Introduction
n a natural gait, the static phase plays an important role due to the foot contact with the ground [1] and the application of the Ground Reaction Force (GRF) [2]. Because according to Newton’s third law, the amount of this reaction force is equal to the body weight and in the opposite direction to the force that the body exerts on the ground as a result, the articulated moments created are directly related to the amount of GRF and the vertical distance from the center of force to the joint [3]. The vertical component of ground reaction force to the joint force is of greater biomechanical importance due to its greater value than the anterior-posterior and internal-external components, as well as its shape of stability in individuals (ground reaction force increases by up to 120% of body weight from the static phase [4, 5]. Improper absorption of the impact forces of the foot on the ground is one of the biomechanical parameters that can cause problems for the foot due to the misalignment of different parts in the lower limb. According to the graph ofthe graph ground reaction force (Figure 1) there are to pwo peak points, there are two peak points [6, 7]; the first peak force is produced by the foot hitting the ground, and the second peak force is produced by the pressure of the toe to the ground [2, 8]. The ground reaction is a good criterion for identifying and classifying people based on the pattern of forces when walking, which can be different in the peak of forces.
In previous studies, the relationship between increasing the vertical component of the GRF and increasing the risk of injury to the body [6, 5, 4] has been investigated. If these forces are repeated, there is a possibility of joint damage, especially in continuous weighting [9, 10, 11, 12, 13, 14].
Changes in the insoles can also change the lever arm of the GRF [15]. Although based on biomechanical principles and several studies, the use of insoles reduces the first peak of reaction force and loading rate, other studies have reported that insoles increase the first peak of reaction force in healthy people. Even in a study, it has been shown that different insoles have little effect on the kinetic variables of people while walking [16, 17, 18]. However, part of these differences is due to the kinetic and kinematic effects of displacement of the pressure center [19].
Effect of factors, such as insole material on the vertical reaction force of the earth has been considered by researchers [15] in some studies. With decreasing insole stiffness, the vertical reaction force of the earth increased [20]; however, in some studies, no significant difference was observed in the effect of insole stiffness [14]. It is unavoidable that vertical forces can be reduced by making interventions in shoes and insoles for easier walking, but it is worth considering that in various studies, there is insufficient instability in different methods and factors on the reaction forces. This study aimed was done to determine the various factors and methods affecting the vertical components of reaction force and loading rates when walking in healthy individuals.
Materials and Methods 
The present review study was conducted with extensive search in databases, such as ISI web of knowledge, Embase, PubMed, and Proquest from 1995 to 2019 using the keywords of ground reaction force, foot orthosis, walking, and impact force of long-term standing. To find more relevant articles, the sources of the selected articles were also examined. After reviewing the titles and abstracts of articles focusing on orthoses and GRF by the authors, in addition to answering the characteristics and questions of researchers in the next phase, the following inclusion criteria were considered for selecting and reviewing articles:
1. The article must be in English.
2. The article must be published from 1995 onwards.
3. The parameter measured in each article should be at least one of the parameters considered in the study (walking, foot orthoses, GRF, and vertical components).
The control level of each article was rated 5 to 1 for randomized control trials, prospective controlled trials (cohort), case-control studies, studies with pre/post-test design, and observational consensus-based clinical studies, respectively. Then, the quality of the articles was examined by the Down and Black tool and Pedro index [21 22]. It should be noted that the accuracy and validity of this test have been proven as a criterion for reviewing and evaluating the quality of research studies. The quality of each article was evaluated separately by two experts, and then, using the Pearson correlation test, the relationship between the results was checked through SPSS V 16 software.
Results
Of the 82 articles obtained, 21 articles met the inclusion criteria (Figure 1). According to Table 1, four domains, including the effect of molding, post, material type, and gender were obtained. 




Seven articles examined the effect of molding on the GRF, and five articles, directly and indirectly, had examined the effect of the insole with a post on the GRF. The effect of material type on the GRF had been investigated in seven studies. Regarding gender, only two articles had focused on GRF without orthoses.
Some studies on the GRF had focused on unhealthy humans. There was an obvious discrepancy between the molding insoles and the change in the maximum vertical force of the ground reaction loading rate. Regarding the effect of the material, it can also be said that it may lead to a further reduction in the pressure of the sole of the foot. Regarding gender, although women have a higher loading rate in the GRF than men without the use of a device, no study was found on the use of insoles and the effect of gender.
According to the results of the quality evaluation of articles, about half of the studies were randomized clinical trials (rate of 5). The high scoring, external validity, internal validity, and Pedro index all indicated the relatively good level of trust and credibility of the articles.
Discussion and Conclusion
This study was done to investigate the changes in GRF produced by foot orthoses due to different effective factors and methods. Determining the effect of different components on GRF in the foot will help us in prescribing orthoses to increase the quality of life of people. The results showed that the four domains have different effects on the vertical component of ground reaction force.
Impact of molding
One of the features of foot molding for making insoles and shoes is that it creates a relatively complete contact with the sole of the foot, which is effective in reducing the forces and its proper distribution [6]. Molded orthoses with more contact between the foot and orthoses increase shock absorption and decrease GRF. A reduction in the first peak of the vertical ground reaction force by the semi-rigid insole has been able to prevent joint destruction damage to some extent [23] and on the other hand, reduce the pressure of the heel and the inner area of ​​the front of the foot and the impact of forces to a quarter percent compared with without insole [6]. Significant reduction in leg fatigue after wearing these insoles [24] and a reduction in loading rate are evident [23, 25]. In contrast to these studies, Sloss reported a 3% -5% increase in the vertical force component in the first peak by carbon-molded orthoses [26] or MacLean did not observe significant changes in maximum impact force and loading rate with this type of orthosis [27]. The reason why these two studies are different despite proving the biomechanical alteration of the foot in the correct position with the insole may be related to other factors, such as the material and shape of the orthosis.
Post effect
One of the purposes of using foot orthoses is to place the foot and lower limb in a better functional position while maintaining a proper alignment and to improve the absorption impact [16, 28]. In a study combining the two methods of molding and post, a decrease in vertical loading rate was observed. Also, a significant difference in the reduction of the maximum vertical reaction force of the earth and the loading rate of the vertical reaction force of the earth during the static phase was proved by post [29, 30, 31]. Rome et al., along with the post, changed the insole material and density and the shape of the insole, and subsequently observed a decrease in loading change [32].
Impact of soft materials
Insole material is one of the factors with a great impact on changing reaction forces. Visco-elastic insoles, by following the physiological structure of the body in the face of excessive forces, have the best efficiency in absorbing impact and reducing the reaction forces of the ground [9]. Supporting softness next to a high heel shoe is recommended to control lateral movements and prevent foot injury [33, 34]. In some studies, a significant difference in fatigue and discomfort of people wearing visco-elastic insoles was observed [14, 21, 29]. The reduction of the maximum impact force of the ground and the loading rate has been more than the use of shoes [35], especially at the beginning of the static phase [36]. Alirezaie announced a reduction in the vertical load of GRF with a hard cushion insole and without soft shoes [37].
Contrary to these reports, some studies did not change the characteristics of the GRF in the use of viscoelastic insoles [38] and even soft orthoses, such as silicone pads did not reduce the impact force as expected, and we cannot support the hypothesis that heel cushions provide better shock absorption [39] or orthoses with softer materials than semi-rigid ones do not necessarily lead to a further reduction in sole pressure [40].
This difference in studies suggests that changes in gait parameters, including the GRF, depend on various factors, such as age, type of insole, the material of the shoe, and even the longevity of the soft insole.
The effect of gender
Although the gait parameters in men and women are somewhat different, the gait symmetry, which indicates the level of GRF in each limb, is slightly different between both genders [41]. Min-Chi Chiu stated that because women are shorter than men, they need more muscle activity to achieve a predetermined constant speed, which increases the maximum loading rate compared with men [42]. In one study, the loading component of GRF was higher in women than men despite different speeds [43].
Finally, to achieve more accurate results and resolve the contradiction between studies on the effect of material type and orthosis type mechanism on the vertical components of the reaction force, more research should be done with modern motion analysis systems [44]. One of the reasons for comfort and improvement of life indicators is related to the effect of insole comfort in terms of gender and proper distribution of GRF in the balanced use of leg muscles and absorption of blows caused by walking on feet and ankles. GRF may be affected by foot orthosis, indicating the existence of various and influential factors, such as age, gender, and orthosis preparation method, that should be further studied.
Conclusion
Determining the effect of different components on GRF in the sole of the foot can help us to better prescribe and understand the factors affecting the quality of life. Our results indicated that factors, such as gender, post, molding, and type of material, despite the effect on the GRF, provide different sign. Different variables should be considered in designing and manufacturing foot orthoses using different methods, and also changes in shoes and orthoses are needed o be considered to change the GRF.

Ethical Considerations
Compliance with ethical guidelines
This article is a meta-analysis with no human or animal sample. There were no ethical considerations to be considered in this research.

Funding
This research did not receive any grant from funding agencies in the public, commercial, or non-profit sectors. 

Authors' contributions
Conceptualization and supervision: Ali Reza Taheri; Methodology: Mohammad Mousavi; Investigation, writing – original draft, and writing – review & editing, data collection, data analysis: Both author. 

Conflict of interest
The authors declared no conflict of interest.

Acknowledgments
We would like to thank MS Rahmani, the head of the library of the Faculty of Rehabilitation Sciences, for providing us with resources.


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