Respiratory Physiology & Neurobiology 180 (2012) 223–229
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Respiratory Physiology & Neurobiology
j our nal ho me p age: www.elsevier.com/locate/resphysiol
The effect of baseline metabolic rate on pulmonary O
2
uptake kinetics during
very heavy intensity exercise in boys and men
Brynmor C. Breese
a
, Alan R. Barker
a
, Neil Armstrong
a
, Andrew M. Jones
b
, Craig A. Williams
a,∗
a
Children’s Health and Exercise Research Centre, College of Life and Environmental Sciences, University of Exeter, UK
b
Sport and Health Sciences, College of Life and Environmental Sciences, University of Exeter, UK
a r t i c l e i n f o
Article history:
Accepted 23 November 2011
Keywords:
Phase II time constant
˙
VO
2
slow component
Electromyogram
Youth
a b s t r a c t
This study tested the hypothesis that pulmonary
˙
V
O
2
kinetics would be slowed during ‘work-to-work’
exercise in adults but not in children. Eight boys (mean age = 12.5 ± 0.5 years) and nine men completed
very heavy step transitions initiated from either ‘unloaded’ pedalling (U → VH) or unloaded-to-moderate
cycling (i.e. U → M to M → VH). The phase II was significantly (p < 0.05) lengthened in M → VH compared
to U → M and U → VH in boys (30 ± 5 vs. 19 ± 5 vs. 21 ± 5 s) and men (49 ± 14 vs. 30 ± 5 vs. 34 ± 8 s). In
U → VH, a greater relative
˙
V
O
2
slow component temporally coincided with an increased linear iEMG slope
in men compared boys (
˙
V
O
2
slow component: 16 ± 3 vs. 11 ± 4%; iEMG slope: 0.19 ± 0.24 vs. -0.06 ± 0.14%,
p < 0.05). These results suggest that an age-linked modulation of
˙
V
O
2
kinetics might be influenced by
alterations in muscle fibre recruitment following the onset of exercise.
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
Following the onset of step exercise, the arrival of more deoxy-
genated blood from the contracting muscles at the pulmonary
capillaries signals an exponential rise in pulmonary O
2
uptake (
˙
V
O
2
)
that has been shown to reflect the kinetics of muscle O
2
con-
sumption (m
˙
V
O
2
) in humans (Grassi et al., 1996; Krustrup et al.,
2009) and is reported to be speeded in children relative to adult
or teenage counterparts [see Armstrong and Barker (2009) for a
review]. Furthermore, there is an elevated O
2
cost or ‘gain’ during
the initial exponential phase that coincides with an attenuated slow
rise in
˙
V
O
2
above the primary phase amplitude (i.e. the
˙
V
O
2
slow
component) in young people exercising above the gas exchange
threshold (GET) (Armon et al., 1991; Fawkner and Armstrong, 2004;
Williams et al., 2001). The physiological factors that mediate these
age differences in
˙
V
O
2
kinetics are, however, unresolved with few
experimental data available.
Previous studies using muscle biopsies has revealed that cycling
at higher work rates mandates the recruitment of muscle fibre
pools with increasing metabolic diversity (i.e. types I, IIa and IIx)
in order to meet the requirements for muscle force production
(Gollnick et al., 1974; Krustrup et al., 2004; Vollestad and Blom,
1985). For example, type II muscle fibres are purported to have a
∗
Corresponding author at: Children’s Health and Exercise Research Centre, Sport
and Health Sciences, University of Exeter, St. Luke’s Campus, Heavitree Road, Exeter
EX1 2LU, UK. Tel.: +44 01392 264890; fax: +44 01392 724726.
E-mail address: C.A.Williams@exeter.ac.uk (C.A. Williams).
reduced microvascular O
2
pressure head, slower
˙
V
O
2
kinetics and
an increased metabolic cost of force production compared to type I
fibres (Behnke et al., 2003; Crow and Kushmerick, 1982; Krustrup
et al., 2008; McDonough et al., 2005). The recruitment of type II
muscle fibres has therefore been implicated in extending the over-
all
˙
V
O
2
and gain during step transitions spanning the heavy and
very heavy domains compared to moderate exercise [see Jones
et al. (2005) for a review]. Conversely, the characteristic
˙
V
O
2
pro-
file reported in young people during heavy-intensity exercise is
similar to conditions in which type I fibre activation is presumably
enhanced, for example, following selective glycogen depletion of
type II fibres (Carter et al., 2004), during pedalling at slow cadences
(Breese et al., 2011; Pringle et al., 2003b), and in adult subjects
with a greater proportion of type I fibres (Barstow et al., 1996;
Pringle et al., 2003a). However, the possibility that children’s faster
phase II
˙
V
O
2
kinetics and reduced
˙
V
O
2
slow component might be
linked to differences in muscle fibre recruitment compared to older
counterparts has yet to be examined experimentally.
In adults, it has been suggested that a lengthened phase II
and increased
˙
V
O
2
gain when step exercise transitions are initiated
from an elevated baseline work rate (i.e. work-to-work exercise)
might reflect the metabolic properties of higher-order (type II)
muscle fibres that are revealed on the pulmonary O
2
signal under
these circumstances (Brittain et al., 2001; DiMenna et al., 2008,
2010; Wilkerson and Jones, 2006, 2007). This supposition is based
on the size principle (Henneman and Mendell, 1981) which posits
an orderly recruitment from smaller motor units with a greater
muscle fibre oxidative capacity (i.e. type I) to larger motor units (i.e.
types IIa and IIx) as the requirement for muscle force production is
1569-9048/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.resp.2011.11.013