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Title: H reflex inhibition during muscular fatigue in man
Author: Olyaei, Gholamreza
Awarding Body: University of Glasgow
Current Institution: University of Glasgow
Date of Award: 1995
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It is well known that the motoneurone firing rates associated with muscle fatigue are reduced during sustained voluntary contraction and it has been suggested that one reason for this decline might lie in reflex inhibition of motoneurone pools by muscle afferents sensitised by the fatiguing contraction. Garland and McComas (1990) demonstrated that fatigue of soleus causes a depression of its H reflex excitability. Soleus is difficult to fatigue and so the objective of the present study was to extend this work by examining the reflex effects of fatiguing contiactions in other muscles. The effects of fatiguing activity of the anterior tibial muscles on their own H reflex excitability and on the H reflex of their antagonist muscle, soleus also were examined. Alternatively quadriceps femoris muscle was fatigued to see how extensive the effect is. The main aim of this research was to determine the effects of fatiguing activity on the excitability of anterior tibial and soleus H reflexes. In addition, the differences between voluntary fatiguing activity and involuntary exercise by direct muscle stimulation were investigated. Experiments were performed on 50 neurologically normal subjects. Their ages ranged from 17 to 42 years. Subjects were seated in a semi-reclined position with their knee and ankle supported at 110° and 90°. Maximal voluntary dorsiflexions were recorded at intervals through the experiment. EMG was recorded with surface electrodes placed over the anterior tibial muscles or over soleus. H reflexes of about half maximal amplitude were elicited by stimulation of the tibial nerve in the popliteal fossa or common peroneal nerve on the head of fibula with single pulses, 0.1-0.5 ms duration, at intervals of 5 sec. The intensities of the stimulus were adjusted to deliver approximately half maximal H reflex amplitudes. In most experiments it was necessary for subjects to make a weak voluntary contraction of the anterior tibial muscles before an H reflex could be elicited. However, consistent H reflexes were recorded in relaxed soleus in all subjects. The mean of 10-15 successive tests was compared before and after anterior tibial muscle activity. At least 30 seconds elapsed after the end of contraction before the first test was made. In addition, a wider range of stimulus intensities was employed to identify maximal M and H waves. Fatigue was induced by intermittent voluntary isometric contractions at 30% of MVC, 7 sec on, 3 sec off, sustained for up to 9 minutes. These periods of muscle activity were performed with and without an arterial occlusion cuff round the midthigh inflated at least 150 mmHg to obstruct the circulation. Alternatively, the anterior tibial muscles were fatigued by transcutaneous electrical stimulation of the muscle belly at 20 Hz at the highest intensity the volunteer found tolerable. Stimulation was delivered for 7 seconds and then turned off for 3 seconds. This stimulation cycle was repeated until the tetanic force had fallen to about half its original value. Approximately equivalent fatigues, as judged by reductions in MVC were produced by electrical and voluntary exercise. H reflexes in anterior tibial muscles were depressed significantly by fatiguing activity of anterior tibial muscles. Soleus H reflexes were depressed significantly by fatiguing activity of anterior tibial and quadriceps muscles. No significant difference was observed between the reflex changes accompanying voluntary and electrically induced muscle activity. The result does not depend on the nature of exercise but the extent of the depression of H reflexes increases as the fatigue increases. All the changes in H reflexes, forces were restored progressively to control values after a 10 minute recovery period. The fatigue developed during experiments was relatively severe, particularly when the blood flow was occluded. In experiments using direct muscle stimulation, the mean force was reduced by approximately 40% and the half relaxation time increased by more than 70%. It seems that a reflex inhibitory system is active during fatigue, which decreases the motoneurone excitability in conjunction with the reduction in force. This reflex is mediated, due to metabolic or chemical changes which could be substantially projected by small diameter afferent fibres from fatigued muscles. This agree with Garland and McComas (1990) who stated that the relative stability of the M and Mmax waves and the H and Hmax reflexes were decrease, suggests that this reduction could be a reflex phenomenon rather than neuromuscular junction failure.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available