Application of the neuromuscular fatigue threshold treadmill test to muscles of the quadriceps and hamstrings

Clyton L.Cmi*,Attil J.KovsTrish A.VnDusselorp,Ethn C.Hill,Evn A.Enquist

aHuman Performance Laboratory,Exercise and Sport Science,University of Wisconsin-La Crosse,La Crosse,Wisconsin 54601,USA bDepartment of Exercise Science and Sport Management,Kennesaw State University,Kennesaw,Georgia 30144,USA cDepartment of Nutrition and Health Sciences,University of Nebraska-Lincoln,Lincoln,Nebraska 68583,USA dDepartment of Kinesiology,University of Alabama,Tuscaloosa,Alabama 35487,USA

Abstract Purpose:The purposes of the present study were:(1)to determine whether the physical working capacity at the fatigue threshold(PWCFT)model that has been used for estimating the onset of neuromuscular fatigue in the vastus lateralis(VL)during incremental treadmill running could also be applied to the vastus medialis(VM),biceps femoris(BF),and semitendinosus(ST)muscles;and(2)if applicable,to compare the running velocities associated with the PWCFTamong these muscles.Methods:Eleven subjects(age 21.7±1.8 years)performed an incremental treadmill test to exhaustion with electromyographic signals recorded from the VL,VM,BF,and ST.Results:The results indicated there were no significant(p＞0.05)mean differences in the running velocities associated with the PWCFTfor the VL(14.4±2.0 km/h),VM(14.3±1.9 km/h),BF(13.8±1.8 km/h),and ST(14.7±2.3 km/h).In addition,there were significant inter-correlations(r=0.68-0.88)among running velocities associated with the PWCFTof each muscle.Individual results also indicated that 9 of the 11 subjects exhibited identical PWCFTvalues for at least 3 of the 4 muscles,but there were no uniform patterns for any intra-individual differences.Conclusion:The findings of the present study suggested that the PWCFTtest is a viable method to identify neuromuscular fatigue in the quadriceps and hamstrings during incremental treadmill exercise and results in consistent PWCFTvalues among these muscles.

Keywords:EMG amplitude;Muscle activation;PWCFT

1.Introduction

The physical working capacity at the fatigue threshold(PWCFT)test of deVries et al.1estimates the maximal power output that can be sustained for an extended period of time without evidence of neuromuscular fatigue during cycle ergometry.Specifically,the PWCFTtest is based on within-stage increases in electromyographic(EMG)amplitude that re flect fatigue-induced increases in muscle activation(i.e.,motor unit recruitment and firing rates)required to maintain the desired power output.It has been demonstrated that the PWCFTprovides an accurate measurement of the highest non-fatiguing workload2and results in consistent values among super ficial muscles of the quadriceps(i.e.,vastus lateralis(VL),rectus femoris(RF),and vastus medialis(VM).3Previous studies have used the PWCFTtest to examine physiological factors associated with neuromuscular fatigue,4-6to assess physical fitness7and factors related to the degeneration of neuromuscular function8,9in the elderly,to prescribe exercise training intensities,10as well as to determine the effectiveness of exercise training programs11and various nutritional supplements as ergogenic aids.12-16Collectively,these findings illustrate that the PWCFTserves as a valid and reliable tool for estimating the onset of neuromuscular fatigue during cycle ergometry with athletic performance and clinical applications.

Recently,Camic et al.17applied the PWCFTmodel used for cycle ergometry to incremental treadmill exercise to derive a new neuromuscularfatigue threshold testforrunning.Utilizing the method of deVries et al.1and recording the EMG signal from the VL,Camic et al.17reported that the PWCFTmodel was able to identify fatiguing from non-fatiguing running velocities by examining the slope coefficients for the EMG amplitudevs.time relationships during each 2-min stage of an incremental treadmill test to exhaustion.Theoretically,the PWCFTtreadmill test estimates the fastest running velocity that can be sustained without progressive increases in muscle activation to compensate for the development offatigue.It was also reported that the running velocity associated with the PWCFTfrom the VL was significantly correlated(r=0.70)and occurred at the same running velocity(14.0±2.3 km/h,mean±SD)as the respiratory compensation point(RCP)(14.0±1.8 km/h).17These findings suggested that the PWCFTdetermined from the VL during incremental treadmill exercise,like the RCP,(1)can be used to identify the boundary between the heavy and severe domains of exercise intensity,and(2)represents the maximal exercise intensity that can be achieved with oxygen uptake(VO2)and lactate still maintaining a steady state.18It has recently been demonstrated,however,that the patterns(linear,quadratic,cubic)of responses for muscle activation(i.e.,EMG amplitude)across exercise intensity(i.e.,VO2)are unique among muscles of the thigh and may be due to variations in muscle architecture, fiber type,or biomechanical differences.19A number of other studies20-23have also reported differences in muscle-activation patterns between the quadriceps and hamstring groups with increases in running velocity or the development offatigue.For example,Kyröl¨ainen et al.21showed that biarticular muscles(e.g.,biceps femoris)exhibited changes in muscle activity across running phases with increases in velocities from 4.0 m/s to 8.5 m/s that were distinct from the activation patterns of monoarticular muscles(e.g.,VL).Thus,it is currently unknown whether the PWCFTmodel is applicable to other muscles of the quadriceps as well as the hamstrings while running due to these variations in activation.Based on the anatomical and biomechanical differences that exist among these muscles,an assessment of their fatigue-induced activation strategies is warranted.Therefore,the purposes of the present study were:(1)to determine whether the PWCFTmodel that has been used for estimating the onset of neuromuscular fatigue in the VL during incremental treadmill running could also be applied to the VM,biceps femoris(BF),and semitendinosus(ST)muscles;and(2)if applicable,to compare the running velocities associated with the PWCFTamong these muscles.

2.Methods

2.1.Subjects

Nine college-aged males(age=22.0± 1.7 years,body mass=75.5±9.0 kg,height=178.2±5.8 cm)and 2 females(20.5±2.1 years,58.7±3.1 kg,165.8±9.8 cm)volunteered to participate in this investigation.These subjects were selected based on their diverse running backgrounds,which included regular participation in recreational races(i.e.,5 km,10 km)(n=3),marathons(n=4),ultramarathons(n=1),triathlons(n=1),collegiate track(n=1),and collegiate soccer(n=1).Each subject visited the laboratory on 2 occasions(separated by at least 48 h)and was instructed to:(1)maintain normal dietary habits and sleep patterns during the course of the study and(2)avoid exercise for 48 h,caffeine and alcohol for 24 h,and food intake for 3 h prior to each visit.The study was approved by the University of Wisconsin-La Crosse Institutional Review Board for Human Subjects,and all subjects completed a health history questionnaire and signed a written informed consent prior to testing.

2.2.Incremental Treadmill Tests

The first laboratory visit was structured as an orientation session to familiarize the subjects with the testing procedures(i.e.,measurement of gas exchange and EMG while running on a treadmill).During the second laboratory visit,each subject performed an incremental test to exhaustion for the determination of PWCFT,RCP,and peak oxygen uptake(VO2peak).The incremental treadmill test involved a standard warm-up of walking at 4.8-6.4 km/h for 4 min.Immediately following the warm-up,the test began at 9.7 km/h and increased 1.6 km/h every 2 min until volitional fatigue.This increment of 1.6 km/h was consistent with the original protocol17and was selected for the practical purpose of estimating the PWCFTacross a wide range of running velocities in the diverse sample.The grade remained constant at 1.0%during the test and was selected to represent the energy cost that is typically experienced by running outdoors.24

2.3.EMG Measurements and Signal Processing

During the second laboratory visit,bipolar(10 mm centerto-center)wireless surface electrode sensors(Tringo Lab Wireless EMG System;Delsys,Natick,MA,USA)were placed on the right thigh over the VL,VM,BF,and ST muscles according to the recommendations of the SENIAM Project.25Because of the lack of reliability of the EMG signal for the RF,26,27this muscle was not examined.Prior to electrode sensor placement,the skin at each electrode site was dry-shaved,lightly abraded with gauze,and cleaned with alcohol.The EMG signals were amplified(gain:×1000)(Tringo Lab Wireless EMG System,bandwidth=20-450 Hz),sampled at 2000 Hz,recorded continuously throughout the incremental test,and stored in a personal computer(Latitude E6540;Dell Inc.,Round Rock,TX,USA)for subsequent analyses.All signal processing was performed using custom programs,which were written with MATLAB programming software(Version 8.2;Mathworks,Natick,MA,USA).The EMG signals were digitally bandpass filtered(fourth-order Butterworth)at 20-450 Hz.

2.4.Determination of PWCFT

The PWCFTvalues were determined using the model of deVries et al.1During each 2-min stage of the incremental treadmill test,six 10-s EMG samples were selected from the signal(10-20 s,30-40 s,50-60 s,70-80 s,90-100 s,and 110-120 s).The EMG amplitude(microvolts root mean square,μVrms)values were calculated for each of the 10-s epochs(MATLAB)and separately plotted across time for each stage(i.e.,running velocity)of the test(Fig.1).The PWCFTfor each muscle was defined as the average of the highest running velocity that resulted in a nonsignificant(p＞0.05,single-tailedttest)slope coefficient for the EMG amplitudevs.time relationship and the lowest running velocity that resulted in a significant(p＜ 0.05)positive slope coefficient(Fig.1).

Fig.1.Illustration of the method used to determine the physical working capacity at the fatigue threshold(PWCFT)during treadmill running.The PWCFTin this example(15.3 km/h)was calculated by averaging the highest running velocity(14.5 km/h)that resulted in a non-significant(p＞0.05)slope coefficient for the EMG amplitude vs.time relationship and the lowest running velocity(16.1 km/h)that resulted in a significant(p＜0.05)slope coefficient.*Slope significantly(p＜0.05)greater than 0.

2.5.Measurements of Gas Exchange

All subjects wore a nose clip and breathed through a 2-way valve(2700,Hans Rudolph,Kansas City,MO,USA)during the incremental tests.Expired gas samples were collected and analyzed using a calibrated metabolic cart AEI Moxus(AEI Technologies,Pittsburgh,PA,USA)with O2,CO2,and ventilatory parameters expressed as 30-s averages.Each subject was also fitted with a Polar Heart Watch system(Polar Electro,Lake Success,NY,USA)to monitor heart rate throughout the test.VO2peakwas defined as the highest VO2value in the last 30 s of the exercise test that met the criteria of Day et al.28The test-retest reliability for VO2peaktesting from our laboratory indicated that the intraclass correlation coefficient wasR=0.95,and the standard error of measurement29(SEm)=97 mL/min,with no significant(p＞0.05)mean difference between test and retest values.The RCP was determined by noninvasive gas-exchange measurements using the method of Beaver et al.30For each subject,running velocities from the incremental treadmill test attained during the second laboratory visit were plotted against VO2values,and the regression equation derived was used to determine the running velocity that corresponded to their RCP.The test-retest reliability for RCP testing from our laboratory indicated that the intraclass correlation coefficient wasR=0.93,and SEm=103 mL/min,with no significant mean difference between test and retest values.

2.6.Statistical Analyses

Mean±SD values were calculated for the PWCFTfrom the VL,VM,BF,and ST as well as RCP and VO2peak.For determination of PWCFTvalues,the relationships for EMG amplitudevs.time for each individual muscle and running velocity were examined using linear regression(IMB SPSS Statistics,Version 24;IMB Corp.,Armonk,NY,USA).A 1-way repeated-measures ANOVA was also used to determine whether there were significant mean differences in running velocities among the PWCFTfrom each muscle and the RCP.Follow-uppost hocanalyses included pairedttests with Bonferroni correction.A 0-order correlation matrix was used to determine the relationships among the PWCFTfrom each muscle and the RCP.An α＜0.05 was considered statistically significant for the 1-way repeated-measures ANOVA and all 0-order correlations.

3.Results

The results of the 1-way repeated-measures ANOVA andpost hocanalyses indicated there were no significant(p＞0.05)differences among the running velocities associated with the VL PWCFT,VM PWCFT,BF PWCFT,ST PWCFT,and RCP(Table 1).In addition,the individual running velocities associated with the PWCFTwere identical for all 4 muscles in 2 subjects,3 of the 4 muscles in 7 subjects,and 2 of the 4 muscles in 2 subjects(Table 2).There were no consistent intra-subject patterns for the PWCFTvalues that distinguished the VL,VM,BF,and ST muscles(Table 2).Furthermore,there were significant(p＜0.05)0-order correlations(r=0.60-0.88)among the running velocities associated with the VL PWCFT,VM PWCFT,BF PWCFT,ST PWCFT,and RCP,except for the VMvs.RCP(r=0.52)(Table 3).

Table 1Physical characteristics,running velocities,and metabolic parameters of the subjects(n=11).

Table 2Individual PWCFTvalues(km/h)for each muscle and subject.

Table 3Correlations among running velocities associated with the PWCFTof each muscle and respiratory compensation point.

4.Discussion

One of the main findings of the present study was that the PWCFTmodel that has been used to estimate neuromuscular fatigue in the VL17during incremental treadmill running was also applicable to other muscles of the quadriceps femoris(VM)and hamstring(BF,ST)groups.That is,the PWCFTmethod of deVries et al.1was able to identify fatiguing from non-fatiguing running velocities during the incremental treadmill test by statistically examining the slope coefficient of the EMG amplitudevs.running velocity relationship of each stage for all muscles.Specifically,the EMG amplitude values at the velocities associated with fatigue increased across time for the VL(r=0.74-0.99),VM(r=0.78-0.99),BF(r=0.76-0.98),and ST(r=0.77-0.99)for all subjects,whereas the nonfatiguing running velocities resulted in non-significant relationships.These findings illustrated that the PWCFTtest is a viable tool for estimating the running velocities associated with neuromuscular fatigue in individual muscles of the thigh.Previous studies20,23,31using other methods have also identified neuromuscular fatigue in various lower-limb muscles based on increases in EMG amplitude during running.For example,Hanon et al.20,23examined the difference in EMG amplitude values associated with 5-10 running bursts of activation at 0:45(min:s)and 3:40(min:s)of each 4-min stage for the VL,BF,gluteus maximus,RF,tibialis anterior,and gastrocnemius muscles during discontinuous,incremental treadmill running to exhaustion.In these studies,20,23running velocities were defined as“fatiguing”if they exhibited significant increases in EMG amplitude at the end compared to the beginning of a stage.One of the major advantages of the PWCFTmodel1,17used in the present study,however,is the evaluation of change in EMG amplitude across the entire stage,compared to only 5-10 running bursts of activation at 2 different time points.Thus,the use of the PWCFTmodel1,17may provide greater insight into the evolution offatigue during incremental treadmill running and may be less susceptible to outliers that exist among EMG activation bursts.

The findings of the present investigation also indicated that there were no significant mean differences in running velocities associated with the VL PWCFT(14.4±2.0 km/h),VM PWCFT(14.3±1.9 km/h),BF PWCFT(13.8±1.8 km/h),and ST PWCFT(14.7±2.3 km/h)(Table 1).In addition,there were significant inter-correlations for the PWCFTvalues that existed among all muscles(r=0.68-0.88)(Table 3).Therefore,the identification of neuromuscular fatigue for the VL,VM,BF,and ST was associated with the same running velocity and was consistent among all muscles.These findings were similar with those of Housh et al.,3which indicated that the PWCFToccurred at the same power output for the super ficial muscles of the quadriceps during incremental cycle ergometry.Using the PWCFTmodel and 30-W incremental stages,the authors3reported:(1)no significant differences in the power output associated with neuromuscular fatigue for the VL(226±58 W),VM(223±58 W),and RF(203±54 W),and(2)significantinter-correlations (r=0.78-0.92) among the muscles.In conjunction,the findings of the present study and those of Housh et al.3suggested that the PWCFTserves as a reliable tool to identify neuromuscular fatigue during incremental treadmill running and cycle ergometry.It is important to note,however,that the running velocities associated with the PWCFTin the current investigation were identical for:(1)all 4 muscles in 2 subjects,(2)3 muscles for 7 subjects,and(3)2 muscles for 2 subjects(Table 2).As proposed by Housh et al.,3it is possible that these intra-subject differences in the PWCFTamong muscles may be due to variations in fiber-type distribution,training status or protocols,and biomechanical differences.Thus,the current findings suggested that an examination into intra-subject variability in the PWCFTcan be used to identify muscle imbalances among the quadriceps and hamstring groups during running as well as to determine individual training strategies for athletes.For example,the potential uses for PWCFTidentification in various muscles of interest include:(1)to determine the effectiveness of training programs through pre-and post-assessments,(2)to prescribe exercise training intensities(i.e.,running velocities)based on%PWCFT,(3)to assess the impact of running-form adjustments on fatigue-related aspects of muscle activation,and(4)to assess the rehabilitative progress in recovering from injury.It is also possible that the PWCFTprotocol could be modified to identify muscle fatigue across differing velocities and grades,depending on performance requirements.In particular,deVries et al.1suggested that the PWCFTtreadmill test could be customized for the elderly by using slower velocities to assess fatigue-related aspects of neuromuscular function during walking.Therefore,the PWCFTtreadmill test has practical applications that can be potentially useful in both athletic and special populations.

The PWCFTvalues for each muscle(VL=14.4±2.0 km/h,VM=14.3±1.9 km/h,BF=13.8±1.8 km/h,and ST=14.7±2.3 km/h)also occurred at the same running velocity as the RCP(14.5±1.7 km/h)(Table 1).In addition,there were significant inter-correlations among the PWCFTvalues for each musclevs.the RCP(r=0.60-0.88),except the VM(r=0.52)(Table 3).As described previously,17the relationships among neuromuscular(PWCFT)and ventilatory-based thresholds(ventilatory threshold and RCP)provide information related to the practical applications and validity of the PWCFTtreadmill test.That is,the running velocity associated with neuromuscular fatigue in muscles of the thigh,like the RCP,demarcates the border between the heavy and severe domains of exercise intensity and,thus,represents the maximal running velocity that can be maintained for an extended period of time with VO2and lactate still reaching a steady state.18In particular,it has been established that continuous exercise above the heavyintensity domain results in VO2and lactate values that do not stabilize,and VO2reaches its maximum.18These findings offer physiological validation that the PWCFTis an accurate estimate of the highest non-fatiguing running velocity.Practical validation of the PWCFTmodel during treadmill exercise through constant runs to exhaustion at,below,and above the estimated PWCFT,however,has not been examined.

5.Conclusions

In summary,the findings of the present investigation illustrated that the PWCFTthat has previously been used to identify the onset of neuromuscular fatigue in the VL is also applicable to other muscles of the thigh(i.e.,VM,BF,and ST)during incremental treadmill running.The non-significant mean differences and significant correlations among the running velocities of the VL PWCFT,VM PWCFT,BF PWCFT,and ST PWCFTsuggested that this neuromuscular fatigue threshold results in consistent estimates across muscles of the quadriceps femoris and hamstring groups and may be used to identify muscle imbalances as well as to determine individual training strategies for athletes.In addition,the development offatigue in these muscles as indicated by the PWCFTcoincided with the running velocity associated with the RCP.These findings provide physiological validation for the PWCFTmodel during incremental treadmill running.

Acknowledgment

This study was funded by a Faculty Research Grant through the University of Wisconsin-La Crosse,La Crosse,Wisconsin,USA.

Authors’Contributions

CLC conceived the study design,data collection,and analysis and drafted the manuscript;AJK assisted with study design and carried out the electromyographic analyses;TAV,ECH,and EAE were involved with study design and coordination,data collection and analysis.All authors have read and approved the final manuscript,and agreed with the order of presentation of the authors.

Competing Interest

The authors declare that they have no competing interests.