Quadriceps Muscle Fatigue and Comfort Generated by

Neuromuscular Electrical Stimulation with Current Modulated

Waveforms

Tiago Araújo

, Ana Anjos

, Neuza Nunes

, Pedro Rebelo

and Hugo Gamboa

Department of Physics, Faculty of Sciences and Technology, New University of Lisbon, Lisbon, Portugal

Schientific Area of Physiotherapy, Lisbon School of Health Tecnology, Polytecnic Institute of Lisbon, Lisbon Portugal

PLUX - Wireless Biosignals S.A., Lisbon, Portugal

Keywords: Fatigue, Neuromuscular Electrical Stimulation, Waveform.

Abstract: Introduction: Neuromuscular electrical stimulation (NMES) is used by physical therapists in the clinic. The

efficacy of NMES is limited by the rapid onset muscle fatigue. The role of NMES parameters is muscle

fatigue is not clear. Objective: To determine the effects of shape waveform on muscle fatigue, during

NMES. Methods: Twelve healthy subjects participated in the study. Subjects were assigned to 1 of 3 groups,

randomly. Group assignment determined the order in which they were tested using 3 different shape

waveforms. Maximal voluntary isometric contraction (MVIC) was measured during the first session.

Fatigue test was applied with amplitude required to elicit 50% of the MVIC. In each 3 testing sessions

torque of contraction and level comfort were measured, and percent fatigue was calculated. Analysis of

variance tests for dependent samples was used to determine the effect of shape waveform on muscle fatigue

and comfort scores Results: The results showed no one shape waveform was most fatigable and that SQ

wave induced more uncomfortable stimulus.

1 INTRODUCTION

Neuromuscular electrical stimulation is a commonly

used tool by physical therapists in sports and clinical

conditions characterized by motor impairments such

as stroke, cerebral palsy, and spinal cord injury

(Glinsky, Harvey and Van, 2007; Maffiuletti et al.,

2000; Newsam and Baker, 2004; Stackhouse et al.,

2007; Snyder-Mackler, Delitto and Stralka, 1994).

The common neuromuscular adaptations that

characterize the aforementioned conditions are

muscle weakness and atrophy resulting from disuse

or neurological injury (Maffiuletti, 2010; Snyder-

Mackler et al., 1994).

However, during electrical stimulation, skeletal

muscles fatigue more rapidly during repetitive

stimulation than during voluntary contractions

(Riener, 1999; Vanderthommen et al., 2003)

Muscle fatigue is defined as a reduction in the peak

force, with continuous and repeated activation

(Mulla, Sepulveda and Colley, 2011)

Rapid fatigue

during NMES is thought to result from the

differences in motor unit recruitment order, higher

activation frequencies and imprecise control of

muscle force comparing voluntary contractions

(Peckham and Knutson, 2005). The problem of

muscle fatigue is aggravated by the fact that

paralyzed muscle show greater fatigability than

healthy muscle. Muscle fatigue is an important

factor limiting the clinical use of NMES (Gerrits et

al., 2003).

New commercial stimulators provide many

different waveforms and pulse settings (Snyder-

Mackler et al., 1994; Kantor, Alon and Ho, 1994).

Researchers have attempted to identify preferred

stimulation settings in terms of force contraction

(Doucet, Lam and Griffin, 2012; Laufer, Ries and

Leninger, 2000), fatigue (Binder-Macleod and

Snyder- Mackler, 1993; Gorgey, Black, Elder and

Dudley, 2009; Kesar and Binder-Macleod, 2006)

and comfort (Kantor et al., 1994). The stimulation

variables that are thought to have the greatest impact

on muscle fatigue include pulse amplitude and

duration and pulse train frequency (Binder-Macleod

and Snyder Mackler, 1993; Doucet et al., 2012).

However, because the number of factors considered

in the different studies is extremely variable, is it

difficult to take definitive conclusions concerning

the optimal settings that can elicit the strongest

contractions with minimal fatigue.

Araújo T., Anjos A., Nunes N., Rebelo P. and Gamboa H..

Quadriceps Muscle Fatigue and Comfort Generated by Neuromuscular Electrical Stimulation with Current Modulated Waveforms.

DOI: 10.5220/0005267400920098

In Proceedings of the International Conference on Bio-inspired Systems and Signal Processing (BIOSIGNALS-2015), pages 92-98

ISBN: 978-989-758-069-7

 2015 SCITEPRESS (Science and Technology Publications, Lda.)

The independent effects of these 3 parameters on

muscle fatigue are still controversial. Conflicting

results exist on the role of current amplitude on

muscle fatigue: for instance while one study

demonstrates fatigue increasing with amplitude

(Binder-Macleod, Halden and Jungles, 1995), others

show no change in fatigue with increasing current

amplitude (Slade, Bickel, Warren and Dudley,

2003). In certain cases the frequency of pulses has

been shown to accelerate muscle fatigue (Gorgey et

al., 2009; Lieber and Kelly, 1993). Nonetheless, a

full understanding of the role of pulse duration on

muscle fatigue has not been reached.

Gorgey et al. (2009) concluded that altering the

pulse duration does not appear to influence fatigue

in NMES. Compared to the influence of current

amplitude, frequency and pulse duration, the role of

shape waveform on muscle fatigue is even less well

established.

According to the literature, the most efficient

stimulation of the nerve fiber is carried out using

square shape waveform (Robertson, Ward, Low and

Reed, 2006)

However, if pulse duration is

sufficiently short (250 µs or less) triangular shape

waveforms are functionally equivalent to square

waves (Robertson et al., 2006). Laufer et al. (2000)

showed stimulation with sinusoidal polyphasic

waveform resulted in more rapid muscle fatigue than

stimulation with square monophasic and biphasic

waveforms, both tested on quadriceps muscle in

individuals without impairment. In addition,

stimulation with polyphasic waveform elicited

weaker electrically induced contractions.

Another factor to take into consideration during

NMES, is the subjective comfort of stimulation.

Many previous studies attempted to determine

waveform parameters or combination of parameters

which caused the least subjective discomfort. In one

study, there was no simple answer, individuals

having their own preference for sinusoidal,

triangular, or square waveform (Delitto and Rose,

1986). A symmetric biphasic square waveform was

generally preferred for the large quadriceps muscle

group, whereas an asymmetric biphasic square

waveform was preferred for the smaller forearm

musculature when compared to a monophasic paired

spike and three medium frequency waveforms

(Baker, Bowman and McNeal, 1988).

The main aim of our study was to investigate the

effect of shape waveform on quadriceps muscle

fatigue with three different shape waveforms, in

individuals without impairments. . A second purpose

was to determine whether changing the shape

waveform (square, triangular and quadratic) could

improve the comfort level of subjects.

2 METHODS

2.1 Subjects

The subjects who volunteered to participate in the

study were the following: six female averaging 25

years old (SD=2,7); six male averaging 24 years old

(SD=2,8) All subjects reported having no known

neuromuscular, skeletal, vascular or dermatological

impairment. Each one received a detailed

explanation of the study and gave informed consent

prior to participation. The Scientific Committee of

Health School of Technology of Lisbon approved

the present study.

2.2 Instrumentation

A portable electrical stimulator with wireless data

communication was used to elicit muscle contraction

(Bio Signals Plux). This device allows the

modulation of different shape waveforms. Table 1

presents a characteristics summary of the electrical

stimulation settings used. The current charge

difference from each waveform was taken into

account in the data analysis. The stimulation charge

was computed accordingly to the stimulation

intensity and waveform, based on the following

formula:

Q= ∫t1I dt (1)

Where I represent the current intensity and t1 to t2 is

the stimulus time range, being considered in this

case constant. The charge of the different waveform

types have been equalized, for each current intensity,

maintaining the amplitude and varying the pulse-

duration time.

Table 1: Summary of stimulation characteristics for 3

waveforms.

Stimulation

arameters

SQ TR QU

Type of

wavefor

Monophasic Monophasic Monophasic

Pulse

duration (µs)

175 375 500

Frequency

(Hz)

50 50 50

Maximal

peak intensity

(mA)

100 100 100

Abbreviations: SQ - Square waveform TR - Triangular waveform

QU - Quadratic waveform

QuadricepsMuscleFatigueandComfortGeneratedbyNeuromuscularElectricalStimulationwithCurrentModulated

Waveforms

Two 10 x 5 cm rectangular, reusable, self-adhering

electrodes were used in each participant; a isokinetic

dynamometer (Biodex)

[25]

was used to assess torque

generated by right quadriceps muscle group during

MVIC and during all electrically induced isometric

quadriceps muscle contractions. The reliability of

the Biodex system dynamometers for knee extensors

and knee flexors peak torque measurements in

isometric, concentric and eccentric tests has been

studied. Interclass correlation coefficients indicated

high to very high reproducibility for isometric,

concentric and eccentric peak torques (0,88-0,92),

and moderate to high reliability for agonist-

antagonist strength ratios (0,62-0,73) (Araújo et al.,

2014).

2.3 Protocol

Each subject participated in 3 sessions, separated by

at least a 48 hours period. Subjects were assigned to

1 of 3 groups, determining the order in which they

were tested using 3 different electrical stimulation

shape waveforms. Group assignment was random.

Participants were not informed of the shape

waveform being used during each testing session.

The right quadriceps muscle was used for all

tests.

During the initial session, MVIC of the right

quadriceps muscle was measured. This measurement

was followed by determination of the current

amplitude required to elicit 50% of the MVIC test

and by a fatigue test using 1 of the 3 shape

waveforms. The procedure for each the ensuing

sessions was similar to the initial procedure, with the

following exceptions: MVIC was determined at the

initial session only; the type of shape waveform

being used for the electrical stimulation component

of the protocol differed and was determined by

group assignment. Testing was carried out in the

FMH Exercise Physiology Laboratory.

MVIC test. Biodex was used to measure MVIC at a

60 degree knee flexion. Subjects’ leg, thigh, and

pelvis were stabilized by seating system pads and

belts. Backrest was set at 110 degree posterior

incline. The fulcrum of the lever arm was aligned

with the most inferior aspect of the lateral

epicondyle of the right femur. The inferior portion of

the shin pad was adjusted superior to the medial

malleolus. Before the test, participants warmed up

and then stretched the major muscle groups of the

lower extremity, holding each stretch for 15 seconds.

Subjects did 3 consecutive 5-second MVIC trials of

the right quadriceps muscle group, with 60 seconds

of rest between trials. They were asked to keep their

arms crossed over their chest and to contract knee

extensors as fast and forcefully as possible, while

verbal encouragement was provided.

Participants were not allowed to view the

measurements on the computer screen. The highest

measured torque was used to calculate 50% of the

MVIC level.

Determination amplitude required to elicit 50% of

the MVIC. A 5 minute rest period was allowed

between the MVIC test and the determination

current amplitude required to elicit 50% of the

MVIC. Each subject’s right thigh was cleaned with

alcohol. The distal electrode was placed on the

vastus medialis approximately 5 to 7 inches from the

top margin of the patella; the proximal electrode was

placed on the lateral border of the femoral rectus

muscle approximately 2/3 of its length above the top

edge of the patella (Figure 1).

Figure 1: Frontal view: Two electrodes are positioned over

vastus medialis and on the lateral border of the femoral

rectus. The stimulated limb is maintained in isometric

conditions.

The electrodes being set, subjects were placed in the

Biodex in the same position used for the MVIC

testing. The current amplitude was determined by

delivering 3-second trains of progressively greater

amplitude. At least 1 minute separated each train.

Three to four trials per participant were performed to

determine the amplitude of the current in milliamps

(mA).

Fatigue test. After a 15-minute rest period, to ensure

muscle fatigue recovery, the fatigue component othe

protocol was performed, using the current amplitude

previously identified and an identical positioning in

the Biodex. The fatigue test consisted in nineteen 3-

second contractions were evoked 2-minute period

(work-to-rest cycle of 3 seconds on and 3 seconds

off) As in other portions of the study, subjects were

BIOSIGNALS2015-InternationalConferenceonBio-inspiredSystemsandSignalProcessing

instructed to keep their arms crossed over their chest

and to try to relax during the electrically stimulated

muscle contractions. They were not able to view the

torque measurements displayed on the computer

screen. After the three first contractions, the subjects

were asked to express their perceived discomfort on

a visual analogue scale (VAS) 10 cm long labeled at

the left extreme “maximum discomfort” and at the

right “no discomfort”.

The fatigue index was measured and reflects the

difference between the torques of the initial and final

contractions divided by the torque of the initial

contraction.

2.4 Data Analysis

The mean value of each contraction was extracted

for all subjects and for all the signals resulting from

the different waveforms. For that, we computed the

mean and standard deviation (SD) value of the

contraction above 90% of its maximum value. The

results were extracted automatically and visually

validated manually by two experts.

Descriptive analysis was performed. Frequency

or means and standard deviations were calculated for

each of the demographic variables.

To check if the variables approached a normal

distribution we used Shapiro-Wilk test. The level of

significance used was α = 0,01.

Data was analyzed using ONE-WAY ANOVA,

comparing groups in age and Body Mass Index

(BMI), discomfort and fatigue percent or Kruskal-

Wallis test if the assumption of normality or

homogeneity was not verified.

To examine the effects of the 3 shape waveforms

on muscle fatigue and discomfort we used ANOVA

for 3 dependents samples or ANOVA Friedman test.

The independent variables were the shape

waveforms (SQ, TR and QU), the contraction

number were 1, 9, 13, 19 and dependent variables

were peak torque, perceived discomfort and fatigue

percent. When applying variance analysis led to the

identification of differences, post-hoc tests were

used.

The level of significance used to inferential

statistics was α = 0,05.

3 RESULTS

Of the 12 subjects who participated, two of them

have not reached values close to50% MVIC with TR

and QU shape waveforms. There was no differences

between the 3 groups in the age (F

2,9

=5,42, P =

0,029)

BMI (X

(2) = 3,500, P = 0,174) and sex

(Table 2). The groups are also similar in physical

exercise practice, not being given the intensity of the

exercise.

Table 2: Demographic variables in 3 groups.

The mean ± SD current amplitudes for SQ, TR

and QU shape waveforms were 77± 14; 92± 8 and

99 ± 2, respectively. The SQ, TR and QU

waveforms evoked mean ± SD percents of MVIC of

50,5% ± 2,4%, 47,2% ± 11,1% and 45,6% ± 10,1%,

respectively (Figure 2). It was noted the SQ wave

generates higher percent of MVIC than the others

with lower current amplitudes. FRIEDMAN’S test

did not reveal statistical difference between evoked

percents of MVIC by 3 shape waveforms in the first

contraction of the fatigue test (X

(2) = 3,362, P =

0,186).

Figure 2: Declining torque of a subject, during fatigue test

with SQ shape waveform.

The 1-way ANOVA did not reveal significant

difference in fatigue percent for SQ (F

2,9

= 1,063, P

Variables

Groups

Group 1

(n = 4)

Group 2

(n= 4)

Group 3

(n = 4)

Sex

Female 2 (33,3%) 2 (33,3%) 2 (33,3%)

Male 2 (33,3%) 2 (33,3%) 2 (33,3%)

Age* 22 ± 3,2 25 ± 1,3 27 ± 2,7

BMI* 21 ± 2,8 22 ± 2,0 19 ± 0,8

Dominant leg

Right 4 (33,3%) 4 (33,3%) 4 (33,3%)

Left 0 0 0

Exercise

practice

Yes 3 (37,5%) 3 (37,5%) 2 (25,0%)

No 1 (25,0%) 1 (25,0%) 2 (50,0%)

Group 1: SQ, TR and QU; Group 2: QU, SQ and TR; Group 3:

TR, QU and SQ.

*Data presented in form of means and standard deviations.

QuadricepsMuscleFatigueandComfortGeneratedbyNeuromuscularElectricalStimulationwithCurrentModulated

Waveforms

= 0,385), TR(F

2,9

= 0,201, P = 0,821) and QU (F

2,9

0,317 P = 0,736) waveform; comfort perception

score of SQ ( F

2,9

= 0,66, P = 0,537), TR ( F

2,9

= 1,

725, P = 0, 232) and QU (F2,9 = 2,189, P = 0,168)

protocols , among the 3 groups, suggest that the

sequence of application of the waves does not

interfere with the results.

Figures 2 and 3 illustrate the decline in the

evoked torque for the 3 shape waveforms. For all 3

waveforms, there was significant reduction in torque

from the initial contraction (F

3,2

= 189,143, P =

0,000; X

(3) = 32,700, P = 0,00; X

(3) =

32,500, P = 0,000 for SQ, TR and QU waveform,

respectively).

No significant differences were observed in

fatigue percent among the 3 shape waveforms (F

2,2

2,677, P = 0,091). The TR wave resulted in a lower

fatigue when compared to the other 2 shape

waveforms (mean ± SD fatigue percent 34,8% ±

12,4% versus 40,3% ± 5,5% and 41,9% ± 9,3%).

Relative to comfort score there were significant

difference between 3 shape waveforms (X

9,500 , P = 0,009). We verified on Table 3 that SQ

wave differs significantly from TR wave (P =

0,007). The most comfortable shape waveform was

the TR wave (mean 5,0 ± 2,5), then the QU wave (

mean 5,6 ± 2,9) and finally the SQ shape waveform

(mean 6,7 ± 2,7).

Table 3: Pairwise comparisons among waveforms, relative

comfort scores.

4 DISCUSSION

The current study investigated the influence of the

waveform (SQ, TR and QU) on muscle fatigue,

during NMES. A second purpose was to determine

how the wave shape could alter subject comfort

level during stimulation.

The results of our study demonstrated that there

are no major differences between 3 waves on muscle

fatigue. However TR wave (mean 34,8% ± 12,4%)

has shown lowest percentage of fatigue than

standard wave SQ (mean 40,3% ± 5,5%) and QU

waveform (mean 41,9% ± 9,3).

Figure 3: Values are mean ± SD. No significant

differences between QU, SQ and TR (P > 0,05). Decline

in torque over repeated contractions for QU, SQ and TR.

The difference between the fatiguing effect of

TR waveform and that of the other 2 waveforms is

difficult to explain. Three of the variables

considered to have the greatest effect on muscle

fatigue (frequency, electrical charge and time on-off

ratios) were the same in all protocols (Binder-

Macleod and Snyde-Mackler, 1993; Doucet et al.,

2012).

Concerning QU wave, it may be thought that the

largest fatigue percent is due to higher current

amplitude (99 mA ± 2 vs 92 mA ± 8). A previous

study showed that increasing the current amplitude

while keeping other NMES parameters constant

modestly increased fatigue (Binder-Macleod et al.,

1995). This suggests that as the current amplitude

increases, more fast-twitch motor units are recruited,

resulting in greater fatigue due their higher

metabolic demand in comparison to slow-twitch

motor units.

Another possibility relates to high pulse duration

(500 µs). Literature reports with increasing pulse

duration there is an increase in the evoked torque,

possibly increasing motor unit activation (Gorgey,

Mahoney, Kendall and Dudley, 2006). A pulse

duration of 450 µs elicited 22% and 55% greater

torque output compared to pulse durations of 250

and 150 µs, respectively (Gorgey et al., 2006;

Gorgey and Dudley, 2008). Because increasing

pulse duration increases the evoked torque per unit

of activated muscle (Gorgey et al., 2006), it causes a

Shape

waveforms

Value test P

SQ vs TR 9,500 0,007*

SQ vs QU 9,500 0,662

QU vs TR 9,500 0,199

F: Value of ANOVA Friedman test

*Significantly different P < 0,05

1 9 13 19

Torque (% MVIC)

Contraction number

BIOSIGNALS2015-InternationalConferenceonBio-inspiredSystemsandSignalProcessing

higher energy demand and thus leads to faster

muscle fatigue.

However, neither amplitude or pulse duration

appears to influence the increased percent fatigue of

SQ wave, since it presented lower amplitude and

pulse duration than that with the best results (77 mA

and 195 µs vs 92 mA and 375 µs). A recent study

showed no change in fatigue with increase of current

amplitude or pulse duration (Gorgey et al., 2009).

These parameters cause an increase of active area in

stimulated muscle (Gorgey et al., 2009). These

findings may indicate that, although the differences

were not statistically significant, wave shape might

be responsible for the slight variations.

We cannot neglect to mention that there are few

works that study the effect of the waveform on

muscle fatigue and the independent study of the

effect of NMES parameters (frequency, pulse

duration and amplitude) on fatigue were made using

exclusively the standard wave, which gives rise to

reserves as to whether generalizations can be made

for other waves.

Relatively to level comfort, results indicate that

TR wave (mean 5 ± 2,5 on VAS score) is

significantly more comfortable than SQ wave (mean

6,7± 2,7, on VAS score). Our work does not

confirms the conclusions of Delitto and Rose

(1986), who found that individual differences exist

as far as tolerance to various waveforms (square,

triangular and sinusoidal) is concerned and that no

waveform tested can be considered optimally

comfortable than the others.

The TR waveform, as used in our study,

consisted of higher amplitude and pulse duration

than that of the SQ wave, but evoked lower percent

of maximum voluntary isometric contraction

(MVIC). With longer duration of this pulse within

the interval 200-400 µs clinical is which according

to some research is a good relationship between

efficiency and comfort to the patient (Lyons,

Sinkjaer, Burridge and Wilcox, 2002). Best results

of the TR wave could suggest that the subjects may

find an electrically elicited contraction more

comfortable at current amplitude producing lower

percentages of the MVIC than amplitude producing

higher percentages of the MVIC.

The use of the visual analogous scale (VAS) in

this study offers an alternative for measuring the

comfort levels (Kersten, 2012). VAS has been

reported to be accurate, sensitive, and reproducible

instruments for patients to report the degree of pain

they are experiencing.

No significant difference in the comfort scores

was seen on 3 groups, suggesting the order of the

administrations of waves do not interfere with

comfort perception, contrary to what Delitto and

Rose (1986) suggested.

From the results obtained in this study it is

possible to propose that the waveform does not

significantly influence muscle fatigue in healthy

individuals and that TR waveform provides more

comfort than the standard contraction wave, during

NMES.

Despite minor differences, the best results of the

TR wave compared to the SQ wave (also at the level

of muscle fatigue) can be quite relevant for clinical

practice. Literature indicates that more efficient

stimulation of the nerve fiber is made using the

square wave, with pulse durations of less than 250s,

but the triangular wave shows equal (Robertson et

al., 2006) and perhaps better results in improved

comfort and reduced fatigue levels for patients.

The new systems of NMES on the market are

modifiable, so therapists can set parameters

(frequency, pulse duration, shape waveform, etc.)

and design custom electrical stimulation programs

for patients to use. Therefore future studies should

focus on the study of the waveform and its

combination with the other parameters, using larger

number of subjects. This study should be performed

in healthy and clinical populations.

REFERENCES

Araújo JB, Rodrigues R, Azevedo R, Silva BG, Pinto RS,

Vaz MA, Baroni BM. (2014). Inter-machine reliability

of the Biodex and Cybex isokinetic dynamometers for

knee flexor/extensor isometric, concentric and

eccentric tests. Phys Ther Sport 15: 131-216.

Baker LL, Bowman BR, McNeal DR.(1988). Effects of

waveform on comfort during neuromuscular electrical

stimulation. Clin Orthop, 233: 75-81.

Binder-Macleod SA, Halden EE, Jungles KA. (1995).

Effects of stimulation intensity on the physiological

responses of human motor units. Med Sci Sports

Exerc.,27:556-565.

Binder-Macleod SA, Snyder-Mackler L. (1993). Muscle

fatigue: clinical implications for fatigue assessment

and neuromuscular electrical stimulation. Phys

Ther.,73:902–910.

Biodex. Website. http://www.biodex.com/

Biosignalsplux. Website. http://www.biosignalsplux.com/.

Delitto A, Rose SJ. (1986). Comparative Comfort of three

waveforms used in electrically eliciting quadriceps

femoris muscle. Phys Ther., 66: 1704-1707.

Doucet DM, Lam A, Griffin L. (2012). Neuromuscular

electrical stimulation for skeletal muscle function.

Yale J Bio Med., 85: 201-215.

Gerrits HL, Hopman MT, Offringa C, Engelen BG,

QuadricepsMuscleFatigueandComfortGeneratedbyNeuromuscularElectricalStimulationwithCurrentModulated

Waveforms

Sargeant AJ, Jones DA, Haan A. Variability in fibre

properties in paralysed human quadriceps muscles and

effects of training. Pflugers Arch. 2003; 445: 734–740.

Glinsky J, Harvey L, Van Es. (2007). Efficacy of electrical

stimulation to increase muscle strength in people with

neurological conditions: a systematic review.

Physiotherapy Research International., 12:175–194.

Gorgey AS, Black CD, Elder CP, Dudley GA. (2009).

Effects of electrical parameters on fatigue in skeletal

muscle. J Orthop Sports Phys Ther, 39(9): 684-692.

Gorgey AS, Dudley GA. (2008). The role of pulse

duration and stimulation duration in maximizing the

normalized torque during neuromuscular electrical

stimulation. J Orthop Sports Phys Ther., 38: 508-516.

Gorgey AS, Mahoney E, Kendall T, Dudley GA. (2006).

Effects of neuromuscular electrical stimulation

parameters on specific tension. Eur J Appl Physiol.,

97: 737-744.

Kantor G, Alon G, Ho HS. (1994). The effects of selected

stimulus waveforms on pulse and phase characteristics

at sensory and motor thresholds. Phys Ther.,74:951–

962.

Kersten P. (2012). The use of Visual Analogue Scale

(VAS) in rehabilitation. J Rehab Med.,44: 609-610.

Kesar T, Binder-Macleod S. (2006). Effect of frequency

and pulse duration on human muscle fatigue during

repetitive electrical stimulation. Exp Physiol., 91: 967-

976.

Laufer Y, Ries JD, Leininger PM. (2000). Quadriceps

femoris muscle torque and fatigue generated by

neuromuscular electrical stimulation with three

different waveforms. Phys Ther., 81: 1307-1316.

Lieber RL, Kelly MJ. (1993). Torque history of

electrically stimulated human quadriceps: implications

for stimulation therapy. J Orthop Res,11:131-141.

Lyons GM, Sinkjaer T, Burridge Jh, Wilcox DJ. (2002). A

review of portable FES-based neural orthoses for

correction of drop foot. IEE Transactions on Neural

Systems and Rehabilitation Engineering, 10: 260-279.

Maffiuletti NA. (2010) Physiological and methodological

considerations for the use of neuromuscular electrical

stimulation. European Journal of Applied

Physiology.,110: 223-234.

Maffiuletti NA, Cometti G, Amiridis IG, Martin A,

Pousson M, Chatard JC. (2000). The effects of

electromyostimulation training and basketball practice

on muscle strength and jumping ability. International

Journal of Sports Medicine., 21: 437-443.

Mulla MR, Sepulveda F, Colley M. (2011). A review of

non-invasive thecniques to detect and predict localized

muscle fatigue. Sensors.,11: 3545-3594.

Newsam CJ, Baker LL. (2004). Effect of an electric

stimulation facilitation program on quadriceps motor

unit recruitment after stroke. Archives of Physical

Medicine and Rehabilitation., 85:2040–2045.

Peckham PH; Knutson JS. (2005) Functional electrical

stimulation for neuromuscular applications. Annu Rev

Biomed Eng., 7:327–360.

Riener R. (1999). Model-based development of

neuroprosthesis for paraplegic patients. Philos Trans R

Soc Lond B Biol Sci., 354: 877–894.

Robertson V, Ward A, Low J. Reed A. (2006).

Electrotherapy Explained: Principles and Practice. 4

Edition. Oxford: Elsevier; Chapter 3, Electrical

stimulation: currents and parameters; 59.

Slade JM, Bickel CS, Warren GL, Dudley GA.

(2003).Variable frequency trains enhance torque

independent of stimulation amplitude. Acta Physiol

Scand.,177:87-92.

Snyder-Mackler L, Delitto A, Stralka SW, Bailey SL.

(1994). Use of electrical stimulation to enhance

recovery of quadriceps femoris muscle force

production in patients following anterior cruciate

ligament reconstruction. Physical Therapy., 74: 901–

907.

Stackhouse SK, Binder-Macleod SA, Stackhouse CA,

McCarthy JJ, Prosser, LA, Lee SC. (2007). Neuro-

muscular electrical stimulation versus volitional

isometric strength training in children with spastic

diplegic cerebral palsy: a preliminary study.

Neurorehabilitation and Neural Repair., 21 (6):475-

485.

Vanderthommen M, Duteil S, Wary C, Raynaud JS,

Leroy-Willig A, Crielaard JM, et al. (2003). A

comparison of voluntary and electrically induced

contractions by interleaved 1H- and 31P-NMRS in

humans. J Appl Physiol., 94:1012–1024.

BIOSIGNALS2015-InternationalConferenceonBio-inspiredSystemsandSignalProcessing