The Ankle Energetic Effect of Functional Insoles on Walking


 Background：Although insoles made of various materials and shapes have been developed to improve performance in sport activities, few objective evaluations on their effectiveness have been conducted. We investigated the effect of insoles supporting the cuboid bone and anterior part of the calcaneus in healthy individuals.Methods:The subjects included 18 healthy males and females. They walked in standardized shoes with a flat insole (a flat insole made of polyurethane without an arched shape on the surface) and a functional insole (made of carbon and supporting the cuboid and anterior part of the calcaneus). We used a three-dimensional motion analysis device and a force plate to analyze gait and quantitatively compared the effect of functional insoles.Results:There was no difference in the parameters of gait analysis (walking speed, cadence, step length, stride length) between flat insoles and functional insoles. The functional insoles reduced ankle power without reducing walking ability. A comparison between Group A (n = 7), in which the left-right difference in ankle power was more than 20%, and Group B (n = 11), in which the left-right difference in ankle power was less than 20%, indicated that the use of functional insoles reduces the left-right difference of ankle power in the group with a larger difference in power.Conclusion：We believe that the use of functional insoles reduced ankle power without reducing walking ability and equalized left-right power. It may therefore reduce the burden on the muscles of the unilateral lower limbs and improve sport performance.Trial registration：The medical research ethics review committee for individuals at Gunma University (study number HS2017-229)　Registered 20 febluary 2018, https://upload.umin.ac.jp/cgi-open-bin/ctr_e/ctr_view.cgi?recptno=R000034362


Abstract Background
Although insoles made of various materials and shapes have been developed to improve performance in sport activities, few objective evaluations on their effectiveness have been conducted. We investigated the effect of insoles supporting the cuboid bone and anterior part of the calcaneus in healthy individuals.

Methods:
The subjects included 18 healthy males and females. They walked in standardized shoes with a at insole (a at insole made of polyurethane without an arched shape on the surface) and a functional insole (made of carbon and supporting the cuboid and anterior part of the calcaneus). We used a threedimensional motion analysis device and a force plate to analyze gait and quantitatively compared the effect of functional insoles.

Results:
There was no difference in the parameters of gait analysis (walking speed, cadence, step length, stride length) between at insoles and functional insoles. The functional insoles reduced ankle power without reducing walking ability. A comparison between Group A (n = 7), in which the left-right difference in ankle power was more than 20%, and Group B (n = 11), in which the left-right difference in ankle power was less than 20%, indicated that the use of functional insoles reduces the left-right difference of ankle power in the group with a larger difference in power.

Conclusion
We believe that the use of functional insoles reduced ankle power without reducing walking ability and equalized left-right power. It may therefore reduce the burden on the muscles of the unilateral lower limbs and improve sport performance. Background Although insoles made of various materials and shapes have been developed to improve performance in sport activities, few objective evaluations on their effectiveness have been conducted. Moreover, the functions required for insoles are not only shock absorption and support for the foot arch, but also improvement of sport performance in sport activities. Although sport performance is often evaluated through breaking competition records, etc., few kinematic quantitative evaluations have been conducted, so the effects thereof remain unclear.
Regarding the effects of insoles other than in terms of sport activities, reports have been made on whether the use of insoles affects the standing balance of the elderly. Several studies on at and textured insoles have reported no difference in standing balance using textured insoles compared to at insoles. Among these, insoles with a pyramid-shaped projection on the surface 1) , those with raised edges 2.3 , those with round plastic bumps 4 , those with granules 5) , and sandals equipped with spike insoles are reported to improve standing balance. As a reason for this, one common effect of aging is a loss of skin sensation, which is thought to be correlated with impaired posture control and an increased risk of falling.
Clinically, insoles have been prescribed to protect the plantar fascia and reduce pain in patients suffering from plantar pain 8) . Lateral wedge insoles are prescribed to prevent bowlegs in patients with varus arthrosis deformans 8) . As a treatment for at feet, insoles are also prescribed as arch support to prevent the medial arch from being attened and everted 8) . These cases are often subject to subjective evaluations such as comfort and pain reduction, with few quantitative analyses having been conducted.
It has been reported that regarding the left-right difference in movement of the lower limbs of healthy individuals, movement is symmetrical while walking on a treadmill, with the left-right difference decreasing with increased speed among individuals suffering from vertical displacement of the knee 9) . Regarding left-right differences in the percentage of time standing on each foot during one walking cycle, it has also been reported that the time standing on the non-dominant leg was signi cantly longer than that on the dominant leg during the standing phase III (from heel landing to toe rising) 10) . In other words, it appears that left-right differences exist in the movement of the lower limbs during normal walking. From these reports, although a dynamic evaluation has been conducted on the balance of the lower limb movement, no evaluations into further details have been conducted.
In this study, we captured an opportunity to evaluate functional insoles (made of carbon and supporting the cuboid and anterior part of the calcaneus) developed to improve performance in sport activities. The functional insoles used herein are expected to improve the balance of the standing posture via the structure of foot support. We will verify the improvement of left-right balance when walking by measuring the ankle moment and power on sagittal plane. Assuming that there is a difference in the left-right moments and power on sagittal plane during normal walking of healthy individuals, we conducted a kinematic analysis on the effects of using insoles from the data of the three-dimensional motion analysis device and the force plate to investigate the effects on walking.

Participants
The subjects included healthy individuals aged 18 to 60, excluding those who require walking aids such as canes, as well as those with neuromuscular diseases, cardiovascular diseases, respiratory diseases, or motor diseases that may in uence their walking. The study was discontinued if a subject complained of pain or discomfort when using the insole. 18 males and females (9 males, 9 females, from 20 to 63 years old, average 43.9 ± 14.4 years old) satisfying the above criteria were selected.

Procedures
For comparison of the insoles among subjects, we asked subjects to walk wearing standardized shoes (no heel counter and shank) with normal at insoles (hereinafter, " at insole") without an arched shape on the surface and made of polyurethane and wearing carbon insoles (BMZ insole, BMZ. Inc, Gunma) (hereinafter, "functional insole") (Figs.1 and Additional le 1 ) and conducted a gait analysis using a threedimensional motion analysis device (Vicon MX, Vicon Motion Systems Oxford. UK,) and a force plate (AMTI, Watertown, MA, USA).
For the three-dimensional movement analysis, analysis markers were attached to a total of 28 points: 4 points at the head, C7, Th8 spinous processes, the midpoint of the left and right superior posterior iliac spines, the acromion, the external humerus condyle, the radial styloid process, the superior anterior iliac spine, the point 1/3 from the greater trochanter on the line between the superior anterior iliac spine and the greater trochanter, the external femoral condyle, the midpoint of the line shape between the external femoral condyle and the ankle lateral malleolus, the ankle lateral malleolus, the midline of the facies posterior to the calcaneus, and the head of the second metatarsal bone.
Patients walked normally in standardized shoes on a walkway of approximately 10 m without knowing which insole they are wearing. The insoles to be worn were randomly chosen. No instructions on walking speed were given and the subjects walked at their optimal speed. After two trials each with both the functional insoles and the at insoles, a total of three measurements were taken.
In order to investigate any correction due to the insoles, a past study compared the control conditions wearing the shoes the subjects regularly wore with 3 mm at insoles and wearing standardized shoes with the same insoles 11) . As a result, the conclusion recommends using the footwear of the participants as the control condition, because standardized shoes, compared to the usual shoes, signi cantly affect the knee adduction impulse, ankle abduction moment, and vertical grounding reaction load factor at the time of knee abduction. However, since walking needs to be evaluated with "shoes + insole", this study focused on the difference in the function of insoles by using the same shoes (no heel counter and no shank).

Data analysis
All subjects completed the measurements without any adverse events. The obtained data was imported into the VISUAL 3D Ver. 6 Visual 3D software program, ver. 6 (C-Motion, Inc., Germantown, MD, USA) to calculate the walking parameters and kinematics data.
Walking speed, cadence, step length, and stride length were recorded as walking parameters. Walking speed was de ned as the speed of movement of the center of gravity in the direction of travel, calculated as the average of 5 m as the middle point during walking. Cadence was also calculated at 5 m as the midpoint during walking. The step length and stride length were also calculated as the average of 5 m as the midpoint during walking.
We calculated hip and ankle moment on the sagittal plane at terminal stance, and analysed them. The peak values of hip extion moment, hip power on the sagittal plane, ankle planter extion moment, and ankle power on the sagittal plane were recorded.

Statistical analysis
We compared the data when wearing at insoles and that when wearing functional insoles. Group A (n = 7) included those with a left-right difference of more than 20% in ankle power when using at insoles, while Group B (n = 11) included those with a difference of less than 20%. We compared the subjects within each group.
Regarding statistical processing, we performed a paired t-test for the walking parameters and Wilcoxon's signed rank test for left-right differences in joint moment, power, and ankle power, with <0.05 considered to be a signi cant difference.

Results
The three-dimensional motion analysis revealed no signi cant difference in the walking parameters between the at insoles and the functional insoles. No differences in hip extion moment or ankle plantar extion moment were observed between the two groups. The functional insoles signi cantly reduced ankle power on sagittal plane compared to the at insoles (P < 0.05). There was no signi cant difference in hip power on sagittal plane between the functional insoles and the at insoles (Table1).
In Group A, in which the left-right difference in ankle power on the sagittal plane is large with at insoles, walking with carbon insoles signi cantly decreased the left-right difference in the power compared to walking with at insoles (Fig.2). However, the insoles caused no difference in Group B, in which the leftright difference in power was small (Fig.3). Table 1 Comparison of spatiotemporal data, hip and ankle sagittal plane kinetic data between at insole and functional insole (Mean ± SD) There was no difference in the walking parameters gait velocity (m/s), cadence (steps/minute), step length (m), stride length (m), peak hip moment (Nm/kg), peak hip power (w/kg), peak ankle plantar extion moment (Nm/kg) , with the exception of peak ankle power (w/kg) which had a signi cant difference P<0.05 .

Discussion
We found that the functional insoles reduce ankle power without affecting walking ability. The reduced power immediately before raising the toe means reduced power of the gastrocnemius muscle and the soleus muscle, which create the ankle's plantar exion force. Reduced muscle power may reduce muscle fatigue and prevent a decline in sport performance during prolonged sport activities.
Reduced ankle power during "kicking the ground to move the foot forward" is known to Increase the hip joint power as a trade-off 12) . However, no change in hip power was observed in this study. Reduced ankle power without increased hip power may also lead to a decrease in overall energy consumption.
Since walking is performed symmetrically, the ankle power while walking is considered to involve no leftright difference. However, it is known that dominant and non-dominant legs have different reaction time and muscle strength 13) . In this study, 7 out of 18 subjects had a left-right difference of more than 20% in ankle plantar exion power during walking with the at insoles. In these 7 subjects, we con rmed that the use of functional insoles reduced the difference in ankle plantar exion power. In other words, the use of functional insoles equalized the left-right power. They may reduce the burden on the muscles of the unilateral lower limbs and prevent the deterioration of sport performance.
The insoles used in this study provide support to the cuboid bone and the anterior part of the calcaneus and are made of light and thin highly rigid carbon. Directly pushing the medial longitudinal arch up, as with polyurethane insoles, is effective for postural stability at rest, but it may hinder the original functions of the arch, such as shock absorption, by changing the arch height and momentum to move forward. The carbon insole used in this study is designed to directly support the cuboid bone. Especially for athletes, it is more useful to hold the arch by supporting the cuboid bone than directly pushing the medial longitudinal arch up.
A recent study concluded that two unique arches on the human foot enabled bipedal walking 14) . Most previous studies have focused on the medial longitudinal arch (hereinafter, MLA), which extends from the heel to the ball of the foot. However, it was revealed that the transverse tarsal arch (hereinafter, TTA) transecting the foot is related to more than 40% of the rigidity of the foot. Only the genus Homo have a fully developed MLA and TTA. These nding suggests that the combination of two adjacent arches creates the longitudinal rigidity of the foot. The ndings that two unique arches in human feet enabled e cient upright walking indicate that support of the arch by the insole not only affects the MLA but also the TTA derived therefrom.
In this way, the joint group around the heel plays a role of a torque converter (force converter) that controls the height of the arch, the actions of the toes of the foot, and the direction and inclination of the lower leg. Therefore, we believe that the foot can do its job more effectively when the major joints in the foot work at their full potential.
For the treatment of at feet, one common deformation in young individuals, a previous study evaluated the hardness of insole materials and the height of the arch support. The results indicated that correction of the height of the arches involved increases in both the hardness of the material and the height of the support 15) . Therefore, while static insoles are suitable for the treatment of at feet, such insoles are inappropriate for sport because they are related to the movement of the articulatio subtalaris. The reason for this is, if the insoles are hard, the subtalar joint may open (the arch is bent and absorbs shock) or close, making it di cult to evoke the locking system of the midtarsal joint and stabilize the foot. In addition, sport insoles require thinness and high rigidity to improve performance. For this reason, we used carbon insoles (made from carbon ber consisting primarily of carbon) that combines the two functions as functional insoles.
There are two types of carbon bers: PAN-based, that is produced by carbonizing polyacrylonitrile bers; and pitch-based, that is made by carbonizing and graphitizing the pitch as residues of the coal and petrochemical industries after melting prevention 16) . In this study, we used PAN-based carbon insoles. Too much pressure on the sustentaculum tali of the calcaneus may limit movement. To avoid this, and also taking into consideration anatomical and kinematic perspectives, this study used insoles with nothing in the forefoot part. We believe the use of such insoles may equalize the work of the left and right ankles, decrease the work of the lower limbs using much power, and reduce the fatigue of the unilateral lower limbs.

Limitation
Going forward, we need to think about "What made the adjustment?". Why did the left-right difference of Group A become smaller? While the at insoles required large force, the carbon insoles required less power. Although we found the use of carbon insoles lead to the equalization of left-right power, further quantitative evaluation is necessary.

Conclusion
We evaluated the effects of functional insoles on walking in healthy individuals using the threedimensional motion analysis device and a force plate. The functional insoles reduced the power of the ankle plantar exion muscles. Regarding the ankle power obtained from the force plate, the use of functional insoles in the group with a large difference in left-right power signi cantly reduced the left-right difference in power, equalizing the left-right power of the ankle plantar exion muscles. In sport activities, the functional insoles are believed to function to reduce the fatigue of the lower leg muscles as well as the burden on the unilateral legs, in addition to improving long-time sport performance. This study was approved by the medical research ethics review committee for individuals at Gunma University (study number HS2017-229).

Consent for publication
Not applicable

Availability of data and materials
The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.

Competing interests
A part of the research cost was funded by BMZ and Gunnma sports doctor council.
Its functional insoles, at insoles and shoes used in this study were provided free of charge by BMZ.
Funding Figure 1 Functional insole (top view and side view) (thin and rigid insole made of carbon) The functional insole is made of carbon, which is thin and highly rigid, with a structure that supports the cuboid bone and the anterior part of the calcaneus. Differences ancle power on sagittal plane in right and left ankle at Group A In Group A, in which the leftright difference in power is large with polyurethane insoles, walking with carbon insoles signi cantly decreased the left-right difference in power compared to walking with polyurethane insoles.