Changes in ventilation related to changes in ... - Springer Link

Eur J Appl Physiol (1996) 72:195-203

© Springer-Verlag 1996

I. V o g i a t z i s • N . C . S p u r w a y • S. J e n n e t t • J. W i l s o n J. S i n c l a i r

Changes in ventilation related to changes in electromyograph activity during repetitive bouts of isometric exercise in simulated sailing

Accepted: 24 July 1995

This study examined the control of ventilation during repetitive bouts of isometric exercise in simulated sailing. Eight male sailors completed four successive 3-rain bouts of similar isometric effort on a dinghy simulator; bouts were separated by 15-s rest intervals. Quadriceps muscle integrated electromyograph activity (iEMG) was recorded during each bout and expressed as a percentage of activity during maximal voluntary contraction (%iEMGmax). From the first to the fourth bout, the 3-min mean averages for ventilation and for %iEMGmax increased from 19.8 (SEM 1.1) to 37.5 (SEM 3.0) l m i n -1 and from 31 (SEM 4) to 39 (SEM 4)% respectively; also, ventilation and %iEMGm~x over each minute throughout the four bouts were significantly correlated (r = 0.85; P < 0.05). Progressive hyperventilation reduced the mean endtidal partial pressure of carbon dioxide from 5.0 (SEM 0.3) kPa during bout 1 to 4.3 (SEM 0.4)kPa during bout 4 [-37.7 (SEM 2.0) to 32.4 (SEM 3.0)mmHg]. From the first to the fourth bout the end-of-bout blood lactate concentration did not increase significantly although the concentration from the third bout onwards was significantly greater than at rest. The results suggested that the development of muscle fatigue, which was enhanced by the insufficiency of recovery during the 15-s intervals and mirrored in the progressive increase in iEMG, was linked with stimuli causing progressive hyperventilation. Though these changes in ventilation and iEMG could not be associated with changes in blood lactate concentration, they could both have been related to accumulating metabolites within the muscles themselves. Abstract

I. Vogiatzis • N.C. Spurway - S. Jennett. J. Wilson - J. Sinclair Institute of Biomedical and Life Sciences, West Medical Building, University of Glasgow, Glasgow G12 8 QQ, Scotland, UK I. Vogiatzis ([E~) School of Health Sciences, University of Sunderland, Chester Road, Sunderland SR 1 3 SD, England, U K

Isometric exercise Surface electromyography • Hyperventilation • Lactate

Key words Sailing •

Introduction The major physical challenge of dinghy sailing is the effort required to keep the boat upright in strong winds. This is mainly achieved by hooking the feet under straps attached to the bottom of the hull, supporting the back of the legs on the deck and suspending the rest of the body over the water (hiking). Hikin9 is a different kind of physical exercise from those which predominate in almost every other sport, in that it imposes essentially static stresses on quadriceps, abdominal and other muscle groups for periods of many minutes with only a few short intervals of relief. (These most commonly occur as the sailor crosses the boat when tacking). Consistent with the isometric nature of hiking, published reports from simulated sailing investigations (Harrison et al. 1988; Spurway and Burns 1993; Blackburn 1994) have shown that relatively large increases in mean arterial blood pressure (BPa) and heart rate (HR) were accompanied by modest increments in oxygen uptake (1)O2). Recently, measurements under actual sailing conditions in strong winds have revealed that, in addition to high HR, minute ventilation (I/E) was also disproportionately high in relation to 1/O2 (Vogiatzis et al. 1994). Physiological studies over the last quarter-century (e.g. Myhre and Andersen 1971; Goodwin et al. 1972; Duncan et al. 1981; Muza et al. 1983) have suggested that the hyperventilation accompanying isometric exercise may be due both to reflexes arising within the exercising muscles and to central command. More recently, Poole et al. (1988) have provided evidence that although hyperventilation during the period of sustained contraction could not be attributed to exerciseinduced lactacidaemia, a reduction in pH contributes to recurrent hyperventilation very shortly after the contraction ceases.

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However, in most of the above studies isometric effort has been sustained in a single muscle group until local muscle exhaustion, whilst hiking uses multiple muscle masses in repetitive bouts of moderate isometric effort. Re-examination of the mechanisms in this applied context therefore seemed appropriate. The purpose of the present study was to investigate the pattern of changes in I)E, blood lactate concentration ([la-]b), electromyograph (EMG) activity and the rating of perceived exertion (RPE) associated with the muscle effort during a series of rapidly-successive hiking bouts. It was hypothesised that incomplete recovery during the brief intervals between the successive bouts would enhance the degree of muscle fatigue during each subsequent bout and that the cumulative effects of fatigue would have a major effect upon ventilation during the hikin9 exercise. Since standardisation of environmental conditions for all subjects was impossible on the water, hiking was performed on a laboratory dinghy simulator in a way that closely mimicked actual sailing. A preliminary account of this work has been presented as a poster to the Physiological Society (Vogiatzis et al. 1995).

Fig. 1 Diagram of the dinghy simulator used to study the isometric posture sailors adopt while sailing

simulated trimming of the sail by applying additional force, via a nylon rope and spring-loaded block assembly, to the end of the deck supporting the weight system. A computer program provided visual information to the subjects to assist them in maintaining the simulator level and the force exerted on the strap constant.

Experimental protocol

Methods Subjects Eight male sailors of Laser-class dinghies (members of the Scottish National Squad) gave informed consent to participation in this study, which had received approval by the local Ethics Committee. The subjects' mean age, height and body mass were 23 (SEM 2) years, 1.78 (SEM 0.05) m and 74 (SEM 4) kg, respectively. Each subject practised on the equipment to become familiar with the required procedure and to establish optimal settings for his individual physique, 5 days prior to undertaking the experimental protocol.

Simulation procedure A dinghy simulator (custom built at Glasgow University), together with a computer-generated protocol, was used to simulate in the laboratory the principal physical demands of single-handed dinghy sailing. The deck of the simulator was constructed using the same dimensions as that of a Laser dinghy, so that the posture normally adopted by the subjects during actual sailing could be closely reproduced. By applying a load to the opposite side of the simulator from the subject and using,a spring damper mechanism, the simulator was made to pivot about a central point. The appropriate load was determined during the familiarisation tests and was based on the relationship between each subject's body mass and the applied load which balanced it when the simulator was level. This relationship was found to be linear (r = 0.96, P < 0.05) and the regression equation used to determine the optimal load was: y = - 23.5 + 0.595x, where y is the predicted load and x is each subject's body mass. The subjects were required to maintain the simulator level by hooking their feet under a toe-strap and by holding their bodies over the edge of the deck (hiking), thus balancing the heeling moment (i.e. the tendency of the simulated boat to tip) (Fig. 1). A load-cell amplifier (attached to the toe-strap) monitored the force applied at this point and the output was recorded on a pen recorder. The subjects also

Each subject was asked to complete four successive 3-rain hiking bouts, separated by approximately 15-s intervals to simulate tacking. Simulation of tacking involved sitting in from the hiking posture and 15 s later moving back out to a full hiking position. While hiking, the subjects were required to keep the force exerted on the toe-strap as constant as possible (and so keep the boat effectively level) and to reproduce the same force during all four bouts.

Measurements of respiratory variables The subjects breathed through a mouthpiece attached to a low resistance respiratory valve (Hans-Rudolph 2600). Expired gases were collected for successive 3-min periods into Douglas bags. Immediately after collection fractional concentrations of 02 and CO2 from each gas sample were analysed using a paramagnetic 02 analyser (Servomex 570, Sussex) and an infrared CO2 analyser (P.K. Morgan, Kent). A dry gas meter (Parkinson Cowan, London) was used to measure the volume of expired gas, which was corrected to standard temperature and pressure, dry, These values were used to calculate the mean values of I)~, 1/O2 and the ventilatory equivalent for oxygen (12E/I)O2) for each period of 3-min: immediately before the start of exercise, throughout each of the four bouts, and during the first 3 min of recovery. Percentage CO2 at the mouth was measured throughout by drawing a continuous sample from the mouthpiece through a rapid CO2 analyser (Instrumentation Laboratory Inc. End-rid IL200, Warrington). The end-tidal percentages were converted to partial pressures (PETCO2) by correcting for water vapour and barometric pressure. A flow head placed on the inspiratory side of the valve allowed breath-by-breath integration of inspired volume by pneumotachography (Computing Spirometer CS7, Mercury Electronics, Glasgow). Cumulative inspired volume (V0 and %CO2 were recorded throughout on the same multichannel recorder as EMG activity (see below). Inspired minute volume (l/i) was calculated from cumulative V1 over 15-s periods. All instruments for gas analysis were calibrated before and after each experiment with appropriate standard gas mixtures (British Oxygen Company).

197 EMG sampling and analysis The E M G activity was recorded from rectus femoris muscle (RF) of the right leg and biceps brachii muscle (BB) (short head) of the right arm respectively. The EMG activity was picked up by pairs of skin electrodes with contact areas of 65 mm 2, placed 15 mm apart in a line parallel to the muscle axis. After the skin had been carefully prepared for their placement, the electrodes were filled with commercial electrode jelly and secured on the skin with adhesive tape. Amplification of the EMG signals was performed by high gain high impedance differential amplifiers (gain 60 dB, bandwidth 10-5000 Hz). The amplified signals were full-wave rectified, integrated (time constant 100 ms) and recorded on a chart recorder and the area under the integrated signals was determined over each 3-rain period throughout each exercise bout. The average integrated EMG (iEMG) activity recorded during each bout was expressed as a percentage of the maximal activity (%iEMGma×) recorded during the strongest of three brief voluntary maximal contractions (MVC) of that muscle group performed by the subject before the experiment. The area under the iEMG trace was also determined for the initial 15 s of each hiking bout, and this was used as the baseline value for that bout. The iEMG activity recorded throughout the four successive hiking bouts was taken as an indicator of changes in the neurogenic drive to the exercising muscles, since it has been demonstrated that this computation can be used as a quantitative estimate of the motor neuron pool activation (Komi and Viitasalo 1976).

Blood sampling and analysis Capillary blood samples (50 gl) were taken from a finger tip of the subject's left hand immediately before the beginning of the first hiking bout, during the 15-s interval following each bout, immediately after the end of the last bout and after 3 rain of recovery. The samples were drawn into capillary tubes (containing fluoride, hepafin and nitrite), mixed thoroughly for 3 rain and analysed for [la-]b in duplicate using an enzymatic membrane method (Analox GM7 lactate analyser, London). Prior to blood analyses the analyser was calibrated with 5 and 8 mmol. 1-1 sodium lactate standards.

Other measurements The HR was monitored throughout by means of short-range telemetry (Sports Tester PE 3000). Blood pressure was measured immediately before the first bout and immediately after the last bout using a standard clinical sphygmomanometer cuff (Accoson mercury manometer). The BP~ was calculated as the diastolic pressure plus one-third of the pulse pressure. The RPE was assessed by the subject immediately after the end of each bout using the category-ratio scale (Borg 1982).

Statistical methods The probability plot correlation coefficient test for normality revealed that data recorded for all variables were normally distributed. One-way ANOVA with repeated measures, followed by Tukey's pairwise comparison tests where appropriate, were used to determine whether changes in the recorded physiological variables were significant. Pearson's product moment was applied to express correlations between variables. Statistical significance for all statistical procedures was established at P < 0.05.

Results Mechanical performance The subjects succeeded in maintaining the simulator level throughout each hiking bout and no significant changes in force exerted on the toe-strap were observed during the four bouts.

Respiratory variables From the first to the fourth bout, the 3-min 1202 and 1)E increased significantly with the greatest increases occurring predictably over the first two bouts. The PETCOz decreased progressively and significantly across the four bouts, indicating hyperventilation; this was also reflected in a progressive and significant increase in I/E/1202 (Fig. 2). The average value for 1/O2 over all four bouts was only 1.04 (SEM 0.05) 1" rain- 1 The continuous breath-by-breath record (Fig. 3) revealed more detail of the pattern of ventilatory changes within and between bouts. Hyperventilation increased to reach the maximum for each bout at the end of its 3-rain period. During the 15-s intervals, there was an immediate decrease in V~, which often started (as in the trace illustrated) with a prolonged expiratory interval. In the same 15-s periods PETCO2 increased by 0.4-0.7 kPa (3-5)mmHg), (Figs. 2D, 3). The particular example in Fig. 3 (top panel) shows increasing again within the first 25 s of resuming V~ exercise, bringing down the PETCO2 from a peak at the end of the rest to a level similar to that at the end of the preceding (third) bout. During the next 15 s this hyperventilation was transiently reversed, with a consequent small increase in PETCO2; thereafter hyperventilation progressed to a new maximum at the end of the fourth bout. This pattern was typical, with minor variations in timing, such that the group data for each of the bouts 2, 3 and 4 showed the average 12I over the whole of the 1st min to be lower than that over the first 15 s (Fig. 4A). The final cessation of exercise was initially characterised by an immediate fall in Vj and consequent rise in PETCO2 (Fig. 3, bottom panel), as in the 15 s rest periods described above. However, after 20 to 30s there was usually a transient secondary increase in V~, before a variable return towards the resting state.

E M G analysis The average %iEMGmax activity recorded from the RF muscle was significantly increased over the four bouts (Fig. 5A). During the 1st min of each bout, the activity did not increase significantly above the baseline level (that of the initial 15 s of the bout) (Fig. 4B). By the end of the 2nd and 3rd min of hiking exercise, in bouts 3 and

198

in parallel until the end of the last bout, a direct and significant (P < 0.05) relationship existed between these variables (Fig. 6). The correlation coefficient between the two parameters for the group overall was 0.85.

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exercise continues, greater motor unit activity (associated with increased voluntary effort) has been required to maintain the given force (Goodwin et al. 1972). Increase in the i E M G activity of the RF muscle during hiking has indeed been reported before, and attributed to recruitment of additional motor units (Vogiatzis et al. 1993). Furthermore, in that study spectrum analysis of the myo-electric signals was available, and a shift towards lower frequencies was e v i d e n t - a strong indication of developing fatigue (Fallentin et al. 1993). Fatigue in the present study was clearly cumulative, as manifested by the increases in the i E M G activity during each successive hiking, bout confirming that the short intervals separating the bouts were not sufficient for much recovery to take place (Funderburk et al. 1974). Although the present experiments did not provide direct information as to the cause of the fatigue the subjects experienced, it is likely that the level of isometric contraction of the quadriceps muscles restricted their perfusion by mechanically compressing the intramuscular blood vessels (see Edwards et al. 1972). The Fig. 4 A For inspired minute volume (V~, body temperature and pressure, saturated) and B for the percentage maximal integrated electromyogram (%iEMGmax) recorded from the rectus femoris muscle, points are the mean values (and SEM) over each minute of each bout, expressed as changes from the first 15 s (baseline period) of each bout (bout 1 filled circles, bout 2 open circles, bout 3 filled triangles and bout 4 open triangles). "Significantlydifferent from bout 1 (P < 0.05). bSignificantlydifferent from bout 2 (P < 0.05) ~Significantlydifferent from bout 3 (P < 0.05)

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Time (miu) Fig. 5 A The percentage of maximal integrated electromyogram; (% iEMGma~)and D heart rate (HR) vales refer to means (and SEM) over 3-min periods. B The rating of perceived exertion (RPE), and C blood lactate concentration ( [ l a - ] b ) refer to the end of each bout. aSignificantly different from bout 1 (P < 0.05). USignificantly different from pre-exercise (P < 0.05)

magnitude of the increase in BPa during the hiking bouts and the steady increase of HR indicated levels of isometric tension where others have shown that blood flow through the muscles is markedly impaired (Lind and McNicol 1967). This contraction-induced ischaemia has been found to result in an increase in lactic acid production which in turn diminishes the muscle pH (Sahlin et al. 1975) and decreases the muscle's contractile force (Sahlin 1986). It has been found that other substances released by active fibres, notably

potassium ions, will also tend to be trapped within the muscle interstitial fluid and contribute to force-decline (Hnik et al. 1986) instead of being distributed throughout the body. A decrease in muscle pH and an elevated extracellular potassium concentration could, however, explain more than simply the need for further recruitment. Both are likely to stimulate the receptive fields of chemosensitive groups III and IV afferent fibres which have been shown to form functional connections with the respiratory centre (McCloskey and Mitchell 1972; Tibes 1977; Tallarida et al. 1981) and their activation could reflexly increase l/E. In addition to this enhancement of respiratory drive by muscle chemoreflexes, the increase in the iEMG activity directly reflected increases in the efferent neural activity (central command) required to maintain the force exerted on the toe-strap. Enhanced central command descending from higher centres to exercising muscles has itself been considered to provide a drive to increase I/E (Goodwin et al. 1972). The present investigation suggests perhaps such a direct association between the neural output to the muscles and ventilation since significant increases in 12~ across the four hiking bouts were strongly correlated with increases in the iEMG activity (r = 0.85; see Fig. 6). The fact that HR and I/I were in turn significantly correlated (r = 0.79, P < 0.05) during all successive hiking bouts could be interpreted as indicating that a part of the enhanced motor command signal from the cortex stimulated brainstem cardiovascular as well as respiratory centres (Goodwin et al. 1972). Although several investigations have demonstrated a distinct link between the increases in 17E and in iEMG activity during incremental exercise tests (Nagata et al. 1981; Viitasalo et al. 1985; Mateika and Duffin 1994), to our knowledge this is the first study to demonstrate

201 a strong association between iEMG activity and ventilation during successive bouts of isometric exercise. Clearly, however, these correlations, although suggestive, do not prove that central command predominated in the respiratory drive: parallel chemoreflexes, originating in the muscle, could elicit increases in both iEMG and ventilation. What the data from this study do show unequivocally is that the total respiratory drive during these periods of hiking, as found in other forms of isometric exercise (Myhre and Andersen 1971; Duncan et al. 1981; Muza et al. 1983; Poole et al. 1988), exceeded that required by metabolism. The increase in l/E recorded throughout the hiking bouts were disproportionately greater than the increments in 1/O2, and the concurrent hypocapnia confirmed that marked hyperventilation was taking place in these laboratory simulations, as has been shown to occur in actual sailing (Vogiatzis et al. 1994). Hyperventilation was increasingly pronounced during each successive bout, and especially during the last minutes of the bouts-parallelling fatigue. The excess of I/E, increases over those in VO2 is in keeping with the concept that respiratory drive during the main parts of each exercise-bout involved factors other than those which match I?E to metabolic rate in moderate dynamic exercise. The two categories of neurogenic stimulus discussed above (chemoreflexes and central command) are clearly candidates for such excess drive. The RPE also increased significantly across the four bouts, due predominantly to increased sensations of fatigue and pain in the ischaemic quadriceps muscle groups. These sensations might have activated the respiratory neurons via the cortex, adding to the hyperventilation (Duncan et al. 1981). In addition, the pain and fatigue could have resulted in increased outflow of catecholamines, which are also capable of inducing hyperventilation (Whelan and Young 1953). Concerning events within the individual hiking periods, iEMG activity did not increase during the first minute of any bout, even though it increased markedly thereafter in bouts 3 and 4; the delayed increase in these bouts could be attributed to reactive hyperaemia occurring during the interval phases and partially restoring blood flow through the muscles (Sjogaard et al. 1988). Averaged data from the cumulative volume records showed that the increase in 1/~ at the start of bouts 2, 3 and 4 was followed at first by a decrease, before the steep increase towards the end of the bout. This is indicated in Fig. 4A, where the I?~ in litres per minute for each successive minute is expressed as the difference from that in the first 15 s of each bout. The triphasic nature of this pattern suggests that the increase at the start of these bouts was not solely evoked by the restart of exercise, although it might have been associated with altered posture and psychological factors. Alternatively, we suggest that stimuli related to the preceding bout might have been acting at this time, some 10 to

30 s after it ended, when substances released into the circulation from the muscles during the intermission could have been having an effect. This interpretation is supported by the absence of the phenomenon in the first bout, and by the similar delay before the secondary increase in 17~after the end of the final bout. The increase in PETCO2 seen during the intervals between bouts, and during, early recovery, always just preceded an increase in V~. It has been shown that transient venous CO2 loading is reflected in an increase in alveolar and arterial PETCO2 and can increase I/E through stimulation of peripheral chemoreceptors (Phillipson et al. 1981). It is doubtful that lactic acid, acting either directly or by displacement of CO; from bicarbonate buffers, contributed substantially during the main part of each sequence, since ]-la-lb did not increase significantly from its respective pre-exercise concentrations until the end of the bouts 3 and 4; however, the small increase in [la-lb observed over the four bouts could reflect a small degree of seepage from regions of the contracted muscles where the blood flow was not completely impeded (Saltin et al. 1981). The moderately raised concentrations seen at the end of bout 3 [2.0 (SEM 0.3) mmol. 1- z] was, however, similar to that described by Poole et al. (1988), who have suggested that a fall in arterial pH might contribute to hyperventilation during recovery. Blood with high CO2 and low 02 content together with lactate and other metabolites such as potassium ions, having been partly trapped in the contracting muscles, may have surged into the circulation when each bout ceased and transiently stimulated ventilation. Similarly, the partial restoration of hyperventilation during final recovery could be attributed to stimulation of arterial and/or central chemoreceptors by substances released into the circulation. Finally, it must not be overlooked that 1702 rose over the four hiking bouts, particularly but not solely in the first two. This suggests firstly that blood flow through the quadriceps and other contracting muscles, though substantially impeded, was not totally so; and secondly that additional motor units or synergistic muscles were recruited (Myhre and Andersen 1971; Wiley and Ling 1971; Kilbom and Person 1982), albeit to low percentages of maximal activity, as fatigue developed. Such additional muscle activation was perhaps present in the BB muscle, which showed a nonsignificant trend towards an increased activity over the four exercise bouts. The physiological demands of dinghy sailing are highly specific and vary considerably with the type of boat (Shephard 1990) and the counterbalance force it requires (Gallozzi et al. 1993). Therefore, the results of the present investigation can be compared quantitatively only with those of other studies which involved hiking performance in the same class__Rofdinghy - the Laser. Average values for 1?O2, HR BPa and post-test [la-]b recorded from the present investigation are in

202

agreement with those [-1.12 (SEM 0.22) l'min -1, 118 (SEM 25) beats, min-1, 16.4 (SEM 2.0) kPa [123 (SEM 14) mmHg] and 2.3 (SEM 0.8) mmol' 1-1, respectively obtained in a recent study of hiking on a Laser simulator (Blackburn 1994). Also the average post-test [la-]b derived from the present study is identical to that [2.3 (SEM 0.4) mmol.1-1] recorded after a 10-min hiking test in Lasers under actual sailing conditions (Vogiatzis et al. in press), thus suggesting that the real hiking effort was well simulated in the present investigation. Furthermore, according to the findings of these other sailing investigations (Blackburn 1994; Vogiatzis et al. in press) the major factor which dominates the physiological responses during hiking is the degree of isometric tension developed in quadriceps muscle. The principal new information which the present study offers is the evidence of a direct association between the onset and magnitude of muscle fatigue in the quadriceps muscle, and ventilatory drive. In conclusion, it was shown that the insufficient recovery separating the repetitive hiking bouts had a cumulative effect upon fatigue in the RF muscle. The development of muscle fatigue, which was mirrored in the progressive increase in the iEMG activity, is likely to be linked, directly or indirectly, to the progressive hyperventilation. These changes in ventilation and iEMG activity occurred in the absence of comparably progressive or significant changes in [la-]u. Acknowledgements This work was supported by the Scottish Sports Council (Grant no. SEP/9D 1548.089). The financial support given to I. Vogiatsiz by the Greek Scholarship Foundation is acknowledged.

References Blackburn M (1994) Physiological responses to 90 minutes of simulated dinghy sailing. J Sports Sci 12:383-390 Borg GAV (1982) Psychophysical base of perceived exertion Med Sci Sports Exerc 14:377 Duncan G, Johnson RH, Lambie DG (1981) Role of sensory nerves in the cardiovascular and respiratory changes with isometric forearm exercise in man. Clin Sci 60:145 155 Edwards RG, Lippold OCJ (1956) The relation between force and integrated electrical activity in fatigued muscle. J Physiol (Lond) 132:677-681 Edwards RHT, Hill DK, McDonnell M (1972) Myothermal and intramuscular pressure measurements during isometric contractions of human quadriceps muscle. J Physiol (Lond) 224:58-59 Fallentin N, Jorgensen K, Simonsen EB (1993) Motor unit recruitment during prolonged isometric contractions. Eur J Appl Physiol 67:335-341 Funderburk GF, Hipskind SG, Welton RC, Lind AR (1974) Development of and recovery from fatigue induced by static effort at various tensions. J Appl Physiol 37:39~396 Gallozzi C, Fanton F, De Angelis M, Dal Monte A (1993) The energetic cost of sailing. Med Sci Res 21:851-53 Goodwin GM, McCloskey DI, Mitchell JH (1972) Cardiovascular and respiratory responses to changes in central command during isometric exercise at constant muscle tension. J Physiol (Lond) 226:173-190

Harrison J, Bursztyn P, Coleman S, Hale T (1988) A comparison of heart rate, oxygen uptake relationship for cycle and dinghy ergometry. J Sports Sci 6:160 Hnik P, Vyskocil F, Ujec E, Vejsada R (1986) Work-induced potassium loss from skeletal muscles and its physiological implications. In: Saltin B(ed) Biochemistry of exercise, VI Human Kinetics, Ill. Champaign, pp 345-364 Kilbom A, Person K (1982) Leg blood flow during static exercise. Eur J Appl Physiol 48:367-377 Komi PV, Viitasalo JHT (1976) Signal characteristics of EMG at different levels of muscle tension. Acta Physiol Scand 96:267-276 Lind AR, McNicol GW (1967) Muscular factors which determine the cardiovascular responses to sustained and rhythmic exercise. Can Med Assoc J 96:706-715 Lind AR, Petrofsky JS (1979) Amplitude of the surface electromyogram during fatiguing isometric contractions. Muscle Nerve 2:257-264 Lloyd AJ (1971) Surface electromyography during sustained isometric contractions. J Appl Physiol 30:713-719 Marchetti M, Figura F, Ricci B (1980) Biomechanics of two fundamental sailing postures. J Sports Med 20:325-332 Mateika JH, Duffin J (1994) Coincidental changes in ventilation and electromyographic activity during consecutive incremental exercise tests. Eur J Appl Physiol 68:54 61 McCloskey DI, Mitchell JH (1972) Reflex cardiovascular and respiratory responses originating in exercising muscle. J Physiol (Lond) 224:173-186 Muza SR, Lee LY, Wiley RL, McDonald S, Zechman FW (1983) Ventilatory responses to static handgrip exercise. J App Physiol 54:1457-1462 Myhre K, Andersen KL (1971) Respiratory responses to static muscular work. Respir Physiol 12:77-89 Nagata A, Muro M, Moritani T, Yoshida T (1981) Anaerobic threshold determination by blood lactate and myoelectric signals. Jpn J Physiol 31:585-597 Phillipson EA, Bowes G, Townsend ER, Duffin J, Cooper JD (1981) Carotid chemoreceptors in ventilatory responses to changes in venous carbon dioxide load. J Appl Physiol 51:1398 1403 Poole DC, Ward SA, Whipp BJ (1988) Control of blood-gas and acid-base status during isometric exercise in humans. J Physiol (Lond) 396:365-377 Sahlin K (1986) Muscle fatigue and lactic acid accumulation Acta Physiol Scand 128 [Suppl 556] :83-91 Sahlin K, Harris RC, Hultman E (1975) Creatine kinase equilibrium and lactate content compared with muscle pH in tissue samples obtained after isometric exercise. Biochem J 152:173 180 Saltin B, Sjogaard G, Gaffiney FA, Rowell LB (1981) Potassium, lactate, and water fluxes in human quadriceps in muscle during static contractions. Circ Res 48 [-Suppl 1] : 18 25 Shephard RJ (1990) The biology and medicine of sailing J, Sports Med 9: 86-99 Sjogaard G, Savard G, Juel C (1988) Muscle blood flow during isometric activity and its relation to muscle fatigue. Eur J Appl Physiol 57:327-335 Spurway NC, Burns R (1993) Comparison of dynamic and static fitness-training programmes for dinghy sailors and some questions concerning the physiology of hiking. Med Sci Res 21:865-867 Tallarida G, Baldoni F, Peruzzi G, Raimindi G, Massaro M, Sangiorgi M (1981) Cardiovascular and respiratory reflexes from muscles in dynamic and static exercise. J Appl Physiol 50:784-791 Tibes U (1977) Reflex inputs to the cardiovascular and respiratory centres from dynamically working canine muscles. Circ Res 41:332-341 Viitasalo JT, Luhtanen P, Rahkila P, Rusko H (1985) Electromyographic activity related to aerobic and anaerobic threshold in ergometer bicycling. Acta Physiol Scand 124:287 293

203 Vogiatzis I, Roach NK, Trowbridge EA (1993) Cartiovascular, muscular and blood lactate responses during dinghy "hiking". Med Sci Res 21:861 863 Vogiatzis I, Spurway NC, Wilson J (1994) On-water oxygen uptake measurements during dinghy sailing. J Sports Sci 12:153 Vogiatzis I, Spurway NC, Jennett S, Wilson J, Sinclair J (1995) Respiratory and metabolic responses during successive bouts of isometric exercise (simulated dinghy sailing) in humans. J Physiol 483:132 133P

Vogiatzis I, Spurway NC, Wilson J, Boreham C Assessment of aerobic and anaerobic demands of dinghy sailing at different wind velocities. J Sports Med Phys Fitness (in press) Whelan RF, Young IM (1953) The effect of adrenaline and noradrenaline infusions on respiration in man. Br J Pharmacol 8:98 102 Wiley RL, Lind AR (1971) Respiratory responses to sustained static muscular contractions in humans. Clin Sci 40:221 234