Electromyographic activity of quadriceps muscle during sit-to-stand in patients with unilateral knee osteoarthritis
Al Amer et al. BMC Res Notes
Electromyographic activity of quadriceps muscle during sit-to-stand in patients with unilateral knee osteoarthritis
Hamad S. Al Amer 0
Mohamed A. Sabbahi 2
Hesham N. Alrowayeh 1
William J. Bryan 3
Sharon L. Olson 2
0 Department of Physical Therapy, Faculty of Applied Medical Sciences, University of Tabuk , Tabuk 71491 , Saudi Arabia
1 Physical Therapy Department, Faculty of Allied Health Sciences, Kuwait University , 90805 Sulaibekhat , Kuwait
2 School of Physical Therapy, Texas Woman's University , 6700 Fannin Street, Houston, TX 77030 , USA
3 Department of Orthopedics, The Methodist Hospital , 6565 Fannin Street, Houston, TX 77030 , USA
Objective: The sit-to-stand (STS) is a simple test to evaluate the functional performance of the quadriceps muscle in patients with knee osteoarthritis (OA). The aim was to evaluate the electromyographic (EMG) activity of the ipsilateral quadriceps during STS task at different seat heights and feet positions in patients with severe unilateral OA. The EMG activity was recorded in a group of eight participants with unilateral OA during the performance of STS task in four conditions: (1) knee-height seat with feet together, (2) knee-height seat with feet askew (feet side by side and heelto-toe), (3) low-height seat (25% lower than knee-height seat) with feet together, and (4) low-height seat with feet askew. Results: There was a statistically significant difference among the four conditions in the EMG activity (p =0.004). Particularly, the EMG activity of the quadriceps was significantly higher when participants rose from the low height with their feet askew than when they rose from the knee height with their feet placed together (p =0.004) or askew (p =0.002). These results recommend considering initial feet position and seat height when evaluating the functional activity of the quadriceps in patients with unilateral OA using STS test.
Knee; Osteoarthritis; Quadriceps muscle; Sit-to-stand; Electromyography
The knee joint is the joint most commonly affected by
osteoarthritis (OA) [
]. Patients with knee OA usually
suffer from pain, limited range of motion, stiffness and
muscle weakness [
]. Therefore, knee OA has been
recognized as a major source of disability and physical
impairment in older adults [
Quadriceps muscle weakness is a common clinical
feature of knee OA [
]. Persistent weakness of the
quadriceps plays a major role in increasing the stress over the
knee joint and progression of joint damage [
improving the functional strength of the quadriceps in
patients with knee OA has received great attention in the
The assessment of the quadriceps muscle is warranted
to determine the functional status of patients with OA.
The sit-to-stand (STS) test is a performance-based
measure frequently used in patients with knee OA to measure
the functional performance of the quadriceps muscle.
Sufficient quadriceps force is required to complete the
STS movement. Therefore, quadriceps weakness was
found to have a significant impact on STS performance
Electromyography (EMG) is commonly used to obtain
information about the effects of chair seat height and
initial feet positions on the activity of the lower limb
muscles during STS movement [
]. Measuring EMG
activity of the knee extensors during these tasks would
reflect the amount of loading applied to the quadriceps.
This is a key muscle to be targeted during the
rehabilitation program of patients with knee OA. Therefore, the
purpose of this study was to evaluate the EMG activity
of the quadriceps muscle, specifically the vastus lateralis
(VL), during STS task at different seat heights and feet
positions in individuals with severe unilateral OA. The
data presented in this study is a side product of another
unpublished research project investigating the activity of
thigh musculatures during selected functional activities
before and after total knee arthroplasty (TKA).
Eight participants (five males and three females)
volunteered for the study 1–2 weeks before undergoing
unilateral elective TKA. The mean age of the participants
was 64.61 ± 11.01 years and the body mass index was
34.06 ± 8.89 kg/m2. Participants were included based on
the following criteria: no other musculoskeletal disorders
or neurological pathologies; and no previous hip, knee,
spine or neck surgery within the past year.
EMG activity was sampled at 1000 Hz and sweep speed
of 100 points/s. using the Myosystem 1200 version 2.11
(Noraxon USA, Inc., Scottsdale, AZ) via the Telemyo
900 telemetry unit (Noraxon USA, Inc., Scottsdale, AZ).
Two adhesive surface electrodes were placed over the
mid-muscle belly of the VL of the arthritic knee. The
electrodes were placed longitudinally in a bipolar
configuration with inter-electrode distance of 2 cm. A ground
electrode was affixed over the fibular head. The VL was
chosen in this study as a representative of the quadriceps
muscles based on its several unique characteristics. The
VL is considered the largest among the four quadriceps
] and the main generator of extension torque
at the knee [
]. Although there is no difference in time
of onset among the four muscles of the quadriceps during
closed-chain movements, the VL has the largest amount
of EMG activity during that type of movement [
The participants performed STS tasks in the
following order: STS at normal height (knee-height seat) with
feet together (Fig. 1a), STS at normal height with feet
askew (feet side by side, heel-to-toe with foot of arthritic
knee behind the other) (Fig. 1b), STS at low height (25%
lower than knee-height seat) with feet together (Fig. 1c),
and STS at low height with feet askew (Fig. 1d) (tasks
hereafter will be identified as NHFT, NHFA, LHFT and
LHFA, respectively). The starting position was sitting on
an armless, backless chair, and maintaining feet flat on
the floor and thighs at hip width. To perform the tasks,
participants were instructed to stand while holding arms
across the chest with weight equally distributed on both
feet in NHFT and LHFT. In NHFA and LHFA, they had
the chance to load their feet as they wish to complete the
task. Two trials of each STS task were performed and
used for analysis.
For normalizing the EMG data, participants completed
two maximal voluntary isometric contractions (MVIC)
using an electromechanical dynamometer (Biodex Inc.,
Shirley, NY) while EMG was recorded. The examined
limb was stabilized with the hip and knee flexed to 90°
and 15°, respectively. Two 10-s trials were recorded with
2 min of rest in between.
To analyze EMG signals of quadriceps, raw signals were
full-wave rectified and smoothed with a 10 ms window.
The obtained linear envelope was analyzed to determine
the peak EMG signal amplitudes during the STS task
trials and during MVIC trials. Because it is common to find
high levels of EMG activity during dynamic tasks
versus MVIC [
], the two trials of each STS task were also
examined for the maximum EMG signal. Subsequently,
the peak EMG signal obtained during the STS tasks trials
was normalized to the maximum EMG signal obtained
either during the MVIC or during the STS trials,
whichever had higher activity. This normalization method is
common in EMG studies [
13, 21, 22
] and was suggested
as a more accurate method since all the normalized peak
EMG activity would fall below or equal to 100% of the
maximum signal [
]. The normalized peak EMG activity
of VL during the two trials of each task was averaged and
used as the final outcome score for the performed task.
Statistical analysis included one independent variable:
the STS task, with four levels (NHFT, NHFA, LHFT
and LHFA). The dependent variable was the normalized
peak EMG activity of the quadriceps muscle measured
in percent of maximum activation. A one-way analysis
of variance (ANOVA) for repeated measures
(univariate approach) was conducted to test the main effect of
the independent variable with alpha level set at 0.05. The
univariate approach was selected due to the small
sample size. Regarding the sphericity assumption,
GeisserGreenhouse epsilon hat (εˆ) of 0.764 was found. Therefore,
the degrees of freedom (factor and error) were adjusted
according to this value to prevent inflation of alpha.
The means and standard deviations of the normalized
EMG for each condition are illustrated in Fig. 2. The
result of the one-way ANOVA showed a statistically
significant difference among the four conditions in the
normalized EMG activity, F (2.29, 16.04)=7.54, p =0.004.
All pairwise comparisons were conducted to
examine the difference among tasks using Bonferroni tests.
Alpha level was adjusted to .05/6 (number of
comparisons) = 0.0083 to prevent type I error. All pairwise
comparisons revealed the normalized EMG activity was
significantly higher in LHFA (M = 76.93%) than in NHFT
(M = 51.16%) or NHFA (M = 59.44%) (Fig. 2). No further
differences were found among the rest of conditions.
Table 1 displays the t-statistic, degrees of freedom and p
value for each comparison.
This study was conducted to evaluate the effect of four
different conditions of STS task on the EMG activity of
the quadriceps muscle in individuals with severe
unilateral OA. The findings showed that during STS
movement, the activity of the quadriceps is modulated by the
chair height and feet position.
A possible explanation of the difference in EMG
activity of the quadriceps between the LHFA and NHFT
positions is that the participants could be trying to avoid
loading the arthritic knee during the latter due to possible
weakness of the quadriceps muscle, pain, or both. When
the arthritic and unaffected sides’ feet were parallel,
participants had the chance to compensate for the arthritic
side which might be weaker than the unaffected side. This
phenomenon has been observed in patients with
unilateral TKA [
]. Farquhar et al. . found the activity
of the quadriceps muscle on the involved side to be
significantly lower than the uninvolved side during STS task
up to 3 months following the surgery. Due to significant
weakness in the knee extensors, patients avoided
loading of the involved limb by shifting the load to the
uninvolved limb. However, because the EMG activity of the
quadriceps at the uninvolved side was not recorded in the
present study, we are not sure if the participants used the
NHFT normal height feet together, NHFA normal height feet askew, LHFT low
height feet together, LHFA low height feet askew
a Indicates significant difference at α = 0.0083
same compensatory approach during the performance
of the STS task. Nevertheless, quadriceps weakness is a
common feature in patients with knee OA and patients
with TKA in the early phases following the surgery [
Repositioning the foot of the unaffected side anterior
to the foot of the arthritic side significantly increased
the activity of the quadriceps. Generally, placing the
feet posteriorly moves the ground reaction force vector
further posteriorly with respect to the knee, leading to
a higher external flexion moment applied on that knee
]. In this study, the relatively posterior position of the
arthritic knee’s foot produced higher demand on the
ipsilateral quadriceps to overcome the increase in the ground
reaction force. Additionally, this position retained the
arthritic knee closer to the center of gravity [
]. As a
result, the arthritic knee was the principal leg to perform
the upward displacement of the body. For that reason, the
relatively posterior position of the foot of the arthritic
knee required the participants to use that knee instead
of the unaffected one as compensation, due to possible
weakness or pain avoidance of the affected side.
The reported increase in the EMG activity of the
quadriceps during STS movement from a low height in
comparison to those with knee height (with feet askew in
both tasks) suggests increasing the demand on the knee
extensors. This finding is in agreement with previous
14, 15, 25
]. Arborelius et al. [
] examined the
effect of rising from two different seat heights in healthy
individuals and found a significant increase in the
activity of the VL muscle with rising from a lower seat height
in comparison to higher seat height. As the seat height
decreases, the knee flexion angle and the knee flexion
moment will increase. This would lead to higher demand
on the quadriceps muscle to extend the knee in lifting the
body weight [
Performing the STS test with placing both feet together
provides an opportunity for the patients to use the
uninvolved side to compensate for the possible weakness of
the arthritic side. This may not reflect the true status of
the quadriceps performance on the affected side.
Conversely, repositioning the unaffected side anterior to the
arthritic side imposes more demand on the patient to use
the involved side instead of compensating with the
uninvolved side. This task better demonstrates the true
functional performance of the ipsilateral quadriceps muscle.
Furthermore, lowering the seat height will add greater
difficulty to the test as it places more demand on the side
being tested. Therefore, starting positions with regard to
chair height and initial feet position need to be
standardized in order to avoid misleading results.
To conclude, the results of this study indicate that the
modification of seat height and feet position during STS
movement plays an important role in clinically evaluating
patients with knee OA. Lowering the seat height and
placing the foot of the unaffected side anterior to the foot
of the arthritic side increase the demand on the
quadriceps muscle of the arthritic knee. This starting position
would prevent patients from utilizing some strategies to
avoid using their arthritic side to complete the STS task.
This, in turn, may reflect the true functional condition
of the knee extensors in patients with knee OA and the
potential need of additional intervention.
A limitation of this study is the small sample size, which
may have affected the significance of the results.
Particularly when Bonferroni adjustment was used. Another
potential source of type II error is the high variability of
the EMG data. In fact, some electrophysiological studies
used a liberal level of significance when analyzing EMG
data in order to avoid type II error e.g. [
12, 13, 21, 22
Another limitation is the lack of EMG testing for the
unaffected limb, and lack of quadriceps strength
measurements. Those recordings could have supported the
OA: Osteoarthritis; STS: Sit-to-stand; EMG: Electromyography; VL: Vastus
lateralis; TKA: Total knee arthroplasty; NHFT: Normal height with feet together;
NHFA: Normal height with feet askew; LHFT: Low height with feet together;
LHFA: Low height with feet askew; MVIC: Maximal voluntary isometric
contraction; ANOVA: Analysis of variance.
SLO, MAS, HNA and WJB conceptualized and designed the study. WJB
recruited the participants. MAS, HNA, and HSA collected the data. HSA & SLO
analyzed the data. MAS and SLO interpreted the data. HAS wrote the initial
draft. MAS & SLO revised the draft. All authors read and approved the final
The authors would like to acknowledge the participants who volunteered for
The authors declare that they have no competing interests.
Availability of data and materials
The datasets generated and/or analyzed during the current study are available
from the corresponding author on reasonable request.
Consent for publication
Ethics approval and consent to participate
The study was approved by the Institutional Review Board of Texas Woman’s
University-Houston Center and The Methodist Hospital for the Protection of
Human Subjects. Subjects’ approvals for participation were obtained using
written informed consent prior procedures.
This study was supported by grants from the Methodist Hospital Research
Springer Nature remains neutral with regard to jurisdictional claims in
published maps and institutional affiliations.
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