Spinal reflexes and coactivation of ankle muscles during a submaximal fatiguing contraction

This study examined the involvement of spinal mechanisms in the control of coactivation during a sustained contraction of the ankle dorsiflexors at 50% of maximal voluntary contraction. Changes in the surface electromyogram (EMG) of the tibialis anterior and of two antagonist muscles, the soleus and lateral gastrocnemius, were investigated during and after the fatigue task. Concurrently, the compound action potential (M-wave) and the Hoffmann reflex of the soleus and lateral gastrocnemius were recorded. The results showed that the torque of the ankle dorsiflexors and the average EMG of the tibialis anterior during maximal voluntary contraction declined by 40.9 +/- 17.7% (mean +/- SD; P < 0.01) and 37.0 +/- 19.9% (P < 0.01), respectively, at task failure. During the submaximal fatiguing contraction, the average EMG of both the agonist and antagonist muscles increased, leading to a nearly constant ratio at the end of the contraction when normalized to postfatigue values. In contrast to the monotonic increase in average EMG of the antagonist muscles, the excitability of their spinal reflex pathways exhibited a biphasic modulation. The amplitude of the Hoffman reflexes in the soleus and lateral gastrocnemius increased to 147.5 +/- 52.9% (P < 0.05) and 166.7 +/- 74.9% (P < 0.01), respectively, during the first 20% of the contraction and then subsequently declined to 66.3 +/- 44.8 and 74.4 +/- 44.2% of their initial values. In conclusion, the results show that antagonist coactivation did not contribute to task failure. The different changes in voluntary EMG activity and spinal reflex excitability in the antagonist muscles during the fatiguing contraction support the concept that the level of coactivation is controlled by supraspinal rather than spinal mechanisms. The findings indicate, however, that antagonist coactivation cannot simply be mediated by a central descending "common drive" to the motor neuron pools of the agonist-antagonist muscle pairs. Rather, they suggest a more subtle regulation of the drive, possibly through presynaptic mechanisms, to the motoneurons that innervate the antagonist muscles.Clinical TrialJournal ArticleResearch Support, Non-U.S. Gov'tinfo:eu-repo/semantics/publishe

The effect of knee joint angle on the coactivation of prepubertal boys and adult males

The purpose of this study was to investigate the influence of knee joint angle on the maximum moment and activation level of the knee extensors and flexors for untrained pre-pubertal boys (n=25; age 9.8±1.1) and adult males (n=25; age 29.6±5.0). Three maximal isometric voluntary knee extensions and flexions were performed at 3 different joint angles (5%, 50% and 95% of the maximal range of knee flexion), while the integrated averaged electromyographic (iaEMG) activity of the vastus medialis (VM), vastus lateralis (VL), and long head of biceps femoris (BF) muscles were recorded. The results indicated that absolute and relative torque was higher in adults compared to children. Agonist activity was not affected in both groups by joint angles. Antagonist activity was higher in extreme angles in both groups and higher in children in all selected joint angle. These findings suggest that the joint angles caused a deficit in neuromuscular performance of children and it could be attributed to their higher antagonist activity

Electromyographical examination of human muscle fatigue of maximal intensity : two different approaches

Etude, chez 7 hommes et 3 femmes (âge : 28 ans) de l'activité EMG du muscle soléaire lors d'une flexion plantaire isométrique d'intensité maximale selon deux méthodes : en fonction de la diminution de la force entre 100% et 50% par échelon de 10% et en fonction de l'endurance musculaire de 10 à 100% par échelon de 10

Submaximal fatigue and recovery in boys and men

We examined the effects of a sustained submaximal isometric contraction on fatigue and recovery rates in untrained prepubescent boys and men. Fifteen prepubescent boys and 15 men executed an isometric plantar flexion at 20% of their maximal voluntary contraction for 10 min. During the fatigue protocol, surface electromyogram of the soleus, medial gastrocnemius, and tibialis anterior muscles were obtained. Following the fatigue protocol, maximal voluntary contraction data were also obtained every 3 min throughout a 15-min recovery period. During the fatigue protocol, agonist and antagonist surface electromyogram increased gradually to a similar extent in both groups. Following fatigue, torque and surface electromyogram during a maximal voluntary contraction decreased compared to prefatigue values and recovered in a similar manner in both groups. However, boys showed faster recovery in torque and surface electromyogram during the third minute of recovery period. It is concluded that a low-intensity sustained isometric fatigue protocol induces similar fatigue levels in boys and men. However, there is evidence that boys can recover faster than men

Electromyographical examination of human muscle fatigue of maximal intensity : two different approaches

Etude, chez 7 hommes et 3 femmes (âge : 28 ans) de l'activité EMG du muscle soléaire lors d'une flexion plantaire isométrique d'intensité maximale selon deux méthodes : en fonction de la diminution de la force entre 100% et 50% par échelon de 10% et en fonction de l'endurance musculaire de 10 à 100% par échelon de 10