Changes in single motor unit behaviour contribute to the increase in contraction speed after dynamic training in humans.

1. The adaptations of the ankle dorsiflexor muscles and the behaviour of single motor units in the tibialis anterior in response to 12 weeks of dynamic training were studied in five human subjects. In each training session ten series of ten fast dorsiflexions were performed 5 days a week, against a load of 30-40% of the maximal muscle strength. 2. Training led to an enhancement of maximal voluntary muscle contraction (MVC) and the speed of voluntary ballistic contraction. This last enhancement was mainly related to neural adaptations since the time course of the muscle twitch induced by electrical stimulation remained unaffected. 3. The motor unit torque, recorded by the spike-triggered averaging method, increased without any change in its time to peak. The orderly motor unit recruitment (size principle) was preserved during slow ramp contraction after training but the units were activated earlier and had a greater maximal firing frequency during voluntary ballistic contractions. In addition, the high frequency firing rate observed at the onset of the contractions was maintained during the subsequent spikes after training. 4. Dynamic training induced brief (2-5 ms) motor unit interspike intervals, or 'doublets'. These doublets appeared to be different from the closely spaced (+/-10 ms) discharges usually observed at the onset of the ballistic contractions. Motor units with different recruitment thresholds showed doublet discharges and the percentage of the sample of units firing doublets was increased by training from 5.2 to 32.7%. The presence of these discharges was observed not only at the onset of the series of spikes but also later in the electromyographic (EMG) burst. 5. It is likely that earlier motor unit activation, extra doublets and enhanced maximal firing rate contribute to the increase in the speed of voluntary muscle contraction after dynamic training.

Mechanical properties and behaviour of motor units in the tibialis anterior during voluntary contractions.

The present work was carried out to analyse the properties and behaviour of Tibialis anterior motor units (MUs) during voluntary contractions in humans. A total of 528 single MU mechanical properties was recorded in 10 subjects by means of the spike-triggered averaging (STA) technique. MU recruitment thresholds and discharge frequencies were recorded during linearly increasing maximal voluntary contraction (MVC). The results indicate a mean (+/- SD) MU torque of 25.5 +/- 21.5 mN.m. and a mean time-to-peak of 45.6 +/- 13.6 ms. A comparison of the average MU twitch torque with that of the muscle allowed an estimate of about 300 MUs in the Tibialis anterior. A positive linear relationship was recorded between the MU twitch torque and the recruitment threshold. The mean minimal and maximal discharge frequencies of MUs were 8.4 +/- 3.0 Hz and 33.2 +/- 14.7 Hz, respectively. The results of the present work indicate that MU behaviour during voluntary contractions is different in the tibialis anterior and in the adductor pollicis.

Determinants of right ventricular failure after heart transplantation.

Predictive factors of right ventricular failure after heart transplantation are not well identified. Clinical and hemodynamic data from 20 patients who developed right heart failure were compared to those of 20 matched patients who did not experience this complication after cardiac transplantation. Preoperative systemic and pulmonary hemodynamics were comparable in the two groups. Patients with posttransplant right ventricular failure had longer waiting time (27 +/- 6 vs 16 +/- 3 weeks, mean +/- SE, P < 0.05), no regression of pulmonary hypertension (0 +/- 0.1 vs 2.3 +/- 0.3 Wood units reduction in pulmonary vascular resistance after transplantation, P < 0.01), and had been ventilated with higher levels of positive end-expiratory pressure (5 +/- 1 vs 1.5 +/- 0.5 cm H2O, P < 0.05). One-month postoperative evolution (mortality, hospital stay, radionuclide ejection fractions) was similar in the two groups. These results suggest that a lesser reversibility of pulmonary hypertension (possibly due to a longer evolution of the cardiac disease, as indicated by the longer waiting time) is the main determinant of right ventricular failure after heart transplantation.