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Year : 2022  |  Volume : 22  |  Issue : 2  |  Page : 51-60

Gait during simulated and natural leg length discrepancy: Inconsistencies in the literature

Department of Physical Education and Sports Sciences, University of Delhi, New Delhi, India

Date of Submission19-Jun-2022
Date of Decision25-Jul-2022
Date of Acceptance27-Jul-2022
Date of Web Publication30-Aug-2022

Correspondence Address:
Prof. Dhananjoy Shaw
Indira Gandhi Institute of Physical Education and Sports Sciences, University of Delhi, B.Block, Vikaspuri, New Delhi - 110 018
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/sjsm.sjsm_14_22

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For convenience, researchers have simulated leg length discrepancy (LLD) by installing an insole or a heel lift in one foot of healthy participants. However, the results of these studies have contradicted the results found in the studies that investigated naturally occurring LLD. The objective of this article was to discuss the methodological considerations of simulated and natural LLD conditions in detail and highlight the inconsistencies in their findings. The studies simulating LLD demonstrate the acute effect of LLD for this reason they should be disparaged for not being physiologic, leading to non-LLD characteristic gait patterns in terms of kinetic and kinematic parameter's. This puts a question mark when it comes to generalizing the results of simulated studies to those that have true LLD.

Keywords: Biomechanics, gait, kinematics, leg length discrepancy, simulated leg length discrepancy

How to cite this article:
Andrabi SM, Shaw D. Gait during simulated and natural leg length discrepancy: Inconsistencies in the literature. Saudi J Sports Med 2022;22:51-60

How to cite this URL:
Andrabi SM, Shaw D. Gait during simulated and natural leg length discrepancy: Inconsistencies in the literature. Saudi J Sports Med [serial online] 2022 [cited 2023 Sep 29];22:51-60. Available from: https://www.sjosm.org/text.asp?2022/22/2/51/355192

  Introduction Top

Leg length discrepancy (LLD) is a condition where the length of the right and left legs varies noticeably in the same individual. When LLD is sufficient, it can cause many functional (related to gait and posture) as well as musculoskeletal problems.[1]

To assess the effects of LLD, screening of the subjects is necessary. There are various ways to evaluate the amount of LLD in an individual which can be broadly classified into radiographic and clinical measurements. The radiographic method is considered the gold standard for evaluating LLD;[2] however, this measurement technique is expensive and exposes the subjects to harmful radiation. On the other hand, clinical methods are cost-effective and have an acceptable validity when compared to the radiographic method.[3] For this reason, the researchers have adopted the clinical methods, with tape measurement being frequently used in many studies as a tool to evaluate the degree of LLD.

One author had to screen 30 subjects to find an LLD of >1.5 cm, meaning that a large sample needs to be screened before incorporating them into a study.[4] To overcome this problem, the researcher adopted a method to simulate LLD by installing an insole or a heel lift in healthy subjects, thus mimicking the condition acutely where the two legs have varied lengths.[5],[6],[7],[8],[9],[10],[11],[12],[13],[14],[15] This method has one advantage, i.e., the degree of LLD can be increased to as much as they want. Furthermore, it seemingly allows the researchers to draw tangible conclusions by saying that a particular degree of LLD is significant or insignificant.

On the other hand, many efforts have also been made to research subjects who have inherent LLD.[12],[16],[17],[18],[19],[20],[21],[22] The researchers have usually divided the subjects into control group and experimental group, with subjects having minor LLD in the control group and subjects with significant LLD in the experimental group.

The literature is divided on the amount of discrepancy that is significant to cause deviation in an able-bodied gait with inconsistencies between simulated and natural LLD studies. Hence, the objective of this study was to look into the methodological consideration of both simulated and natural LLD studies to see how their findings stand out.

  Methods Top

Inclusion/exclusion criteria

The studies investigating gait and LLD with no other musculoskeletal problems or neurological problems were included in this review. Studies that simulated the LLD by applying raises were also included in this review since they are the focus of this study.

Search strategy

The search was made in the databases such as ScienceDirect, PubMed, and Europe PMC. The following keywords were used to search for the article: “leg length discrepancy,” “leg length inequality” and were mixed with “gait,” “walking” to find the purposeful articles using Boolean operators such as OR. Eleven and eight studies investigating simulated and natural LLD, respectively, were identified for this review study. [Figure 1] shows the scheme of the process through which studied were included for this review study.
Figure 1: Flow chart of selection process of studies (source of data)

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Inherent leg length discrepancy interference

Equal leg length in the human population is a rear occurrence,[23] which means that most of the population (90%) has some amount of LLD.[24] This develops into a problem of underestimation or overestimation in the simulated studies with the inherent LLD either increasing or decreasing the LLD depending upon the intervention side and the inherent side of the short leg. The inherent LLD can either add to the artificial LLD if the lift is applied to the Long leg, or it can offset the artificial LLD if the heel lift is applied to the short leg, thereby partly correcting the induced LLD. This inherent LLD can interact and potentially fiddle with the results of the simulated results and will be referred to as inherent LLD interference (ILLDI) from now on.

Prior leg length discrepancy screening of subjects in the simulated studies

There are many studies that did not account for ILLDI,[5],[7],[8],[13],[14] while other studies have controlled this factor with prior screening with the tape measurement method[6],[10],[12],[15] and palpation meter method.[9] Most of the studies that screened their subjects for LLD set the acceptable LLD at <0.5 mm,[9],[10],[12] while two studies set the acceptable LLD at <1 cm [Table 1].[6],[15]
Table 1: Studies investigating simulated LLD

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It however does not make much sense to allow a discrepancy of up to 1 cm and then go on to study the intervention effect of 1 cm or even 2 cm LLD. The reason is that if you are simulating a condition of 1 cm, the allowed LLD which is up to 1 cm will chip in, and thus, the researcher will overestimate or underestimate the amount of discrepancy.

Material used/side of intervention

For their study, studies investigating simulating LLD have used different kinds of material to mimic the condition of LLD. The materials that are commonly used are pair of sandals (high-density ethylene vinyl acetate),[7],[8],[9],[10],[11] insole,[5] soft insole,[6] rubberized compound,[12] flexible polyurethane soles,[13] heel-raising orthotic device,[14] and Crepe material [Table 1].[15] The ethylene vinyl acetate seems to be predominantly used because of its lightweight and tough nature; these properties would not allow this additional material to affect any of the gait parameters.

In regard to the side of intervention, studies have maintained a trend of applying the raises to the right side,[5],[6],[9],[11],[14],[15] thus mimicking a shorter left side, while other studies have not mentioned their side of intervention.[7],[8],[9],[10],[12],[13] Why studies are mimicking a shorter left side as opposed to a shorter right side has not been justified in the literature. Especially when studies have shown that up to 75% of cases have a shorter right leg in a sample consisting of LLD individuals.[24] This adds a limitation to the simulated studies.

Magnitude of leg length discrepancy

Simulated studies have considerably varied in the amount of LLD induced, ranging from 0.5 cm up to 5 cm. However, the most commonly induced magnitude of LLD is 1 cm, 1.5 cm, and 2 cm.[5],[6],[7],[8],[9],[10],[11],[12],[13],[14],[15]

In the case of the studies examining natural LLD, the subjects were divided into a control group and a discrepant group after the screen for LLD was done.

Kinetic deviations associated with leg length discrepancy

[Table 2] displays the common kinetic deviations associated with LLD. The table is further divided to show the how those deviation occur at a anatomical location or during a specific phase of gait. The studies show that kinetic deviations begin to develop in the subjects with LLD even when it is as small as 0.5 cm in the joint contact forces[5] and also in vertical ground reaction force (VGRF)[20] in both the short and long limbs. The joint contact forces in the individuals having ranging from 0.5 cm to 1.5 cm and whereas joint contact forces increased in the individuals having LLD ranging from 1.5 cm up to 4 cm. The fact that there was a decline initially and then an increase as the level of LLD was increased is surprising. One possible reason could be the ILLDI since the study[5] did not show any prior screening record. Thus, the possible hypothesis is that up to 1.5 cm of induced LLD is actually correcting the inherent LLD, and so, we see the kinetic deviations up to 1.5 cm as a result of the correction of inherent LLD.
Table 2: Common kinetic deviations associated with leg length discrepancy during walking

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In regard to the variable weight distribution the weight shifts to the shorter limb of the LLD, the effect is pronounced from 1 cm and continues to shift up to 4 cm toward the shorter limb of the LLD.[8] From an LLD level of 2 cm, there is an increased center of mass displacement in the mediolateral direction which continues up to 4 cm of LLD.[8]

The flexion moment of the ankle, knee, and hip joint at 2 cm LLD has shown to be significantly different when they were compared to a control group with LLD of < 0.5 cm. During the time of loading and mid-stance, there was an increased ankle plantar flexion moment, while smaller ankle plantar flexion was found during the late phase in the short limb of LLD. In the long limb, there was a smaller plantar flexion moment in the late stance.[10] In regard to knee flexion moment, there was a smaller knee flexion moment during the early stance in the short limb, while there was a greater knee extension moment in the long limb during the early stance phase of the gait cycle. There was an increased hip flexion moment in the short leg during early stance phase while a greater hip flexion moment during the terminal stance of the gait cycle [Table 3].[10]
Table 3: Between-condition comparison map of LLD (kinetic variables)

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Spatiotemporal deviations associated with leg length discrepancy

The data from various studies suggest that the spatiotemporal variables are not affected until there is an LLD of 1.5 cm or greater [Table 4]. The stance phase was decreased at 1.5 cm in the short limb, while in the long leg, the stance phase was increased at 2 cm of LLD. The single support phase decreased in the short leg while the same variable increased in the long leg. In regard to the variable swing phase, there was an increased swing phase in the long leg at 1.5 cm;[6] however, in a contradiction, another study found a decreased swing phase in the short limb at 2 cm of LLD.[9] On the other hand, there was a decreased swing phase in the long leg at 1.5 cm[6] and 2 cm[9] of LLD. In the double support, phase, gait velocity, and cadence decreased when the LLD was 1.5 cm.[6]
Table 4: Between-condition comparison map of LLD (spatiotemporal parameters)

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Kinematic deviations associated with leg length discrepancy

[Table 5] displays the common kinematic deviations associated with LLD. It further displays the kinematic deviations at specific anatomical locations for both legs during stance phase and swing phase. The kinematic variables, more specifically the range of motion variables, are also affected when the LLD level is 1 cm and more so when the LLD level is 2 cm or greater. At 1 cm of LLD, there is an increased amount of hip abduction and adduction in the short limb, while there is a decreased amount of hip adduction in the long limb [Table 6].[6] At the ankle joint of the long limb, there is an increased amount of dorsiflexion, pronation, eversion, and inversion as well as decreased supination.
Table 5: Common kinematic deviations associated with leg length discrepancy during walking

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Table 6: Between-leg comparison of LLD

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At 2 cm LLD, there is increased hip adduction and abduction in the short leg.[6] However, in another study, hip abduction was not significant in naturally occurring LLD.[16] In addition, there was a smaller hip flexion and adduction during the first half of the stance phase.[10] In the long leg there were increased hip adduction, greater hip flexion, and adduction angle during the Frist half of the stance phase and a higher hip addiction angle in comparison to non-LLD individuals. At the knee joint, there was greater knee extension during the early stance phase of gait in the short leg;[10] in the long leg, there was an increased knee flexion,[6],[10] knee extension, and knee internal rotation [Table 7].[6]
Table 7: Between-leg comparison of LLD

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At the ankle joint, there was greater rear foot plantar flexion and inversion throughout stance in the long limb.[10] In the long limb, increased dorsiflexion,[10],[6] pronation, eversion,[6] and inversion[6],[10] and decreased supination[6] were observed during the stance phase of the gait.

  Discussion Top

The objective of this study was to look critically into the methodological aspects of the simulated and natural studies which are the two ways to investigate the biomechanics of LLD during gait. First, it should be bought to attention that while simulating LLD, prior screening is an essential part of the study since it allows ruling out the danger of including participants who have a considerable amount of LLD who can potentially tamper with the results of the study.

White et al. demonstrated that the effects of a simulated LLD and true LLD on ground reaction forces were similar during the first half of the gait cycle (weight acceptance). However, it was pointed out that the short- and long-term adaptations could differ in the last phase of the stance of the gait cycle.[12] The findings of this study imply that when LLD is simulated, the deviations in gait are reported earlier on, as opposed to when studying subjects with congenital LLD who seem to adopt an LLD up to 2 cm fairly well [Table 8] and [Table 9].
Table 8: Between-condition comparison map of LLD (kinetic variables)

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Table 9: Between-condition comparison map of LLD (kinematic variables)

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To point out a few inconsistencies, increased hip abduction (short limb) was found at 2 cm in a simulated study,[6] which was not confirmed by a study investigating congenital LLD.[16] Similarly, increased VGRF (short limb) was found at 2 cm of LLD in a simulated study[8] while a study investigating congenital LLD in runners could not confirm the finding [Table 1].[20] There is clearly a distinction between short- and long-term adaptations which are often not recognized when concluding. Therefore, it is suggested that the finding of the simulated studies cannot and should not be generalized to those with congenital LLD.

There seems to be a breakpoint between 1.5 cm and 2 cm at which LLD substantially affects the gait parameters of an individual [Table 10]. Many studies have investigated the immediate effects of correcting LLD with an insole, and the kinematic deviations are instantly corrected.[16] This seems like an appropriate measure to treat mild LLD (>1.5 cm).
Table 10: Studies investigating natural LLD

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  Conclusions Top

It is a common practice for studies to recruit healthy subjects to participate in studies simulating LLD with LLD level below a pre-set level (<1 cm or <0.5 cm). Studies not screening for LLD before commencing a simulated study may end up recruiting participants with significant LLD which will affect the outcome of the study. It is evident that simulated studies report deviations in gait parameters early on, while the subjects with congenital LLD seem to adapt an LLD of up to 2 cm fairly well during unloaded gait. The findings extend the conclusion that he studies simulating LLD demonstrate the acute effect of LLD for this reason they should be disparaged for not being physiologic, leading to non-LLD characteristic gait patterns in terms of kinetic and kinematic parameter's. Hence, generalization becomes impossible.

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Conflicts of interest

There are no conflicts of interest.
  References Top

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  [Figure 1]

  [Table 1], [Table 2], [Table 3], [Table 4], [Table 5], [Table 6], [Table 7], [Table 8], [Table 9], [Table 10]


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