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A mes filles Clémence et Salomé,

A ma femme Barbara,

A mes parents,

A Mariella.

Respiratory Physiology & Neurobiology171 (2010) 128...134

Contents lists available atScienceDirect

Respiratory Physiology & Neurobiology

journal homepage:www.elsevier.com/locate/resphysiol Determinants of oxygen consumption during exercise on cycle ergometer: The effects of gravity acceleration

Julien Bonjour

a , Carlo Capelli b , Guglielmo Antonutto c , Stefano Calza d

Enrico Tam

a e , Dag Linnarsson f , Guido Ferretti a d a Département de Neurosciences Fondamentales, Université de Genève, Geneva, Switzerland b

Dipartimento di Scienze Neurologiche e della Visione, Facoltà di Scienze Motorie, Università di Verona, Verona, Italy

c

Dipartimento di Scienze e Tecnologie Biomediche, Facoltà di Medicina, Università di Udine, Udine, Italy

d

Dipartimento di Scienze Biomediche e Biotecnologie, Facoltà di Medicina, Università di Brescia, Brescia, Italy

e Facoltà di Scienze Motorie, Università di Bologna, Bologna, Italy f Department of Physiology and Pharmacology, Karolinska Institutet, Stockholm, Sweden article info

Article history:

Accepted 24 February 2010

Keywords:

Internal power

Pedalling frequency

Humans

Centrifuge

abstract The hypothesis that changes in gravity acceleration ( a g ) affect the linear relationships between oxygen consumption ( V O 2 ) and mechanical power (w) so that at anyw,V O 2 increases linearly witha g was tested under conditions where the weight of constant-mass legs was let to vary by inducing changes ina g in a humancentrifuge.Theeffectsofa g ontheV O 2 frequencies ( f p , 1.0 and 1.5Hz), during four work loads on a cycle ergometer (25, 50, 75 and 100W) and at foura g levels (1.0, 1.5, 2.0 and 2.5 times normal gravity).V O 2 increased linearly withw. The slope did not differ signi“cantly at variousa g andf p , suggesting invariant mechanical ef“ciency during cycling, independent off p anda g . Conversely, they-intercept of theV O 2 /wrelationship, de“ned as constantb, increased linearly witha g . Constantbis the sum of restingV O 2 plus internal metabolic power (E i ). Since the former was the same at all investigateda g , the increase in constantbwas entirely due to an increase in E i . Since theV O 2 versuswlines had similar slopes, the changes inE i entirely explained the higherV O 2 at eachw,asa g was increased. In conclusion, the effects ofa g onV O 2 are mediated through changes inE i and not in wor in restingV O 2

© 2010 Published by Elsevier B.V.

1. Introduction

In future human exploration of the Moon and Mars, humans will be weightless during transit, will be exposed to fractions of the normal gravity on the Moon and Mars, and will be exposed to intermittent hypergravity during anticipated sessions of training in arti“cial gravity ( di Prampero, 2000 ). The complex logistics and limited onboard resources of long-term space travel necessitate a ing that of exercise training in a variable gravity environment. In addition we expect that studies of the metabolic cost of a stan- dardized exercise in differing gravity environments would provide of dynamic leg exercise. Corresponding author at: Département de Neurosciences Fondamentales, Cen- tre Médical Universitaire, 1 rue Michel Servet, CH-1211 Genève 4, Switzerland. Tel.: +41 22 3795363/39 030 3717440; fax: +41 22 3795402. E-mail address:Guido.Ferretti@unige.ch(G. Ferretti). During exercise on a cycle ergometer, the metabolic power E) is a linear function of the mechanical power (w) set by the resistance imposed on the ergometer (Dickinson, 1929; Garry and Wishart, 1931; Henry and De Moor, 1950; Banister and Jackson,

1967; Whipp and Wasserman, 1969; Gaesser and Brooks, 1975),

which is barely affected by factors such as age, sex, “tness status and training. The slope of that line (E/w) is the reciprocal of the delta-ef“ciency of exercise (=w/E)(Gaesser and Brooks, 1975
). They-intercept of the same line was traditionally consid- those variations to the cost of lifting the mass of the legs during pedalling. It was then demonstrated that the relationship between Eandwtakes differenty-intercepts when weights are added to the legs (

Kamon et al., 1973

). This “nding led to the concept of metabolic internal power during cycling ( E i ), which became the object of several investigations (Seabury et al., 1977; Kaneko and Yamazaki, 1978; Wells et al., 1986; Luhtanen et al., 1987; Widrick et al., 1992; Francescato et al., 1995; Girardis et al., 1999; Martin et

Hirakoba, 2007, 2008

E i includes all sources of metabolic energy

1569-9048/$ ... see front matter© 2010 Published by Elsevier B.V.

doi:

10.1016/j.resp.2010.02.013

J. Bonjour et al. / Respiratory Physiology & Neurobiology171 (2010) 128...134129 expenditure other than the performance of external mechanical power ... incidentally, external power is the power against air and the cycle ergometer. The frequency of pedalling (f p ) and the mass of the legs ( M L ) were identi“ed as two of the determinants ofE i .A more complete model of E i during cycling, which adds the gravity acceleration ( a g )to f p andM L as an independent determinant of it, was then proposed (

Girardis et al., 1999

). In fact, due to asymme- tries in the pedalling cycle, the rotational movement of the legs is viscous-elastic forces. The last are related to the physico-chemical characteristics of the muscle...tendon complex. Inertia accounts for a small fraction of the overall acceleration, and can be neglected in the present context. Gravity acceleration, however, cannot be ignored as a determi- nant of E i . In fact the relationship betweenEandwwas displaced downward in microgravity (0G, as in space "ight) with respect to Earth (1G, where the relative unit G designatesa g =9.81ms 2

Girardis et al., 1999

). On the other side, data obtained in a human tionship with respect to that obtained on the same subjects at 1G

Rosenhamer, 1968

). Although these displacements appeared to be coherent with the predictions based on the models of E i

Girardis

et al., 1999 ), a systematic analysis of the effects ofa g onEandE i duringcyclingisstilllacking.SinceE i ispredictedtobedirectlypro- portional toa g , and thus the vertical displacements of theEversus wrelationships shown byGirardis et al. (1999)may be entirely accounted for by changes in E i , we may hypothesize that at any given submaximal wthere ought to be a positive linear relation- ship between Eanda g , and thus between steady state submaximal oxygen consumption ( V O 2 ) anda g . In order to test this hypothesis, we carried out the present study, the aim of which was to deter- mine the effects ofa g on theV O 2 versuswrelationship at differing f p values.

2. Methods

2.1. Subjects

The study was conducted on 14 young healthy subjects. They were 25.8±1.4 years old, 181±1cm tall and their body mass was

75.8±2.2kg. They had no history of cardiopulmonary disease and

were not taking medications at the time of the experiments. They were also instructed not to drink coffee or use nicotine-containing products on the day of the experiment. The subjects received writ- ten information about the procedure and signed a consent inform holm.

2.2. Maximal oxygen consumption

The individual maximal oxygen consumption (

V O 2 max ) was determined during an incremental exercise test on a cycle ergome- ter, using a metabolic cart (K4b 2 , Cosmed, Italy). Each submaximal work load lasted 3min. The lowest power was 100W. Power was then progressively increased by 50-W steps. The step increase was

Heart rate (

f h ) was determined at the steady state, at the end of each work load. Individual V O 2 max was established from the plateau attained by the relationship between V O 2 and power. If such a plateau was not observed, subsidiary criteria for V O 2 max establishment were: (1) a lack of increase inf h between successive work loads ( f h <5min 1 ); and (2) values for gas exchange ratio higher than 1.1. The maximal aerobic mechanical power ( w max was identi“ed from the individual relations of V O 2 towas the min-imum wrequiring aV O 2 equal toV O 2 max . Although these criteria were recently criticized for use with unsteady state ramp proto- cols (

Poole et al., 2008

), they are still accepted as far as the classical intermittent steady state protocols are concerned.

2.3. The centrifuge

The study was conducted in the human centrifuge of Karolin- ska Institute, Stockholm, Sweden. The subjects were located in the centrifuge gondola, sitting on a seat, which could be adjusted to be perpendicular to the resultant of the normal G vector and the centrifugal G vector. They were secured on the seat by a “ve- point safety belt. The feet were “xed on an electrically braked cycle ergometer (Model 380, Siemens-Elema, Sweden) that was locatedquotesdbs_dbs21.pdfusesText_27

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