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Students are to have read the Knee research article (presented via a link below), and then are to summarize & analyze their reading using the provided questions below.

https://praxis.mrooms.net/pluginfile.php/37932/mod_assign/intro/TKA%20Research%20Article.pdf

Answer the following questions in your own words to reflect your interpretation of the information to address the following questions.

1.      What is the purpose of the article? Why is this topic important? [2 points]

2.      How was the research conducted? [Summarize the methods and procedures- 4 points]

3.      What did the authors find? [Summarize the results & conclusions- 4 points]

4.      What are at least TWO strengths of the study? What are at least TWO weaknesses of the study? Explain or defend each point. [4 points total]

5.      What are at least TWO clinical implications from this research? Describe the correlation to clinical practice, then explain & defend whether you found the evidence useful. [2 points]

Research Report
Relationship Between Intensity of
Quadriceps Muscle Neuromuscular
Electrical Stimulation and Strength
Recovery After Total Knee
Arthroplasty
Background. Neuromuscular electrical stimulation (NMES) can facilitate the
recovery of quadriceps muscle strength after total knee arthroplasty (TKA), yet the
optimal intensity (dosage) of NMES and its effect on strength after TKA have yet to
be determined.
Objective. The primary objective of this study was to determine whether the
intensity of NMES application was related to the recovery of quadriceps muscle
strength early after TKA. A secondary objective was to quantify quadriceps muscle
fatigue and activation immediately after NMES to guide decisions about the timing of
NMES during rehabilitation sessions.
Design. This study was an observational experimental investigation.
Methods. Data were collected from 30 people who were 50 to 85 years of age and
who received NMES after TKA. These people participated in a randomized controlled
trial in which they received either standard rehabilitation or standard rehabilitation
plus NMES to the quadriceps muscle to mitigate strength loss. For the NMES intervention group, NMES was applied 2 times per day at the maximal tolerable intensity
for 15 contractions beginning 48 hours after surgery over the first 6 weeks after TKA.
Neuromuscular electrical stimulation training intensity and quadriceps muscle
strength and activation were assessed before surgery and 3.5 and 6.5 weeks after
TKA.
Results. At 3.5 weeks, there was a significant association between NMES training
J.E. Balter, MS, Physical Therapy
Program, Department of Physical
Medicine and Rehabilitation, University of Colorado.
P. Wolfe, MS, Department of Preventive Medicine and Biometrics,
University of Colorado.
D.G. Eckhoff, MD, Department of
Orthopedics,
University
of
Colorado.
R.S. Schwartz, MD, Division of
Geriatric Medicine, University of
Colorado.
M. Schenkman, PT, PhD, FAPTA,
Physical
Therapy
Program,
Department of Physical Medicine
and Rehabilitation, University of
Colorado.
W.M. Kohrt, PhD, Division of Geriatric Medicine, University of
Colorado.
intensity and a change in quadriceps muscle strength (R2⫽.68) and activation
(R2⫽.22). At 6.5 weeks, NMES training intensity was related to a change in strength
(R2⫽.25) but not to a change in activation (R2⫽.00). Furthermore, quadriceps muscle
fatigue occurred during NMES sessions at 3.5 and 6.5 weeks, whereas quadriceps
muscle activation did not change.
[Stevens-Lapsley JE, Balter JE,
Wolfe P, et al. Relationship
between intensity of quadriceps
muscle neuromuscular electrical
stimulation and strength recovery
after total knee arthroplasty. Phys
Ther. 2012;92:1187–1196.]
Limitations. Some participants reached the maximal stimulator output during at
© 2012 American Physical Therapy
Association
least 1 treatment session and might have tolerated more stimulation.
Conclusions. Higher NMES training intensities were associated with greater
quadriceps muscle strength and activation after TKA.
Published Ahead of Print:
May 31, 2012
Accepted: May 23, 2012
Submitted: December 20, 2011
Post a Rapid Response to
this article at:
ptjournal.apta.org
September 2012
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Jennifer E. Stevens-Lapsley, Jaclyn E. Balter, Pamela Wolfe, Donald G. Eckhoff,
Robert S. Schwartz, Margaret Schenkman, Wendy M. Kohrt
J.E. Stevens-Lapsley, PT, PhD,
Physical
Therapy
Program,
Department of Physical Medicine
and Rehabilitation, University of
Colorado, Mail Stop C244, 13121
East 17th Ave, Room 3116,
Aurora, CO 80045 (USA). Address
all correspondence to Dr StevensLapsley at: jennifer.stevens-lapsley
@ucdenver.edu.
Intensity of Quadriceps Muscle Neuromuscular Electrical Stimulation and Strength Recovery After TKA
T
Neuromuscular electrical stimulation has been widely used in both
research and clinical settings to preserve or restore muscle mass and
function during prolonged periods
of disuse or immobilization.18 –20
Neuromuscular electrical stimulation causes a muscle contraction
through transcutaneous application
of electrical current to terminal
branches of motoneurons.20,21 One
of the unique features of NMES is
that it elicits a less orderly recruitment of motoneurons than voluntary
exercise, favoring the activation of
large
(type
II),
higher-forceproducing motor units at relatively
low levels of stimulation.22 To date,
most NMES training studies have
focused on people who are healthy
rather than on those with compromised muscle function,23 yet the
greatest benefits of NMES may be
seen with impaired muscle function,20 such as after TKA.
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With voluntary muscle strengthening, higher intensities of muscle
overload translate to greater strength
gains.24 Similarly, the higher the
NMES training intensities, the greater
the strength gains for both
healthy25,26 and impaired17 muscles.
However, to date, no studies have
evaluated how NMES training intensity is related to the preservation of
quadriceps muscle strength when
NMES is initiated early after TKA.
A substantially limiting factor with
NMES use is patient tolerance
because of the discomfort associated
with intense stimulation. Therefore,
evidence that higher training intensities translate to greater muscle function is necessary to justify the
approach of encouraging patients to
undergo the highest tolerable NMES
intensity. Furthermore, the timing of
NMES application during rehabilitation (before or after voluntary contraction– based exercises) depends
on the degree of acute muscle
fatigue with NMES use and immediate changes in muscle activation. For
example, in the presence of substantial quadriceps muscle fatigue without improvement in muscle activation, NMES application might be best
timed after other exercises. In contrast, a marked improvement in muscle activation after NMES application
without substantial quadriceps muscle fatigue would encourage NMES
use before the initiation of other
exercises to take advantage of
enhanced muscle recruitment.
In our recently published randomized controlled clinical trial, we demonstrated that NMES application to
the quadriceps muscle early after
TKA effectively attenuated quadriceps muscle strength loss and
improved functional performance.14
However, given the discomfort associated with high NMES training
intensities, it is important to determine whether higher NMES training
intensities produce greater strength
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and performance gains than lower
intensities. Therefore, the purpose
of this investigation was to determine whether higher NMES training
intensities resulted in greater recovery of quadriceps muscle strength
within the first 6.5 weeks after TKA;
the data were derived from a parent,
prospective randomized clinical trial
of NMES application. We hypothesized that higher NMES training
intensities would be associated with
greater recovery of quadriceps muscle function after TKA. We further
sought to quantify quadriceps muscle fatigue and activation immediately after an NMES session. We
hypothesized that quadriceps muscle activation would improve immediately after an NMES session,
despite some acute fatigue of the
quadriceps muscle.
Method
Design Overview
This investigation was an observational substudy of data derived from
a randomized, controlled, parallelgroup intervention trial to evaluate the benefits of adding NMES
to a postoperative TKA rehabilitation program.14 This substudy
included only participants randomized to the NMES intervention
arm. Participants were assessed 1
to 2 weeks before TKA and at 3.5
and 6.5 weeks after TKA at the
Clinical and Translational Research
Center, University of Colorado.
Informed consent was obtained from
all participants.
Setting and Participants
People who had osteoarthritis and
were undergoing a primary unilateral TKA by 1 of 3 orthopedic
surgeons at the University of
Colorado Hospital were consecutively recruited between June 2006
and June 2010. Volunteers were
recruited by referral or advertisement at preoperative educational
sessions. All participants underwent a similar tricompartmental,
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otal knee arthroplasty (TKA)
successfully relieves pain and
improves function in patients
with knee osteoarthritis, although
the recovery of quadriceps muscle
force and function is suboptimal
compared with that in people who
are healthy and predisposes patients
to disability with increasing age.1–3
Previous studies demonstrated quadriceps muscle strength deficits of
50% to 60% relative to preoperative
levels, despite the initiation of physical therapy within 48 hours of surgery.4 –7 Early quadriceps muscle
strength deficits after TKA have been
attributed largely to central activation deficits (also referred to as
“reflex inhibition”).5,6,8 Therefore,
neuromuscular electrical stimulation
(NMES) has been used as an adjunct
to traditional rehabilitation for
patients after TKA9 –14 because it may
provide a more effective means of
mitigating quadriceps muscle central
activation deficits and increasing
quadriceps muscle strength than voluntary exercise alone.9,14 –17
Intensity of Quadriceps Muscle Neuromuscular Electrical Stimulation and Strength Recovery After TKA
cemented TKA with a medial parapatellar surgical approach.
NMES Intervention
A portable Empi 300PV stimulator
(Empi Inc, a DJO Global company, St
Paul, Minnesota) was used for the
NMES intervention because it produces comparable levels of average
peak torque with similar levels of
discomfort as a VersaStim 380 clinical stimulator (Electro-Med Health
Industries, Miami, Florida), which
was used in previous NMES investigations but is not practical for home
use.9,27–29 During NMES treatment at
home, the lower limb was secured
with Velcro straps (Velcro USA Inc,
Manchester, New Hampshire) to a
stable chair to allow for approximately 85 degrees of hip flexion and
60 degrees of knee flexion.11,14 Selfadherent, flexible rectangular electrodes (7.6 â«» 12.7 cm; Supertrodes
[SME Inc, Wilmington, North Carolina]) were placed on the distal
medial and proximal lateral portions
of the anterior thigh and marked to
facilitate consistent reapplication by
the participant.
The size of the electrodes used for
NMES is important because it
directly influences the density of
the current. A high current density
with small electrodes can cause
painful stimulation before a sufficient muscle contraction to allow for
muscle strengthening is reached.30
Therefore, selecting an appropriate electrode size is important for
September 2012
Neuromuscular electrical stimulation from the portable electrical
stimulator was applied to the resting
muscle, and the participant was
instructed to relax during the electrically induced muscle contraction.
Importantly, NMES application with
the muscle at rest or NMES application superimposed on voluntary
contraction does not appear to
influence training-induced strength
gains.20,23,31–33 Therefore, NMES
was applied to the muscle at rest
to allow for the quantification of
NMES training intensity as a percentage of quadriceps muscle maximal voluntary isometric contraction
strength. The intensity was set to
the maximal intensity tolerated
during each session, and participants
were repeatedly encouraged to
increase the intensity as tolerated.
The stimulator delivered a biphasic
current with a symmetrical waveform at 50 pulses per second for
15 seconds (including a 3-second
ramp-up time) and a 45-second off
time (250-microsecond pulse duration). Participants performed 15 contractions per session, 2 sessions per
day, 6 or 7 days per week.
Participants randomized to the
NMES intervention group received
about 9 weeks of standardized rehabilitation as previously described14
after TKA in addition to the NMES
intervention. Initial familiarization
with the NMES device (Empi 300PV)
occurred during preoperative testing
to facilitate the application of NMES
early after surgery. Participants used
the NMES unit a few times at home
before surgery to become familiar
with the device, a strategy that has
been recommended to increase participant tolerance of NMES.20 Partic-
The Bottom Line
What do we already know about this topic?
Quadriceps muscle weakness after total knee arthroplasty (TKA) is profound and often persists years after surgery. Using neuromuscular electrical stimulation (NMES) early after TKA surgery may enhance recovery of
physical function such as walking.
What new information does this study offer?
Higher intensities of NMES to the quadriceps muscle after TKA increases
muscle strength and activation more than lower intensities.
If you’re a patient, what might these findings
mean to you?
The findings suggest that during your rehabilitation after your surgery, you
should make every effort to tolerate the highest intensity of NMES possible
in order to maximize your quadriceps muscle strength. Although people
may need a few sessions to get used to the stimulation, they often learn to
tolerate the stimulation well.
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People were included in the study
if they were 50 to 85 years of
age. Exclusion criteria for the study
were uncontrolled hypertension,
uncontrolled diabetes, a body mass
index greater than 35 kg/m2, significant neurologic impairments, contralateral knee osteoarthritis (as
defined by a pain level of greater
than 4/10 with activity), and other
unstable lower-extremity orthopedic
conditions.
comfortable stimulation. Furthermore, applying the electrodes
over the motor point of the muscle reduces the current threshold
required. In the present study, large,
rectangular electrodes were used to
maximize treatment tolerance and
effectiveness.
Intensity of Quadriceps Muscle Neuromuscular Electrical Stimulation and Strength Recovery After TKA
Participant setup for assessment of neuromuscular electrical stimulation (NMES) training intensity at the 3.5- and 6.5-week testing
sessions (left) and sample torque data from a representative participant at the 3.5-week testing session (right). Average (Avg) torque
was the average peak quadriceps muscle torque across 10 electrically elicited contractions during the NMES training intensity
assessment. MVIC⫽maximal voluntary isometric contraction, Pre-op⫽preoperative.
ipants were encouraged to use the
stimulator at an intensity that was
tolerable but slightly uncomfortable,
although no minimum intensity was
required for participation in the
study protocol. In addition, participants were repeatedly instructed to
continue to increase the intensity to
their maximal tolerance within and
between sessions. Most participants
demonstrated safe and proper use of
the stimulator during their inpatient
stay in the hospital. When there
were concerns about participant
implementation or tolerance of
NMES, a study physical therapist visited the participant at home within
the first week of discharge to monitor a home treatment session. Participants were given paper logs to track
adherence.
Outcome Measures
Isometric quadriceps muscle
torque and activation testing.
Isometric quadriceps muscle torque
and activation were assessed at the
preoperative and 3.5- and 6.5-week
testing sessions as previously
described.11,34,35 A HUMAC NORM
(CSMi, Stoughton, Massachusetts)
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electromechanical
dynamometer
was used to measure quadriceps
muscle torque. Data were collected
with a Biopac Data Acquisition System (BIOPAC Systems Inc, Goleta,
California) and analyzed with AcqKnowledge software, version 3.8.2
(BIOPAC Systems Inc). Participants
were positioned in an electromechanical dynamometer with 60
degrees of knee flexion as previously
described (Fig. 1).14 Participants
were asked to perform a maximal
voluntary isometric contraction
(MVIC) of the quadriceps muscles
using both visual and verbal feedback up to 3 times unless the first 2
attempts were within 5% of each
other. The trial with the highest
MVIC torque output was then normalized to body weight (in kilograms) for data analysis.
A Grass S48 stimulator with a Grass
model SIU8T stimulus isolation unit
(Grass Instruments, West Warwick,
Rhode Island) and self-adherent, flexible electrodes (7.6 â«» 12.7 cm;
Supertrodes) were used to determine voluntary muscle activation as
previously described.14 Voluntary
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activation of the quadriceps muscle
was assessed with the doublet interpolation technique, in which a
supramaximal stimulus was applied
to the quadriceps muscle during an
MVIC and again, immediately afterward, to the quadriceps muscle at
rest (stimulus parameters: 2 pulses,
pulse duration of 600 microseconds,
and electrical train of 100 pulses per
second).11,34,35 Values of less than
100% represented incomplete motor
unit recruitment or decreased motor
unit discharge rates.34 –36
NMES training intensity and
immediate fatigue assessment.
Neuromuscular electrical stimulation training intensity was assessed
at 3.5 and 6.5 weeks after surgery for
participants in the NMES intervention group because assessment of
training intensity in the home (during treatment) was not feasible.
While seated in the electromechanical dynamometer (Fig. 1), participants were asked to use the NMES
stimulator at the same intensity as
that used at home. Once participants
had reached their typical NMES treatment intensity (usually within the
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Figure 1.
Intensity of Quadriceps Muscle Neuromuscular Electrical Stimulation and Strength Recovery After TKA
first 2 or 3 contractions), the average
electrically elicited (rather than voluntary) torque while the stimulator
was on was recorded across 10 contractions. This average torque was
then expressed as a percentage of
the MVIC during the preoperative
session to minimize the potential for
activation deficits to confound
torque measurements.
Data Analysis
For analyses of NMES training intensity at 3.5 and 6.5 weeks, quadriceps
muscle torque and activation were
represented as percent changes from
preoperative torque26 and activation. The NMES training intensities at
3.5 and 6.5 weeks were represented
as [(average torque/MVIC torque
before surgery) â«» 100], where average torque was the average peak
quadriceps muscle torque across 10
electrically elicited contractions during the NMES training intensity
assessment. Changes in torque and
activation immediately after an
NMES session were represented as
[(value after NMES ⫺ value before
NMES)/value before NMES] â«» 100.
All statistical analyses were performed with SPSS for Windows, version 16.0 (SPSS Inc, Chicago, Illinois). Linear regression was used to
estimate the association between
NMES training intensity and percent
changes in quadriceps muscle
torque and activation at 3.5 and 6.5
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Role of the Funding Source
This study was supported by the
National
Institute
on
Aging
(K23AG029978), an American College of Rheumatology New Investigator Award, a Foundation for
Physical Therapy Marquette Challenge Grant, and a Clinical and
Translational Science Award Grant
(UL1 RR025780). A peer-reviewed
research grant from Empi Inc, a DJO
Global company, was used to support the purchase of Empi 300PV
electrical stimulators and recruitment and transportation costs for
participant visits. None of the sponsors had any influence on the study
design, implementation, or data analysis and interpretation.
Results
Participant Characteristics
This substudy included only the
NMES intervention group, comprising 30 participants (12 men and 18
women) who were 65.6 (SD⫽8.5)
years of age and had a body mass
index of 29.0 (SD⫽5.0) kg/m2.
Adherence to NMES
Adherence to NMES treatment was
assessed at the 3.5- and 6.5-week
time points by use of participant
adherence logs and reported as the
percentage of expected NMES treatment at home (15 contractions per
session, 2 sessions per day, 6 or 7
days per week). At 3.5 weeks, 23
participants (76.7%) were adherent
(completed ⬎80% of prescribed contractions), 6 participants (20.0%)
were partially adherent (completed
50%– 80% of prescribed contractions), and 1 participant (3.3%) had
no adherence records but was
included in NMES training intensity
assessments and analyses. At 6.5
weeks, 17 participants (56.7%) were
adherent (completed ⬎80% of prescribed contractions), 6 (20.0%)
were partially adherent (completed
50%– 80% of prescribed contractions), 2 (6.7%) were not adherent
(completed ⬍50% of prescribed contractions), and 5 (16.6%) had no
adherence records but were
included in NMES training intensity
assessments and analyses. All participants, regardless of their individual
adherence index, were included in
the analyses. There was no relationship between NMES adherence and
change in quadriceps muscle
strength or NMES adherence and
change in activation.
NMES Training Intensity
The mean stimulation intensities
were 83.7 (SD⫽3.1) mA at 3.5 weeks
and 82.1 (SD⫽3.3) mA at 6.5 weeks.
These values generally corresponded
to a visible muscle contraction that
was at least equivalent to that
achieved during a straight leg raise.
Ten participants (32.3%) reached the
maximal voltage output of the stimulator (100 mA) during at least 1
week of treatment; 3 participants set
the stimulator at 100 mA for all 6
weeks of treatment.
Overall, NMES training intensity
ranged from 1.6% to 76.7% of the
maximal quadriceps muscle strength
achieved during the preoperative
MVIC (X⫽16.1% [SD⫽14.8%] at 3.5
weeks; X⫽17.7% [SD⫽11.3%] at 6.5
weeks). Overall, the results indicated
the presence of a relationship
between NMES training intensity and
quadriceps muscle function (Fig. 2).
At 3.5 weeks, there were significant
associations of NMES training intensity with a percent change in torque
from
the
preoperative
value
(adjusted R2⫽.68, P⬍.001) and acti-
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Changes in quadriceps muscle
torque and activation immediately
after NMES were also assessed at 3.5
and 6.5 weeks. Methods identical to
those described above were used to
measure quadriceps muscle MVIC
torque and activation immediately
after the NMES training intensity
assessment. Measurements obtained
after an NMES session were compared with measurements obtained
before NMES application to determine whether fatigue or changes in
activation were present after NMES.
weeks. Paired sample t tests were
used to assess differences between
quadriceps muscle torque and activation immediately after an NMES
session and quadriceps muscle
torque and activation before the
NMES session at 3.5 and 6.5 weeks.
No adjustment was made for multiple comparisons because this was a
secondary analysis and should be
considered hypothesis generating.
Intensity of Quadriceps Muscle Neuromuscular Electrical Stimulation and Strength Recovery After TKA
Relationship between neuromuscular electrical stimulation (NMES) training intensity and percent change in quadriceps (Quad)
muscle strength and activation at 3.5 and 6.5 weeks after total knee arthroplasty (TKA). Neuromuscular electrical stimulation (NMES)
training intensity is represented as a percentage of the maximal voluntary isometric contraction (MVIC) at the preoperative (Pre-op)
session. Percent changes in quadriceps muscle strength and activation at 3.5 and 6.5 weeks after TKA were calculated as [(value after
NMES ⫺ value before NMES)/value before NMES] ⫻ 100. Solid lines represent data at 3.5 and 6.5 weeks after TKA, and dashed lines
represent data without a potential outlier’s data point at 3.5 weeks after TKA. Adjusted R2 values are shown on each graph, and
asterisks indicate significance (P⬍.05).
vation (adjusted R2⫽.22, P⫽.006).
At 6.5 weeks, there was a significant
association of NMES training intensity with a percent change in torque
from
the
preoperative
value
(adjusted R2⫽.25, P⫽.003) but not
with activation (adjusted R2⫽.00,
P⫽.31). In 1 participant, low preoperative strength made the postoperative strength relative to training
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intensity appear unusually great.
Therefore, data are presented with
and without this potential outlier.
Muscle Fatigue and Activation
After NMES
Quadriceps muscle fatigue was
apparent immediately after NMES
application, such that quadriceps
muscle torque was significantly
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lower after NMES application than
before NMES application at 3.5
weeks (P⬍.001) and 6.5 weeks
(P⬍.001) (Fig. 3). There were no significant changes in NMES quadriceps
muscle activation immediately after
NMES application relative to activation before NMES application at 3.5
and 6.5 weeks (P⫽.67 and P⫽.55,
respectively).
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Figure 2.
Intensity of Quadriceps Muscle Neuromuscular Electrical Stimulation and Strength Recovery After TKA
Immediate changes in normalized quadriceps (Quad) muscle torque and activation after an NMES session. Asterisks indicate
significant differences between values before NMES application (Pre NMES) and values after NMES application (Post NMES) (P⬍.05,
paired t test).
Discussion
Neuromuscular electrical stimulation offers an effective approach for
mitigating quadriceps muscle central
activation deficits early after TKA
and restores normal quadriceps muscle function more effectively than
voluntary
exercise
alone.9,10,14
Patients with large quadriceps muscle central activation deficits from a
variety of causes have shown negligible improvements in strength even
after intensive rehabilitation focused
on traditional, voluntary exercise
paradigms.37 It appears that these
patients, including those who have
had TKA, may have difficulty training
their muscles at intensities sufficient
to promote strength gains. Neuromuscular
electrical
stimulation
appears to help counter these muscle activation deficits and reeducate
the quadriceps muscle to facilitate
the recovery of muscle function
early after TKA. Results from the previously published parent study14
indicated that, at the 3.5-week visit,
NMES treatment resulted in greater
improvements in quadriceps muscle
strength and a trend toward greater
quadriceps muscle activation than
the control treatment. The results of
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the present study expanded these
findings by demonstrating that stronger, electrically induced muscle
forces during training resulted in
greater improvements in quadriceps
muscle strength. Importantly, the
relationship between NMES dose
and muscle strength was more
marked at 3.5 weeks than at 6.5
weeks, suggesting that NMES dose
may have a greater impact during
early rehabilitation than during later
rehabilitation after TKA. Furthermore, muscle activation also was
related to training intensity at 3.5
weeks but not at 6.5 weeks, likely
because activation deficits without
NMES were most pronounced at 3.5
weeks (X⫽73.6% [SD⫽17.9%]) and
had substantially diminished by 6.5
weeks (X⫽86.3% [SD⫽10.6%]) in
the parent study (ceiling effect). Also
interesting was the finding that muscle activation and NMES training
intensity were not as strongly related
as were muscle strength and NMES
training intensity, possibly because
of more general variability in muscle
activation outcomes than in muscle
strength outcomes. Finally, the relatively high degree of adherence to
NMES may have precluded finding a
relationship between NMES adherence and changes in quadriceps muscle strength or activation.
Few studies have explored how
NMES training intensity is related to
muscle strength gains,25,26 especially
in patients.28 Although NMES effectiveness is strongly influenced by the
intrinsic neuromuscular properties
of the tissue, such as motor nerve
branching,20,38,39 results from the
few studies investigating NMES training intensity have found that NMESinduced strength gains are positively
correlated with NMES training intensity. In particular, Snyder-Mackler et
al28 found that the recovery of quadriceps muscle strength after anterior
cruciate ligament reconstruction
was positively correlated with NMESinduced contraction intensity. They
also found that there was a minimum threshold for NMES training
intensity to elicit an increase in quadriceps muscle strength (10% of the
MVIC of the uninvolved quadriceps
muscle). Selkowitz26 explored NMES
in the quadriceps muscle in people
who were healthy using maximally
tolerable isometric contractions 3
days per week for 4 weeks. Selko-
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Figure 3.
Intensity of Quadriceps Muscle Neuromuscular Electrical Stimulation and Strength Recovery After TKA
witz also found that the relative
strength improvement with NMES
was positively and significantly correlated with the training contraction
intensity.
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Electrically elicited muscle contractions allow for the activation of a
larger proportion of type II muscle
fibers than volitional exercise at a
comparable intensity.46 – 48 Type II
muscle fibers are larger than type I
fibers but also are more fatigable;
therefore, greater activation of these
fibers maximizes force production,
but at the expense of increased muscle fatigue.49 With volitional exercise, type II fibers are typically activated only during high-intensity
voluntary contractions.50 This activation pattern occurs with voluntary
contractions as a result of a more
orderly recruitment of motoneurons
because smaller motoneurons have
lower activation thresholds than
larger motoneurons; therefore,
smaller motoneurons (which innervate type I muscle fibers) are
recruited before larger motoneurons
(which innervate type II muscle
fibers).50 With electrically elicited
muscle contractions, a less orderly
recruitment of motor units occurs
because factors such as the size of
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the axonal branches and their
orientation toward the current field
influence motor unit recruitment,
such that a larger proportion of
motor units that are activated only
during high-level voluntary muscle
contractions may be recruited earlier
during electrically elicited muscle
contractions.22,46,51,52 Additionally,
less fatigue occurs during voluntary
muscle contractions because muscle
force is sustained through modulation of the rates of firing of active
motor units53 or through alternate
recruitment of new motor units
when active motor units become
fatigued.20 In contrast, electrically
elicited contractions rely on sustained recruitment of the same
(ie, spatially fixed) motor units at
a fixed rate that is temporally
synchronous.20
With the myriad factors that contribute to increased muscle fatigue with
NMES, it is not surprising that quadriceps muscle fatigue immediately
after NMES application was found in
the present study, even though the
average NMES training intensity was
relatively low compared with that in
a previous investigation.17 More surprising was the lack of improvement
in quadriceps muscle activation
immediately after a single session of
NMES. One possible explanation is
that the consistent use of NMES after
TKA facilitated a trend toward
greater overall quadriceps muscle
activation in the parent study (11%
more activation; P⫽.09),14 such that
a single session of NMES was not
sufficient to further boost quadriceps muscle activation. Nevertheless, in the presence of substantial
quadriceps muscle fatigue without a
concurrent increase in muscle activation, NMES application may be
best timed after other exercises prescribed for rehabilitation for patients
using NMES daily.
One of the most substantial limitations for NMES use is patient tolSeptember 2012
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An increasing body of literature has
explored mechanisms that might
explain how larger, electrically elicited muscle contractions confer
greater strength gains. These studies
have suggested that NMES appears to
influence motor performance by
influencing motor cortex excitability.40 – 43 Smith et al44 found a relationship between quadriceps muscle
NMES intensity and activation of sensorimotor cortex regions of the
brain, such that higher current intensities increased cortical activity.
Adams et al39 mapped patterns of
muscle activation after repeated
NMES-evoked isometric contractions
and found that with increasing intensity, the number of motor units activated also increased and contributed
to greater force output. Furthermore, the NMES-evoked contractions were able to recruit muscle
fibers deep within the muscle even
at lower training intensities because
the muscle fibers innervated by the
nerves stimulated by the transcutaneous application of NMES are distributed throughout the muscle.
Nevertheless, evidence more generally suggests that NMES at the stimulation intensities achieved in the
present study largely targets superficial nerves more than deep ones
because of proximity to the stimulating electrodes, especially at lower
training intensities.20 However, even
lower levels of stimulation that target peripheral afferent nerves can
induce prolonged changes in the
excitability of the human motor cortex.42 Subthreshold peripheral stimulation of the afferent nerves of the
hand resulted in an increase in the
functional magnetic resonance imaging signal intensity in the primary
and secondary motor and somatosensory areas of the cortex.40
In addition to the limitation of relatively superficial recruitment of
nerve fibers with NMES, another limitation of NMES is muscle fatigue,
although muscle fatigue is thought to
be a necessary stimulus for muscle
hypertrophy. Muscle fatigue with
NMES occurs because of the great
metabolic demand on the muscle,
which results in greater muscle
fatigue than would occur with voluntary contractions.20,21,39,45 This
increased muscle fatigue may be a
result of several factors. In part, muscle fatigue with NMES occurs more
rapidly than muscle fatigue with volitional exercise because of the
repeated contractile activity within
the same muscle fibers with NMES.39
Muscle fatigue also may be influenced by the less orderly recruitment of motor units with electrically
elicited muscle contractions than
with voluntary muscle contractions.21
Intensity of Quadriceps Muscle Neuromuscular Electrical Stimulation and Strength Recovery After TKA
When familiarization before surgery
is not possible, allowing patients a
few sessions after surgery to become
accustomed to NMES application
often allows them to gradually tolerate higher levels of NMES. Also,
when patients have control over the
intensity of NMES and understand
the importance of increasing the
stimulation intensity as much as is
tolerable, they appear to increase the
stimulation intensity more readily
than when a physical therapist has
control over the stimulator. Adherence to NMES treatment is influenced by patient tolerance of NMES.
In the present study, all participants
were included in the analyses,
regardless of adherence to NMES, to
allow for more generalizability of the
results in clinical settings.
Despite the strong association
between NMES training intensity and
quadriceps muscle function after
TKA, especially for the first few
weeks after TKA, we were unable to
determine a minimum threshold
September 2012
for NMES application. A reasonable
guideline for NMES application may
be the presence of a visible muscle contraction, yet evidence suggests that even subthreshold peripheral stimulation of afferent nerves
results in increased activity of the
primary and secondary motor and
somatosensory areas of the cortex.40
Therefore, even lower training intensities could result in altered motor
unit recruitment and corresponding
improvements in quadriceps muscle function. Additional investigation is necessary to better establish
a minimum threshold for NMES
application.
Dr Stevens-Lapsley, Ms Balter, Dr Eckhoff,
Dr Schwartz, Dr Schenkman, and Dr Kohrt
provided concept/idea/research design. Dr
Stevens-Lapsley, Ms Balter, Dr Schwartz,
Dr Schenkman, and Dr Kohrt provided writing. Dr Stevens-Lapsley and Ms Balter provided data collection, data analysis, and project management. Dr Stevens-Lapsley, Dr
Schenkman, Dr Schwartz, and Dr Kohrt provided fund procurement. Dr Eckhoff provided study participants. Dr Stevens-Lapsley,
Dr Eckhoff, Dr Schwartz, Dr Schenkman,
and Dr Kohrt provided facilities/equipment.
Dr Schwartz provided institutional liaisons.
Ms Balter, Ms Wolfe, Dr Eckhoff, Dr
Schwartz, Dr Schenkman, and Dr Kohrt provided consultation (including review of manuscript before submission). The authors
acknowledge Dana Judd, PT, DPT, for assistance with participant testing and treatment; Roger Enoka, PhD, and John Kittelson,
PhD, for consultation and guidance on
research design and implementation; and
Tami Struessel, PT, DPT, for consultation and
guidance on physical therapy interventions.
They also thank the physical therapists at the
University of Colorado Hospital.
This study was approved by the Colorado
Multiple Institutional Review Board.
Some of these results were presented at the
American Academy of Orthopaedic Surgeons Meeting; February 7–11, 2012; San
Francisco, California.
This study was supported
by the National Institute on
Aging (K23AG029978),
an American College of
Rheumatology New Investigator Award, a
Foundation for Physical Therapy Marquette
Challenge Grant, and a Clinical and Transla-
tional Science Award Grant (UL1 RR025780).
A peer-reviewed research grant from Empi
Inc, a DJO Global company, was used to
support the purchase of Empi 300PV electrical stimulators and recruitment and transportation costs for participant visits.
This work was part of a clinical trial
with ClinicalTrials.gov registry number
NCT00800254.
DOI: 10.2522/ptj.20110479
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