(VE), respiratory frequency (f), tidal volume (VT) and end-tidal PCO2 progressively

Transcription

1 Journal of Physiology (1988), 396, pp With 4 text-figures Printed in Great Britain EFFECTS OF PEDAL RATE ON RESPIRATORY RESPONSES TO INCREMENTAL BICYCLE WORK BY NARIKO TAKANO From the Physiology Laboratory, Department of School Health, Faculty of Education, Kanazawa University, Kanazawa 920, Japan (Received 7 May 1987) SUMMARY 1. The influence of pedal rate on ventilatory response and breathing pattern during cycle exercise was studied in twelve untrained female subjects performing 15 W/min incremental work on a bicycle at 30 and 60 r.p.m. Comparisons were made within the range of aerobic work rate to avoid additional influences of a developing lactic acidosis. 2. At each pedal rate, CO2 excretion (VC702) increased progressively to a level of 1-2 1/min with incrementally loaded cycling. With increasing fco, minute ventilation (VE), respiratory frequency (f), tidal volume (VT) and end-tidal PCO2 progressively increased. The inspiratory (TI) and expiratory (TE) durations decreased sharply on the transition from rest to unloaded cycling; further decreases occurred during incrementally loaded cycling. 3. Compared to 30 r.p.m., cycle exercise at 60 r.p.m. resulted in greater increases in VE and lower levels of end-tidal Pco2 at any given levels of Vco2. The greater ventilatory responses were due mostly to greater increases in f, which were in turn due to greater decreases in TE. The decrease in TI during cycling was little affected by changes in pedal rate. 4. The different magnitudes of ventilatory and Pco0 responses under the two pedal rate conditions suggest that neurogenic stimuli, central and/or peripheral in origin, participate in the control of exercise hyperpnoea in the non-steady-state phase. The possibility that the ventilatory response to cycle exercise is affected by the way that pedal rate is changed is discussed. INTRODUCTION It remains uncertain whether neurogenic influences, originating in the exercising limbs and/or cortical and subcortical regions, contribute to exercise hyperpnoea (see Dejours, 1964; Wasserman, Whipp & Casaburi, 1986). A way of challenging this problem in humans is to examine whether or not respiratory responses to limb movement can be altered with variations of movement frequency and/or force while humoral influences remain constant. Thus, it is hypothesized that if any neurogenic stimuli contribute to exercise hyperpnoea in humans, an increase in the frequency and/or force of the limb movement at a constant CO2 excretion rate (V00) will

2 390 N. TAKANO increase pulmonary ventilation (VE) and decrease arterial Pco2 (P. co2)* Some studies favour this hypothesis (Hanson, Claremont, Dempsey & Reddan, 1982; McMurray & Ahlborn, 1982; McMurray & Smith, 1985) while others do not (Sipple & Gilbert, 1966; Kay, Petersen & Vejby-Christensen, 1975a; Casaburi, Whipp, Wasserman & Koyal, 1978). Any concurrent change in the humoral stimuli during limb movement, such as development of metabolic acidosis in heavier exercise, leads to a fall in P. co2 and complicates the interpretation of VE and Pco2 responses. Previous investigations have been performed at work rates below the anaerobic threshold (AT) that is the work level at which blood lactate concentration begins to increase (Casaburi et al. 1978; McMurray & Ahlborn, 1982) or without taking this into account (Sipple & Gilbert, 1966; Kay et al. 1975a; McMurray & Smith, 1985). Recently, Coast, Cox & Welch (1986) and Hagberg, Mullin, Giese & Spitznagel (1981) have shown that the increase in blood lactate during heavy bicycle exercise (80 % V02 max' maximum rate of 02 uptake) is least at the optimal pedal rate at which the mechanical efficiency is maximal, while it is greater in either slower or faster pedalling. Coast & Welch (1985) also showed that the optimal pedal rate increases with work rate. These results seem to suggest that AT may be dependent on pedal rate, being highest at the optimal pedal rate. In addition, the optimal pedal rate varies among subjects, partly dependent on cycling experience (Coast et al. 1986). For the study of bicycle exercise at different pedal rates and at work rates below AT, determination of AT would be required for each pedal rate. On these lines, we reexamined ventilatory and PCO2 responses during cycle exercise at different pedal rates. In rhythmic exercise, the breathing pattern tends to be entrained to the rhythm of movement (Bechbache, Chow, Duffin & Orsini, 1979; Bramble & Carrier, 1983), the tendency possibly being higher in trained cyclists and runners (Kohl, Koller & Jager, 1981; Bramble & Carrier, 1983). The present study was performed on untrained female subjects. METHODS Subjects Twelve healthy female volunteers (age: years, weight: kg, height: cm) were studied. The subjects were ignorant of the precise purpose of the present study. They were trained on a mechanically braked cycle ergometer (Monark, Denmark) while wearing a respiratory mask on one occasion. All subjects gave informed consent. Incremental work test Each subject performed cycle exercise at two pedal rates (30 and 60 r.p.m.), for each of the pedal rates a different day being allotted. A 3 min rest period on the cycle ergometer was followed by 2 min unloaded cycling and then loaded cycling in which the work rate was increased progressively by 15 W every minute until the subject was exhausted. The pedal rate was paced by a metronome. The tests on each subject were performed at the same time of the day and at least 3 h after the last meal. The two test days were arranged to be in the same phase of the menstrual cycle (Dombovy, Bonekat, Williams & Staats, 1987). Measurements of respiratory and metabolic parameters Throughout the periods of rest and work, the subject breathed through a respiratory mask (dead space: 200 ml) which was connected to a hot-wire flowmeter for continuous measurement of

3 RESPIRATORY RESPONSE DURING BICYCLE EXERCISE respiratory flow. Respiratory gases were sampled continuously into a thin tube (1 mm i. d.) placed 1 cm within the nostril and passed to a gas analyser (Medical-gas analyser, MG-360, Minato Med. Sci. Co., Japan) for measurement of 02 (by a zirconium reaction) and CO2 (by infra-red absorption). Signals from the flowmeter and gas analyser were fed to a minicomputer (Respiromonitor, RM-200, Minato Med. Sci. Co., Japan), which was capable of computing, on a breath-by-breath basis, tidal volume (VT), respiratory frequency (f ), minute ventilation ( VE), inspiratory and expiratory 2-0 r I r.p.m. / I#11 I0' 60 r.p.m. 0*5 F ol Reest r 1.5[ 1 0oe 30 r.p.m. I 6,0 60 r.p.m. 0-5 Rest Work rate (W) Fig. 1. Relationships of VO. (lower panel) and VCO, (upper panel) to work rate during 15 W/min incremental bicycle work at two different pedal rates. Values are mean + S.E.M. of twelve subjects except at the higher work rates, for which the number of subjects is given beside the symbols. durations (TI and TE, respectively), 02 intake (VO2), CO2 excretion (Vco,) and end-tidal Pco. On computation of VO and Vco,, corrections were mace for transport delay and dynamic response of the gas analyser (9oguchi, Ogushi, Yoshiya, Itakura & Yamabayashi, 1982). The accuracy of measurement was within + 3 % of the reading for VT, ± 1% for end-tidal Pco, and + 5 % for Vco, and VO. In each subject, the average values for each variable were calculated using the breath-by-breath data obtained during the last 1 min at rest and during unloaded cycling, and during the last 20 s of loaded cycling at each work rate. The work rate just below that at which the respiratory compensation (RC) for metabolic acidosis occurred was determined according to the following criteria: (1) the VE/VCO0 value, having been declining, was steadily increasing and (2) the end-tidal PC02, having been slowly rising, was steadily falling, as the work rate was increased (Beaver, Wasserman & Whipp, 1986).

4 392 N. TAKANO Results for all subjects were obtained as mean + S.E.M. The responses at the two pedal rates were compared by a paired Student's t test. RESULTS Figure 1 shows the mean respiratory gas exchange rates during incremental work tests. At work rates above ca. 75 W, the efficiency (work rate/ fo2) was lower in E Eo 38 '- 1;- f F,;-w~ ~ ~ n 60 r.p.m. 34 OW Rest 301., ~~~~60r.p.m.4 mi / 3 ta 20r i0 r.p.m O Rest 0 0w / n m4 Vco, (I min-') Fig. 2. Relationship± of P/ (lower panel) and end-tidal PCO (PC upper panel) tod co within the aerobic Vcos range at two pedal rates. *, significant difference between the two pedal rates at P < 0-05, and *,at P < Values are mean + S.E.M. for the number of subjects studied at each level of Vco,. Twelve subjects were studied at Vco,, below 0-8 I/min, eight at 1-0 I/min and five at 1-2 I/min. 30 r.p.m. cycling than in 60 r.p.m. The mean work rate just below that at which RC occurred for twelve subjects was W (range:7ed120) in 30 r.p.m. cycling and 110c 5 W (range: ) in 60 r.p.m. Menycli at threetofivevdif renlevel tova 'aerobic range') were 1 a24 vau08/min (range: 0i88-1e69) in 30r.p.m. cycling and I/min (range: ) in 60 r.p.m. cycling. For individuals, the aerobicvo ranges were not always similar (within+0-03 I/min) under the two cycling conditions. For each subject, the respiratory data obtained -during 30 r.p.m. cycling were compared with those during 60 r.p.m. cycling at three to five different levels of fco2 within the aerobic range. When a fco2 value obtained in one cycling test differed by

5 RESPIRATORY RESPONSE DURING BICYCLE EXERCISE r.p.m r.p.m. Rest 0-5 w * 60 r.p.m. c 30 - E.-.30 r.p.m. 0 oẇ 20- Rest 10 a VCo (I min-,) Fig. 3. Relationships of f (lower panel) and VT (upper panel) to VC02. see Fig. 2. For explanations, VT/VC (%) 50 VCo (I min-') * r.p.m. 30 r.p.m. *+,P-.* 30 S 30 r.p.m. R05 60 r.p.m OW OW - Rest 10 Rest TE (S) T, (s) Fig. 4. VT-TI and VT-TE relations at rest and during unloaded cycling (O W) followed by incrementally loaded work at two pedal rates. The results during loaded work are shown VT values are normalized by vital capacity (VC). For significance level in relation to Vc 2. and the number of subjects at each level of Vc02, see Fig. 2. less than 006 1/min from that in the other test, the respiratory data at those two ico2 levels were compared. When fco2 in one cycling test differed by more than /min from that in the other, an intermediate value was calculated by averaging adjacent Vco2 data, and the respiratory data corresponding to such a calculated Vco2 level were used for comparison. Figures 2 and 3 show the results for respiratory variables plotted against VCo2. The

6 394 N. TAKANO resting values were similar for both runs. The increase of Vco2 with unloaded cycling at both pedal rates was associated with an increase of VE (Fig. 2), which was almost entirely due to an increased f (Fig. 3). During loaded cycling, all the respiratory variables increased progressively with Vco2. At all levels of V 60 r.p.m. cycling resulted in greater ventilatory responses (Fig. 2), which were largely due to greater increases in f (Fig. 3). End-tidal PCO. increased progressively with increasing VC02. However, the level of PCO2 was significantly lower at 60 r.p.m. than at 30 r.p.m., the difference progressively increasing from 1-7 to 3-4 mmhg with increasing VC02 from 0-3 to 12 1/min. Figure 4 shows changes in the VT-TI and VT-TE relations in association with the changes in VC02. At both pedal rates, TI and TE were reduced sharply with unloaded cycling; these changes were followed by a further shortening during loaded cycling. The magnitude of decrease during exercise was greater in TE than in TI. The decrease in TI during cycling was almost similar at 30 and 60 r.p.m., while the decrease in TE tended to be greater at 60 r.p.m. than at 30 r.p.m., the difference being significant at two higher levels of Vc02. DISCUSSION Effect of pedal rate on ventilatory and end-tidal PCO2 responses to bicycle exercise Since in the present study the work rate was increased every 1 min, the results were obtained in the non-steady state (phase II, Wasserman et al. 1986). Under this exercise condition, cycling at 60 r.p.m. produced greater ventilatory responses and lower end-tidal PCO2 at any given levels of aerobic VC02 than cycling at 30 r.p.m. (Fig. 2). The range of aerobic work rate was assessed by determining the RC point. In incremental work tests, the RC point is located above the AT point in terms of work rate. However, the initial linear relationship of VE vs. Vco2 is held up to the RC point, above which it changes to a steeper slope, due to respiratory compensation for metabolic acidosis (Wasserman et al. 1986). In the present study, end-tidal PCO2 was measured in place of Pa co. A question arises as to whether the difference in end-tidal Pco, between the two pedal rates is comparable to that in Pa C02 Previous studies have consistently shown that the difference between end-tidal and arterial PCO2 in exercise increases with increasing work rates and is predictable from VT, f and Vc02 (Jones, McHardy, Naimark & Campbell, 1966; Whipp & Wasserman, 1969; Jones, Robertson & Kane, 1979). A prediction of Pa co2 from our end-tidal PC02 data was made using the prediction equations described by Jones et al. (1966, 1979). Thus, estimated Pa co2 during cycling at 60 r.p.m. was mmhg lower than at 30 r.p.m. This difference is comparable to that in end-tidal PC02 (Fig. 2). The results of our own are in accordance with those of other workers, who studied treadmill running vs. walking in the steady state considering the development of metabolic acidosis in heavier work (Hanson et al. 1982) or without such consideration (McMurray & Ahlborn, 1982; McMurray & Smith, 1985). In contrast, Kay et al. (1975a) in steady-state exercise and Casaburi et al. (1978) during sinusoidal exercise found the ventilatory response during cycle exercise to be closely coupled to changes in VC02 but unaffected by changes in pedal rate. Although development of metabolic

7 RESPIRATORY RESPONSE DURING BICYCLE EXERCISE acidosis during limb movement might explain the disagreement among investigators, we suspect that different procedures for varying movement frequencies may be another reason. When the limb movement starts from rest, as in the studies of Hanson et al. (1982), McMurray & Ahlborn (1982), McMurray & Smith (1985) and our own, the ventilatory response to exercise may be greater, the higher the movement frequency. If so, a positive result regarding the effect of movement frequency would be obtainable in studies using the rest-cycling transition. On the other hand, when the movement frequency is varied more smoothly, sinusoidally as in the study of Casaburi et al. (1978) or between pedal rates of 50 and 70 r.p.m. as in the study of Kay, Petersen & Vejby-Christensen (1975a), the effect on ventilatory responses might not be appreciable. Influence of pedal rate on breathing pattern during cycle exercise In either cycling run of 30 and 60 r.p.m., marked decreases in TI and TE (Fig. 4) and hence a steep increase in f (Fig. 3) were seen in the rest-unloaded cycling transition. In the study of Bechbache et al. (1979), a similar result has been observed for subjects who entrained their f to pedal rate. No such decreases in TI and TE have been found in the study of Lind & Hesser (1984). These aspects led us to suspect that our subjects might have displayed an entrainment off to pedal rate. Except for the case of one subject during 60 r.p.m. cycling, in whichf was locked at 15 at lower levels of VC02 and 30 or more at higher Vco2, f during cycling at both pedal rates increased progressively with increasing VCo2 (Fig. 3), without having an integer or half-integer multiple relationship to pedal rate. Thus, the steep increase inf in the rest-unloaded cycling transition is likely to be due to a corticogenic drive but not due to any mechanism related to the entrainment. Entrainment has been reported to more readily occur by the use of a metronome for pacing (Bechbache & Duffin, 1977; Bechbache et al. 1979). However, it is unknown whether the metronome signal is responsible for the marked increase in f in the rest-unloaded cycling transition. Compared to 30 r.p.m. cycling, the greater increase in VE in 60 r.p.m. cycling was mostly due to the greater increase in f (Fig. 3), which was in turn due to the greater decrease in TE (Fig. 4). TI during cycling was less affected by the pedal rate change. In dog experiments, Agostoni & D'Angelo (1976) have demonstrated a greater decrease in TE than in TI with single electrical stimulation of the hindlimbs, suggesting a role of peripheral neurogenic stimuli in the control of respiratory durations during exercise. However, in the study of Kay et al. (1975 b), in which sharp decreases in TI and TE have been observed in the rest-cycling transition as in our study, VT-TI and VT-TE relations during steady-state cycle exercise were similar at two pedal rates of 50 and 70 r.p.m., contrary to our study. In their study, the protocol called for a transition of pedal rate from rest to 50 r.p.m., which was followed by 70 r.p.m. Again, the different ways of pedal rate change may explain the disagreement on TI and TE responses with pedal rate change between Kay et al. (1975b) and us. In conclusion, at a given level of aerobic Vco2 produced by 15 W/min incremental work on bicycle, a fast limb movement results in a greater ventilatory response, accompanying a lower end-tidal Pco2, compared to a slow limb movement. These 395

8 396 N. TAKANO changes would be due to an increase in neurogenic respiratory stimuli, central and/or peripheral in origin, with the fast limb movement. This suggests that in addition to humoral stimuli (Wasserman et al. 1986), neurogenic ones are involved in the control of exercise hyperpnoea in the non-steady-state phase (phase II). The ways of movement frequency change, whether a rest-movement transition or a slow movement-fast movement transition, may be important in affecting breathing pattern and ventilatory response during limb movement but this problem remains to be resolved. REFERENCES AGOSTONI, E. & D'ANGELO, E. (1976). The effect of limb movements on the regulation of depth and rate of breathing. Respiration Physiology 27, BEAVER, W. L., WASSERMAN, K. & WHIPP, B. J. (1986). A new method for detecting anaerobic threshold by gas exchange. Journal of Applied Physiology 60, BECHBACHE, R. R., CHOW, H. H. K., DUFFIN, J. & ORSINI, E. C. (1979). The effects of hypercapnia, hypoxia, exercise and anxiety on the pattern of breathing in man. Journal of Physiology 293, BECHBACHE, R. R. & DUFFIN, J. (1977). The entrainment of breathing frequency by exercise rhythm. Journal of Physiology 272, BRAMBLE, D. M. & CARRIER, D. R. (1983). Running and breathing in mammals. Science 219, CASABURI, R., WHIPP, B. J., WASSERMAN, K. & KOYAL, S. N. (1978). Ventilatory and gas exchange responses to cycling with sinusoidally varying pedal rate. Journal of Applied Physiology 44, COAST, J. R., Cox, R. H. & WELCH, H. G. (1986). Optimal pedalling rate in prolonged bouts of cycle ergometry. Medicine and Science in Sports and Exercise 18, COAST, J. R. & WELCH, H. G. (1985). Linear increase in optimal pedal rate with increased power output in cycle ergometry. European Journal of Applied Physiology 53, DEJOURS, P. (1964). Control of respiration in muscular exercise. In Handbook of Physiology, section 3: Respiration, vol. I, ed. FENN, W. 0. & RAHN, H., pp Washington D.C.: American Physiological Society. DOMBOVY, M. L., BONEKAT, H. W., WILLIAMS, T. J. & STAATS, B. A. (1987). Exercise performance and ventilatory response in the menstrual cycle. Medicine and Science in Sports and Exercise 19, HAGBERG, J. M., MULLIN, J. P., GIESE, M. D. & SPITZNAGEL. E. (1981). Effect of pedalling rate on submaximal exercise responses of competitive cyclists. Journal of Applied Physiology 51, HANSON, P., CLAREMONT, A., DEMPSEY, J. & REDDAN, W. (1982). Determinants and consequences of ventilatory responses to competitive endurance running. Journal of Applied Physiology 52, JONES, N. J., MCHARDY, G. J. R., NAIMARK, A. & CAMPBELL, E. J. M. (1966). Physiological dead space and alveolar-arterial gas pressure differences during exercise. Clinical Science 31, JONES, N. J., ROBERTSON, D. G. & KANE, J. W. (1979). Difference between end-tidal and arterial Pco2 in exercise. Journal of Applied Physiology 47, KAY, J. D. S., PETERSEN, E. S. & VEJBY-CHRISTENSEN, H. (1975a). Breathing in man during steady-state exercise on the bicycle at two pedalling frequencies, and during treadmill walking. Journal of Physiology 251, KAY, J. D. S., PETERSEN, E. S. & VEJBY-CHRISTENSEN, H. (1975b). Mean and breath-by-breath pattern of breathing in man during steady state exercise. Journal of Physiology 251, KOHL, J., KOLLER, E. A. & JAGER, M. (1981). Relationship between pedalling and breathing rhythm. European Journal of Applied Physiology 47, LIND, F. & HESSER, C. M. (1984). Breathing pattern and lung volumes during exercise. Acta physiologica scandinavica 120,

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