Subsequent studies however have demonstrated that the contribution of group

Subsequent studies however have demonstrated that the

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Subsequent studies, however, have demonstrated that the contribution of group III–IV afferents to the development of fatigue can vary across muscles. Martin et al. (2006) examined the contribution of feedback by group III–IV afferents to the fatigue experienced during sustained 2 min MVCs with the elbow flexor and extensor muscles. The protocols involved comparing the amplitude of potentials evoked in muscle with transmastoid stimulation of the corticospinal tract to assess the excitability of motor neurones during MVCs and during recovery when blood flow to the muscles was occluded and when it was intact. The influence of these afferents on motor neurone excitability was estimated by comparing the amplitude of the evoked potentials in the presence and absence of ischaemia that altered the feedback delivered by group III–IV afferents. When the fatiguing contraction was performed with the triceps brachii muscle, the amplitude of the evoked response decreased during the MVC and remained depressed while blood flow was occluded, but recovered within 15 s with the restoration of blood flow. The amplitude of the potentials evoked in triceps brachii also decreased after the fatiguing contraction was performed with the elbow Figure 3. Decreases in MVC torque and voluntary activation during a fatiguing contraction A , decrease in MVC torque after three sets of maximal shortening contractions (60 deg s 1 over a 75 deg range of motion) and three matched maximal isometric contractions with the knee extensor muscles. The two tasks were performed on separate days and the isometric contractions were sustained to produce the same relative decrease in MVC torque. The data correspond to mean + S . E . M . for the shortening contractions and isometric contractions for the first, second and third sets of shortening contractions and the three isometric contractions. Adapted with permission from Babault et al. (2006). B , corresponding decreases in voluntary activation as assessed with paired stimuli (10 ms apart) during selected maximal contractions. Initial values for voluntary activation were 88.3 ± 3.0% for the session in which the shortening contractions were performed and 89.4 ± 3.1% in the session for the isometric contractions. Data from Babault et al. (2006). C 2008 The Authors. Journal compilation C 2008 The Physiological Society
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16 R. M. Enoka and J. Duchateau J Physiol 586.1 flexor muscles. In contrast, the amplitude of the potentials evoked in biceps brachii increased after a fatiguing contraction with triceps brachii. These results indicate that group III–IV afferents depressed the excitability of triceps brachii motor neurones, but facilitated those that innervate biceps brachii. Given the known differences in central connections between muscles that can exert different actions (Pierrot-Deseilligny & Burke, 2005), it seems likely that fatiguing contractions can involve other differences in adjustments between motor neurone pools.
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