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This study examined the evolution of H-reflex and V-wave responses of soleus muscle during maximal voluntary plantar-flexor contraction. We also investigated the relationship between the V response and force level and between V-wave during maximal voluntary contraction (MVC) and the maximal H reflex at rest. The H-reflex and the V-wave responses are measures of motoneuron excitability and also reflect the magnitude of presynaptic inhibition on Ia afferents and the magnitude of descending motor drive. Both may be influenced by postsynaptic inhibition. Twenty male subjects participated in the study and were assigned to one of two groups. The maximal M wave (Mmax) was evoked at rest in the 20 subjects, who then performed 10 maximal voluntary contraction. During MCV performance, a stimulus was delivered at supra-maximal intensity, which allowed us to record the superimposed M wave (Msup) and V wave of the soleus muscle. These parameters were also recorded during sub-maximal contractions (20, 40, 60, 80% of one MVC) in 10 subjects. The maximal H reflex (Hmax), was evoked at rest in the other 10 subjects. These subjects then performed 10 MVC and the Hsup (superimposed H, evoked by means of stimulus at Hmax intensity) was recorded. The results show that the amplitude of maximal M wave increased during MVC (gain 44.52 ± 10.71%). No significant difference between Hmax/Mmax at rest and the Hsup/Msup ratios during MVC was observed, while an effect of force level on the V/Msup ratio was found. V/Msup and Hmax/Mmax were linearly correlated (r2 = 0.81), but V/Msup was significantly lower (P < 0.01) than Hmax/Mmax. In conclusion, the present study shows that maximal voluntary contractions potentiate some reflex responses. The V wave, which reflects motoneuron excitability presynaptic inhibition of Ia afferents and the magnitude of descending central motor drive to spinal motoneurons, may be a relatively simple method to analyse the modulation adaptive neural alterations at spinal and supraspinal level during voluntary contractions. [Copyright &y& Elsevier]

Abstract: The aim of this study was to determine whether patients with cystic fibrosis (CF), despite their ventilatory limitation, would develop neuromuscular fatigue of quadriceps muscles following a maximal cycling exercise. Eleven adults with CF (age=26.8±6.9years; forced expiratory volume in 1s=54.1±12.8% predicted) and 11 age-matched healthy subjects performed a maximal incremental cycle test with respiratory gas exchange measurements. Maximal voluntary contraction (MVC) and electromyographic (EMG) activity of the vastus medialis muscle were recorded before and after exercise. Neural and contractile properties of the quadriceps were also investigated using femoral nerve electrical stimulation. Patients had lower exercise capacity, peak oxygen uptake and MVC than controls. MVC fell significantly postexercise in both groups (CF: −20±10%, controls: −19±6%; p <0.01). EMG root mean square values, M-wave amplitude and duration were unchanged in both groups. Peak twitch (−46.9±13.8%), maximal rate of twitch torque development (−50.3±13.8%) and relaxation (−35.2±19.5%) were all reduced after exercise in CF patients. The control group followed the same pattern (−38.4±14.4%, −42.1±14.7% and −15±20.4%) but the statistical significance was not reached for the maximal rate of twitch torque relaxation. In conclusion, CF patients demonstrated lower limb fatigue following symptom-limited cycle exercise, which was comparable to that exhibited by healthy controls. This fatigue may be due to contractile impairments and not to transmission failure. Further studies should be conducted in a larger sample to confirm these preliminary results. [Copyright &y& Elsevier]