***TASK*** the task is to summarize an input biomedical literature in six sentences 
    ***INPUT*** the input is a biomedical literature 
    ***OUTPUT*** the output is the summary of an input biomedical literature in six sentences 
    ***DOCUMENTATION*** 
    ***EXAMPLES*** 
    Input: The normal respiratory muscle effort at maximal exercise requires a significant fraction of cardiac output and causes leg blood flow to fall . We question ed whether the high levels of respiratory muscle work experienced in heavy exercise would affect performance . Seven male cyclists [ maximal O(2 ) consumption ( VO(2 ) ) 63 + /- 5 ml . kg(-1 ) . min(-1 ) ] each completed 11 r and omized trials on a cycle ergometer at a workload requiring 90 % maximal VO(2 ) . Respiratory muscle work was either decreased ( unloading ) , increased ( loading ) , or unchanged ( control ) . Time to exhaustion was increased with unloading in 76 % of the trials by an average of 1.3 + /- 0.4 min or 14 + /- 5 % and decreased with loading in 83 % of the trials by an average of 1.0 + /- 0.6 min or 15 + /- 3 % compared with control ( P < 0.05 ) . Respiratory muscle unloading during exercise reduced VO(2 ) , caused hyperventilation , and reduced the rate of change in perceptions of respiratory and limb discomfort throughout the duration of exercise . These findings demonstrate that the work of breathing normally incurred during sustained , heavy-intensity exercise ( 90 % VO(2 ) ) has a significant influence on exercise performance . We speculate that this effect of the normal respiratory muscle load on performance in trained male cyclists is due to the associated reduction in leg blood flow , which enhances both the onset of leg fatigue and the intensity with which both leg and respiratory muscle efforts are perceived Fatigue of the respiratory muscles during intense exercise might compromise leg blood flow , thereby constraining oxygen uptake ( Vo(2 ) ) and limiting exercise tolerance . We tested the hypothesis that inspiratory muscle training ( IMT ) would reduce inspiratory muscle fatigue , speed Vo(2 ) kinetics and enhance exercise tolerance . Sixteen recreationally active subjects ( mean + or - SD , age 22 + or - 4 yr ) were r and omly assigned to receive 4 wk of either pressure threshold IMT [ 30 breaths twice daily at approximately 50 % of maximum inspiratory pressure ( MIP ) ] or sham treatment ( 60 breaths once daily at approximately 15 % of MIP ) . The subjects completed moderate- , severe- and maximal-intensity " step " exercise transitions on a cycle ergometer before ( Pre ) and after ( Post ) the 4-wk intervention period for determination of Vo(2 ) kinetics and exercise tolerance . There were no significant changes in the physiological variables of interest after Sham . After IMT , baseline MIP was significantly increased ( Pre vs. Post : 155 + or - 22 vs. 181 + or - 21 cmH(2)O ; P < 0.001 ) , and the degree of inspiratory muscle fatigue was reduced after severe- and maximal-intensity exercise . During severe exercise , the Vo(2 ) slow component was reduced ( Pre vs. Post : 0.60 + or - 0.20 vs. 0.53 + or - 0.24 l/min ; P < 0.05 ) and exercise tolerance was enhanced ( Pre vs. Post : 765 + or - 249 vs. 1,061 + or - 304 s ; P < 0.01 ) . Similarly , during maximal exercise , the Vo(2 ) slow component was reduced ( Pre vs. Post : 0.28 + or - 0.14 vs. 0.18 + or - 0.07 l/min ; P < 0.05 ) and exercise tolerance was enhanced ( Pre vs. Post : 177 + or - 24 vs. 208 + or - 37 s ; P < 0.01 ) . Four weeks of IMT , which reduced inspiratory muscle fatigue , result ed in a reduced Vo(2 ) slow-component amplitude and an improved exercise tolerance during severe- and maximal-intensity exercise . The results indicate that the enhanced exercise tolerance observed after IMT might be related , at least in part , to improved Vo(2 ) dynamics , presumably as a consequence of increased blood flow to the exercising limbs Inspiratory muscle training ( IMT ) has been shown to improve time trial performance in competitive athletes across a range of sports . Surprisingly , however , the effect of specific IMT on surface swimming performance remains un-investigated . Similarly , it is not known whether any ergogenic influence of IMT upon swimming performance is confined to specific race distances . To determine the influence of IMT upon swimming performance over 3 competitive distances , 16 competitive club-level swimmers were assigned at r and om to either an experimental ( pressure threshold IMT ) or sham IMT placebo control group . Participants performed a series of physiological and performance tests , before and following 6 weeks of IMT , including ( 1 ) an incremental swim test to the limit of tolerance to determine lactate , heart rate and perceived exertion responses ; ( 2 ) st and ard measures of lung function ( forced vital capacity , forced expiratory volume in 1 s , peak expiratory flow ) and maximal inspiratory pressure ( MIP ) ; and ( 3 ) 100 , 200 and 400 m swim time trials . Training utilised a h and -held pressure threshold device and consisted of 30 repetitions , twice per day . Relative to control , the IMT group showed the following percentage changes in swim times : 100 m , −1.70 % ( 90 % confidence limits , ±1.4 % ) , 200 m , −1.5 % ( ±1.0 ) , and 400 m , 0.6 % ( ±1.2 ) . Large effects were observed for MIP and rates of perceived exertion . In conclusion , 6 weeks of IMT has a small positive effect on swimming performance in club-level trained swimmers in events shorter than 400 Background Whether or not isolated endurance training of the respiratory muscles improves whole-body endurance exercise performance is controversial , with some studies reporting enhancements of 50 % or more , and others reporting no change . Twenty fit ( VO2 max 56.0 ml/kg/min ) , experienced cyclists were r and omly assigned to three groups . The experimental group ( n = 10 ) trained their respiratory muscles via 20 , 45 min sessions of hyperpnea . The placebo group ( n = 4 ) underwent " sham " training ( 20 , 5 min sessions ) , and the control group ( n = 6 ) did no training . Results After training , the experimental group increased their respiratory muscle endurance capacity by 12 % . Performance on a bicycle time trial test design ed to last about 40 min improved by 4.7 % ( 9 of 10 subjects showed improvement ) . There were no test-re-test improvements in either respiratory muscle or bicycle exercise endurance performance in the placebo group , nor in the control group . After training , the experimental group had significantly higher ventilatory output and VO2 , and lower PCO2 , during constant work-rate exercise ; the placebo and control groups did not show these changes . The perceived respiratory effort was unchanged in spite of the higher ventilation rate after training . Conclusions The results suggest that respiratory muscle endurance training improves cycling performance  in fit , experienced cyclists . The relative hyperventilation with no change in respiratory effort sensations suggest that respiratory muscle training allows subjects to tolerate the higher exercise ventilatory response without more dyspnea . Whether or not this can explain the enhanced performance is unknown BACKGROUND --The sensation of respiratory effort may increase as expiratory muscles become fatigued during expiratory loading . A study was performed to determine whether expiratory muscle training ( EMT ) affects the sensation of respiratory effort during exercise in healthy subjects . METHODS --Six subjects performed EMT for 15 minutes twice daily for four weeks using a pressure threshold device ;  another six subjects served as a control group .  The expiratory threshold was set at 30 % of the individual 's maximum expiratory mouth pressure ( PEmax ) . The sensation of respiratory effort was evaluated during a progressive exercise test using the Borg scale . RESULTS --After EMT  PEmax increased by 25 % in the training group . The Borg score increased as exercise grade increased before and after EMT , but scores for each grade were lower after EMT .  Minute ventilation during exercise decreased after EMT , as did the breathing frequency during exercise , while the expiratory time increased . Although there was no difference in the relationship between Borg score and minute ventilation before or after EMT , the curve shifted to a lower Borg score after EMT . There were no changes in PEmax , Borg score , minute ventilation , or breathing pattern after the four week study period in the control group . CONCLUSION --These findings suggest that EMT increases expiratory muscle strength and reduces the sensation of respiratory effort during exercise , presumably by reducing minute ventilation Several studies have reported that improvements in endurance performance following respiratory muscle training ( RMT ) are associated with a decrease in blood lactate concentration ( [Lac]B ) . The present study examined whether pressure threshold inspiratory muscle training ( IMT ) elicits an increase in the cycling power output corresponding to the maximum lactate steady state ( MLSS ) . Using a double-blind , placebo-controlled design , 12 healthy , non-endurance-trained male participants were assigned in equal numbers to an experimental ( IMT ) or sham training control ( placebo ) group . Cycling power output at MLSS was initially identified using a lactate minimum protocol followed by a series of constant power output rides ( 2.5 % increments ) of 29.5 min duration ; MLSS was reassessed following six weeks of IMT or sham IMT . Maximum inspiratory mouth pressure increased significantly ( 26 % ) in the IMT group , but remained unchanged in the placebo group . The cycling power output corresponding to MLSS remained unchanged in both groups after the intervention . After IMT , [Lac]B decreased significantly at MLSS power in the IMT group [ −1.17 ( 1.01 ) mmol l−1 after 29.5 min of cycling ; mean ( SD ) ] , but remained unchanged in the placebo group [ + 0.37 ( 1.66 ) mmol l−1 ] . These data support previous observations that IMT results in a decrease in [Lac]B at a given intensity of exercise . That such a decrease in [Lac]B was not associated with a substantial ( > 2.5 % ) increase in MLSS power is a new finding suggesting that RMT-induced increases in exercise tolerance and reductions in [Lac]B are not ascribable to a substantial increase in the ‘ lactate threshold ’ Abstract . Isolated respiratory muscle endurance training ( RMT ) can prolong constant-intensity cycling performance . We tested whether RMT affects O2 supply during exercise , i.e. whether the partial pressure of oxygen in arterial blood ( PaO2 ) and /or its oxygen saturation ( SaO2 ) are higher during exercise after RMT than before . A group of 28 sedentary subjects were r and omly assigned to either an RMT ( n=13 ) or a control group ( n=15 ) . The RMT consisted of 40 × 30 min sessions of normocapnic hyperpnoea . The control group did not perform any training . Breathing and cycling endurance time as well as PaO2 and SaO2 during cycling at a constant intensity of 70 % maximum power output were measured before and after the RMT or the control period . Mean breathing endurance increased significantly after RMT compared to control [ RMT 5.2 ( SD 2.9 ) vs 38.1 ( SD 6.8 ) min , control 6.5 ( SD 5.7 ) vs 6.4 ( SD 7.6 ) min ; P<0.01 ] , as did mean cycling endurance [ RMT 35.6 ( SD 11.9 ) vs 44.0 ( SD 17.2 ) min , control 32.8 ( SD 11.6 ) vs 31.4 ( SD 14.4 ) min ; P<0.05 ] . The RMT did not affect PaO2 which ranged from 11.6 to 12.3 kPa ( 87–92 mmHg ) , and SaO2 which ranged from 96 % to 98 % throughout all tests . In conclusion , RMT  substantially increased breathing and cycling endurance in sedentary subjects . These changes , however , can not be attributed to increased O2 supply , as neither PaO2 nor SaO2 were increased during exercise after RMT We evaluated the effects of specific inspiratory muscle training on simulated time-trial performance in trained cyclists . Using a double-blind , placebo-controlled design , 16 male cyclists ( VO 2max = 64 - 2 ml·kg -1 · min -1 ; mean - sx ¥ ) were assigned at r and om to either an experimental ( pressure-threshold inspiratory muscle training ) or sham-training control ( placebo ) group  . Pulmonary function , maximum dynamic inspiratory muscle function and the physiological and perceptual responses to maximal incremental cycling were assessed . Simulated time-trial performance ( 20 and 40 km ) was quantified as the time to complete pre-set amounts of work . Pulmonary function was unchanged after the intervention , but dynamic inspiratory muscle function improved in the inspiratory muscle training group ( P h 0.05 ) . After the intervention , the inspiratory muscle training group experienced a reduction in the perception of respiratory and peripheral effort ( Borg CR10 : 16 - 4 % and 18 - 4 % respectively ; compared with placebo , P h 0.01 ) and completed the simulated 20 and 40 km time-trials faster than the placebo group [ 66 - 30 and 115 - 38 s ( 3.8 - 1.7 % and 4.6 - 1.9 % ) faster respectively ; P = 0.025 and 0.009 ] . These results support evidence that specific inspiratory muscle training attenuates the perceptual response to maximal incremental exercise . Furthermore , they provide evidence of performance enhancements in competitive cyclists after inspiratory muscle 
    Output:
    
RMT improves endurance exercise performance in healthy individuals with greater improvements in less fit individuals and in sports of longer duration s. The two most common types of RMT ( inspiratory muscle strength and respiratory muscle endurance training ) do not differ significantly in their effect , while combined inspiratory/expiratory strength training might be superior .
Improvements are similar between different types of sports .
Changes in performance can be detected by constant load tests , time trials and intermittent incremental tests only .