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Experimental studies of spinal mechanisms associated with muscle fatigue
University of Gävle, Belastningsskadecentrum.
2004 (English)Doctoral thesis, monograph (Other scientific)
Abstract [en]

Title Experimental stodies of spinal mechanisms associated with muscle fatigue Abstract Muscle fatigue is ubiquitous in every day life.Muscle fatigue might be considered as an altered state of motor behaviour, which impairs motor performance. By contrast, muscle fatigue could also be considered a positive phenomenon, which protects muscle tissue from damage that might be incurred to it by overose.

The principal aim of the thesis was to explore some of the mechanisms ofmuscle fatigue at the spinallevel in animal models. The activation of multiple motor units of a single calf muscle may influence contractile properties of its neighbouring, otherwise inactive units, providing evidence for spatial spreading of fatigue between different muscle parts. The release of metabolites, their action on inactive muscle units and the effects oflocal hypoxia are the most likely causes. Fatigue-induced metabolite shift in the interstitium provokes excitation and/or sensitisation of high-threshold afferent fibers, with complex effects on the spinal premotoneuronal network involved in the modulation of motoneuronal output. This was examined by stoding the intrasegmentallamellar distribution of the lumbar spinal intemeurons following fatiguing contractions of the triceps surae muscle. Furthermore, fatigue ofcalfmuscles enhanced the activity of fusimotor neurons to these muscles irrespective of the regime of muscle activity (isometric vs. lengthening) in conditions that simulate locomotion. Altered fusimotor activity, through increased or maintained muscle spindIe arrerent responsiveness may be advantageous, providing support to the skeletomotor activity and enhanced information about muscle periphery to higher nervous centres. The particular effects of intemeuronal network at motor input (presynaptic inhibition system) and output (recurrent inhibition system) stages were then addressed. Fatigue of triceps surae muscle induced a suppression of the monosynaptic reflex. The intensity ofpresynaptic inhibition increased, while the intensity of recurrent inhibition decreased. Post fatigue-evoked changes in monosynaptic reflexes and presynaptic inhibition indicate the possibility that high-threshold afferents inhibit group la terminals pre-synaptically, which would allow fatigue-induced signals from the muscle to reduce the relevance of proprioceptive feedback. Besides intrasegmental, intersegmental spreading of nociceptive signals was explored. Activation of sensory arrerents from dorsal neck muscles by capsaicin induces powerful activation ofintemeurons located in the cervical spinal cord, as weIl as a widespre ad activation of cells in lumbar spinal cord segments.

The results confirm the pivotal role of small diameter muscle afferents in the orchestration of segmental responses to fatigue and show complex interactions that may lead to limited accuracy of motor output. They also depict processes that may be related to, and even become precursors of chronic muscle pain.

Place, publisher, year, edition, pages
Arbetslivsinstitutets tryckeri, Umeå , 2004.
Series
Umeå University Medical Dissertations, ISSN 0346-6612 ; 910
Keyword [en]
Muscle fatigue, Monosynaptic reflex, Presynaptic Inhibition, Recurrent Inhibition, Fusimotor System, Fos-immunoreactivity, Referral Pain
Identifiers
URN: urn:nbn:se:hig:diva-2849ISBN: 91-7305-703-7 (print)OAI: oai:DiVA.org:hig-2849DiVA: diva2:119511
Available from: 2007-11-05 Created: 2007-11-05

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