The aims of this study are to characterize the contamination of EMG signals by artifacts generated by the delivery of spinal cord transcutaneous stimulation (scTS) and to evaluate the performance of ...an Artifact Adaptive Ideal Filtering (AA-IF) technique to remove scTS artifacts from EMG signals. Methods : In five participants with spinal cord injury (SCI), scTS was delivered at different combinations of intensity (from 20 to 55 mA) and frequencies (from 30 to 60 Hz) while Biceps Brachii (BB) and Triceps Brachii (TB) muscles were at rest or voluntarily activated. Using a Fast Fourier Transform (FFT), we characterized peak amplitude of scTS artifacts and boundaries of contaminated frequency bands in the EMG signals recorded from BB and TB muscles. Then, we applied the AA-IF technique and the empirical mode decomposition Butterworth filtering method (EMD-BF) to identify and remove scTS artifacts. Finally, we compared the content of the FFT that was preserved and the root mean square of the EMG signals (EMGrms) following application of the AA-IF and EMD-BF techniques. Results : Frequency bands of ~2Hz width were contaminated by scTS artifact at frequencies nearby the main frequency set for the stimulator and its harmonics. The width of the frequency bands contaminated by scTS artifacts increased with current intensity delivered using scTS (p<0.001), was lower when EMG signals were recorded during voluntary contractions compared to rest (p<0.05), and was larger in BB muscle compared to TB muscle (p<0.001). A larger portion of the FFT was preserved using the AA-IF technique compared to the EmD-BF technique (96±5% vs. 75±6%, p<0.001). Conclusion : The AA-IF technique allows for a precise identification of the frequency bands contaminated by scTS artifacts and ultimately preserves a larger amount of uncontaminated content from the EMG signals.
During fatiguing voluntary contractions, the excitability of motoneurons innervating arm muscles decreases. However, the behavior of motoneurons innervating quadriceps muscles is unclear. Findings ...may be inconsistent because descending cortical input influences motoneuron excitability and confounds measures during exercise. To overcome this limitation, we examined effects of fatigue on quadriceps motoneuron excitability tested during brief pauses in descending cortical drive after transcranial magnetic stimulation (TMS). Participants (n=14) performed brief (~5 s) isometric knee extension contractions before and after a 10-min sustained contraction at ~25% maximal EMG of vastus medialis (VM) on one (n=5) or two days (n=9). Electrical stimulation over thoracic spine elicited thoracic motor evoked potentials (TMEP) in quadriceps muscles during ongoing voluntary drive and 100ms into the silent period following TMS (TMS-TMEP). Femoral nerve stimulation elicited maximal M-waves (Mmax). On the two days, either large (~50% Mmax) or small (~15% Mmax) TMS-TMEPs were elicited. During the 10-min contraction, VM EMG was maintained (P=0.39) whereas force decreased by 52% (SD 13%) (P<0.001). TMEP area remained unchanged (P=0.9), whereas large TMS-TMEPs decreased by 49% (SD 28%) (P=0.001) and small TMS-TMEPs by 71% (SD 22%) (P<0.001). This decline was greater for small TMS-TMEPs (P=0.019; n=9). Therefore, without the influence of descending drive, quadriceps TMS-TMEPs decreased during fatigue. The greater reduction for smaller responses, which tested motoneurons that were most active during the contraction suggests a mechanism related to repetitive activity contributes to reduced quadriceps motoneuron excitability during fatigue. By contrast, the unchanged TMEP suggests that ongoing drive compensates for altered motoneuron excitability.
Rapid force production is critical to improve performance and prevent injuries. However, changes in rate of force/torque development caused by the repetition of maximal contractions have received ...little attention. The aim of this study was to determine the relative influence of rate of torque development (RTD) and peak torque (T(peak)) on the overall performance (i.e. mean torque, T(mean)) decrease during repeated maximal contractions and to investigate the contribution of contractile and neural mechanisms to the alteration of the various mechanical variables. Eleven well-trained men performed 20 sets of 6-s isokinetic maximal knee extensions at 240° · s(-1), beginning every 30 seconds. RTD, T(peak) and T(mean) as well as the Rate of EMG Rise (RER), peak EMG (EMG(peak)) and mean EMG (EMG(mean)) of the vastus lateralis were monitored for each contraction. A wavelet transform was also performed on raw EMG signal for instant mean frequency (if(mean)) calculation. A neuromuscular testing procedure was carried out before and immediately after the fatiguing protocol including evoked RTD (eRTD) and maximal evoked torque (eT(peak)) induced by high frequency doublet (100 Hz). T(mean) decrease was correlated to RTD and T(peak) decrease (R(²) = 0.62; p<0.001; respectively β=0.62 and β=0.19). RER, eRTD and initial if(mean) (0-225 ms) decreased after 20 sets (respectively -21.1 ± 14.1, -25 ± 13%, and ~20%). RTD decrease was correlated to RER decrease (R(²) = 0.36; p<0.05). The eT(peak) decreased significantly after 20 sets (24 ± 5%; p<0.05) contrary to EMG(peak) (-3.2 ± 19.5 %; p=0.71). Our results show that reductions of RTD explained part of the alterations of the overall performance during repeated moderate velocity maximal exercise. The reductions of RTD were associated to an impairment of the ability of the central nervous system to maximally activate the muscle in the first milliseconds of the contraction.
In vitro studies have shown that alterations in redox state can cause a range of opposing effects on the properties of the contractile apparatus in skeletal muscle fibers. To test whether and how ...redox changes occurring in vivo affect the contractile properties, vastus lateralis muscle fibers from seven healthy young adults were examined at rest (PRE) and following (POST) high-intensity intermittent cycling exercise. Individual mechanically skinned muscle fibers were exposed to heavily buffered solutions at progressively higher free Ca
to determine their force-Ca
relationship. Following acute exercise, Ca
sensitivity was significantly decreased in type I fibers (by 0.06 pCa unit) but not in type II fibers (0.01 pCa unit). Specific force decreased after the exercise in type II fibers (-18%) but was unchanged in type I fibers. Treatment with the reducing agent dithiothreitol (DTT) caused a small decrease in Ca
-sensitivity in type II fibers at PRE (by ∼0.014 pCa units) and a significantly larger decrease at POST (∼0.035 pCa units), indicating that the exercise had increased
-glutathionylation of fast troponin I. DTT treatment also increased specific force (by ∼4%), but only at POST. In contrast, DTT treatment had no effect on either parameter in type I fibers at either PRE or POST. In type I fibers, the decreased Ca
sensitivity was not due to reversible oxidative changes and may have contributed to a decrease in power production during vigorous exercises. In type II fibers, exercise-induced redox changes help counter the decline in Ca
-sensitivity while causing a small decline in maximum force.
This study identified important cellular changes occurring in human skeletal muscle fibers following high-intensity intermittent exercise:
) a decrease in contractile apparatus Ca
sensitivity in type I but not type II fibers,
) a decrease in specific force only in type II muscle fibers, and
) a redox-dependent increase in Ca
sensitivity occurring only in type II fibers, which would help maintain muscle performance by countering the normal metabolite-induced decline in Ca
sensitivity.
Force-velocity tests performed on stationary cycle ergometers are widely used to assess the torque and power generating capacities of the lower limbs. The aim of this study was to identify how ...testing and modeling procedures influence the assessment of individual torque-cadence and power-cadence relationships. Seventeen males completed 62 {plus minus} 16 pedal cycles from six 6-s all-out efforts interspersed with 5-min rest. True measures of maximal power for a particular cadence were obtained for 24 {plus minus} 3 pedal cycles, while power was only 94 {plus minus} 3% of the true maximum in 19 {plus minus} 5 pedal cycles. Pedal cycles showing maximal levels of power also displayed higher levels of electromyography (EMG: 89 {plus minus} 7 vs. 87 {plus minus} 7%) and co-activation (34 {plus minus} 11 vs. 31 {plus minus} 10 a.u.), as well as lower variability in crank torque and EMG profiles. Compared to the linear and second order polynomial models that are traditionally used; a better goodness of fit was obtained when the torque-cadence and power-cadence relationships were predicted using second and third order polynomials, respectively. The later modelling procedures also revealed an asymmetry in the power-cadence relationship in most participants (i.e. 15 out of 17) and provided a better estimation of maximal cadence (Cmax: 214 {plus minus} 20 rpm) from the x-intercept of power-cadence relationships (C0: 214 {plus minus} 14 rpm). Therefore, we recommend predicting the individual shapes of torque- and power-cadence relationships using 2nd and 3rd order polynomial regressions after having selected pedal cycles during which true measures of cadence-specific maximal power were recorded.
The aim of the study was to compare the Force-Velocity profiles of track sprint cyclists obtained in seated and non-seated positions. Athletes were tested on a stationary cycle ergometer for the ...seated position and on a racing bike at the velodrome for the non-seated position. We modelled torque and power vs. cadence relationships and extracted maximal force (F
0
), optimal cadence (C
opt
), maximal power (P
max
), maximal cadence (C
0
) and C
opt
/C
0
ratio. Torque/power production was larger in the non-seated position for cadences ranging from 20 to 120 rpm, while more torque and power were produced in the seated position at cadences above 160 rpm. The effective pedal force increased by 0.2 times bodyweight at 50 rpm, and the power production increased by 2.5 W. kg
−1
at 90 rpm in the non-seated position. C
opt
(-14 ± 8 rpm, P < 0.05) and C
0
(−55 ± 32 rpm, P < 0.05) were lowered, while P
max
(+1.7 ± 1.1 W. kg
−1
, P < 0.05) and C
opt
/C
0
ratios (+0.07 ± 0.04, P < 0.05) were increased in the non-seated position when compared with the seated position. Our results show that adopting a non-seated position allows sprint cyclists to maximise torque/power production at lower cadences, while torque/power production was maximised at higher cadences when athletes adopted a seated position.
Abstract The value of electromyography (EMG) is sensitive to many physiological and non-physiological factors. The purpose of the present study was to determine if the torque–velocity test (T–V) can ...be used to normalize EMG signals into a framework of biological significance. Peak EMG amplitude of gluteus maximus (GMAX), vastus lateralis (VL), rectus femoris (RF), biceps femoris long head (BF), gastrocnemius medialis (GAS) and soleus (SOL) was calculated for nine subjects during isometric maximal voluntary contractions (IMVC) and torque–velocity bicycling tests (T–V). Then, the reference EMG signals obtained from IMVC and T–V bicycling tests were used to normalize the amplitude of the EMG signals collected for 15 different submaximal pedaling conditions. The results of this study showed that the repeatability of the measurements between IMVC (from 10% to 23%) and T–V (from 8% to 20%) was comparable. The amplitude of the peak EMG of VL was 99 ± 43% higher ( p < 0.001) when measured during T–V. Moreover, the inter-individual variability of the EMG patterns calculated for submaximal cycling exercises differed significantly when using T–V bicycling normalization method (GMAX: 0.33 ± 0.16 vs. 1.09 ± 0.04, VL: 0.07 ± 0.02 vs. 0.64 ± 0.14, SOL: 0.07 ± 0.03 vs. 1.00 ± 0.07, RF: 1.21 ± 0.20 vs. 0.92 ± 0.13, BF: 1.47 ± 0.47 vs. 0.84 ± 0.11). It was concluded that T–V bicycling test offers the advantage to be less time and energy-consuming and to be as repeatable as IMVC tests to measure peak EMG amplitude. Furthermore, this normalization method avoids the impact of non-physiological factors on the amplitude of the EMG signals so that it allows quantifying better the activation level of lower limb muscles and the variability of the EMG patterns during submaximal bicycling exercises.
To investigate how quadriceps muscle fatigue affects power production over the extension and flexion phases and muscle activation during maximal cycling.
Ten participants performed 10-s maximal ...cycling efforts without fatigue and after 120 bilateral maximal concentric contractions of the quadriceps muscles. Extension power, flexion power and electromyographic (EMG) activity were compared between maximal cycling trials. We also investigated the associations between changes in quadriceps force during isometric maximal voluntary contractions (IMVC) and power output (flexion and extension) during maximal cycling, in addition to inter-individual variability in muscle activation and pedal force profiles.
Quadriceps IMVC (-52 ± 21%,
= 0.002), voluntary activation (-24 ± 14%,
< 0.001) and resting twitch amplitude (-45 ± 19%,
= 0.002) were reduced following the fatiguing task, whereas vastus lateralis (
= 0.58) and vastus medialis (
= 0.15) M-wave amplitudes were unchanged. The reductions in extension power (-15 ± 8%,
< 0.001) and flexion power (-24 ± 18%,
< 0.001) recorded during maximal cycling with fatigue of the quadriceps were dissociated from the decreases in quadriceps IMVC. Peak EMG decreased across all muscles while inter-individual variability in pedal force and EMG profiles increased during maximal cycling with quadriceps fatigue.
Quadriceps fatigue induced by voluntary contractions led to reduced activation of all lower limb muscles, increased inter-individual variability and decreased power production during maximal cycling. Interestingly, power production was further reduced over the flexion phase (24%) than the extension phase (15%), likely due to larger levels of peripheral fatigue developed in RF muscle and/or a higher contribution of the quadriceps muscle to flexion power production compared to extension power during maximal cycling.
We investigated the association between changes in vastii electromyography (EMG) and knee extensor fatigue during high-intensity cycling, and the subsequent effect on lower-limb power and ...intermuscular coordination during all-out cycling. On two separate days, participants completed 30-s all-out cycling or 10-min of high-intensity cycling followed by 30-s all-out cycling. EMG for gluteus maximus (GMAX), rectus femoris (RF), vastii (VAS), hamstrings (HAM) and gastrocnemius (GAS); co-activation for GMAX/RF, VAS/HAM and VAS/GAS; isometric maximal voluntary force (IMVF) and resting twitch (RT) of the knee extensors were measured. VAS EMG increases during high-intensity cycling (6% to 14%, P < 0.05) were negatively correlated (r = −0.791, P < 0.05) with knee extensor IMVF decreases (−2% to−36%, P < 0.05) following the exercise. Knee extensor IMVF decreases were positively correlated (r = 0.757, P < 0.05) with all-out cycling power reductions (0% to −27%, P < 0.05). VAS/GAS co-activation did not change (P > 0.05) during all-out cycling while VAS and GAS EMG decreased. Larger increase in VAS EMG during high-intensity cycling was associated with greater knee extensor fatigue and larger power reduction during all-out cycling. High VAS/GAS co-activation potentially limited power reduction induced by knee extensor fatigue during all-out cycling.
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