk.

Then, we defined the center of mass (COM) of each part of the body with the following formula^[31]:

$${\displaystyle \begin{array}{l}{\displaystyle X_{\mathrm{C}\mathrm{O}\mathrm{M}}=X_{\mathrm{p}}{L}_{\mathrm{p}}+X_{\mathrm{d}}{L}_{\mathrm{d}}}\end{array}}$$

$${\displaystyle \begin{array}{l}{\displaystyle Y_{\mathrm{C}\mathrm{O}\mathrm{M}}=Y_{\mathrm{p}}{L}_{\mathrm{p}}+Y_{\mathrm{d}}{L}_{\mathrm{d}}}\end{array}}$$

where X_COM $X_{CM}$ and Y_COM are the coordinates of COM; X_p and Y_p are the coordinates of the proximal end; X_d and Y_d are the coordinates of the distal end; and L_p and L_d are the percentages of segmental length from the proximal and distal ends, respectively. COG is the weighted average of the calculated COM of 7 segments, and was calculated by using the following formulae:

$${\displaystyle \begin{array}{l}{\displaystyle X_{\mathrm{C}\mathrm{O}\mathrm{G}}=\frac{{\sum }_{i=1}^7{m}_i{x}_i}{M}}\end{array}}$$

$${\displaystyle \begin{array}{l}{\displaystyle Y_{\mathrm{C}\mathrm{O}\mathrm{G}}=\frac{{\sum }_{i=1}^7{m}_i{y}_i}{M}}\end{array}}$$

where X_COG and Y_COG are the coordinates of COG, x_i and y_i are the COM coordinates of the $i$ th segment, m_i is the mass of the $i$ th segment, and $M$ is the body mass.

In order to obtain the plantar pressure in STS transfer, we used flexible membrane pressure sensor, MY2901 module and TTL to USB module to construct the measurement system. The measurement system can connect 4 membrane pressure sensors and obtain 4 groups of pressure data at the same time. The TTL to USB module can be connected to the computer to read the AD value data directly. We obtained the calibration data of MD30-60 sensor sample, and established the relationship between AD value and pressure through multi segments linear fitting.

Dynamic margin of stability were adapted from the references.^[32–34] Extrapolated center of mass was calculated as:

$${\displaystyle \begin{array}{l}{\displaystyle X_{\mathrm{E}\mathrm{C}\mathrm{O}\mathrm{M}}=x+\frac{v}{{\omega }_0}}\end{array}}$$

where × was the COM position, same with the COG position, v was the COM velocity. ω₀ was calculated as:

$${\displaystyle \begin{array}{l}{\displaystyle {\omega }_0=\sqrt{\frac{\mathrm{g}}{l}}}\end{array}}$$

where g was the gravitational constant, and g = 9.81 m/s². l was the equivalent pendulum length, which in this study was taken as the mean distance from the ankle marker to the COM while finishing standing. The dynamic margin of stability was taken as:

$${\displaystyle \begin{array}{l}{\displaystyle X_{\mathrm{M}\mathrm{O}\mathrm{S}}=X_{\mathrm{C}\mathrm{O}\mathrm{M}}-\mathrm{B}\mathrm{O}\mathrm{S}}\end{array}}$$

where base of support (BOS) was the boundary of the base of support, which in this study was taken as the foot length.

We screened all the data obtained and excluded the data with obvious errors in order to ensure accuracy of the results. As different subjects took different time to complete STS transfer, we normalized the time to calculate the mean value and standard deviation of the data. All curves were drawn by spline fitting using Origin 2018.

2.5. STS phase division

The STS motion was divided into 4 phases, as shown in Figure 2. Phase I, designated the momentum forward phase, began with initiation of the motion (the change of tilt angle of trunk Δθ₄ > 0.1°) and ended just before the buttocks lift-off the seat (the change of hip angle Δθ₂ > 0.6°). During this phase, the COG inclines forward, and the buttocks and shank are still at rest. Phase II, designated the momentum transfer phase, began with the buttocks lift-off the seat and ended at maximum ankle dorsiflexion (the tilt angle of trunk θ₄ reached maximum). During this phase, the trend of trunk motion changes from forward flexion to backward extension, and the displacement of COG changes from Anterior-Posterior to Vertical direction. The phase III was designated the extension phase, which started after the ankle joint reaches the maximum dorsiflexion, and ended at the completion of the first hip extension (the trunk angle θ₃ reached maximum). During this phase, the shank extends backward, the hips continue to rise, and the trunk extends backward. The phase IV was designated the stabilization phase (the change of trunk angle Δθ₃ < 0.1°). In this phase, the body of subjects will have Anterior–Posterior and lateral sway slightly, and gradually change to a stable state. Since the end time of swaying process cannot be determined, it is difficult to define this phase. For the purposes of this paper and for calculations we have considered only phases I, II, and III.

Figure 2.

Four phases of STS. STS = sit-to-stand.

3. Results

3.1. Kinematic analysis

The kinematic parameters under different IFAs were obtained, mainly including the percentage of duration in each phase of the STS transfer, the peak velocity of each joint, peak tilt angles of trunk and shank and the peak angular velocity of each joint, as shown in Table 2. We find that percentage of duration in each phase of the STS transfer is different with different IFA, and the main differences are in phase I and phase II. The phase I of U15 takes 24.5% T, while phase I of N, U0 and U30 takes about 20% T, and the maximum difference is (U15-U0) 5.4%. The phase II of U15 takes the least time, about 30.8% T. In phase III, different IFAs have no significant effect on the percentage of duration of the phase, accounting for about 44% T.

Table 2.

Kinematic parameter.

	N	U0	U15	U30
Percentage of completion time
Phase I (%)	20.2	19.1	24.5	20.2
Phase II (%)	35.1	37.3	30.8	36.2
Phase III (%)	44.7	43.6	44.7	43.6
Peak velocity of joint
$V_{Shoulder}$ (mm/s)	767.2	769.8	772.5	788.6
$V_{Hip}$ (mm/s)	572.7	577.1	567.9	616.3
$V_{Knee}$ (mm/s)	170.7	166.9	201.7	206.6
Peak tilt angles of trunk and shank
${\theta }_{4max}$ (°) *	35.9 (4.4)	43.8 (7.1)	38.2 (5.6)	46.1 (6.2)
${\theta }_{5max}$ (°)	22.2 (4.0)	21.2 (4.1)	22.8 (4.2)	23.7 (4.5)
Peak angular velocity of joint
$w_{3max}$ (°/s)	−73.2	−83.8	−76.9	−83.7
$w_{2max}$ (°/s)	85.9	85.5	86.3	81.9
$w_{1max}$ (°/s)	−30.2	−27.3	−25.4	−30.1

Variables showing significant differences across conditions are indicated with an *.

${\omega }_1$ = angular velocity of knee, ${\omega }_2$ = angular velocity of hip, ${\omega }_3$ = angular velocity of shoulder.

We calculated the peak velocity of shoulder, hip and knee joint during the STS (Table 2). The data show that different IFAs will have a greater impact on the joint velocity. Under 4 different experimental conditions of IFAs, the peak velocity of shoulder joint has little difference. The peak velocities of hip joint have little difference of N, U0 and U15, but the peak velocity of hip joint of U30 is relatively high, about 616.3 mm/s. The peak velocities of knee joint have little difference of N and U0, and the peak velocities of knee joint have also little difference of U15 and U30. Apparently, the peak velocities of knee joint of U15 and U30 are larger than that of N and U0.

Through the significance analysis of the peak tilt angle of trunk (Table 2), we find that there is a significant difference between N and U0 (P = .000 < 0.05). There is no significant difference between N and U15 (P = .141 > 0.05). There is significant difference between N and U30 (P = .000 < 0.05). Among them, the biggest difference is N and U30, and the biggest difference is (U30-N) 10.2 °. At the same time, we make a significant analysis of the peak tilt angle of the shank, and find that different IFAs has little effect on the tilt angle of the shank.

We find that the angular velocity difference of shoulder joint of N and U0 is the maximum, about 10.6 °/s (Table 2). The angular velocity difference of hip joint of U15 and U30 is the maximum, about 4.4 °/s. The angular velocity difference of knee joint of N and U15 is the maximum, about 4.8 °/s.

3.2. Center of gravity (COG)

Through the calculation, we obtained the trajectories of the COG under 4 different experimental conditions of IFAs, as shown in Figure 3, and the overall trend presents an “L” shape. We divide the whole process into 3 phases. In phase I, the displacement of COG moves forward along the X direction. In phase II, the displacement of COG transfer from X to Y direction gradually. In phase III, the displacement of COG moves mainly upward along the Y direction. Comparing the COG trajectories of different experimental conditions of IFAs, we find that the COG trajectories of N and U15 almost coincide. The COG trajectories of U0 and U30 have a large forward displacement in the X direction, especially for U30.

Figure 3.

COG trajectories of different experimental conditions of IFAs. COG = center of gravity, IFAs = initial foot angles.

3.3. Plantar pressure parameters

We defined the plantar pressure parameter as the ratio of plantar pressure to steady-state plantar pressure during STS. The stability value represents the plantar pressure value of the subject after fully stand. The plantar pressure parameters of different experimental conditions of IFAs are shown in Figure 4. Through comparative analysis, we find that the plantar pressure parameter is the smallest for U30, and the value is 1.16; the plantar pressure parameter is the largest for U0, and the value is 1.47; the plantar pressure parameters of N and U15 are approximately equal, and the values are 1.29 and 1.35, respectively. In order to further compare the data of plantar pressure, we made statistics on the plantar pressure parameters, as shown in Table 3. In the table, T* represents the time when the plantar pressure parameters reach the peak.

Figure 4.

Plantar pressure parameters of different experimental conditions of IFAs. IFAs = initial foot angles.

Table 3.

Data of plantar pressure of different experimental conditions of IFAs.

IFA	T* (%)	Plantar pressure/ plantar pressure of steady-state	Plantar pressure of steady-state (N)
N	46.85	1.35	8.88
U0	38.84	1.47	7.49
U15	47.22	1.29	7.98
U30	39.78	1.16	9.46

IFAs = initial foot angles.

3.4. Dynamic margin of stability

We calculated the dynamic margin of stability under different IFAs, and the calculation results are shown in Figure 5. The dynamic margin of stability at different IFAs all showed a trend of first rising and then decreasing during the STS motion. In Phase I, the trunk leans forward and the COG moves forward. At this time, the dynamic margin of stability is < 0, and the body balance is mainly supported by the seat in this stage. Starting from stage II, the COG continues to move forward, and the dynamic margin of stability rises first and then falls, but always > 0, with the body is balanced on the feet. By comparing the dynamic margin of stability at different IFAs, we found that the trajectory of N and U15 are almost coincide, which was consistent with the previous results of the COG, and indicating that the states of U15 and N are basically consistent.

Figure 5.

Dynamic margin of stability of different experimental conditions of IFAs. IFAs = initial foot angles.

4. Discussion

4.1. Comparison with previous studies

The results obtained in this paper were compared and discussed with some similar previous studies. In the research of Tanaka R et al,^[35] Vicon motion capture system and Microsoft Kinect motion capture system were used to collect the central coordinates of the joint node, and the center of mass of each body part was synthesized to calculated the COG trajectory. In this paper, the coordinate of the joint node was obtained by high-definition camera. The same method is used to calculate the COG trajectory. Compared with the results calculated in this paper, the basic trend is roughly the same as those obtained by Tanaka,^[35] which proved the feasibility of this method, and provided a low-cost way to obtain the change of the COG position.

Some researchers measured the fall risk of the elderly by measuring the ground reaction force with a force plate, which divided the change process of the whole ground reaction force parameter into 3 stages for detailed analysis.^[36] In this paper, we obtained the plantar pressure parameters, we found that the overall trend was the same, but the way of phase division was different, and the time proportion of each phase was also different. This paper divided the phases according to the kinematic characteristics.

This paper explored the process of STS motion by controlling the IFA as a condition. Turcot^[37] studied for the first time the influence of toe positioning on body dynamics during STS transition. They conducted STS transition experiments based on the symmetry or not between feet and different IFAs, found that toe-out foot position affects knee flexion torque. In this paper, we also found that different IFAs can cause greater trunk forward inclination, resulted in forward center of gravity, increased ground reaction force, and affected stability performance and knee flexion moment.

4.2. The effect of IFA on kinematics

The kinematic parameters can reflect the motion characteristics of human STS transfer. Compared with the kinematic parameters obtained under 4 different experimental conditions of IFAs, we found that there were significant differences in the percentage of duration in each phase, the peak velocity of hip and knee joint, and the peak tilt angle of trunk and the angular velocity of shoulder joint. In the study of Turcot^[37] on how the IFA affects the body dynamics, they divided the STS process into 3 phases: T0 to T1, T1 to T2, and T2 to T3. They found that different IFAs lead to different percentages of duration in each phase, and the difference is mainly concentrated in T2 to T3. In this study, the percentage of duration in each phase is also calculated. We found that different IFAs will affect the percentage of duration in each phase, the difference is mainly concentrated in phase I and phase II. This is not consistent with the results of Katia Turcot study, which may be due to the different IFAs or the different phase division methods. The peak velocity of each joint of the subjects is affected by the IFA, especially for the hip and knee joint. We found that the peak velocity of the knee joint increases with the increase of the IFA, the knee joint motion is mainly concentrated in phase II and III. Therefore, we can infer that a larger IFA will accelerate the completion of phase II and III. The forward tilt angle of the trunk will directly affect the position of the COG. Therefore, we calculated the maximum forward tilt angle of the trunk under 4 different experimental conditions of IFAs. We found that if the IFA is large (U30) or small (U0), the peak tilt angle of trunk will increase accordingly. The difference of the peak tilt angle of trunk was small for N and U15. The effect of IFAs on trunk motion can provide a reference for the analysis of COG.

4.3. The effect of IFA on plantar pressure

The plantar pressure parameter can reflect the load on each joint of human body. When the plantar pressure parameter is larger, the load on each joint will increase accordingly. We found that the difference of plantar pressure parameters of N and U15 is small, which indicates that the IFA of N is about 15 °. In addition, with the increase of the IFA (U0-U15-U30), the peak of plantar pressure parameters decreased (1.47−1.29−1.16), which indicates that the larger the IFA, the less the load on each joint. Through these results, we believe that when developing rehabilitation training protocols for patients with lower limb disorders, clinicians can formulate different STS transfer strategies according to the specific condition of patients, and use different IFAs for STS transfer training, so as to increase or reduce the joint torque of patients and achieve better rehabilitation effect.

4.4. The effect of IFA on stability

BOS is the area on which the body rests and the area that provides support for the body. The larger the distance between feet, the wider the scope of BOS and the higher the stability of human body. Limits of stability (LOS) express that the maximum distance an individual is able or willing to move their center of mass in any direction without loss of balance or changing the BOS. The position relation of COG and LOS of subjects is shown in Figure 6, in which the purple area is LOS. We can see that the shape is an inverted cone with the foot as the vertex. When the COG of the human body is located in the LOS, the feet can effectively support the human body in a stable state. When the COG is located outside the LOS, the feet cannot support the human body to achieve balance and will be in a dangerous state. From Figure 6, we can see that the COG of the subjects is outside the LOS in the sitting position, and the seat and feet support the human body. In phase I, the trunk leans forward, the COG moves forward, and gradually approaches the LOS. In this phase, the body is mainly supported by the seat. At the beginning of phase II, when the buttocks are about to leave the seat, the COG is located at the edge of LOS. At this moment, the subject’s body is in the most unstable state. Since then, the weight of the body transfer to foot, the COG is always in LOS, and the body is in balance. The closer the COG is to the center of LOS, the better the stability is. This result is consistent with the calculation result of dynamic margin of stability in Figure 5. By comparing the COG of different experimental conditions of IFAs, we found that if the IFA is small or large (U0 or U30), the COG trajectories will be closer to the edge of LOS, resulting in the decrease of stability. In this study, we found that the larger the IFA, the smaller the plantar pressure parameters. This may be because larger IFA results in decreased stability of the body, the body sways slightly and the plantar also sways slightly, so the plantar pressure decreases. When the IFA is 15° (U15), the COG is closer to the center of LOS and has better stability. When clinicians develop rehabilitation training protocols for patients, they could position the IFA at about 15°, so as to increase the stability of patients in the STS transfer, and help patients with better STS transfer training, and gradually restore the lower limb function.

Figure 6.

Position relation of COG and LOS of subjects. COG = center of gravity, LOS = limits of stability.

4.5. Limitation

Through this study, we found that the IFAs will have a certain impact on the kinematics and dynamics of the STS transfer of the human body, which can provide clinicians with relevant judgment for the motion performance of patients. However, this study is based on healthy young people, so the results of the experiment cannot be directly applied to the elderly and patients with lower limb disorders. In the future research, we will recruit the elderly and patients with lower limb disorders as the research object, and deeply explore the influence of different IFAs on the kinematics and dynamics of these population during the STS transfer. In addition, there are some limitations in this study. First of all, this experiment only collected and analyzed the kinematic and dynamics parameters of the left side of the body, without considering the differences between the 2 sides of the body. Secondly, because it is difficult for phase IV to be defined, we only studied the phase I, II and III of STS motion, and the motion characteristics of phase IV are not clear. We can further explore phase IV in the future.

5. Conclusion

This paper mainly studies the effects under 4 different experimental conditions of IFAs on the kinematics and dynamics of human STS transfer. We carried out 4 different IFA experiments to obtain the human motion characteristics, including kinematics parameters, plantar pressure parameters and COG trajectory. By comparing the kinematic parameters, we found that there were significant differences in the percentage of duration in each phase during STS, the peak velocity of hip and knee joint, and the maximum tilt angle of trunk and the angular velocity of shoulder joint under 4 different experimental conditions of IFAs. The change of the maximum tilt angle of trunk is the most obvious. A larger or a smaller IFA (U0 or U30) will cause a larger tilt angle of the trunk. Through the analysis of plantar pressure parameters, we found that the larger the IFA, the smaller the plantar pressure parameters. In addition, we also obtain the effect of IFA on the COG trajectory. The larger or smaller IFAs will result in a larger displacement of COG in horizontal direction, thus reducing the stability. When the IFA is 15°, the COG is close to the center of LOS, which can provide a better stability. This paper summarizes the influence under 4 different experimental conditions of IFAs on STS transfer, so as to provide a starting point and bases for clinicians to develop rehabilitation training protocols and STS motion strategies for patient. The main limitation of this study is that the subjects were all healthy young people, not the elderly and patients with lower limb disorders, so the results cannot be directly applied to these groups.

Author contributions

Conceptualization: Shuo Yang, Zicheng Yi, Binwei Zhou, Qiang Xue.

Formal analysis: Shuo Yang, Binwei Zhou.

Methodology: Shuo Yang, Binwei Zhou.

Supervision: Qiang Xue.

Visualization: Zicheng Yi.