Effects of footrest heights on muscle fatigue,kinematics,and kinetics during prolonged standing work

Abstract

BACKGROUND

: Among the tools for relieving lower back pain, footrests are commonly recommended. Few studies have investigated the effects of footrest and the proper application of footrest height.

OBJECTIVE

: The purpose of this study was to compare the effects of the normalized footrest height on muscle fatigue, kinematics, kinetics, and pain intensity.

METHODS

: In total, 13 males who had a history of non-specific lower back pain during prolonged standing were recruited. The experimental conditions were 2-hour prolonged standing with no footrest and with footrests of 5%, 10%, and 15% of body height. Muscle fatigue was investigated through measurements of the median frequency ratio and the muscle activity ratio (post/pre) in lumbar erector spinae. The lumbo-pelvic angles, and the external moment in the lumbar region were investigated. A visual analog scale was used to investigate the intensity of the pain.

RESULTS

: The footrests at 10% and 15% of the body height caused a lower change in the median frequency ratio and the muscle activity ratio than the other conditions. The footrest at 10% of the body height placed the lowest external moment on the lumbar region among all the conditions. The pain intensity was significantly lower in with footrest conditions than with no footrest condition.

CONCLUSIONS

: The results suggests that a footrest height of 10% of the body height can be recommended as a normalized height for prolonged standing work in subjects with a history of non-specific lower back pain during prolonged standing.

Keywords

Footrest lower back pain normalization prolonged standing

1. Background

Lower back pain (LBP) is defined as discomfort or pain in the area from the posterior ribs to above the margins of the buttocks; LBP is a common work-related disability [1, 2, 3]. Within LBP, ‘non-specific’ LBP is defined as LBP without a specific pathology, such as an infection, fracture, structural deformity, or cauda equina syndrome [4]. Many factors contribute to non-specific LBP, including mechanical factors.

Within mechanical factors, much research has suggested that LBP is related to occupational environments requiring prolonged periods of static standing [1, 2, 3, 5]. Moreover, the strong association between LBP and prolonged standing has been attributed to many factors, such as muscle fatigue from the effort required to maintain an upright posture, fatigue in passive structures of the spine and intervertebral disc stress from excessive lordosis [6, 7, 8]. Thus, many researchers have investigated tools or methods to relieve LBP during prolonged standing.

Tools investigated to relieve LBP in prolonged standing include anti-fatigue mats, sloped surfaces, and footrests [6, 9, 10, 11]. In particular, footrests are commonly recommended to reduce LBP during prolonged standing [9, 10]. The use of footrests has been suggested to decrease intervertebral disc stress and to flatten lumbar lordosis with increased pelvic posterior tilt [8, 10, 12]. A previous study that investigated the effects of footrests during prolonged standing suggested positive effects, such as decreased pain and co-contraction index of the trunk muscles [12]. However, previous studies investigating the effects of footrests typically used only a 15 cm high footrest and they suggested as a limitation that they did not normalize footrest height according to subject characteristics [10, 12]. Although previous studies recommended recommend footrest heights of 10 $\sim$ 20 cm [9, 10], this height was not suggested based on experimental research.

Thus, the purposes of this study were thus to compare the effects of normalized footrest height on muscle fatigue, kinematics, kinetics, and pain, and to recommend a footrest height for use in prolonged standing.

2. Methods

2.1 Subjects

To calculate sample size, the G*Power software (ver. 3.1.2; Franz Faul, University of Kiel, Kiel, Germany) was used based on a pilot study. The calculated sample size was nine, with an effect size of 0.50, an $\alpha$ level of 0.05, and a power of 0.80. In total, 13 males were recruited (Table 1).

Table 1
General characteristics of subjects (N $=$ 13)

Variables	Mean (SD)
Age (year)	24.2 (1.9)
Height (cm)	174.1 (4.8)
Weight (kg)	75.7 (11.8)
Leg length (cm)	88.9 (4.1)
Lumbar lordosis angle ( ${}^{\circ}$ )	21.8 (3.7)

Inclusion criteria were that the subjects were currently asymptomatic and that they had a history of non-specific LBP during prolonged standing. In detail, the criteria were as follows: 1) intermittent unilateral or bilateral non-specific LBP symptoms during prolonged standing, 2) more than one symptomatic episode past 18 months prior to testing, and 3) sufficient pain to impair performance in activities of daily living, such as self-care activities [13]. Additionally, the subjects included had lumbar curvatures of 18 $\sim$ 30 ${}^{\circ}$ , avoiding extreme values, based on previous studies [14, 15, 16]. A bubble inclinometer (Baseline; Fabrication Enterprises, White Plains, NY, USA) was used to measure lumbar curvature in a standing position [17]. Intra-rater and inter-rater reliabilities of this method have been reported to be good (intra-class correlation coefficients were 0.94 and 0.89, respectively) [14]. Exclusion criteria were previous spinal surgery, current spinal pain, any neurological or joint disease affecting the trunk, and significant spinal conditions, such as scoliosis, spina bifida, diagnosed spinal pathologies, tumors, and a present fracture [13].

Prior to the study, the examiner explained the entire procedure to the subjects, and they provided written informed consent prior to participating. This study was approved by the Yonsei University Wonju Institutional Review Board (approval number: 1041849-201606-BM-030-02).

2.2 Experimental protocol

The footrest height was normalized according to subject body height. To meet the recommendation for footrest height [9, 10], the heights were set at 5%, 10%, and 15% of body height and the experimental conditions were with no footrest (level) and footrests of 5%, 10%, and 15% of body height.

Muscle fatigue was investigated through measurements of median frequency (MDF) and muscle activity through electromyography (EMG). The lumbo-pelvic angles, and external moment in the lumbar region were included in the kinematic and kinetic variables. A visual analog scale (VAS) was used to investigate the intensity of pain. The lumbo-pelvic angles, external moment in the lumbar region, and VAS data were collected during a 2-hour prolonged standing task. And the EMG data were collected through pre- and post-functional fatigue tasks to compare the effects of the intervention (2-hour prolonged standing task with footrests of different heights). Thus, muscle fatigue between the conditions was compared by relative changes in the EMG data before and after the 2-hour prolonged standing task.

The experimental protocol started with anthropometric measurements (body weight, body height, and leg length). Next, the subjects performed the functional fatigue task and, during this, pre-task EMG data were collected. Following this task, the subjects were subjected to the 2-hour prolonged standing task under four conditions (with no footrest and with footrests of 5%, 10%, and 15% of body height). For post-task EMG data collection, the subjects conducted the functional fatigue task again after the 2-hour prolonged standing task. All subjects experienced the four conditions and EMG measurements (pre and post) were conducted in each condition with the functional fatigue task. The condition order was random and subjects performed each condition on different days over 2 weeks to avoid fatigue.

Figure 1.

Functional fatigue task (left) and the 2-hour prolonged standing task with a footrest (right).

2.2.1 Functional fatigue task

The functional fatigue task was an isometric back fatigue protocol with an external load. The external load was set at 10% of body weight for each subject and was achieved using a simple grocery basket [18]. The subjects handled the basket close to the body and stood straight with their shoulders in a neutral position and elbows in semi-flexion for 1 min (Fig. 1). The investigator provided feedback regarding maintaining the correct posture to all subjects [18].

2.2.2 2-hour prolonged standing task

During the 2-hour prolonged standing task, the subjects performed light office work, such as simple document work. The desk height was adjusted so that the work surface was just below elbow height [19] and the monitor was set at each subject’s eye level. The subjects were restricted in leaning their weight on the desk [3] and their feet were maintained on the force plate, one on each. In conditions using the footrest, the footrest was placed on one force plate and the force plate was calibrated with the footrest in place. Subjects placed their dominant leg on the footrest while maintaining the leg perpendicular to the ground [12]. The distance between the subjects and footrest was set at 10 cm and the subjects switched their leg on the footrest every 15 min during the 2-hour prolonged standing [12] (Fig. 1).

2.3 Instrumentation

2.3.1 Surface electromyography

The Noraxon Telemyo DTS surface EMG system (Noraxon Inc., Scottsdale, AZ, USA) was used to collect EMG activity and MDF data. Prior to electrode placement, the skin was shaved and swabbed with alcohol. Disposable, self-adhesive Ag/AgCl surface electrodes were used and placed at locations 2 cm apart on the muscle belly of the lumbar erector spinae (LES). The LES electrodes were placed bilaterally, above and below, 2 cm apart from the L3 spinous process [5].

The EMG signals were collected at 1000 Hz, and filtered with bandpass filter (Lancosh FIR) between 20 and 450 Hz with a 60 Hz notch filter. The MDF data were collected through a fast Fourier transform algorithm using MyoResearch Master Edition 1.07 XP (Noraxon Inc., Scottsdale, AZ, USA). The MDF data are presented as the MDF ratios of pre- and post- 1-min functional fatigue task measurements in each muscle (post/pre) to show changes in MDF in each condition [20]. The MDF ratios are presented as averaged bilateral values.

The EMG activity level is a sign of a fatigue response in a sustained muscle contraction, thus the EMG activity was measured during the functional fatigue tasks. For the EMG activity measurement, the EMG signals were processed into root-mean-square values, calculated from 50 ms data windows. To investigate relative changes in the EMG activity in each condition, the EMG activity data is presented as the EMG activity ratios of pre- and post- 1-min functional fatigue task measurements in each muscle (post/pre) [21]. The EMG activity ratios are presented as averaged bilateral values.

2.3.2 Motion analysis system

The VICON MX system, with six infrared cameras (Oxford Metrics, Ltd., Oxford, UK) and two force plates (AMTI-OR6-7-2000 model; Advanced Mechanical Technology, Inc., Watertown, MA, USA), was synchronized and used to measure kinematic and kinetic data. The sampling rate of the VICON MX system was 100 Hz and the sampling rate for the two force plates was 1000 Hz. All data were processed using the VICON Nexus software (Oxford Metrics, Ltd.). For kinematic measurements, in total, 20 reflective markers were attached to the subjects. Four markers were attached to the lumbar spine (T12 and L2 spinous process and bilaterally 3 cm lateral for the L2 spinous process) and 16 markers were attached to the bilateral lower extremities [22]. The lumbar extension-flexion moment was calculated from kinematic angles and the ground reaction force in each subject. The lumbar angle measured using lumbar and pelvic markers was used to estimate the distance to the load point of lumbar region from the line of gravity, and the measured vertical ground reaction force was used as an applied external force of the lumbar region. Through this processing, the lumbar extension-flexion moment was calculated by a customized model in VICON Nexus (Oxford Metrics, Ltd.). The lumbar extension-flexion moment data were analyzed using absolute values of moment data to investigate conditions that least affected the lumbar region. Kinematic and kinetic data were collected at initial 1 min, 60 min, and final 1 min during the 2-hour prolonged standing task session and averaged values of the three measurement were used in analyses.

2.3.3 Visual analog scale

A 100 mm VAS scale was used to assess changes in pain. The ‘0’ point meant ‘no pain’ and the ‘100’ point meant ‘the worst pain imaginable.’ VAS scores have been reported to have high construct validity and reliability [23, 24]. The subjects drew a line on the VAS sheet and changes in VAS between pre and post 2-hour prolonged standing task were used for analyses.

2.3.4 Footrest

Two footrests with adjustable heights were made for this study (Fig. 2). The height variation in one footrest was from 7 to 12 cm and it was used for the 5% of body height condition. The height variation in the other footrest was from 15 to 27 cm and it was used for the 10% and 15% of body height conditions. The width and length were 40 cm and 25 cm, respectively.

Figure 2.

Height-adjustable footrest.

2.4 Statistical analysis

The SPSS software (ver. 23.0; SPSS Inc., Chicago, IL, USA) was used to analyze data. A one-sample Kolmogorov-Smirnov test was used to assess the normality of the distribution of variables. All variables showed normal distributions. Thus, one-way repeated-measures analyses of variance were used to analyze all variables according to the four conditions (with no footrest and with footrests at three heights). The $\alpha$ level was set at 0.05 and the modified Bonferroni correction (Holm’s method) was used as a post-hoc test. The calculated $P$ values are listed from minimal to maximal; the significance was determined by comparison with the adjusted $\alpha$ level. The adjusted $\alpha$ level was calculated according to { $\alpha$ /(number of pair-wise comparisons – position in the sequence $+$ 1)} [25].

3. Results

3.1 LES MDF ratios and EMG activity ratios

To present data as a ratio, it was necessary to satisfy the condition that significant differences in the pre-MDF and pre-EMG activity data did not exist. Thus, the pre-MDF and pre-EMG activity levels for each condition, shown in Table 2, were statistically analyzed like the other variables. In both the pre-MDF and pre-EMG activity levels, there were no significant differences between all of conditions.

Table 2
Pre-MDF and Pre-EMG activity levels

	Level	5%	10%	15%	P
Pre-MDF ${}^{\rm a}$	65.17	64.86	61.12	62.42	0.127
(Hz)	(SD 12.72)	(SD 13.05)	(SD 13.64)	(SD 14.47)
Pre-EMG ${}^{\rm b}$	15.05	15.54	16.13	15.36	0.285
(%MVIC ${}^{\rm c}$ )	(SD 6.71)	(SD 6.61)	(SD 7.03)	(SD 6.56)

${}^{\rm a}$ Median frequency, ${}^{\rm b}$ Electromyography, ${}^{\rm c}$ Maximal voluntary isometric contraction.

Figure 3.

LES MDF ratios (top) and LES EMG activity ratios (bottom) among the four conditions (LES: lumbar erector spinae, MDF: median frequency, EMG: electromyography, *significant difference between conditions).

There were significant differences in the LES MDF ratios among the four conditions ( $p<$ 0.05) (Fig. 3). The LES MDF ratio in the level condition was significantly lower than in the 10% and 15% of body height conditions ( $p=$ 0.002 and 0.009, respectively). In the 5% of body height condition, the LES MDF ratio was significantly lower than that in the 10% of body height condition ( $p=$ 0.007).

Significant differences were found in the LES EMG activity ratios according to the four conditions ( $p<$ 0.05) (Fig. 3). In the level condition, the LES EMG activity ratio was significantly higher than those in the 10% and 15% of body height conditions ( $p=$ 0.002 and 0.011, respectively). The LES EMG activity ratio in the 5% of body height condition was significantly higher than in the 10% of body height condition ( $p=$ 0.009).

3.2 Lumbar and pelvic angles

Figure 4.

Absolute value of external lumbar moment in the four conditions (*significant difference between conditions).

There were significant differences in the lumbar and pelvic angles among the four conditions ( $p<$ 0.05) (Table 3). For the lumbar angle, there were significant differences in pair-wise comparisons between all conditions ( $p<$ adjusted $\alpha$ value). In the pelvic angle, there were significant difference in post-hoc tests between all of conditions ( $p<$ adjusted $\alpha$ value), except between the 10% and 15% of body height conditions ( $p>$ 0.05).

Table 3

Lumbar and pelvic angles (Unit: ${}^{\circ}$ )

	Level	5%	10%	15%	P
Lumbar	9.72	0.99	$-$ 1.69	$-$ 3.98	0.000
	(SD 4.82) ${}^{\ast}$	(SD 5.47) ${}^{\ast}$	(SD 4.96) ${}^{\ast}$	(SD 5.49) ${}^{\ast}$
Pelvis	4.86	0.74	$-$ 1.56	$-$ 2.97	0.000
	(SD 3.66) ${}^{\ast}$	(SD 3.22) ${}^{\ast}$	(SD 1.83) ${}^{{\dagger}}$	(SD 3.80) ${}^{{\dagger}}$

Positive values mean extension and anterior pelvic tilt, negative values mean flexion and posterior pelvic tilt, ${}^{\ast}$ Significant differences compared with all other conditions, ${}^{{\dagger}}$ Significant differences compared with level and 5% of body height conditions.

3.3 Absolute value of external lumbar moment

There were significant differences in the absolute value of external lumbar moment according to the four conditions ( $p<$ 0.05) (Fig. 4). The results of pair-wise comparisons showed that the lumbar moment in the 10% of body height condition was significantly lower than in the other conditions (level: $p=$ 0.007, 5%: $p=$ 0.006, 15%: $p=$ 0.004). There were no significant differences in lumbar moment between the other conditions.

Figure 5.

Changed VAS scores in the four conditions (VAS: visual analog scale, *significant difference between conditions).

3.4 VAS score changes

There were significant differences in the VAS score changes (pre and post 2-hour prolonged standing task) according to the four conditions ( $p<$ 0.05) (Fig. 5). The results of post-hoc tests showed that the VAS change in the level condition was significantly higher than in the other conditions (5%: $p=$ 0.012, 10%: $p=$ 0.001, 15%: $p=$ 0.005). There was no significant difference in VAS score change between the 5%, 10%, and 15% of body height conditions.

4. Discussion

This study investigated the effects of normalized footrest height on muscle MDF ratios, muscle activity ratios, lumbo-pelvic angle, lumbar moment, and VAS score changes in subjects who were currently asymptomatic and had a history of non-specific LBP during prolonged standing.

Maintenance of low-level contraction during standing and such sustained contraction cause changes in EMG signals, such as decreased MDF and increased EMG activity, as fatigue responses [26, 27]. The MDF ratios and EMG activity ratios of the LES showed significant differences between the conditions. In the LES, the MDF ratios at level, 5%, and 15% of body height conditions were lower than 1.0 (pre $>$ post) and the MDF ratio in the 10% of body height condition was about 1.0. However, the EMG activity ratios of the level and 5% of body height conditions were greater than 1.0 (pre $<$ post), while the EMG activity ratios of the 10% and 15% body height conditions showed values of about 1.0. In the level condition, subjects maintained an upright posture with no intervention. Thus, the decrease in MDF with increased EMG activity in the level condition is expected as the result of sustained contraction of the LES. When subjects used a footrest in the 5% of body height condition, the MDF was decreased and the EMG activity was increased. Use of the footrest was expected to provide alternating rest from sustained contraction of the LES. However, the lumbar angle in the 5% of body height condition was slightly extended. Thus, the footrest height was not sufficient to provide rest from the sustained contraction of the LES. The LES MDF and EMG activity in the 10% of body height condition did not change between the pre and post measurements. Thus, the lowest muscle fatigue occurred in the 10% of body height condition.

The lumbar angles were significantly different between all conditions. The pelvic angles were significantly different in all conditions except between the 10% and 15% of body height conditions. Previous studies investigated lumbofemoral rhythm and suggested that, as hip flexion increased, lumbar flexion movement increased in a standing position [28, 29]. Thus, increased flexion in the lumbar would be expected to increase hip flexion because increased footrest height affected lumbar movement. However, the measured values of pelvic tilt to the posterior in the 15% of body height condition increased more than those in the 10% of body height condition, although the pelvic angles in the 10% and 15% of body height conditions were not significantly different. Many muscles and ligaments are involved in lumbo-pelvic movements and stabilization according to muscle length and movement direction [30]. Because of the contributions of these factors, it was expected that some limitations, such as passive tension according to muscle length and ligament involvement, occurred in the 15% of body height condition. Thus, changes in the pelvic angle in the 15% of body height condition were not enough to show significant differences versus the 10% of body height condition.

The absolute values of lumbar extension-flexion moment (external) were used in this study to identify conditions where the values were closest to zero. The value of lumbar moment in the 10% of body height condition was the smallest of the conditions tested and closest to zero. Excessive lumbar extension or flexion moment is known as a potential contributor to injuries in the lumbar region by increasing compressive loading and shear forces [12, 19, 31]. Thus, it is important to find the conditions where the lowest lumbar moment was recorded in either extension or flexion; the 10% of body height condition satisfied this. A previous study reported that intermittent trunk flexion alleviated LBP during prolonged standing due to a decrease in compression in the lumbar region [32]. In the present study, slight lumbar flexion was caused by footrest use and a decreased lumbar moment was seen. Thus, decreased compression in the lumbar region and decreased LBP would be expected, especially in the 10% of body height condition.

The VAS score changes were significant between the level condition and the other conditions. Pain and muscle fatigue are known to be closely related [33]. In particular, sustained muscle contraction that caused muscle fatigue in this study is a factor that causes pain [26, 27, 33]. In this study, the most painful condition was the level condition, which also showed the most fatigue in the LES. During footrest conditions, where there was alternately release of the LES with alternate lumbar flexion, fatigue in the LES was lower than in the level condition and pain development was also lower. Additionally, a decrease in lumbar moment could affect pain development by decreasing the compressive load in the lumbar region. Thus, the pain relief when using a footrest is probably due to relief of the muscle fatigue caused by sustained contraction and load on the lumbar segments. Clinically, it has been reported that the minimal clinically important difference in VAS scores in LBP is 18 $\sim$ 19 mm [34]. Thus, it would be expected that the LBP data in the level condition represented actual pain development and use of the footrest affected such pain development.

This study has some limitations. First, the subjects had a history of LBP, but they were young and asymptomatic at the time of the experiments. Although LBP development through the experimental protocol was sufficient, because of LBP’s characteristic of recurring, subjects with chronic LBP or older adults could show different results, such as muscle fatigue in gluteal muscles. For generalization, it is important also to investigate subjects with symptomatic conditions and of various age groups in future studies. Second, some variables, such as muscle fatigue in the lower extremity muscles and compensatory movements were not measured. Because of prolonged standing, lower extremity muscles could show fatigue responses. Additionally, a limited range of motion in the hip is commonly associated with lumbar rotation. Although lumbar rotation was limited in the subjects, by investigator feedback, further studies are recommended to assess lumbar rotation and other compensatory movements.

5. Conclusions

The aim of this study was to compare the effects of footrest use according to normalized height and to recommend a footrest height for prolonged standing work. The 10% of body height condition caused the lowest muscle fatigue and placed the lowest load on the lumbar region, with the lowest pain development. Thus, a footrest height of 10% of body height can be recommended as a normalized height for prolonged standing work in subjects who are currently asymptomatic and have a history of non-specific LBP during prolonged standing.

Footnotes

Acknowledgments

This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2015R1D1A1A01057620).

Conflict of interest

None to report.

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Effects of footrest heights on muscle fatigue,kinematics,and kinetics during prolonged standing work

Abstract

BACKGROUND

OBJECTIVE

METHODS

RESULTS

CONCLUSIONS

Keywords

1. Background

2. Methods

2.1 Subjects

Table 1 General characteristics of subjects (N = 13)

2.2.2 2-hour prolonged standing task

2.3 Instrumentation

2.3.1 Surface electromyography

2.3.2 Motion analysis system

2.3.3 Visual analog scale

2.3.4 Footrest

3. Results

3.1 LES MDF ratios and EMG activity ratios

Table 2 Pre-MDF and Pre-EMG activity levels

4. Discussion

5. Conclusions

Footnotes

Acknowledgments

Conflict of interest

References

Table 1
General characteristics of subjects (N $=$ 13)

Table 2
Pre-MDF and Pre-EMG activity levels