Exp Physiol 92.4 pp 779–780 779
Experimental Physiology – Letter to Editor
Relationship between muscular indeed generalized. By using this protocol, the authors did provide an enticing
effort and generalized central Ross et al. (2007) confirm that the decrease hypothesis. That is, in contrast to other
fatigue following marathon running in MVC in the tibialis anterior was not the studies, including our own (Saboisky et al.
in man result of reduced motivation or general 2003; Martin et al. 2005), which show
whole-body fatigue. Figure 1 shows data that the CNS can recover quickly following
redrawn from Ross et al. (2007). The change endurance exercise, cortical drive to the
in the MVC for the tibialis anterior from tibialis anterior, or rather the skeletal
The study by Ross et al. (2007) represents baseline to postexercise is equivalent to muscles ‘used’ during the previous exercise
a significant advancement in the 16.7% reduction and, in terms of voluntary bout, continues to be depressed for at
understanding of muscular fatigue activation, is 13.6% reduction. Interestingly, least 20 min following the marathon. These
following long duration exercise and the MVC remains depressed up to 4 h post findings lend support to the theory that
perhaps finally establishes that the brain marathon but the voluntary activation is generalized CNS fatigue is not a common
‘decides’ how much drive to the muscle is restored by this time. Contrast this to the phenomenon as hypothesized by others
directed in the postexercise period. Ross wrist flexors, which do not reach statistical (Nybo & Nielsen, 2001) but rather there is a
et al. (2007) established that, following significance at any point following the selective attenuation in the CNS motor drive
a marathon, the voluntary activation marathon. The finding that the wrist flexor even during exercise-induced hyperthermia
and force output of the tibialis anterior MVC was maintained is a crucial result, and regardless of contraction type (Martin
immediately postexercise was significantly which supports the proposition that the et al. 2005). It is also worth noting that the
reduced. The authors also reported that the CNS is able to discriminate and attenuate CNS does not only discriminate between
maximal voluntary contraction (MVC) of motor unit recruitment according to which ‘used’ and ‘unused’ muscle groups, but it
the wrist flexors was not reduced following muscles are most likely to experience also most probably provides a priming of
the marathon. This protocol where the high metabolic demands and increase the contralateral fatigue for the upper limbs
subsequent force output and voluntary likelihood of catastrophe. (Todd et al. 2003) and for the lower limbs
activation of the ‘used’ muscles (in this Although Ross et al. (2007) did not (Rattey et al. 2006), with possible differences
case the tibialis anterior) are compared measure the voluntary activation of the between sexes (Rattey & Martin, 2007).
with that of the ‘unused’ muscles (wrist wrist flexors through either cortical or Although the editorial by Rasmussen et al.
flexors) in the preceding exercise has been electrical stimulation and therefore could (2007) in this same issue of the journal
used previously to extrapolate whether or not definitively conclude that prolonged contends that the paper by Ross et al.
not the fatigue induced by the exercise is exercise does not induce generalized fatigue, (2007) does not advance our understanding
60
50
40
Wrist Flexors "unused"
MVC (kg)
30
6 Tibilais. ant. "used"
Figure 1. The data redrawn from Ross et al. (2007) for the 3
MVC of the wrist flexors, representing the ‘unused’
muscles, and the tibilais anterior, representing the ‘used’
muscles, following a marathon MVC = -16.7% (VA = -13.6%)
The measurement intervals represent premarathon or baseline, 0
immediately following the marathon (Post), 4 and 24 h post Baseline Post +4 hr +24 hr
marathon. Note the change in MVC and voluntary activation (VA)
from baseline to post marathon for the tibialis anterior. Measurement Interval
C 2007 The Authors. Journal compilation C 2007 The Physiological Society DOI: 10.1113/expphysiol.2007.038083
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780 F. Marino Exp Physiol 92.4 pp 779–780
of the mechanisms responsible for the Exercise and Sports Science Laboratories Rattey J & Martin PG (2007). Central fatigue
development of central fatigue, even and School of Human Movement Studies explains sex differences in muscle fatigue and
though these authors speculate that brain Charles Sturt University, Bathurst contralateral cross-over effects of maximal
metabolism is a major determinant, from NSW Studies, Charles Sturt University contractions. Pflugers Arch DOI
10.1007/s00424-007-0243-1.
a neurological point of view, consideration Bathurst, NSW 2795, Australia
Rattey J, Martin PG, Kay D, Cannon J & Marino
should be given to a reduced motor drive
Email: fmarino@csu.edu.au FE (2006). Contralateral muscle fatigue in
to the skeletal muscles as a means of human quadriceps muscle: evidence for a
protecting the organism from physiological centrally mediated fatigue response and
catastrophe. cross-over effect. Pflugers Arch 452, 199–207.
Therefore, the conclusion that is most References Ross EZ, Middleton N, Shave R, George K &
important in relation to the findings of Nowicky A (2007). Corticomotor excitability
Ross et al. (2007) and those studies which Martin PG, Marino FE, Rattey J, Kay D & contributes to neuromuscular fatigue
show a selective attenuation in motor drive Cannon J (2005). Reduced voluntary following marathon running in man. Exp
is that generalized muscular fatigue as a activation of human skeletal muscle during Physiol 92, 417–426.
shortening and lengthening contractions in Saboisky J, Marino FE, Kay D & Cannon J
consequence of endurance exercise is not
whole body hyperthermia. Exp Physiol 90, (2003). Exercise heat stress does not reduce
easily identified, at least with the studies
225–236. central activation to non-exercised human
and methodologies currently employed, Nybo L & Nielsen B (2001). Hyperthermia and skeletal muscle. Exp Physiol 88, 783–790.
and that the avoidance of physiological central fatigue during prolonged exercise in Todd G, Petersen NT, Taylor JL & Gandevia SC
catastrophe is possibly the main purpose for humans. J Appl Physiol 91, 1055–1060. (2003). The effect of a contralateral
attenuated central drive to skeletal muscle. Rasmussen P, Secher N & Petersen NT (2007). contraction on maximal voluntary activation
Viewpoints: Understanding central fatigue: and central fatigue in elbow flexor muscles.
Frank Marino where to go? Exp Physiol 92, 369–370. Exp Brain Res 103, 308–313.
C 2007 The Authors. Journal compilation C 2007 The Physiological Society
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