The overall activity of the inspiratory muscles can be evaluated by different invasive techniques. First the tension-time diaphragmatic index ([TT.sub.DI]) proposed by Bellemare and Grassino (Bellemare and Grassino, 1982; 1983) assesses the activity of the diaphragm during breathing given by [TT.sub.DI] = [P.sub.DI]/[P.sub.DImax] X Ti/[T.sub.TOT] where [P.sub.DI] is the transdiaphragmatic pressure, [P.sub.DImax] the maximal transdiaphragmatic pressure and [T.sub.I]/[T.sub.TOT] the duty cycle. A more direct measurement is the analysis of the time and frequency contents of the electromyographic signal. Both techniques are invasive and involve the use of gastric and oesophageal balloon and oesophageal electrodes respectively. A simplified non-invasive index obtained from the mouth occlusion pressure ([P.sub.0.1]) is proposed as an alternative method. This non-invasive tension-time index, validated during exercise in healthy subjects, is derived from the equation [T.sub.T0.1] = [P.sub.0.1]/[P.sub.Imax] x [T.sub.I]/[T.sub.TOT], where [P.sub.Imax] is the maximal inspiratory pressure and [T.sub.I]/[T.sub.TOT] is the duty cycle (Hayot et al., 2000).
The efficiency of the respiratory muscle during exercise is much debated in the literature. Although some authors have argued that respiratory muscle performance does not limit maximal incremental exercise tolerance in healthy trained and untrained adults (Coast et al., 1990; Hayot et al., 2000), heavy submaximal exercise has been shown to impair respiratory muscle performance in humans (Johnson et al., 1993; Mador et al., 1993). To the best of our knowledge, no study has drawn the comparison between these two techniques. The present study was aimed to measure [T.sub.T0.1] and surface EMG of inspiratory muscles of trained cyclists during a maximal incremental exercise, not to assess fatigue, but to check if [T.sub.T0.1] could also be a reliable index of the inspiratory muscle activity and recruitment. We reasoned that the [T.sub.T0.1] could also be a reliable index of the inspiratory muscle activity and recruitment. Though the activity of inspiratory accessory muscle is more and more important to the genesis of increased pressures it was not reflected by [EMG.sub.di]. To assess the overall activity of the inspiratory muscle, as the [T.sub.T0.1], we used the surface EMG (SEMG) and the aim of our study was to determine whether tension-time index reflects the inspiratory muscle activity and recruitment.
Eight trained males cyclist took part in this study. The experimental procedures complied with the ethical standards of the 1975 Helsinki Declaration and approval was received from the appropriate local institutional review board. All qualified participants were familiarized to exercise on the cycle ergometer and instructed to avoid exercise, food, and caffeine for at least 2 h prior to exercise testing. All subjects were assigned to a maximal exercise testing session with tension-time index and SEMG measurements. The anthropometric, spirometry and maximal inspiratory pressures are listed in the table 1. The ATS spirometry interpretation workshop only states that subjects should be "never-smokers, free of respiratory symptoms and disease" (Johannessen et al., 2007; Redlich et al., 2014). Subjects not meeting these guidelines were excluded.
[P.sub.0.1] is the maximum pressure developed during a spontaneous respiratory effort during a 100 ms occlusion at the beginning of inspiration (Kera et al., 2013; Whitelaw et al., 1975). Its timing is such that it is not influenced by the conscious response to occlusion. Moreover, as this index is derived from the ventilatory drive it has the advantage of being independent of the mechanical properties of the lung (Kera et al., 2013). [P.sub.0.1] a valid index of neural output was assessed at the level of functional residual capacity (FRC) (Whitelaw et al., 1975). Subjects were asked to breathe quietly, with the nose occluded, through a mouthpiece connected to the pneumotachograph (Fleish Lausanne, Switzerland) with a two-way low- resistance breathing valve (0.9 cm[H.sub.2]Ox[L.sup.-1] s, dead space of 50 ml, model 9340 occlusion valve, Hans Rudolph inc, Kansas City, Missouri, U.S.A). During the exhalation phase of breathing, a balloon was rapidly inflated in the inspiratory limb of the breathing circuit to occlude the subsequent inspiratory flow. It was closed during expiration and automatically opened about 150 ms after the onset of the subsequent inspiration. Mouth occlusion pressure ([P.sub.0.1]) was measured with a differential pressure transducer (Druck, LPM 9000 series, [+ or -] 50 cm[H.sub.2]O, Leicester, England). The balloon was inflated with helium from a small gas cylinder, and the valve was controlled manually with a small switch. The subject was asked to continue to breathe normally despite the occlusions. Throughout manoeuvres mentioned previously the subject wore headphones and listened to music to dampen any noise from the switching device controlling the balloon, and could see neither the occlusion valve nor the operator and therefore, was unable to anticipate the airway occlusion. The Lab view interface (Lab view, National Instruments Corporation, Austin, Texas, U.S.A.) that provided a visual feedback was used to identify the onset of inspiration. At rest, manoeuvres were made until five technically satisfactory and reproducible measurements were obtained (variation
Maximal inspiratory pressure
Maximum inspiratory pressure ([P.sub.Imax]) is the force that respiratory muscles are able to generate during an occlusive manoeuvre at prefixed volume (Hautmann et al., 2000). Maximal inspiratory pressure ([P.sub.Imax]) was measured at the functional residual capacity (FRC) (the effect of variation of muscle...