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O2 slow component dependent
on progressive recruitment of fast-twitch fibers in trained
runners?
1 Laboratoire Sport Performance et Santé, 34090 Montpellier, Faculté des Sciences du Sport, Université de Montpellier I, France; 2 Laboratoire des Sciences du Sport, 25000 Besançon, Unité de Formation et de Recherche en Science et Techniques des Activités Physiques et Sportives, Besançon, France; 3 Groupe Analyse du Mouvement, Dijon 21000, UFR STAPS Université de Bourgogne, France; and 4 Institut des Sciences du Sport et de l'Education Physique, 1015 Lausanne, Faculté des Sciences Sociale et Politiques, Switzerland
The goal of this study was to use spectral analysis of EMG data
to test the hypothesis that the O2 uptake
(
O2) slow component is due to a
recruitment of fast fibers. Thirteen runners carried out a treadmill
test with a constant speed, corresponding to 95% of the velocity
associated with maximal O2. The
O2 response was fit with the classical
model including three exponential functions. Electrical activity of six
lower limb muscles (vastus lateralis, soleus, and gastrocnemius of both
sides) was measured using electromyogram surface electrodes. Mean power
frequency (MPF) was used to study the kinetics of the electromyogram
discharge frequency. Three main results were observed: 1) a
common pattern of the MPF kinetics in the six muscles studied was
noted; 2) MPF decreased in the first part of the exercise,
followed by an increase for all the muscles studied, but only the
vastus lateralis, and gastrocnemius muscles of both sides increased
significantly (P < 0.05); and 3) the
beginning of the MPF increase of the four muscles mentioned above
corresponded with the beginning of the slow component. Our results
suggest a progression in the average frequency of the motor unit
discharge toward the high frequencies, which coheres with the
hypothesis of the progressive recruitment of fast-twitch fibers during
the O2 slow component. However, this
interpretation must be taken with caution because MPF is the result of
a balance between several phenomena.
O2 kinetics; mean power frequency; electromyogram; power spectrum; O2 uptake
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