Keywords

1 Introduction

Weight-bearing Walking is closely related to people’s lives, for example, March training for troops, workers’ bear-loading, carrying the backpacks to school of students and so on. In the case of weight-bearing walking, the body will make the appropriate adjustments to achieve the body’s balance and stability because of the pressure. Research showed that a series of injury problems will incur if people bear load too much or bear the weight for a long time [1,2,3]. Related medical research also proved that it is easy to cause foot blisters, stress fractures, lower limb joint pain, back muscle strain and the waist dish outstanding disease Under the condition of weight-bearing walking for a long time [4, 5]. Research also found that weight bearing cause many adverse effects to children’s growth and development [6, 7]. Therefore, to study on weight-bearing walking is significant to improve the design of backpack and reduce the body’s fatigue.

In the relevant research field, studying the effect of weight-bearing walking on the human body took a large part, For example, the effects of weight - bearing walking on human gait and plantar pressure. However, weight-bearing walking is a behavior which caused human fatigue and if we carry out the research from the point of view of the composition and causes of fatigue, the results will be more practical significance, especially it provides a theoretical basis for the ergonomic design of weight-bearing. At present, there are four theories of great impact in this field: Levin’s conservation model [8], QquF model [9], theory of Fatigue Motivation [10], and clue competition [11]. Levin’s conservation model is a theoretical model of nursing work fatigue, it holds that fatigue is the behavior of the body to protect itself in short supply, but also a clue to the body’s ability to imbalance. The QquF model is proposed by Angelique et al. The model is the relationship between external stimuli and fatigue. The theory suggests that when the attractiveness of external stimuli increases, people’s feeling of fatigue will be reduced; when the external stimulus load increases, people’s feeling of fatigue will be increase. Attractions of external stimuli mean that quality is negatively correlated with fatigue; Load of external stimuli means that quality and fatigue are positively correlated. The theory of fatigue motivation is a kind of theory about the relationship between motivation, fatigue and ability consumption, which was put forward by the American psychologist Maier RF based on previous experiments. And, the theory possesses a certain dialectical, cognitive color and a powerful explanation to phenomenon of life. Pennebaker proposed the clue competition, which is a theory of external stimulus and fatigue and holds that the result of clue competition is a curve relationship between external stimuli and fatigue. Four theories above fundamentally advance a core idea that fatigue is caused by multiple factors together. But at present, the mechanism of human fatigue in motion state has not been well explained, leading the limitations to describe the fatigue state. And it can’t reflect the interrelationships between the various mechanisms.

In this paper, subjective and objective way was used together and the weight-bearing walking was as the research object. The Borg scale was used to quantify the fatigue caused by weight-bearing walking of young people. And then we established forecasting model of the fatigue of shoulder, waist and back by analyzing the characteristics of them.

2 Method

2.1 Participants

Sixteen undergraduates participated in the experiment who 3 of them are pre-experimental subjects and they were used to verify the feasibility of the experiment and improve the experimental design. Therefore, there were 13 participants in the formal experiment. All 16 participant were male who their age was 22–25 years, height was 170 ± 4.1 cm, weight was 72.5 ± 4.2 kg, and all of them were in good health. Subjects are required to avoid strenuous exercise before 24 h prior to the experiment and have a fully rest for fear that it would cause fatigue accumulation.

2.2 Apparatus and Task

The subjects were asked to walk on the treadmill carrying a 15 kg weight knapsack. The weight of 15 kg was a appropriate load based on pre-experiment and the load could avoid too light to lead subjects’ fatigue and also could avoid too heavy causing body damage. The ground slope was zero degree. The subjects’ walking speed 5 km/h and the walking time was 21 min with zero degree of treadmill’s slope. The walking speed of 5 km/h referred to the speed requirements of troops March. Experiment time was set of 21 min to allow the subject to achieve a high degree of fatigue during the experiment without losing its physical limit. When the subjects asked to stop walking, in other words, the subjects couldn’t continue walking with that walking speed, the experiment was over.

The force values of the shoulder, waist and back during walking were measured by a pressure measuring device. The arrangement of the pressure sensors was shown in Fig. 1. Every 3 min, the Borg scale (see Table 1) was used to inquire the fatigue feeling of the subjects. And the subjects were asked to have an adaptive training before the experiment and wear safety rope during the experiment. When the subjects feel that it’s hard to go on the experiment and appear unsteady gait, pale or chest tightness, etc., the experiment should stop immediately. Therefore, two security guards must be on the scene. One was in front of and the other was behind the platform. The experimental scene was shown as Fig. 2.

Fig. 1.
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The arrangement of force sensors

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Table 1. Comfort evaluation forms of shoulders, waist and backs
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Fig. 2.
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Weight-bearing walking experiment

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2.3 Procedure

When the subjects arrived at the experiment site, they performed the experiment according to the following process:

  1. (1)

    The subjects were instructed to know the procedure and the Borg scale;

  2. (2)

    The pressure sensor was fixed on every subject;

  3. (3)

    Set the slope of treadmill to 0° and launch it to start the experiment;

  4. (4)

    Subjects were asked the degree of fatigue feeling with Borg scale every 3 min and the data was recorded by experimenters.

  5. (5)

    The experiment was stopped After 21 min of walking of subjects.

3 Results

The average pressure on the left side of the shoulder of the 10 subjects was calculated every 3 min, as shown in Table 2.

Table 2. The pressure of subjects’ left shoulder (N)
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Similarly, the average pressure of the right shoulder could be calculated and the result was shown in Table 3

Table 3. The pressure of subjects’ right shoulder (N)
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Tables 2 and 3 showed that the force on subjects’ left shoulder and right shoulder was inconsistent during the weight-bearing walking. But there was a common characteristic of all subjects that the force on one shoulder is smaller and the force of other shoulder is larger. It was also different of the pressure on shoulder of different subjects.

In the same way, the pressure of the waist and back of 10 subjects were calculated as shown in Tables 4, 5, 6, and 7.

Table 4. The pressure of left side of subjects’ waist (N)
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Table 5. The pressure of right side of subjects’ waist (N)
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Table 6. The pressure of left side of subjects’ back (N)
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Table 7. The pressure of right side of subjects’ back (N)
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Seen from the data of Table 4, 5, 6, and 7, it could be found that the waist and back’s force regularities are similar to shoulders’. Furthermore, there was no rule of the force on the same side (left or right) of different people. Specifically, some subjects had a larger stress on left shoulder and a smaller force on right shoulder while others are the opposite. Meanwhile, the pressure on the waist and back of each subject were also different.

Two characteristics could be found from the data of Tables 2, 3, 4, 5, 6, and 7. The first was that the pressure on waist is much smaller than that of shoulder while the force on the back is smaller than waist. The reason for this phenomenon was that the shoulder is the main load-bearing area during weight-bearing walking. The pressure sensor on the shoulder was easier to fix than the waist and back and was always close to the surface of clothing during the experiment. The force on the waist and back was difficult to measure by sensor. On the one hand, during the experiment, limited by sensors’ area, subjects’ slightly shaking caused that the pressure sensor deviates from its original position or slack and it couldn’t be completely contacted with the body. The subject’s subjective fatigue score of the shoulder was higher than back and waist. That is to say, the main cause of fatigue was that the shoulders suffer sustained high-intensity pressure, which leading to the whole body fatigue. This proved that the shoulder is the most vulnerable part in the weight-bearing walking, which is consistent with the relevant research results. The second characteristic was that there is no consistency of the stress on the experimenters’ shoulder, waist, back. In the other words, when the stress on shoulder of one subject was higher than anther subject, it couldn’t make sure that the magnitude of stress on waist of one person is larger than another person. The reason was that each human is an independent individual and there is no exactly same man. Meanwhile, stress-bearing parts were not only the shoulder, waist and back.

4 The Model of Fatigue Evaluation

Impulse of calculus algorithm as follows:

$$ I = \int_{t1}^{t2} {fdt} $$
(1)

Where “I” is the impulse of the force, “t1” and “t2” are the time, and “f” is the force.

When the value of the force f was the average, the impulse of the force I was I = f × (t2 − t1). Taking into account the degree of fatigue would increase with the increase of time, the product of pressure of shoulder, waist, back and time could be calculated every 3 min. The product result could be regarded as a factor in the fatigue evaluation model. The Tables 8, 9, 10, 11, 12, and 13 showed the product which is the result of multiply time by the force of every part on human.

Table 8. The impulse of larger force side of subjects’ shoulder (Ns)
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Table 9. The impulse of smaller force side of subjects’ shoulder (Ns)
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Table 10. The impulse of larger force side of subjects’ waist (Ns)
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Table 11. The impulse of smaller force side of subjects’ waist (Ns)
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Table 12. The impulse of larger force side of subjects’ back (Ns)
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Table 13. The impulse of smaller force side of subjects’ back (Ns)
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The Borg score and impulses of the bigger force value and the smaller force value on each part of human were plotted. As shown in Figs. 3, 4, 5, 6, 7, and 8.

Fig. 3.
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The scatter diagram of Borg score and impulse of larger force side of shoulder

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Fig. 4.
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The scatter diagram of Borg score and impulse of smaller force side of shoulder

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Fig. 5.
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The scatter diagram of Borg score and impulse of larger force side of waist

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Fig. 6.
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The scatter diagram of Borg score and impulse of smaller force side of waist

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Fig. 7.
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The scatter diagram of Borg score and impulse of larger force side of waist

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Fig. 8.
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The scatter diagram of Borg score and impulse of smaller force side of back

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The above graphics indicate that the pressure impulse has a strong linear relationship with the subjective feeling of fatigue. Therefore, single linear regression models of fatigue evaluation of shoulder, waist and back were established that the independent variable is the pressure impulse I of the shoulder, waist and back, and the dependent variable is the subjective feeling of fatigue B, as shown in Eqs. 2–7.

The fatigue model of the larger force side of shoulder:

$$ {\text{B}} = 1.0 \times 10^{ - 3} I + 1.223 \quad {\text{R}} = 0.958 $$
(2)

The fatigue model of the smaller force side of shoulder:

$$ {\text{B}} = 1.0 \times 10^{ - 3} I + 1.271 \quad {\text{R}} = 0.944 $$
(3)

The fatigue model of the larger force side of waist:

$$ {\text{B}} = 1.0 \times 10^{ - 3} I + 1.029 \quad {\text{R}} = 0.947 $$
(4)

The fatigue model of the smaller force side of waist:

$$ {\text{B}} = 2.0 \times 10^{ - 3} I + 1.152 \quad {\text{R}} = 0.907 $$
(5)

The fatigue model of the larger force side of back:

$$ {\text{B}} = 2.0 \times 10^{ - 3} I + 0.859 \quad {\text{R}} = 0.934 $$
(6)

The fatigue model of the smaller force side of back:

$$ {\text{B}} = 4.0 \times 10^{ - 3} I + 0.907 \quad {\text{R}} = 0.925 $$
(7)

The fatigue value of shoulder, waist and back of the remaining three persons were calculated with the above model. The value was compared to the subject feeling of fatigue during the experiment of the corresponding body part. The error could be used to verify the model’s precision or reliability. The fatigue value of model and the value of experiment were shown in Tables 14, 15, and 16. It could be seen that the error of each model is less than ±1.3, which satisfies the accuracy requirement and prove the model’s precision.

Table 14. The verification result of the first subject
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Table 15. The verification result of the second subject
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Table 16. The verification result of the third subject
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Considered the definition of impulse in physics, the impulse of shoulder, waist and back were chose as the fatigue evaluation index of weight-bearing walking. Impulse is the cumulative effect of force on time, which is consistent with the increase in body fatigue over time. At the same time, the independent variable of the model combines the relation of force and time, which fatigue is result from a complex, multi-factor working together. The fatigue evaluation model’s high linearity showed that the model is feasible and it proved a theoretical fact that fatigue is caused by a complicated and multi - factor common.

5 Conclusion

It is very important to carry out load-bearing walking fatigue research, to evaluate the damage caused by different load to human body, to scientifically design load task and to improve the design of backpack. So the specific conclusions of this study are as follows:

  1. (1)

    The shoulder is the main area of bearing pressure and also is the most easily fatigue during the process of weight-bearing walking.

  2. (2)

    During the weight-bearing walking, the force of the different side of the human’s same parts is different and the difference is significant.

  3. (3)

    The fatigue evaluation model combines the subjective feeling of fatigue and the objective index of the impulse of shoulder, waist and back and it avoid the simplification of fatigue evaluation model of current study, so the model of this study is a improvement.