Impact of different heel heights on spinal posture and muscle activity in young adult women

Context High-heeled shoes are frequently used by women. Whether wearing high-heeled shoes can alter the alignment and muscle activity of the spine is still a debatable issue among many clinicians. Aims The purpose of this study was to examine the effect of different heel heights on the spinal posture and muscle activity in adult women, who do not habitually wear high-heeled shoes. Materials and methods A repeated measures design was implemented in this work. A total of 17 healthy women participated in this study. Before collecting data, each participant had a 10-min familiarization period for each heel height. Spinal alignment parameters, sagittal angles, and the amplitude of lumbar erector spinae electromyography were recorded while standing, wearing shoes of different heel heights (0, 6, and 10 cm). Descriptive statistics including mean and SD was applied for dependent variables and participant demographics. Repeated measures analysis of variance was implemented to detect the differences in dependent measures among three heel conditions. Results Sagittal spine angles and alignment parameters were not impacted by wearing high-heeled shoes. Lumbar erector spinae electromyography amplitudes increased with higher heels. However, the difference was not statistically significant. Conclusion Short-term use of high-heeled shoes does not induce changes in spine shape or lumbar spine muscular activation in nonhabitual wearers. Thus, further investigation of the long-term impact is required. The current findings may be important to consider by physiotherapists managing spinal postural dysfunctions.


Introduction
Wearing high-heeled shoes is a common fashion habit that almost every women adopts.Surveys have shown that up to 69% of women regularly wear high-heeled shoes [1].It is noteworthy that several published studies have reported significant adverse effects on different body segments and structures due to prolonged wearing of high-heeled shoes.Yet, women still prefer to wear them.Studies have shown that frequent wearing of high-heeled shoes could contribute to various dysfunctions as ankle instability and sprain [2], increased neck muscular activities and repetitive strain injuries [3], reduced plantar flexors strength [4], shortened gastrocnemius muscle [5], and increased stiffness of Achilles tendon and decreased ankle joint active range of motion on long-term use [5].In addition, walking with highheeled shoes would require higher metabolic functions compared with low-heeled shoes as it increases oxygen consumption and heart rate [6].Furthermore, changes in gait pattern were found associated with wearing high-heeled shoes [7,8], and even potentially knee joint osteoarthritis [9].
Frequent wearing of high-heeled shoes has been linked to back pain [10,11].Several studies have investigated the effect of wearing high-heeled shoes on the spinopelvic posture [11][12][13][14][15][16] or muscle activity [7,10,[17][18][19].However, there is still no consensus regarding the reported findings.The available evidence regarding the impact of high-heeled shoes on spinal curvature is controversial.Previous studies have shown that wearing high-heeled shoes flattens the curvature of the lumbar spine [13,14,20,21] and tilts the pelvis posteriorly [13,16,21].On the contrary, other studies have reported that wearing high-heeled shoes is associated with increased lumber lordosis in women [11,16].Interestingly, another study has reported that the use of foot insole in male participants would increase lumbar lordosis as a result of elevated heels [12].In addition, a previous report has shown that the impact of high-heeled shoes on spinopelvic posture varies according to habituation of wearing high-heeled shoes.Lumbar spine flattens and pelvis tilts posteriorly in nonusers of high-heeled shoes, while the reverse occurred in those using highheeled shoes habitually [16].Further studies have shown that there is no change in the sagittal spine shape while wearing high-heeled shoes [15,22].
The effect of variation of heel height on spinal muscle electromyography (EMG) activity was investigated in some studies.Some evidence exist that standing in high-heeled shoes requires more activation of the cervical and lumbar paraspinal muscles [19].Walking in high-heeled shoes resulted in earlier onset and increased activation of the lumbar paraspinal muscles [17].However, another study has reported that earlier onset of paraspinal muscle activity is not associated with increased amplitude while walking with highheeled shoes [10].
The contradicting results among earlier studies may likely stem from multiple factors.These may include using nonstandardized heel height, or different techniques of heel elevation such as high-heeled shoes [11,16], instrumented heel lift [10], or insole heel lift [12].Additionally, examining participants of different criteria (habitual high-heel wearers or nonwearers), investigating different genders (only men [12], only women [14], or a mixed sample [22]), using different measuring techniques, or even using a small sample size [20] may also contribute to inconsistent findings.The aforementioned factors could possibly confound the quantification of spine curvature angles, or muscle activity, and hence, result in unmatched findings.
The objective of the current work was to investigate the effect of short-term use of shoes of different heel heights on sagittal spinal curvature and lumbar spine EMG activity among nonhabitual wearers of highheeled shoes.It was hypothesized that spinal posture and muscle activity would be impacted by wearing shoes of different heel height.In the current study, nonhigh-heel wearers were selected to exclude the confounding effect of spinal muscles long-term adaptation that would potentially affect the shape of the spine.Further, an examination of the immediate effect of high-heeled shoes on spine posture and muscles would clarify any potentially associated response to high-heeled shoes wearing on the spine.The current work would give some evidence regarding spine posture changes as well as muscle activity in relation to footwear, which might be helpful for the physiotherapists and healthcare providers who deal with spine problems and posture correction protocols.

Design
Repeated measures design was implemented in the current study.The participants underwent two sets of measurements in a single laboratory visit.The first measurement set consisted of photogrammetric assessment of spinal curves and posture performed while wearing three different heel heights (0, 6, and 10 cm).Then, the participants were allowed to rest for 30 min.Afterwards, the second measurement set was carried out to record surface EMG for lumbar erector spinae muscles under similar high-heeled shoe heights.Kyphotic and lordotic spine angles were measured, along with spine alignment parameters including trunk imbalance, pelvic lateral tilt, surface rotation, and lateral deviation.Muscle activation amplitude of lumbar erector spinae muscle was examined.

Sample
This study aimed to recruit a homogeneous sample of young women.Therefore, a convenient sample was selected under strict criteria.A total of 17 healthy adult women met the inclusion criteria.Demographic data are presented in Table 1.Eligibility criteria for participants were being healthy adult women, having a BMI of between 18 and 25.Participants had to not have suffered from musculoskeletal or neurological problems, and used to wear high-heeled shoes occasionally for less than 10 h per week or not at all [4,17].This was estimated as the average working hours of 1 day a week.Participants were excluded if they had spinal deviation, previous spinal surgeries, leg length discrepancy, or foot deformities.The study was approved by the institutional review board.All participants signed a consent form after explanation of the study procedures.

Instruments
Sagittal spine curvatures and posture were measured using the DIERS Formetic 4D spine analysis system (DIERS International GmbH, Dillenbergweg 4 65388 Schlangenbad, Germany).This system quantifies spine angles and shape based on the surface topography scanning method.To measure the spinal posture, each participant was instructed to stand barefoot in front of the device at a marked point on the floor (∼2 m to the device's projector).After projecting light stripes on the back of the participant and recording the spine shape, a software analyzed the captured curvature by means of photogrammetry.Accordingly, the peak kyphotic and lordotic curve angles, trunk imbalance, pelvic tilt, surface rotation, and lateral deviation were measured for each participant [15].In this study, instrumented heel lift was applied by attaching custom-made wooden wedges (6 or 10 cm high) to a standard sports shoes (Fig. 1).
Muscle activation of the lumbar erector spinae was assessed using a Delsys Trigno wireless EMG system (Delsys Inc., Natick, Massachusetts, USA).The recording component consisted of two parallel silver bar electrodes with an interelectrode distance of 1 cm.EMG data was measured at a sampling rate of 2000 Hz using EMGworks software.The recorded surface EMG data was filtered through a bandpass filter of 10-500 Hz and then processed by the rate monotonic scheduling algorithm.
Procedure A total of 48 women were scanned for eligibility.Only 17 participants met the inclusion criteria.Each participant was instructed to remove clothes so that the trunk and pelvis (to gluteal cleft) were exposed.
Participants were given a custom-made gown to wear in order to cover only the front of their trunk and keep the back completely exposed.Then, the participants were instructed to stand still in natural erect posture on a marked point on the floor for photogrammetric capturing of the bared back while wearing a flat sport shoes (supplied by the examiners and available in two sizes) (Fig. 1).After that, the 6 cm wedge was attached to the sport shoes [3,16,20].Participants, then, performed a 10-min adaptation activities including four cycles of: walking along a specified path for 30 s, sitting on a chair for 30 s, carrying a box of 1.5 kg for 30 s, standing on a preselected point for 30 s, and ascending and descending a two-step staircase twice.Photogrammetric assessment of the spine was repeated for the 6-cm heel lift wedge.Afterwards, same procedures were repeated while attaching the 10-cm heel lift wedge.
As the spine posture was assessed by surface topography, which could easily get distorted by surface contours.Therefore, it was preferred to measure the EMG in a separate measurement set to avoid spinal curve distortion created by surface EMG electrodes facing the projecting light stripes of the Formetric system.Surface EMG for the lumbar erector spinae was recorded according to the SENIAM guidelines [23].The participant's skin was prepared and the sensors were attached to lumbar erector spinae on the right and left sides, 2 cm lateral to the spinous process of L4.Electrical activity of the muscles was recorded for each of the three heel height conditions in standing posture directly after conducting the adaptation activities as explained in the first measurement set.

Statistical analysis
Data were statistically analyzed using SPSS, version 21 (SPSS Inc., Chicago, Illinois, USA).Descriptive statistics, including mean±SD, were quantified for all variables.The Shapiro-Wilk's test showed that the data are normally distributed.Repeated measures analysis of variance was applied to the dataset (kyphotic and lordotic spine angles, trunk imbalance, pelvic tilt, surface rotation, sagittal deviation, and right and left EMG rate monotonic scheduling data) among the three measurement conditions.P value was set at 0.05.

Results
Descriptive data for participants' demographics are presented in Table 1.The results have shown that there is no significant effect (P=0.57) of wearing the instrumented heel lift on changing the kyphotic angle of the spine (Table 2).There was reduction of kyphotic angle of 2.3°, and 1.1°between flat shoes and 6 cm height shoes, and 10 cm shoes, respectively.Same results were obtained for the lumbar curve angle showing no significant impact of the three tested conditions of high-heeled shoes on changing the lumbar angle (P=0.54).The lumbar curve angle was decreased with increased heel height.However, the insignificant reduction in mean was only 0.6 between flat and 6 cm height shoes, and 2°between flat shoes and 10 cm heel height.Similarly, spine posture measures including trunk imbalance (P=0.9),pelvic tilt (frontal plane) P=0.07, surface rotation P=0.1, and lateral deviation P=0.8 showed nonsignificant effect as a function of heel height.A trend was noticed in spine posture measures, except the lateral deviation, in which all measured variables increased with higher heels.Similarly, muscular EMG activity of the right (P=0.3), and left (P=0.4)lumbar erector spinae displayed no significant difference when standing in different heel heights (Table 2).

Discussion
The purpose of this study was to investigate the effect of wearing high-heeled shoes of 0, 6, and 10-cm height on spine sagittal curvature, alignment, and lumbar muscles activity while standing.This was performed to seek basic understanding of the impact of wearing high-heeled shoes in nonhigh-heeled shoe wearers.As some studies have investigated a mixed sample of men and women [15,22], the current study selectively examined adult female participants who were characterized as nonhigh-heeled shoes wearers, a factor which could affect and explain our results.The results of the current study indicated that there was no significant impact of wearing high-heeled shoes of 6-cm, or 10-cm height on changing the sagittal spine angles, posture, or lumbar muscle activity.Thoracic kyphotic and lumbar lordosis angle showed no significant change after wearing high-heeled shoes of different heights, which agrees with the previous studies [15,22,24].Concerning spinal posture, the current results showed that standing in high-heeled shoes did not impact spine shape parameters, which agrees with the previous work of Betsch et al. [15], who examined the effect of foot insole on changing the spine posture.
In contrast, other previous studies have reported contradicting results [11][12][13][14]16,20,25]. They concluded that wearing high-heeled shoes would impact lumbar angle.It is essential to highlight that they examined the long-term effect of wearing high-heeled shoes, unlike the current study.It is noticeable that different factors could affect the study results, mostly the adaptation time of wearing high-heeled shoes.Since wearing highheeled shoes for a long time might impact the somatosensory system, different body systems reaction to accommodate that change would likely occur [12].For example, frequent use of high-heeled shoes may impact ankle joint range of motion.Furthermore, calf muscle power is negatively influenced [4].In addition, the duration of habitual wearing high-heeled shoes is found positively correlated with increased pelvic anterior tilt and hyperlordosis angle in high-heeled shoes users.It was previously found that as the time of using high-heeled shoes increased, the lumbar curve increased together with anterior pelvic tilt which might be associated with back pain in frequent wearers of highheeled shoes.Nevertheless, this relationship was not found for the nonhigh-heeled shoes users [16].
A study proposed that the forward shift of the body's line of gravity occurred while wearing high-heeled shoes due to plantar flexion of the ankle joint.This would decrease the sagittal lumbar spine curve a well as the sacral base angle as a compensatory mechanism [13].Such finding would explain the trend noted in lumbar spine angle reduction in the current study.It is possible, however, that when a nonhabitual highheeled shoes wearer wears high-heeled shoes, the primary compensation would occur in the lower limb.As previously reported, knee flexion was found to be the initial compensation to ankle plantar flexion that accompany high-heeled shoes wearing.Later, and after knee muscles fatigue, pelvis and trunk start to compensate for forward shift of line of gravity [21].Furthermore, kinematic chain compensations could primarily take place in the lower extremity and pelvis with no adjustment in spine curves [15].
The current study has shown insignificant impact of high-heeled shoes wearing on lumbar erector spinae, which disagrees with the previous work of Han [19].
It is noteworthy to emphasize that this study examined nonhigh-heeled shoes wearers.However, no information was provided relevant to that specific sample criteria in the previous work of Han [19].Wearing high-heeled shoes was reported to be related to low-back pain in high-heeled shoes wearers due to higher lumbar erector spinae activation during dynamic activity [20].Thus, frequent use of high-heeled shoes could lead to adaptive change in the lumbar curvature [16,20,21].
In addition, as mentioned earlier, spinal changes were found to be positively correlated with the duration of wearing high-heeled shoes [16].Since the muscular force required to stabilize the spine is dependent on the magnitude of spine curvature to produce the tangential stabilizing force [26], it could be expected that spinal muscle activity would adaptively increase or decrease to follow the curvature change.This might explain the presence of adaptive changes in habitual high-heeled shoes wearers in previous studies, which was not found in the present study due to the characteristics of the sample being nonhigh-heeled shoes wearers.
The current study should acknowledge some limitations.The investigation of sagittal pelvic posture would add a complete picture regarding spin pelvic behavior as a result of wearing high-heeled shoes.In addition, examination of ankle and knee muscle activity would add more explanation to the primary compensatory changes in the lower limb that might be linked to spinal muscle activity in elevated heels.Furthermore, a prospective study is recommended to investigate the adaptation time needed to elicit a change in spine posture and spine muscle activity.
In conclusion, the current study showed that shortterm use of high-heeled shoes for study participants does not impact sagittal spine posture or the lumbar erector spinae activity, and that the compensatory effect on the spine angles might need more time to be evident in nonhabitual female users of high-heeled shoes.
Financial support and sponsorship

Conflicts of interest
There are no conflicts interest.

Table 1
Demographic data of the study participants High-Heeled shoes impact on the spine Elsayed et al. 119

Table 2
Mean±SD of kyphotic and lordotic angles, spine posture, and surface electromyography as a function of different heel height High-Heeled shoes impact on the spine Elsayed et al. 121 CI, confidence interval; sEMG, surface electromyography.a F test.