Prior heavy exercise increases oxygen cost during moderate.pdf

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Journal of Electromyography and Kinesiology 18 (2008) 99–107

www.elsevier.com/locate/jelekin

Prior heavy exercise increases oxygen cost during moderate

exercise without associated change in surface EMG

Joaquin U. Gonzales, Barry W. Scheuermann *

Department of Kinesiology, The University of Toledo, Toledo, OH 43607, USA

Department of Health, Exercise, and Sport Sciences, Texas Tech University, Lubbock, TX 79409, USA

Received 22 June 2006; received in revised form 7 September 2006; accepted 7 September 2006

Abstract

The aim of this study was to test the hypothesis that prior heavy exercise results in a higher oxygen cost during a subsequent bout of

moderate exercise due to changes in muscle activity. Eight male subjects (25 ± 2 yr, ±SE) performed moderate–moderate and moderate–

heavy–moderate transitions in work rate (cycling intensity, moderate = 90% LT, heavy = 80% VO 2 peak). The second bout of moderate

exercise was performed after 6 min (C) or 30 s (D) of recovery. Pulmonary gas exchange was measured breath-by-breath and surface

electromyography was obtained from the vastus lateralis and medialis muscles. Root mean square (RMS) and median power frequency

(M D PF) were computed. Prior heavy exercise increased D VO 2 = DWR (C: +2.0 ± 0.8 ml min 1 W 1 , D: +3.4 ± 0.8 ml min 1 W 1 ;

P < 0.05) and decreased exercise eciency (C: 13.3 ± 5.6%, D: 22.2 ± 4.9%; P < 0.05) during the second bout of moderate exercise

in the absence of changes in RMS. M D PF was slightly elevated ( 2%) during the second bout of moderate exercise, but M D PF was not

correlated with VO 2 (r = 0.17). These ﬁndings suggest that the increased oxygen cost during moderate exercise following heavy exercise is

not due to increased muscle activity as assessed by surface electromyography.

Keywords: Electromyography; Prior heavy exercise; Constant work rate exercise; Oxygen cost

1. Introduction

tude for a given increase in work rate. Typically, the gain

(D VO 2 = DWR) of the VO 2 –work rate relationship, whether

determined during ramp or constant work rate exercise,

approximates 10 ml min 1 Æ W 1 except for heavy exercise

where D VO 2 = DWR may approach P12 ml min 1 Æ W 1

( Barstow et al., 1993; Henson et al., 1989; Roston et al.,

1987 ). Through the noninvasive examination of muscle

activity by surface electromyography (sEMG), Bigland-

Ritchie and Woods (1974) has shown that VO 2 increases in

linear fashion with increases in force and motor unit recruit-

ment, a ﬁnding that has since been conﬁrmed by other studies

( Hug et al., 2004; Jammes et al., 1998 ). The recruitment of

motor units for the production of force couples skeletal mus-

cle activity to the metabolic rate (as VO 2 Þ during physical

exercise as energy is required for muscular contraction.

Since Gerbino et al. (1996) ﬁrst reported that a prior

bout of heavy exercise resulted in a speeding of the mean

response time of VO 2 kinetics during a subsequent bout

During the adjustment to an abrupt increase in exercise

intensity, pulmonary oxygen uptake ( VO 2 Þ increases, after

a short delay, towards a new steady-state if the exercise is

of moderate intensity (i.e. below the lactate threshold, LT).

Results from a number of studies ( Barstow and Mol´,

1991; Barstow et al., 1993 ) suggest that the characteristics

of the VO 2 –work rate relationship during moderate exercise

can be described as a linear dynamic system, that is with an

invariant time constant and a proportional change in ampli-

* Corresponding author. Present address: Cardiopulmonary and Metab-

olism Research Laboratory, Department of Kinesiology, MS 119, Health

and Human Services Building, The University of Toledo, Toledo, OH

43606-3390, USA. Tel.: +1 419 530 2741; fax: +1 419 530 4759.

E-mail address: barry.scheuermann@utoledo.edu (B.W. Scheuer-

mann).

doi:10.1016/j.jelekin.2006.09.002

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J.U. Gonzales, B.W. Scheuermann / Journal of Electromyography and Kinesiology 18 (2008) 99–107

of heavy exercise, several investigators have manipulated

this protocol in an eﬀort to identify the factor(s) that regu-

late both the rate of adjustment and amplitude of the VO 2

response to exercise (for review see Jones et al., 2003 ). One

mechanism that has gained considerable support relates

motor unit recruitment patterns to metabolic demands

( Burnley et al., 2001, 2002; Sahlin et al., 2005 ). Burnley

et al. (2001) has demonstrated that prior heavy exercise

increases the amplitude of the primary rise in VO 2 during

a subsequent bout of heavy exercise. The increase in VO 2

was later shown by the same authors to be associated with

a concomitant increase in integrated sEMG but not mean

power frequency ( Burnley et al., 2002 ). These ﬁndings are

consistent with the view that the increase in the amplitude

of VO 2 is a consequence of additional motor units being

recruited in order to generate the required force, but the

extent to which less ecient type II motor units are

recruited remains an issue of debate ( Cleuziou et al.,

2004; Scheuermann et al., 2001 ).

Recently, Sahlin et al. (2005) examined the eﬀect of prior

heavy exercise on VO 2 during a subsequent bout of moder-

ate exercise and found reduction in gross exercise eciency

which the authors related to impaired muscle contractility

induced by the prior bout of heavy exercise. However, that

study did not assess motor unit recruitment patterns and

only speculated that alterations in muscle recruitment

may have lead to the lower eciency. Many previous stud-

ies examining the relationship between metabolic require-

ments and motor unit recruitment patterns have

examined the association during heavy intensity exercise

that has the added complication that steady-state condi-

tions may not be achieved. Constant work rate exercise

in the moderate intensity domain allows for comparisons

to be made between established steady-state VO 2 and

motor unit recruitment conditions. Therefore, the purpose

of the present study is to examine the eﬀect of prior heavy

exercise on the steady-state VO 2 response and sEMG dur-

ing a subsequent bout of moderate exercise. While it might

be predicted that the O 2 cost of moderate exercise remains

independent of prior exercise conditions (i.e. linear

dynamic system), we hypothesized that if prior heavy exer-

cise resulted in a higher absolute VO 2 and D VO 2 = DWR

during a subsequent bout of moderate exercise, the higher

O 2 cost would be associated with changes in motor unit

recruitment patterns reﬂecting either an increase in the

number of motor units (RMS) and/or the type of motor

units recruited (M D PF) as previously suggested ( Jones

et al., 2003; Sahlin et al., 2005 ) and reported during

repeated bouts of heavy exercise ( Burnley et al., 2002 ).

by the Institutional Review Board for Research Involving Human

Subjects at Texas Tech University and is in accordance with

guidelines set forth by the Declaration of Helsinki.

2.2. Experimental protocol

Subjects reported to the Applied Physiology Laboratory at

Texas Tech University on three separate occasions with no less

than 48 h between testing sessions. Each subject was instructed to

consume only a light meal, and to abstain from vigorous exercise

and caﬀeinated beverages for P12 h prior to arriving at the

Applied Physiology Laboratory for testing. Exercise testing was

performed at approximately the same time of the day for each

subject. Prior to exercise testing, seat height and handlebar posi-

tion were adjusted on the cycle ergometer for each subject and

returned to the same position for subsequent testing.

Preliminary exercise testing of each subject was performed to

both familiarize the subject with testing procedures and for the

determination of the estimated lactate threshold (LT) and peak

oxygen uptake ( VO 2 ; peak Þ . The highest mean VO 2 averaged over a

30 s interval was taken as VO 2 ; peak . All exercise testing was per-

formed on an electrically braked cycle ergometer (Corival 400,

Lode, The Netherlands). The initial exercise test involved 4 min of

loadless cycling (0 W) followed by progressive exercise to the limit

of tolerance in which the work rate increased as a ramp function

at a rate of 25 W min 1 . For all testing, the subjects were

instructed to maintain pedal cadence at 70 rpm that was aided by

both visual feedback and verbal encouragement. The estimated

LT was determined by visual inspection from gas exchange indices

using the V-slope approach, ventilatory equivalents and end-tidal

gas tensions. From the results of the ramp test, work rates that

would elicit a VO 2 equivalent to 90% LT (i.e. moderate intensity)

and 80% of VO 2 ; peak (i.e. heavy intensity) were determined.

On each of the second and third exercise sessions, subjects per-

formed two protocols of constant work rate exercise. Each protocol

consisted of alternating step transitions inwork rate froma baseline

of 20 W to moderate exercise followed by either a second bout of

moderate exercise (i.e. moderate–moderate) or by heavy exercise

that was followed by a second bout of moderate exercise (i.e.

moderate–heavy–moderate). In all protocols, bouts of moderate

exercise were 6 min in duration and heavy exercise was performed

for 4 min. The second bout of moderate exercise was initiated after

6 min or 30 s of recovery from either moderate or heavy exercise.

Diﬀerent recovery times were used to examine the relationship

between VO 2 and muscle activity during conditions where quite

diﬀerent metabolic requirements would be expected and thus, the

coupling between VO 2 and motor unit recruitment patterns could

be purposely challenged. Subjects completed one moderate–mod-

erate protocol (Protocol A, 6 min of recovery between exercise

bouts; Protocol B, 30 s of recovery between exercise bouts) followed

after at least 15 min of rest by one moderate–heavy–moderate

protocol (Protocol C, 6 min of recovery from heavy exercise; Pro-

tocol D, 30 s of recovery from heavy exercise) during each visit.

2. Methods

2.3. Measurement of pulmonary gas exchange

2.1. Subjects

Pulmonary gas exchange was measured breath-by-breath using

an automated metabolic measurement system (MedGraphics,

Model CPX/D, Medical Graphics Corp., St. Pauls, MN). Expired

gas ﬂows were measured using a pitot pneumotachograph con-

nected to a pressure transducer. The ﬂow signal was integrated to

yield a volume signal that was calibrated with a syringe of known

Eight healthy, male subjects (24.9 ± 2.4 yr) provided written

informed consent after being explained all experimental proce-

dures, the exercise protocol, and possible risks associated with

participation in the study. The experimental protocol was approved

J.U. Gonzales, B.W. Scheuermann / Journal of Electromyography and Kinesiology 18 (2008) 99–107

101

volume (3.0 l). Prior to each exercise session, the O 2 and CO 2

analyzers were calibrated using gases of known concentrations.

Corrections for ambient temperature and water vapor were made

for conditions measured near the mouth.

cially available software (MatLab, The MathWorks Inc., Natick,

MA). The raw sEMG signal was passed through a bandpass ﬁlter

of 20–450 Hz, a notch ﬁlter of 60 Hz, and full wave rectiﬁed. The

root mean square (RMS), a measure of the recruited muscle

activity required for force generation, and the median power

frequency (M D PF), an indication of the distribution of frequency

content, were computed for each muscle. The M D PF was deﬁned

by the following equation: R fmed

2.4. Measurement of surface electromyography (sEMG)

During each of the protocols, surface electromyography

(sEMG) was obtained from the vastus lateralis and vastus medi-

alis muscle groups using a commercially available data acquisition

system (PowerLab 8SP, ADInstruments, Grand Junction, CO).

The analog sEMG signal was sampled at a rate of 2000 Hz,

ampliﬁed (common mode rejection ratio: 96 dB, input impedance:

1MX, gain: 5000; Model 408 Dual Bio Ampliﬁer-Stimulator,

ADInstruments, Grand Junction, CO), passed through a fre-

quency window of 3-3000 Hz, digitized by a 12-bit A/D converter,

and stored on a computer for later analysis. The raw sEMG signal

was sampled using bipolar (2 · 9 mm discs, 15 mm diameter

sample area) Ag–AgCL surface electrodes (DDN-30 Norotrode,

Myotronics-Noromed, Inc., Tukwila, WA) with a ﬁxed inter-

electrode spacing of 30 mm placed on the right leg. The sEMG

electrodes were positioned over the distal half of the muscle belly

aligned longitudinally to the muscle ﬁbers. A reference electrode

was placed over the tibial tuberosity or over the head of the ﬁbula.

Electrode sites were shaved and cleaned with alcohol prior to

electrode placement in order to reduce inter-electrode resistance

(<10 kX). All wiring attached to the electrodes was securely fas-

tened to prevent motion artifact. The sEMG signal was checked

for motion artifact by moving and tapping the area surrounding

the electrode. The site was cleaned again and a new electrode

applied if motion artifact was detected in the signal.

fmed S m ð f Þ df . S m (f)

is the power density spectrum of the sEMG signal, fmed is the

M D PF of the sEMG signal, and f is the frequency in hertz. The

RMS and M D PF values during exercise were normalized to

baseline cycling at 20 W during the 60 s period prior to the ﬁrst

moderate exercise bout. The normalized RMS and M D PF

responses for the vastus lateralis and vastus medialis muscles were

averaged together to provide an overall representation of muscle

activity during exercise ( Burnley et al., 2002 ). RMS and M D PF

were averaged over 5 s intervals, corresponding to the same time

interval as for VO 2 during moderate exercise.

S m ð f Þ df ¼ R 1

2.7. Statistical analysis

Oxygen uptake ( VO 2 Þ , D VO 2 = DWR, RMS, DRMS/DWR,

DRMS = D VO 2 ,M D PF and DEﬀ were analyzed using a two-way

repeated measures ANOVA design with protocol and time as the

main eﬀects. Student–Newman–Keuls post hoc analysis was used

to further analyze signiﬁcant interactions. One-sample t tests were

used to test for signiﬁcant diﬀerences from steady-state levels.

Statistical signiﬁcance was accepted when P < 0.05. All values are

reported as the mean ± SE.

3. Results

2.5. Measurement of plasma lactate

3.1. Subjects

Prior to testing, subjects rested in a supine position while a

percutaneous Teﬂon catheter (22 gauge, Insyte I.V. Catheters,

Becton Dickinson, Inc.) was placed into a dorsal hand vein. The

blood sample was arterialized by heating the forearm and hand

throughout the exercise protocol by use of a heating lamp.

Samples were obtained at rest and at 2 min intervals during

exercise and recovery in each protocol. Samples were placed in an

ice-water slurry and analyzed for plasma lactate concentration

([Lac]) within 5–10 min (Stat Proﬁle M Blood Gas and Electrolyte

Analyzer, Nova Biomedical, Inc., Waltham, MA).

On average, subjects weighed 72.1 ± 4.2 kg, were 178.1

± 2.4 cm tall, and had a body mass index of 20.2 ± 1.2

kg m 2 . The group mean aerobic capacity ( VO 2 ; peak Þ mea-

sured during the preliminary ramp exercise test was 45.1 ±

3.2 ml kg 1 min 1 and the estimated LT was 52.9 ± 2.4%

of the VO 2 ; peak , corresponding to a VO 2 of 1700 ± 14 ml

min 1 . The mean work rate for moderate and heavy exercise

was 88 ± 10 W and 214 ± 20 W, respectively.

2.6. Data analysis

3.2. O 2 Uptake response

For each subject, VO 2 was averaged over the last 120 s of the

initial baseline cycling (20 W) stage and over the ﬁrst and last 60 s of

each of the steady-state moderate exercise bouts. Since the experi-

mental design required that the recovery duration prior to the

second bout of moderate exercise be of variable duration (i.e. 6 min

or 30 s), D VO 2 = DWR was calculated for each subject using the

baseline VO 2 prior to the ﬁrst bout of moderate exercise for each

respective protocol. Net eciency (DEﬀ), deﬁned as the ratio of the

change in work accomplished to the change in total energy expen-

diture, was calculated by utilizing the respiratory exchange ratio

and converting the average VO 2 response to W ( VO 2 ð W Þ¼½ VO 2

(ml min 1 ) 0.001 ml l 1 Cal Equiv (kcal l 1 O 2 )4185J]/60smin 1 )

( Mallory et al., 2002 ).

Oﬀ-line processing of the sEMG signal was performed using a

computer program developed in our laboratory using commer-

The absolute pulmonary VO 2 response to moderate con-

stant work rate exercise is presented in Table 1 . In spite of

the considerably diﬀerent metabolic rates (6 min vs. 30 s

recovery) at the onset of exercise, steady-state VO 2 during

the second bout of moderate exercise was similar to that of

the ﬁrst bout in the moderate–moderate transitions (Proto-

cols A and B). In the moderate–heavy–moderate transi-

tions (Protocols C and D), prior heavy exercise resulted

in a higher steady-state VO 2 during the second bout of

moderate exercise whether the recovery duration was

6 min or 30 s (1500.2 ± 115.2 ml min 1 and 1584.0 ±

124.3 ml min 1 , respectively). The elevated absolute VO 2

was greater for moderate exercise preceded by 30 s of

recovery as compared to 6 min of recovery from heavy

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J.U. Gonzales, B.W. Scheuermann / Journal of Electromyography and Kinesiology 18 (2008) 99–107

Table 1

Comparison of steady-state VO 2 , DEﬀ, and sEMG between the ﬁrst and second bouts of moderate exercise within each protocol

Protocol A Protocol B Protocol C Protocol D

First Second First Second First Second First Second

VO 2 (ml min 1 ) 1411 ± 126 1438 ± 135 1413 ± 133 1422 ± 139 1420 ± 132 1500 ± 115 * 1418 ± 127 1584 ± 124 * , #

D VO 2 (ml min 1 ) 588 ± 95 616 ± 102 635 ± 107 645 ± 109 633 ± 112 714 ± 89 * 651 ± 115 818 ± 103 * , #

D VO 2 = DWR (ml min 1 W 1 ) 9.7 ± 0.4 10.1 ± 0.4 10.3 ± 0.4 10.4 ± 0.7 10.3 ± 0.4 12.2 ± 0.7 * 10.7 ± 0.4 14.1 ± 0.8 * , #

DEﬀ (%) 29.8 ± 1.3 28.7 ± 1.1 28.0 ± 1.0 28.9 ± 2.9 28.4 ± 1.3 24.4 ± 1.5 * 27.2 ± 1.1 21.0 ± 1.4 *

RMS (lV Æ s 1 ) 0.11 ± 0.02 0.11 ± 0.02 0.10 ± 0.02 0.11 ± 0.01 0.10 ± 0.01 0.11 ± 0.01 0.11 ± 0.02 0.11 ± 0.01

DRMS/DWR (%W) 1.7 ± 1.9 1.8 ± 0.3 2.0 ± 0.3 2.0 ± 0.4 1.8 ± 0.2 2.1 ± 0.3 2.0 ± 0.2 2.2 ± 0.3

DRMS = D VO 2 (lV Æ l 1 min 1 ) 0.09 ± 0.01 0.09 ± 0.02 0.09 ± 0.01 0.10 ± 0.02 0.09 ± 0.02 0.09 ± 0.02 0.10 ± 0.02 0.08 ± 0.02

M D PF (Hz)

69.1 ± 2.9 71.2 ± 3.3 * 68.2 ± 1.8 70.0 ± 2.0 *

70.3 ± 2.5 71.9 ± 2.3 *

72.2 ± 3.5 72.0 ± 3.3

Values are mean ± SE.

* Signiﬁcant diﬀerence (P < 0.05) between bouts of moderate exercise within the same protocol.

# Signiﬁcant diﬀerence (P < 0.05) between the second bout of moderate exercise in Protocols C and D.

exercise (P < 0.05). Interestingly, the higher absolute VO 2

during the second bout of moderate exercise in the moder-

ate–heavy–moderate transitions was negatively correlated

with aerobic capacity (i.e. ﬁtness level) such that individu-

als with the lower VO 2 ; peak exhibited the largest increase in

VO 2 following heavy exercise (r = 0.54, F = 5.87,

P < 0.05).

3.3. Vastus muscle sEMG activity

The average RMS and M D PF response from the vastus

lateralis and vastus medialis muscles during moderate con-

stant work rate cycling exercise are presented in Table 1 .

No diﬀerence in RMS or the percent change in RMS from

20 W cycling was observed between bouts of moderate

exercise within any of the four constant work rate exercise

protocols despite changes in the O 2 cost of exercise ( Fig. 1 ).

In contrast, M D PF was increased by 2–3% during the sec-

ond bout of moderate exercise in the moderate–moderate

transitions (Protocols A and B), and also during the mod-

erate–heavy–moderate transition with the 6 min recovery

(Protocol C). However, M D PF remained unchanged

between the ﬁrst and second bouts of moderate exercise

when the second bout of moderate exercise was preceded

by 30 s of recovery from heavy exercise (Protocol D; see

Table 1 ). Interestingly, M D PF expressed as the percent

change from 20 W cycling was found to be 46.8 ± 8.7%

lower than the average response during the ﬁrst minute

of the second bout of moderate exercise when preceded

by 30 s of recovery from heavy exercise (Protocol D,

Fig. 1 ). The change in M D PF was not correlated with the

increase in VO 2 (r = 0.17, F = 0.80, P = 0.38).

The ratio of RMS to cycling work rate (DRMS/DWR)

showed a constant level during all moderate exercise bouts

indicating a coupling between motor unit recruitment and

cycling work rate ( Table 1 ). The relationship between

RMS and VO 2 was examined by the DRMS = D VO 2 ratio

which was increased during the ﬁrst minute of moderate

exercise when preceded by 6 min of recovery from either

moderate or heavy exercise, but reduced when preceded

by 30 s of recovery when VO 2 (i.e. metabolic rate) was ele-

Fig. 1. Changes in vastus muscle activity during the ﬁrst and last minute

of the second bout of moderate exercise. Upper panel: The increase in

RMS from 20 W cycling (DRMS) reached a stable level of motor unit

recruitment at exercise onset that did not vary or change with prior heavy

exercise. Lower panel: The increase in M D PF from 20 W cycling (DM D PF)

was similar between the ﬁrst and last minute of moderate exercise during

the moderate–moderate transitions, but varied from the average response

during the moderate–heavy–moderate transitions. Dotted lines represent

the average steady-state value reached during the ﬁrst bout of moderate

exercise in the four protocols combined. (*) Signiﬁcant diﬀerence between

ﬁrst and last minute of moderate exercise within each protocol (P < 0.05).

J.U. Gonzales, B.W. Scheuermann / Journal of Electromyography and Kinesiology 18 (2008) 99–107

103

Fig. 2. The ratio of RMS to VO 2 (DRMS = D VO 2 ) was increased at the

onset of exercise when 6 min of recovery was present before the second

bout of moderate exercise. DRMS = D VO 2 showed a reduction under

conditions of high metabolic rate at exercise onset. During the last minute

of exercise, DRMS = D VO 2 was similar to the average steady-state value

calculated during the ﬁrst bout of moderate exercise shown as the dotted

line. (*) Signiﬁcant diﬀerence between ﬁrst and last minute of moderate

exercise within each protocol (P < 0.05).

vated ( Fig. 2 ). During the last minute of the second bout of

moderate exercise, DRMS = D VO 2 ratio returned to the

average steady-state ratio measured during the ﬁrst bout

of moderate exercise in all the exercise protocols irrespec-

tive of the prior exercise conditions.

3.4. Gain and exercise eciency

Fig. 3. Changes in gain (D VO 2 = DWR) and net eciency (DEﬀ ) during the

last minute of moderate exercise for each protocol. Prior heavy exercise

resulted in a higher gain and a decrease in exercise eciency during the

second bout of moderate exercise. (*) Signiﬁcant diﬀerence between ﬁrst

and second bout of moderate exercise within each protocol (P < 0.05). (#)

Signiﬁcant diﬀerence between the second bouts of moderate exercise in

Protocols C and D (P < 0.05).

The gain or D VO 2 = DWR for moderate exercise paral-

leled the VO 2 response and is presented in Table 1 . Moder-

ate–moderate exercise transitions (Protocol A and B) did

not lead to a diﬀerence in D VO 2 = DWR which remained

at 10 ml min 1 W 1 in spite of the diﬀerent recovery met-

abolic rates prior to the onset of the second bout of mod-

erate exercise. In contrast, prior heavy exercise increased

D VO 2 = DWR to 12.2 ± 0.7 ml min 1 W 1 and 14.1 ± 0.8

ml min 1 W 1 during the second bout of moderate exercise

as compared to the ﬁrst bout of moderate exercise when

preceded by 6 min and 30 s of recovery from heavy exer-

cise, respectively (P < 0.05). The increase in D VO 2 = DWR

during the second bout of moderate exercise was greater

after 30 s of recovery (i.e. high metabolic rate) as compared

to 6 min of recovery from heavy exercise (Protocol

D > Protocol C, P < 0.05; Fig. 3 ).

Net eciency or DEﬀ calculated during steady-state

moderate exercise is presented in Table 1 . During the mod-

erate–moderate exercise transitions, DEﬀ was not diﬀerent

between the ﬁrst and second bout of moderate exercise.

However, prior heavy exercise resulted in a decrease in DEﬀ

by 13.3 ± 5.6% and 22.2 ± 4.9% during the second bout

of moderate exercise as compared to the average steady-

state response for Protocols C and D, respectively

( Fig. 3 , P < 0.05).

3.5. Plasma lactate

The plasma [Lac] response following heavy exercise was

analyzed for diﬀerences in the rate of [Lac] clearance into

the blood during the second bout of moderate exercise in

the moderate–heavy–moderate transitions (Protocols C

and D, Fig. 4 ). On average, plasma [Lac] increased from

1.6 mmol Æ l 1 at rest to a peak level of 9.3 ± 1.0 mmol Æ l 1

and 9.6 ± 1.0 mmol Æ l 1 3 min after heavy exercise for Pro-

tocols C and D, respectively. Although the second bout of

moderate exercise was initiated either 6 min or 30 s after

heavy exercise, the recovery proﬁle of plasma [Lac] was

similar between Protocols C and D when time was aligned

to the end of heavy exercise rather than to the onset of the

moderate intensity exercise. Correlation analyses did not

reveal a relationship between plasma [Lac] and VO 2 (r =

0.009, F = 0.003, P = 0.96), M D PF (r = -0.27, F = 2.26,

P = 0.14), D VO 2 = DWR (r = 0.20, F = 1.15, P = 0.29), or

DEﬀ (r = 0.14, F = 0.59, P = 0.45) during the second bout

of moderate exercise in Protocols C and D.

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