Jacques Duchateau

8 mars 2013

Fac des Sciences de la Motricité, Université Libre de Bruxelles, Bruxelles, Belgique

"Changes in motor control with ageing"

Invité par Christiane Rossi-Durand

Senescence is accompanied by a marked decline in capability of the motor system. In addition to the decrease in muscle mass (sarcopenia), there is a loss of motor neurones that reduces the number of motor units. However, surviving motor neurones can reinnervate some denervated muscle fibres, increasing thereby the innervation ratio of the remaining motor units.

Although this profound remodelling of the neuromuscular system, voluntary muscle activation does not appear to contribute greatly to the weakness commonly observed in elderly adults. Indeed, the deficit in voluntary activation during an isometric maximal voluntary contraction (MVC) is often similar in physically active elderly and young adults (Klass et al., 2007). This observation can be surprising because the maximal motor unit discharge rate is significantly reduced in elderly adults during a MVC (Connelly et al., 1999). This age-related reduction in discharge rate is, however, usually accompanied by a slowing of muscle contraction that shifts the muscle force-frequency relation leftward and would permit to attain maximal performance with a lower motor units discharge rate. In contrast, the age-related reduction in motor unit discharge rate at the onset of a fast (ballistic) contraction is more affected than a slow MVC (Klass et al., 2008). The decrease in instantaneous discharge rate tends to be progressively greater for the successive interspike intervals of the discharge train. These data indicate that the slowing of motor unit discharge rate likely limits, in addition to the slowing of muscle contractile properties, the maximal rate of torque development in elderly adults. Therefore, healthy ageing appears to involve neural impairments that influence the intrinsic maximal capacity of motor neurones to discharge at very high rates. Similarly, the remodelling of motor unit has a substantial impact on fine motor skills. For example, the fluctuation of force during steady contractions at different intensities is greater and the precision of our movements during shortening and lengthening contractions is diminished with ageing indicating impairment in motor control (Enoka, 2008). In addition to changes related to motor unit reorganization, the control of movement and balance during upright posture is accompanied by a greater involvement of the supraspinal structure in elderly subjects (Baudry & Duchateau, 2012).

The aim of the talk is to give an overview of the main age-related changes of the neuromuscular system and to evoke briefly the potential capacity of practise and exercise in limiting these alterations.

Baudry S. & Duchateau J. J Physiol. 590 : 5541-5554, 2012 Connelly D.M., Rice C.L., Roos M.R. & Vandervoort A.A. J. Appl. Physiol. 87 : 843-852, 1999 Enoka RM. Neuromechanics of Human Movement. Human kinetics, 549pp, 2008. Klass M., Baudry S. & Duchateau J. Eur. J. Appl. Physiol.100 : 543-551, 2007 Klass M., Baudry S. & Duchateau J. J. Appl. Physiol. 104 : 739-746, 2008

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