ORIGINAL ARTICLE

Jamie S.M. Pringle Æ Andrew M. Jones

Maximal lactate steady state, critical power and EMG during cycling

Accepted: 19 July 2002 / Published online: 19 September 2002� Springer-Verlag 2002

Abstract We hypothesised that: (1) the maximal lactatesteady state (MLSS), critical power (CP) and electro-myographic fatigue threshold (EMGFT) occur at thesame power output in cycling exercise, and (2) exerciseabove the power output at MLSS (P-MLSS) results incontinued increases in oxygen uptake ( _VV O2), blood lac-tate concentration ([La]) and integrated electromyogram(iEMG) with time. Eight healthy subjects [mean (SD)age 25 (3) years, body mass 72.1 (8.2) kg] performed aseries of laboratory tests for the determination of MLSS,CP and EMGFT. The CP was determined from fourexhaustive trials of between 2 and 15 min duration. TheMLSS was determined as the highest power output atwhich the increase in blood [La] was less than 1.0 mMacross the last 20 min of a series of 30-min trials. TheEMGFT was determined from four trials of 2 min du-ration at different power outputs. The surface electr-omyogram was recorded continuously from the vastuslateralis muscle. The CP was significantly higher thanthe P-MLSS [242 (25) vs. 222 (23) W; P<0.05], al-though the two variables were strongly correlated(r=0.95; P<0.01). The EMGFT could not be deter-mined in 50% of the subjects. Blood [La], _VV O2 andminute ventilation all increased significantly with timefor exercise at power outputs above the P-MLSS. Inconclusion, the P-MLSS, and not the CP, represents theupper limit of the heavy exercise domain in cycling.During exercise above the P-MLSS, there is no associ-ation between changes in iEMG and increases in _VV O2

and blood [La] with time.

Keywords Fatigue threshold Æ _VVO2 slowcomponent Æ Endurance exercise

Introduction

It is known that the tolerable duration (time limit, tlim)of high-intensity exercise decreases hyperbolically as afunction of the power output both during exercise withsmall muscle groups (Monod and Scherrer 1965) andduring whole-body exercise such as cycling (Poole et al.1988). This relationship may be transformed into a lin-ear relationship between the tlim and the total amount ofwork performed during the task (work limit, Wlim) asfollows:

Wlim ¼ aþ b� tlimð Þ ð1Þ

where b is the critical power (CP; i.e. the power as-ymptote of the hyperbolic relationship) and a representsthe anaerobic work capacity (i.e. a finite quantity ofwork that can be performed above the CP using energyderived from anaerobic glycogenolyis, and phosphagenand oxygen stores; Monod and Scherrer 1965; Moritaniet al. 1982; Poole et al. 1990). Moritani et al. (1982)proposed that cycling at a power output below the CPcould be sustained for a long time without fatigue,whereas cycling at a power output above CP wouldresult in the accumulation of blood lactate and deple-tion of the stored energy sources at a predictable rateuntil exhaustion. The CP concept has also been appliedto other modes of exercise such as running (Hughsonet al. 1984; Smith and Jones 2001) and swimming(Wakayoshi et al. 1993). However, it should be stressedthat there are a number of assumptions inherent in theCP concept and it should not be applied uncritically, forexample to activities in which the relationship betweenvelocity and metabolic power is not constant (diPrampero 1999).

The maximal lactate steady state (MLSS) has beendefined as the highest constant power output that can bemaintained without a progressive increase in blood lac-tate concentration ([La]) over time (Beneke and vonDuvillard 1996; Jones and Doust 1998). The physio-logical importance of the MLSS is that it defines the

Eur J Appl Physiol (2002) 88: 214–226DOI 10.1007/s00421-002-0703-4

J.S.M. Pringle Æ A.M. Jones (&)Department of Exercise and Sport Science,Manchester Metropolitan University, Hassall Road,Alsager, ST7 2HL, UKE-mail: a.m.jones@mmu.ac.ukTel.: +44-161-2475656Fax: +44-161-2476375

exercise intensity above which anaerobic metabolismmakes an increasingly important contribution to theenergy demand of exercise (Antonutto and di Prampero1995). At power outputs up to and including the MLSS,there is a balance between the rate of lactate productionand the rate of lactate removal, whereas at power out-puts above the MLSS, the rate of lactate productionexceeds the rate of lactate clearance. It can be calculatedthat the net anaerobic energy yield is negligible whenthere is a balance between lactate production and re-moval, even if blood [La] is elevated (Antonutto and diPrampero 1995). The MLSS is determined by measuringthe blood [La] response to a series of constant-load ex-ercise bouts of up to 30 min duration performed ondifferent days, and is identified as the highest poweroutput at which blood [La] increases by <1.0 mM afterbetween 10 and 30 min of exercise (Beneke and vonDuvillard 1996; Jones and Doust 1998). The poweroutput at MLSS (P-MLSS) demarcates the boundarybetween the heavy exercise domain [in which oxygenuptake ( _VV O2), blood [La] and hydrogen ion concentra-tion ([H+]) can be maintained at an elevated but steadylevel] and the severe exercise domain [in which bothblood [La] and _VV O2 increase continuously, and maxi-mum oxygen uptake ( _VV O2max) may be reached unlessvolitional exhaustion ensues earlier; Poole et al. 1988].The CP has also been used to demarcate the boundarybetween the heavy and severe exercise domains (Poole etal. 1988; Vandewalle et al. 1997). However, surprisinglyfew studies have tested the supposition that the P-MLSSand the CP occur at the same power output. Indeed,several studies have shown that exercise at the CP canonly be maintained for some 15–40 min before exhaus-tion occurs or that power output has to be reduced forexercise to continue (Housh et al. 1989; Jenkins andQuigley 1990). This latter observation is clearly at oddswith the concept that the CP and the P-MLSS areequivalent.

The highest power output that can be maintainedwithout an increase in the integrated electromyogramsignal (iEMG) over time has been termed the electro-myogram (EMG) ‘‘fatigue threshold’’ (EMGFT; Mori-tani et al. 1993). This concept is based on theobservation of a linear relationship between externalpower output and the rate of increase in iEMG with

time (Housh et al. 1991), and is determined by measur-ing the iEMG response to four short bouts of high-power exercise (Moritani et al. 1993; de Vries et al.1982). Simultaneous increases in iEMG and pulmonary_VV O2 during high-intensity exercise have been taken asevidence that the _VV O2 ‘‘slow component’’ is related tothe serial recruitment of additional (type II) motor units(Saunders et al. 2000; Shinohara and Moritani 1992).No previous study has determined the relationship be-tween the EMGFT, the CP and the MLSS.

The purpose of this study was to test the hypothesesthat: (1) the MLSS, CP and EMGFT occur at the samepower output in cycling exercise and (2) exercise abovethe P-MLSS results in continued increases in _VV O2, blood[La] and iEMG with time.

Methods

Subjects

Eight healthy subjects (one female) were briefed as to the benefitsand risks of participation and gave their written informed consentto participate in the study, which was approved by the ManchesterMetropolitan University Ethics Committee. The subjects were allinvolved in regular exercise training and were familiar with labo-ratory exercise testing procedures. The subjects’ physical andphysiological characteristics are shown in Table 1. Subjects wereinstructed to avoid strenuous exercise in the 48 h preceding a testsession and to arrive at the laboratory in a rested and fully hy-drated state, and at least 3 h postprandial. For each subject, teststook place at the same time of day (±2 h).

Design of the study

Subjects first completed an incremental exercise test to exhaustionto determine the lactate threshold (Thla) and _VV O2max. Over thesubsequent 2 weeks, each subject visited the laboratory a furthernine times to determine the CP (four trials), P-MLSS (four trials)and the power output at EMGFT (P-EMGFT, one trial). Thesetrials were presented in random order.

All cycle tests were conducted on an electrically braked cycleergometer (Ergoline, Jaeger, Germany), with seat and handlebarheight and angle kept constant for individual subjects. A 5-minwarm-up of pedalling at 50 W was allowed before all tests. At thestart of a test, subjects increased their pedal rate to 90 revÆmin–1

(this pedal rate was used for all tests) and the necessary loading wasapplied, at which point timing commenced. Throughout all tests,heart rate was recorded every 5 s using a telemetric heart ratemonitor (Polar Electro Oy, Kemple, Finland).

Table 1 Subject characteristics.( _VV O2max Maximal oxygenuptake, Thla lactate threshold,F female, M male)

Subjectno.

Gender Age(years)

Mass(kg)

Height(cm)

_VV O2max

(mlÆmin–1)% _VV O2max

at Thla

Trainingbackground

1 F 26 66.2 172 2450 47 Recreational exercise2 M 29 87.0 177 3190 67 General fitness training3 M 25 65.6 174 3141 53 Recreational cyclist4 M 27 73.2 178 3917 57 Competitive duathlete5 M 20 61.2 167 3492 61 Competitive cyclist6 M 25 76.7 180 3920 56 Competitive duathlete7 M 21 70.4 179 4872 57 Competitive cyclist8 M 24 76.8 182 4860 45 Competitive runnerMean 25 72.1 176 3730 55SD 3 8.2 5 844 7

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Incremental exercise test

The starting power output for the incremental test was 50 W, andthis was increased by 25 W at the end of each minute until subjectsreached volitional exhaustion and/or the pedal rate could no longerbe maintained at 90 revÆmin–1. Strong verbal encouragement wasprovided during the latter stages of the test. For each minute ofexercise, pulmonary gas exchange and minute ventilation ( _VV E) weremeasured, and at the end of each stage a fingertip blood sample wastaken to determine whole blood [La] (see later).

The _VV O2 at Thla was determined from plots of blood [La]against _VV O2 as the first clear and sustained increase in blood [La]above the near-resting baseline concentrations. The highest _VV O2

measured in the test was accepted as the _VV O2max. The power out-puts corresponding to various percentages of _VV O2max were esti-mated by extrapolation of the linear regression of _VV O2 versuspower output for the sub-Thla portion of the incremental test. Thepower outputs corresponding to Thla and _VV O2max were determinedwith account taken of the lag in _VV O2 that occurs during incre-mental exercise (Davis et al. 1982).

Determination of the MLSS

To determine the MLSS, subjects completed four 30-min constant-load transitions at power outputs calculated to require between100% of the _VV O2 at Thla and 50% of the difference between the_VV O2 at Thla and _VV O2max (50% D). The difference in power outputbetween the trials used to determine the MLSS was 19 (5) W. Ex-pired air was collected for a timed period every 5 min and fingertipcapillary blood samples were collected at the start and end of ex-ercise and every 5 min throughout exercise. The P-MLSS was de-termined as the highest power output at which the increase in blood[La] was less than 1.0 mM across the last 20 min of the 30-min trial(Beneke and von Duvillard 1996; Jones and Doust 1998). A surfaceEMG signal was recorded from the vastus lateralis muscle over thelast 48 s of each 1-min period during each trial (see later). Theelectrode site was marked with reference to anatomical landmarksand distinguishing skin markers, and in subsequent sessions theEMG electrodes were replaced in the same position. Data collectedduring the determination of MLSS were normalised relative to theaverage iEMG in the 1st min for that particular test, allowing arelative comparison between tests.

Determination of the CP

CP was determined from four exhaustive transitions at poweroutputs calculated to require between 50% D and 110% _VV O2max

and always included a trial at 100% _VV O2max. These trials wereperformed on separate days. Subjects were instructed to maintainthe power output for as long as possible and exercise was termi-nated when the pedal rate dropped below 85 revÆmin–1 for morethan 5 s. In all cases, this drop-off was precipitous. The time toexhaustion was recorded to the nearest second and the CP wascalculated according to the linear model of power output versus 1/time (Fig. 1). Those conditions eliciting volitional exhaustionwithin 2–15 min were included in the CP determination (typicallythis included four trials, but in two of the subjects only three trialswere within this time criterion).

Determination of the EMGFT

The EMGFT was determined from four square-wave transitions,each 2 min in duration and separated by at least 25 min of rest.These transitions required power outputs between 75% D and115% _VV O2max (equivalent to 230–460 W for males and 150–275 Wfor the female). The surface EMG was recorded continuously fromthe vastus lateralis muscle. All EMG data collection was completedwithin one laboratory visit (see below). The power output and therate of increase of the vastus lateralis iEMG for each 2-min trialwere plotted and the P-EMGFT was defined as the intercept on the

y-axis of this graph. The P-EMGFT could not be calculated for allsubjects (see results).

Measurement of pulmonary gas exchange and _VV E

Subjects wore a nose clip and breathed through a Salford low-resistance respiratory valve/mouthpiece assembly (both fitted atleast 30 s before expired air collection began). The mouthpiece wasattached to a 1-m-long piece of 3.75-cm-bore Falconia tubing.Expired air was collected into 150 l Douglas bags (Hans Rudolph,Kansas City, Mo., USA) for a whole number of breaths over ahand-timed period, and closing and opening of the Douglas bagwas synchronised with inspiration. Expired air was collected for atleast 45 s of each 1-min period during the incremental test and for@60 s preceding the withdrawal of each blood sample (i.e. 4–5 min,9–10 min, etc.) during the MLSS trials.

Expired air was analysed for percentage of oxygen and carbondioxide by sampling through a paramagnetic transducer and aninfrared analyser, respectively (Servomex, Crowborough, UK, se-ries 1400). Both gas analysers were calibrated with BOC-certifiedprecision gases immediately before each experimental session. Gasvolume was determined by a dry gas volume meter (Harvard Ap-paratus, Edenbridge, UK) and was frequently calibrated andchecked for linearity with a high-precision 7-l graduated gas syringe(Hans Rudolph). Pulmonary gas exchange variables [ _VV O2, carbondioxide output, _VV E and the respiratory exchange ratio] were de-termined using standard formulae.

Measurement of blood [La]

Finger-prick blood samples were collected every minute during theincremental test and every 5 min during the MLSS determination.The puncture site was cleaned with an alcohol swab, dried withtissue, and a small skin puncture approximately 2 mm in depth wasmade using a disposable safety lancet. The first drops of blood werewiped away, and approximately 20–25 ll of arterialised blood wascollected into capillary tubes containing an anticoagulant agent(Hawksley and Sons, Lancing, UK). Whole-blood [La] was deter-mined using an automated analyser (YSI 1500, Yellow SpringsInstruments, Ohio, USA), which was calibrated prior to the testsessions using a 5 mM lactate standard provided by the manufac-turer. The coefficient of variation for blood [La] measurement was1.3% (0.06 mM) for 20 samples in the physiological range (5 mM).

Electromyography

Surface electrodes were applied to the skin of the right leg over thevastus lateralis muscle. The muscle belly was palpated during afunctional isometric contraction and the site selected at the visualmidpoint of the muscle belly. The skin was shaved, lightly rough-ened with abrasive electroencephalogram gel and cleaned withcotton wool dipped in mild detergent and water. Bipolar silver/silver chloride surface electrode stickers (30 mm·20 mm, BIO-TAB) were placed on the selected site, with a centre-to-centre in-terelectrode distance of 30 mm, along a line approximately parallelto the direction of the underlying muscle fibres. Two 50 mm leadswere used to connect the electrodes to a subminiature preamplifier,which was connected directly on top of an electrocardiogram-typepress-stud electrode located on the skin as far as possible from theEMG electrodes. The electrode wires and preamplifier were furthersecured to the skin using surgical tape where necessary.

The preamplifier was connected to a lightweight transmitter ona waistband worn by the subjects. This eight-channel FM MT8radio telemetry system (MIE Medical Research, Leeds, UK)transmitted the signal to the nearby receiver, which was connectedto an IBM PC via a 12-bit analog-to-digital converter, and thesignal was sampled at 625 Hz (Myo-dat software, MIE). The rawEMG data were band-pass filtered, full-wave rectified and scruti-nised visually for remaining movement artefacts. One subject’s datafor exercise above the P-MLSS was discarded due to technical

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problems. The rectified EMG was integrated with respect to time(iEMG), with an iEMG value computed every 2 s (EMGFT test) or10 s (MLSS trials).

Statistical analysis

Analysis of variance with two-tailed, paired Student t-tests whereappropriate were used to test the significance of differences betweenthe P-MLSS, CP and P-EMGFT. In the trials used for determina-tion of P-MLSS, paired Student t-tests were used to compare thephysiological variables at 10 min with those at the end of exercise.Pearson product moment correlation coefficients were used to as-sess the significance of relationships between selected variables.Bland and Altman plots (Bland and Altman 1986) were used todetermine the bias and limits of agreement where appropriate.Statistical significance was accepted at 5%. Results are presented asmean (SEM) unless stated otherwise.

Results

MLSS, CP, and EMGFT

Table 2 shows the power output at the MLSS, CP andEMGFT for each subject. The CP and the P-MLSS weresignificantly different (P<0.05), although they were verystrongly correlated (r=0.95, P<0.01; Fig. 2; Table 2).In addition, CP occurred at a higher percentage of thepower output at _VV O2max (P- _VV O2max) than did P-MLSS[CP 71 (3)% vs P-MLSS 65 (3)%; P<0.05]. The EMGFT

could not be determined in half of the subjects. On av-erage, these four individuals had a significantly higher

Fig. 1 A Hyperbolic relation-ship between power output andtime to exhaustion for a singlesubject (no. 2). B Critical power(CP) was determined by lineartransformation of the datawhere CP is the y-axis interceptof the linear trend line of powerversus 1/time to exhaustion

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P-MLSS, CP and P- _VV O2max than their counterparts inwhom an EMGFT was determined (Table 2). The an-aerobic work capacity [@19 (1) kJ] and the average timeto exhaustion at 100% P- _VV O2max [@215 (15) s] were notsignificantly different between the two subgroups. Rea-sons for the inability to determine the EMGFT were that:(1) iEMG occasionally decreased over time at somepower outputs and (2) the rate of iEMG increase overtime was sometimes less at higher compared to lowerpower outputs, and thus the slope of the linear regres-sion line was negative. The method for determination ofthe EMGFT is illustrated in Fig. 3.

Physiological responses to exercise relative to the MLSS

Four subjects (nos. 1, 2, 4 and 5) could not complete thefull 30 min for the trial above the P-MLSS. Thus, theaverage exercise time was 21.9 (3.3) min at this exercisepower output. Over the final 20 min of exercise belowthe P-MLSS, blood [La] decreased by @0.4 mM (non-significant; Fig. 4). At the P-MLSS, blood [La] reached aplateau at 3.8 (0.5) mM for the final 20 min. Above theP-MLSS, blood [La] increased significantly by 1.2(0.8) mM from 10 min to the end of exercise (P<0.05).

Figure 4 shows that below and at the P-MLSS, asteady state in _VV O2 and _VV E was achieved. On average,_VV O2 attained 80 (3)% _VV O2max across the final 20 min forexercise at the P-MLSS. This was equivalent to 56 (4)%D and exceeded the _VV O2 values predicted from the linearextrapolation of the power output/ _VV O2 relationshipobtained during the incremental test by 225 (60) mlÆmin–1.During the trial above the P-MLSS, _VV O2 increased sig-nificantly from 3126 (270) mlÆmin–1 [85 (2)% _VV O2max] at10 min to 3301 (274) mlÆmin–1 [89 (2)% _VV O2max] at theend of exercise. At the respective time points, thesevalues were 245 (67) and 371 (62) mlÆmin–1 higher, re-spectively, than the predicted value.

Above the P-MLSS, _VV E increased significantly from80 (8) to 92 (7) lÆmin–1 from 10 min to the end of exercise

(P<0.05), in comparison to the non-significant increasefrom 68 (7) to 73 (7) lÆmin–1 at P-MLSS.

The iEMG response

The large interindividual variability in iEMG responsesmeant that there were no significant changes over time atany of the power outputs studied. Figure 5 shows thatthe normalised iEMG increased by 6 (3) and 12 (7)%from the 1st to the 30th min for exercise just below andat the P-MLSS, respectively. Normalised iEMG in-creased by 3 (1)% (<P-MLSS) and 4 (5)% (at P-MLSS)between 10 and 30 min. Above the P-MLSS, the iEMGresponse was extremely variable across subjects (Fig. 5).On average, the peak increase [11 (8)%] occurred at10 min. After this, iEMG decreased over time, reachinga value 3 (9)% above the initial value by the end ofexercise. This was due to the intersubject variability inboth the time to end exercise and the iEMG response(Fig. 6). When the data were normalised by expressingtime as a percentage of the overall exercise time, theaverage iEMG was remarkably constant and the in-creasing variability in the response can be seen in thedivergence of the iEMG scatter towards the end of ex-ercise (Fig. 6).

Discussion

There were three main findings to this study. Firstly,the CP was significantly higher than the P-MLSS, al-though the two variables were strongly correlated.Secondly, the EMGFT could not be determined in foursubjects, and it was not related to CP or P-MLSS.Thirdly, for subjects cycling at @20 W above the P-MLSS, close to their CP, blood [La], _VV O2 and _VV E allincreased significantly over time. This was not the casefor iEMG due to the large intersubject variability in theEMG response.

Table 2 Power output at theThla, maximal lactate steadystate (MLSS), critical power(CP), the electromyogramfatigue threshold (EMGFT) andat _VV O2max (P- _VV O2max) in thesubjects in whom the EMGFT

could be determined (EMGFT

group, and in those in whom itcould not (Non-EMGFT group)

Subject no. Thla (W) MLSS (W) CP (W) EMGFT (W) P- _VV O2max (W)EMGFT group

1 95 115 118 128 2232 157 180 205 114 2833 105 165 210 228 2804 200 255 237 296 365Mean 139 179 192 192 288SEM 24 29 26 43 29Non-EMGFT group5 161 220 219 – 3086 169 240 289 – 3557 198 310 341 – 4238 150 290 314 – 433Mean 170 265* 291** – 380*SEM 10 21 26 – 30All mean 154 222 242a 192 334All SEM 14 23 25 43 27

aSignificantly different to MLSS(P <0.05);*significantly different toEMGFT group (P<0.05);**significantly different toEMGFT group (P<0.01)

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Difference between the P-MLSS and the CP

The relationship between power output and the inverseof the time to exhaustion was highly linear (r=0.986),indicating that the calculated CP was representative oftrue maximal efforts in each trial. The CP theoreticallyrepresents the highest power output that can be main-tained without fatigue (Monod and Scherrer 1965;Moritani et al. 1981) and has been considered to becoincident with the P-MLSS (Poole et al. 1988, 1990).Experimental data suggesting that the CP or criticalvelocity (CV) and MLSS power/velocity are coincidenthas been presented for cycling (Poole et al. 1988, 1990;Vandewalle et al. 1997), swimming (Wakayoshi et al.1993) and local knee-extension exercise (Le Chevalier etal. 2000), although it should be noted that the MLSSwas not determined directly in any of these studies. Incontrast, a number of studies indicate that the CP or CV

cannot be sustained beyond approximately 10–30 min,presumably due to the accumulation of muscle andblood [La] and [H+] (Housh et al. 1989, 1991; Jenkinsand Quigley 1990; Pepper et al. 1992). In running, theCV and velocity at MLSS (V-MLSS) were shown to besimilar by Sid-Ali et al. (1991) and Smith and Jones(2001). However, Sid-Ali et al. (1991) did not determinethe MLSS directly, and Smith and Jones (2001) stressedthat whilst the V-MLSS [13.8 (1.1) kmÆh–1] and CV [14.4(1.1) kmÆh–1] were, on average, not significantly differentin a group of trained runners, large interindividual dif-ferences in the variables indicated that the terms werenot necessarily interchangeable. It is unclear why thepresent data (significant difference between CP and P-MLSS) differ from those of Smith and Jones (no sig-nificant difference between CV and V-MLSS), when theexperimental methods employed were similar. It is pos-sible that differences in exercise mode or precision in the

Fig. 2 A The relationship be-tween the power output at themaximal lactate steady state(P-MLSS) and the CP (n=8).B The bias and the limits ofagreement between the twovariables using a Bland andAltman analysis (Bland andAltman 1986)

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determination of MLSS influenced the relationship be-tween MLSS and CV/CP in the two studies.

The present study revealed CP to be highly correlatedwith, but significantly higher than P-MLSS. Although,on average, the CP was only 20 W higher than the P-MLSS, the physiological significance of this difference isevident in the subjects’ responses to exercise just above

Fig. 3 Determination of the electromyogram fatigue threshold(EMGFT) in a subject (no. 2) in whom the increase in integratedelectromyogram (iEMG) over time was relatively proportional tothe power output. The EMGFT was determined as the y-interceptof the linear regression relating power output to the slope of theiEMG increase with time at that power output

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Fig. 4 Blood lactate concen-tration ([La]), oxygen uptake( _VV O2) and minute ventilation( _VV E) responses over time forcycling exercise below [201(23) W; closed circles], at [222(23) W; open squares] andabove [241 (24) W; closed tri-angles] the P-MLSS. Values aremean (SEM) (n=8). Above P-MLSS, _VV O2 and _VV E increasedsignificantly from 10 min to theend of exercise

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their P-MLSS. That is, when subjects exercised at apower output >P-MLSS [241 (24) W] and close to theirCP [242 (25) W], blood [lactate], _VV E and _VV O2 increasedsignificantly with time (Fig. 4) and some subjects fa-tigued before 30 min had elapsed. This suggests stronglythat exercise at the CP cannot be sustained without anincreasing lactic acidosis. In summary, our first hy-pothesis that the highest power output at which a steadystate in blood [La] can be maintained coincides with theCP was not supported.

It should be pointed out here that the CP conceptcontains a number of implicit assumptions that may bequestionable (di Prampero 1999; Smith and Jones 2001).For example, it is assumed that: (1) power is infinite astime to exhaustion approaches zero; (2) the anaerobicenergy stores are depleted at exhaustion; (3) the effi-ciency of exercise is constant; (4) _VV O2 reaches the re-quired rate instantaneously at exercise onset (see diPrampero 1999 for discussion). Furthermore, the CPconcept should only be applied to modes of exercisewhere the energy cost per unit of distance covered isindependent of velocity.

One reason for the dissociation between the P-MLSSand the CP in the present study may be the trainingstatus of our subjects. Compared to sedentary subjects,trained subjects may have higher tolerance to exhaustiveexercise (and the ensuing intramuscular and systemicacidosis and high rates of ventilation) and a greater ex-perience of exercising to absolute (i.e. reaching me-chanical failure) rather than voluntary exhaustion.Theoretically, the point of absolute exhaustion mustoccur at or later than the actual or voluntary point ofexhaustion. Therefore, the ‘‘true’’ or ‘‘theoretical abso-lute’’ CP can only be higher than the CP determinedfrom trials to voluntary exhaustion. It should be pointedout that the mean difference between CP and P-MLSS inthis study (20 W) was similar to the precision with whichP-MLSS was determined. Therefore, there might be nodifference between CP and P-MLSS if (but only if) the P-MLSS was systematically underestimated in our study.

On the other hand, however, the criterion for MLSSdetermination we used (<1 mM change in blood [La]from 10 to 30 min) is relatively liberal and may lead toan overestimation of the ‘‘true’’ MLSS. The morestringent criteria applied by some investigators (i.e. de-fining MLSS as the power output at which the slope ofthe linear regression through the [La] values is equal tozero) would have served to lower the P-MLSS in thepresent study, thus further increasing the difference be-tween P-MLSS and CP. The duration of the exhaustivetrials used to determine CP can also influence the cal-culated CP and, therefore, the relationship between CPand P-MLSS. There is some evidence that longer trialsto exhaustion tend to lower the CP (Jenkins et al. 1998;Vandewalle et al. 1997), and in our study this wouldhave reduced the difference between CP and P-MLSS.However, it has been argued that longer trials can beaffected by factors such as motivation, and we thereforefollowed the recommendations of Hill (1993) that thetrials to exhaustion used in the determination of CPshould be in the range of @2–15 min.

Theoretically, the CP should be independent of themathematical model used to express the hyperbolic re-lationship between power output and the time limit forwhich it can be sustained. In the present study, CP wascalculated using the linear model of power output versus1/time. It has been suggested that two-componentmathematical models overestimate CP (Bull et al. 2000)and that a three-component mathematical model wouldbe more appropriate (Morton 1996). Furthermore, thepower output versus 1/time model may yield a higher CPthan the linear model of work done (power · time)versus time (Gaesser et al. 1995). In the present study,however, the CP calculated from the latter model wasalmost identical to that calculated from the power out-put versus 1/time model [238 (26) vs 242 (25) W]. Fur-thermore, the bias and limits of agreement forcomparisons between the CP and P-MLSS using eithermodel of CP determination [work limit model: 16(56) W; time limit model: 20 (46) W] suggest that the CP

Fig. 5 Normalised iEMG re-sponses over time for cyclingexercise below (closed circles),at (open squares) and above(closed triangles) the P-MLSS.Values are mean (SEM). TheiEMG was normalised relativeto the average iEMG in the1st min of exercise for eachcondition. The 6–12% increasesin iEMG below and at P-MLSSwere not significant. After aninitial increase, iEMG de-creased towards the end ofexercise for exercise at intensi-ties above P-MLSS

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Fig. 6 Normalised iEMGresponses over time for cyclingexercise below (lower panel), at(middle panel) and above(upper panel) the P-MLSS. TheiEMG was normalised relativeto the average iEMG in the1st min of exercise for eachcondition, and time wasnormalised relative to the endexercise time. The 6–12%increases in iEMG below and atP-MLSS were not significant

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and P-MLSS are different and should not be used in-terchangeably.

Physiological responses at and above the P-MLSS

For exercise below and at the P-MLSS, a _VV O2 steadystate was attained by 10 min of exercise, which was el-evated by 154 to 225 mlÆmin–1 above the value predictedfrom the linear extrapolation of the _VV O2/power outputrelationship from the incremental exercise test. Abovethe P-MLSS, _VV O2 was elevated above the expected valueby 181 (67) mlÆmin–1 at 5 min and 245 (67) mlÆmin–1 at10 min, and increased by another 126 (32) mlÆmin–1 tothe end of exercise. Therefore, for cycling exercise, itappears that the maximal steady states for blood [La]and _VV O2 occur at approximately the same power output.However, whilst the P-MLSS appears to represent theupper limit of the heavy exercise domain, it should bepointed out that _VV O2max was not attained in thosesubjects reaching volitional exhaustion for exerciseabove the P-MLSS.

Simultaneous measurement of pulmonary and leg_VV O2 demonstrated that @86% of the slow rise in _VV O2

between 3 and 21 min of constant-power heavy cyclingexercise originates from within the exercising limb(Poole et al. 1991). The oxygen cost of systemic supportprocesses such as ventilatory and cardiac work, gluco-neogenesis of lactate, ‘‘extra’’ postural muscle activityand elevations of plasma catecholamines and bodytemperature, therefore makes a relatively small contri-bution to the development of the _VV O2 slow component(Casaburi et al. 1987; Poole et al. 1991). In the presentstudy, it was calculated that the oxygen cost of _VV E couldaccount for @30% of the D _VV O2 from 5 min to the end ofexercise. In contrast to previous studies (e.g. Poole et al.1988), D _VV O2 was negatively related to Dblood [La] in the>P-MLSS trial. The lack of a significant positive rela-tionship between D _VV O2 and Dblood [La] has been re-ported previously (e.g. Carter et al. 2000). The similarityin the time course of the two variables noted in earlierstudies may have been a feature of the use of shorterexercise bouts.

EMG responses to exercise

The highest power output that can be maintainedwithout an increase in iEMG over time has been definedas the EMGFT (P-EMGFT; Moritani et al. 1993). LeChevalier et al. (2000) reported that the CP in local kneeextension exercise was significantly correlated (r=0.96–0.98) and not significantly different (<3%) from thepower output associated with a steady state in _VV O2,blood [La] and iEMG (predicted from the change inthese variables during constant-load, supra-CP trials).However, de Vries et al. (1982) reported that the P-EMGFT was @12% higher than the CP (191 vs 170 W),although they were highly correlated (r=0.87). In the

present study, the EMGFT could only be determined inthe ‘‘less-fit’’ half of the subject group. For these foursubjects, the similar power output at CP and EMGFT

was entirely coincidental and there was no consistenttrend to suggest that the EMGFT was related to theother physiological variables investigated. The poweroutputs used in the four, 2-min trials were adjusted tothe fitness of the subject, as suggested by de Vries et al.(1982) and Moritani et al. (1993), and in all cases werebetween the CP and the power output at 115% _VV O2max.However, in the well-trained subjects exercising at highpower outputs, the linear trend line fitted to the iEMGversus time response over the 2-min bouts was eithernegative or did not describe the response well. Fur-thermore, the proportional linear relationship betweenexternal power output and the rate of increase in iEMGreported previously (Moritani et al. 1982; de Vries et al.1982) could not be reproduced.

It has been proposed that the EMGFT is more closelyassociated with the steady state of lactate metabolism inthe active muscle (i.e. the MLSS) than with the lactate orventilatory threshold, given that it occurred at a _VV O2

just above the ventilatory threshold (Shinohara andMoritani 1992; de Vries et al. 1982). However, the pre-sent study provides little support for this hypothesis.Whilst _VV O2 and blood [La] did not increase during ex-ercise below and at the P-MLSS, iEMG showed a non-significant increase with time. Also, during exerciseabove the P-MLSS, where _VV O2, blood [La] and _VV E in-creased significantly over time, there was generally littlechange in iEMG when the response was normalised totime to exhaustion.

It is thought that the increase in iEMG represents therecruitment of previously inactive motor units and/orincreased firing rate (rate coding) of the activated motorunits to compensate for a decrease in contractility ofimpaired or fatigued motor units (Edwards and Lippold1956). If additional motor units are recruited, the sizeprinciple dictates that they will be larger motor units of ahigher threshold (Beelen et al. 1993; Vollestad and Blom1985). A concurrent slow rise in pulmonary _VV O2 andincreases in iEMG from the exercising muscles duringheavy and severe exercise has been taken as evidencethat the serial recruitment of the less-efficient type IImotor units is related to the _VV O2 slow component(Barstow et al. 1996; Saunders et al. 2000; Shinoharaand Moritani 1992). However, Scheuermann et al.(2001) were unable to detect significant changes in EMGduring heavy cycle exercise that elicited a _VV O2 slowcomponent, and Takaishi et al. (1996) reported an in-crease in iEMG without an increase in _VV O2 in somesubjects.

The finding that iEMG occasionally decreased overtime in both the 2- and 30-min exercise bouts, especiallyin the fitter subjects, suggests that a degree of mechanicalfailure occurred in the most powerful fibres of the vastuslateralis muscle, ‘‘masking’’ any increases in recruitmentor rate coding or synchronisation of fibres lower in therecruitment hierarchy.

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It should be acknowledged that our interpretation ofthe neuromuscular response to heavy and severe exerciseis based solely on our measurement of EMG activity inthe vastus lateralis. Whilst the vastus lateralis probablyproduces a large proportion of the propulsive forcesgenerated in cycling (Broker and Gregor 1994), Houshet al. (1995) reported that for some subjects the P-EMGFT was lower in the rectus femoris muscle com-pared to the vastus lateralis muscle. Thus, it is possiblethat other muscles, including the gluteus maximus,contribute proportionally more to the slow rises in _VV O2

and blood [La] during exercise above the P-MLSS. Thecalculation of EMGFT in the vastus lateralis did notaccurately predict the cardiorespiratory or iEMG re-sponses to longer-term exercise. The usefulness of EMGvariables as indices of fatigue is disputed and the presentstudy indicates that EMG activity and cardiorespiratoryresponses are not related in any simple way. This may beparticularly true in cycling in which the same poweroutput can be generated using different combinations ofthe muscle groups around the hip, knee and ankle, andin which the contributions from the left and right legscan fluctuate. This might also explain the differencebetween our results and those of Le Chevalier et al.(2000), who studied EMG responses during single knee-extension exercise.

In conclusion, theCPwas significantly greater than theP-MLSS. The P-MLSS appears to represent the upperlimit of the heavy exercise domain in cycling. At a poweroutput @6% above the P-MLSS, blood [La] and _VV O2 in-creased significantly with time. The EMGFT could only bedetermined in four of the eight subjects. The large inter-subject variability in the iEMGresponses recorded duringthe determination of both P-EMGFT and P-MLSS sug-gests that surface EMG cannot be used to distinguishbetween increased recruitment, rate coding and mechan-ical failure occurring in the active muscles.

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