The V O2 slow component for severe exercise depends on type of exercise and is not correlated with time to fatigue

Transcription

1 The V O2 slow component for severe exercise depends on type of exercise and is not correlated with time to fatigue VERONIQUE L. BILLAT, 1 RUDDY RICHARD, 2 VALERIE M. BINSSE, 2 JEAN P. KORALSZTEIN, 2 AND PHILIPPE HAOUZI 3 1 Laboratoire Science du Sport, Lille 2, Lille; 2 Institut Coeur Effort Santé, Paris, and Centre de Médecine du Sport Caisse Centrale des Activites Sociales, Paris; and 3 Laboratoire de Physiologie, Faculté de Médecine de Nancy, Vandoeuvre-lès-Nancy, France Billat, Veronique L., Ruddy Richard, Valerie M. Binsse, Jean P. Koralsztein, and Philippe Haouzi. The V O2 slow component for severe exercise depends on type of exercise and is not correlated with time to fatigue. J. Appl. Physiol. 85(6): , The purpose of this study was to examine the influence of the type of exercise (running vs. cycling) on the O 2 uptake (V O2 ) slow component. Ten triathletes performed exhaustive exercise on a treadmill and on a cycloergometer at a work rate corresponding to 90% of maximal V O2 (90% work rate maximal V O2 ). The duration of the tests before exhaustion was superimposable for both type of exercises (10 min 37 s 4 min 11 s vs. 10 min 54 s 4 min 47 s for running and cycling, respectively). The V O2 slow component (difference between V O2 at the last minute and minute 3 of exercise) was significantly lower during running compared with cycling ( vs ml/min). Consequently, there was no relationship between the magnitude of the V O2 slow component and the time to fatigue. Finally, because blood lactate levels at the end of the tests were similar for both running ( mmol/l) and cycling ( mmol/l), there was a clear dissociation between blood lactate and the V O2 slow component during running. These data demonstrate that 1) the V O2 slow component depends on the type of exercise in a group of triathletes and 2) the time to fatigue is independent of the magnitude of the V O2 slow component and blood lactate concentration. It is speculated that the difference in muscular contraction regimen between running and cycling could account for the difference in the V O2 slow component. oxygen slow component; fatigue; running; cycling PREVIOUS STUDIES PERFORMED on cycling have reported that oxygen uptake (V O2 ) can attain a steady state above the lactate threshold but only in the work-rate range where lactate remains constant. Indeed, it is only above what has been termed critical power (17) that V O2 continues to rise until the end of the test or until exhaustion (22, 29). Furthermore, the magnitude of this slow component increases with the level of work rate and eventually takes V O2 to the maximal V O2 (V O2max ), even during submaximal exercise (22, 23). Although the mechanism underlying the continuous rise in V O2 during suprathreshold exercise remains poorly understood, we have recently reported (5) that the V O2 slow component was extremely small during an 18-min exhaustive run compared with that reported during cycling (23). Indeed, in that study, all the distance runners attained only 90% of their V O2max during the last minute before exhaustion, without ever reaching V O2max. Data comparing the magnitude of the V O2 slow component for different types of dynamic exercises are not available in the literature. Kyle and Caiozzo (16) reported that various types of exercise exclusively involving the legs yielded very similar power output, as long as the motion was similar and the same muscle groups were involved. However, cycling and running differ greatly in terms of muscular contraction regimen and, therefore, mechanical efficiency. For instance, the concentric work of cycling may account for a lower mechanical efficiency than running, which relies on a stretch-shortening cycle (6). The higher efficiency of stretch-shortening movements has been attributed to the elastic behavior of the muscles during contact with the ground. Cavagna et al. (8) estimated elastic contributions to be 40 50% of the total power generated during running. The gastrocnemius and soleus muscles also function during cycling on stretch-shortening cycles, although the stretching phases are not as apparent as in running or jumping (14). In addition, it has been suggested that bicyclists could minimize peripheral muscle fatigue by pedaling at a rate that produces a higher than the optimum metabolic rate (the most economical) but that lowers crank forces (torque). Indeed, Patterson and Moreno (19) demonstrated that when the pedaling rate was increased from 60 to 120 rpm, the resultant force on the pedals averaged over a crank cycle was isometric-like and produced no external work. Their results suggested that pedaling at 90 rpm might minimize peripheral forces and therefore peripheral muscle fatigue, even though this rate might result in higher V O2.In running, however, Cavanagh and Williams (9) reported that the freely chosen stride length allows for the most economical run. Whether these differences between running and cycling have a potential effect on the V O2 slow component is unclear. No studies have compared the V O2 slow component for cycling and running in subjects trained for both exercises and with the same V O2max and the same fraction of V O2max at the blood lactate threshold (2). We hypothesized that the type of exercise could influence the changes in the efficiency, i.e., the V O2 slow component, during exhaustive suprathreshold exercise. The purpose of this study was to examine 1) the influence of exercise, running vs. cycling, on the V O2 slow component during exhaustive exercise in triathletes equally trained in cycling and running and 2) whether the magnitude of the V O2 slow component /98 $5.00 Copyright 1998 the American Physiological Society

2 THE V O2 SLOW COMPONENT DURING RUNNING AND CYCLING 2119 Table 1. Physical characteristics, triathlon experience, and training regimen data of 10 triathletes Subject No. Age, yr Weight, kg Height, cm Triathlon Experience, yr No. of Triathlon Competitions Mean Training Distance, km/wk Running Cycling Swimming Mean SD influences the duration of exercise (time limit: t lim )ata velocity (90% V V O2max ) or a work rate (90% WR V O2max ) corresponding to 90% of V O2max. Finally, the relationship between the V O2 slow component and blood lactate accumulation for cycling and running exercise was analyzed. METHODS Subjects. Ten well-trained triathletes gave their informed consent and volunteered to participate in this study, which was approved by the Paris Ethical Committee. The physical characteristics of the subjects are presented in Table 1. All subjects were highly motivated and familiar with treadmill running, ergometer cycling, and with the sensation and symptoms of fatigue during heavy exhaustive cycling and running exercise. Materials. The running tests were performed on a motorized treadmill (Gymrol 1800, Techmachine, St. Etienne, France) kept at a 0% slope for all of the tests, the speed being controlled with a resolution of 0.5-mi./h (controller provided by the Centre d Enseignement et de Développement pour le Montage en Surface, Université Joseph Fourier, Grenoble, France). The cycling tests were carried out on an electronically braked cycle ergometer (ERG 600 Bösch, Berlin, Germany). Respiratory and pulmonary gas exchange variables were measured by using a MedGraphics CPMax cart (Medical Graphics, St. Paul, MN), which was calibrated before each test according to the manufacturer s instructions. Breath-bybreath data were averaged every 15 s. An electrocardiograph was monitored from a three-lead configuration (Jaeger cardioscope), and the output signal was fed to the CPX Medical Graphics system for computing heart rate. The blood samples were analyzed for blood lactate concentration (YSI 27 analyzer, Yellow Springs Instruments, Yellow Springs, OH). Preliminary measurements. The tests were performed on each subject at the same time of day in a climate-controlled laboratory (21 22 C). The subjects were instructed not to train hard or to ingest food and beverages containing caffeine for 3 days before testing. Each subject underwent two preliminary incremental tests on both the treadmill and the cycloergometer to determine 1) V O2max, 2) the work rate associated with V O2max (WR V O2max, and 3) the fraction of V O2max at which the lactate threshold appeared. These two incremental tests (running and cycling) were performed 3 days apart and in a randomized order. Subjects performed a continuous incremental test (3-min stages) to exhaustion. Duration and workload increments were standardized for running and cycling as follows. The workload increments were estimated to demand a V O2 response equal to 2 rest (2 Mets, i.e., ml kg 1 min 1 ). Each work increment ranged between 35 and 50 W for cycling, depending on the weight of the subjects [on the basis of 12 ml O 2 min 1 W 1 according to the recommendations of Astrand and Rodahl (1)]. For example, for a 60-kg subject, the workload increments were 35 W [60 (kg) * 7 (ml kg 1 min 1 )/12 (ml O 2 min 1 W 1 )]. For running, the speed was increased by 2 km/h (33.3 m/min) at each stage, except for the last stage where the increment was only 1 km/h (16.7 m/min). The initial work was set at between 70 and 100 W for cycling and at 10 km/h for running. V O2max was the highest 30-s V O2 reached at the end of an incremental test. The power output or velocity associated with V O2max was defined for running and cycling as the minimal workload at which V O2max occurred (5). All subjects gave a maximum effort and were encouraged to do so. Blood samples were obtained from the fingertip at the end of each stage, immediately after the end of the exercise test, and then 8 min into the recovery period. In this study, the lactate threshold was defined as the V O2 corresponding to the starting point of an accelerated lactate accumulation of 4 mmol/l and expressed in %V O2max (2). Although a ramplike increase in work rate cannot allow precise determination of this blood lactate threshold, the incremental test provides a useful index to delineate the domain at which blood lactate starts to accumulate in the blood, as illustrated for one subject in Fig. 1. Analysis of Fig. 1 shows that, below 70% of the peak work rate, trivial changes in lactate occurred, whereas above 80%, blood lactate increased above 3 mmol/l. This threshold was in accordance to that proposed by Aunola and Rusko (2), which was closer to the onset of blood lactate accumulation as defined by Sjödin Fig. 1. Representative lactate (La) response during an incremental test in 1 subject during running (open symbols) and cycling (closed symbols).

3 2120 THE V O2 SLOW COMPONENT DURING RUNNING AND CYCLING Table 2. Maximal values of metabolic and cardiorespiratory parameters during incremental tests for cycling and running Variable Cycle Ergometry Treadmill Running P Value V O2max, l/min V O2max, ml kg 1 min RER HR, beats/min [La], mmol/l Lactate threshold, % V V or O2max % P V O2max Values are means SD; n 10 subjects. V O2max, maximal O 2 uptake; RER, respiratory exchange ratio; HR, heart rate; [La], lactate concentration; V V and P O2max V O2max, velocity and power associated with V O2max, respectively. and Jacobs (24) rather than to the lactate threshold of Farrel et al. (10). Experimental design and protocols. The subjects ran and cycled to exhaustion at 90% WR V O2max. These two tests were separated by 1 wk. After a 15-min warm-up period at 50% WR V O2max, which was below the lactate threshold for all the subjects, the work rate was increased within 20 s to 90% WR V O2max. All subjects were given verbal encouragement throughout each trial. For running, the time to fatigue at 90% V V O2max was the time at which the subject s feet left the treadmill as he placed his hands on the guardrails. For cycling, this time corresponded to the time at which the subject was no longer able to pedal at a given power output. This time was defined as the time limit at 90% WR V O2max (t lim90% ). Blood samples were obtained from the fingertip before the warm-up run, during the last 30 s of the warm-up, immediately after the end of the exercise test, and then 8 min into the recovery period. The time course of blood lactate during the all-out exercises was not analyzed. Data analysis. Statistical analysis was performed by using t-tests for paired comparisons to determine whether V O2max, blood lactate concentration, respiratory exchange ratio, heart rate, blood lactate threshold (in %V O2 max ), and times to exhaustion were different in the cycling and running tests. The V O2 slow component was computed as the difference between V O2 at the last and the third minute of the exercise. A two-way analysis of variance for repeated measurements tested the overall effect of time on cardiorespiratory and blood parameters during the first minute (onset), the minute at the mid-time to fatigue, and the last minute of t lim (end) during the constant-power test. Scheffé s post hoc anaylsis was then used to locate the diferences. The results are presented as means SD. Statistical significance was set at P RESULTS Incremental tests. Table 2 shows the cycling and running V O2max, heart rate, and blood lactate values reached at the end of the incremental tests. There was no significant difference between running and cycling for V O2max, maximal heart rate, blood lactate level, and lactate threshold. Finally, the V O2 elicited at 90% WR V O2max represented and % of V O2 for cycling and running, respectively, which was not significantly different (P 0.22). Consequently, the intensity of the exhaustive exercise (i.e., 90% WR V O2max ) was well above the lactate threshold and similar for both type of exercises in absolute (V O2 ) and relative intensity (%lactate threshold and %V O2max ). In addition, the selected warm-up period was well below the severe-exercise domain delineated by the lactate threshold. In fact, the end of the 15-min warm-up period was performed at 56 2 and 54 3% of V O2max for running and cycling, respectively (50% WR V O2max ). Constant work rate tests. Table 3 shows the V O2 slow component, the cycling and running V O2max, heart rate, and blood lactate reached at the end of the all-out constant work rate tests performed at 90% WR V O2max. Time to fatigue. The 90% V V O2max and 90% WR V O2max were, respectively, km/h and W. Running and cycling times to fatigue at this velocity or power output were not significantly different at 10 min (Table 3). The V O2 slow component. As shown in Table 3, compared with cycling, the V O2 slow component for running was significantly lower ( vs ml/min, P 0.02). During cycling, V O2 at exhaustion was not significantly different from V O2max determined during the incremental test. In contrast, at the end of the exhaustive run test, the triathletes did not, on average, reach their V O2max (Fig. 2). However, examination of individual responses revealed different patterns of responses (Fig. 3, A-C). Four subjects (subjects 4, 5, 7, and 8) reached their V O2max in the cycling but not in the running test (Fig. 3A); four subjects (subjects 2, 3, 6, and 9) reached their V O2max both in cycling and running Table 3. Delay before exhaustion and maximal values of metabolic and cardiorespiratory parameters during constant workload tests for cycling and running Variable Cycle Ergometry Treadmill Running P Value t lim, min, s 10 min 37 s 4 min 11 s 10 min 54 s 4 min 47 s 0.83 Work rate or speed, W or km/h Relative work rate, %V O2max V O2, l/min * 0.02 V O2, ml kg 1 min * 0.02 Relative V O2,%V O2max * 0.03 HR, beats/min [La], mmol/l [La], mmol/l V O2, ml/min * 0.02 Values are means SD; n 10 subjects. t lim, Delay before exhaustion; V O2,O 2 uptake; [La] and V O2 (l/min), difference in [La] and V O2, respectively, between last minute and minute 3 of exercise. *Significantly different from cycle ergometry, P 0.05.

4 THE V O2 SLOW COMPONENT DURING RUNNING AND CYCLING 2121 Fig. 2. Relationship between O 2 uptake (V O2 ) and time during all-out test for running (open symbols) and cycling (closed symbols) at 90% of work rate at maximal V O2 (V O2max ). Values are means SD. Horizontal dashed lines, level of V O2max for running (r) and cycling (c). (Fig. 3B), the remaining two subjects (subjects 1 and 10) reaching neither their cycling nor running V O2max (Fig. 3C). In other words, of the 10 triathletes studied, 8 reached their V O2 during cycling and only 4 during running. Blood lactate and the V O2 slow component. The subjects ended their warm-up periods with a blood lactate level of mmol/l for running and mmol/l for cycling. Blood lactate level at the end of the exhaustive exercises was not significantly different from that at the end of the incremental test, both for running ( vs mmol/l, P 0.79) and cycling ( vs mmol/l, P 0.85). The magnitude of the V O2 slow component and the level of blood lactate accumulation were correlated for both cycling and running (r 0.48, P 0.03, n 20), for cycling only (r 0.66, P 0.05, n 10) but not for running only (r 0.12, P 0.74, n 10) (Fig. 4). The V O2 slow component and time to fatigue. No significant correlation was found between the magnitude of the V O2 slow component and the duration tolerated in this suprathreshold exercise (r 0.15 for both running and cycling combined and r 0.23 and 0.18 for running and cycling, respectively, considered separately). However, the duration of suprathreshold exercise was positively correlated with the blood lactate accumulation for running (r 0.79, P 0.01) but not for cycling (r 0.32, P 0.05). Fig. 3. Temporal profile of individual V O2 responses for running (open symbols) and cycling (closed symbols) expressed as %V O2max. A: data for 4 subjects (subjects 4, 5, 7, and 8) attaining V O2max during cycling but not running. B: data for 4 subjects (subjects 2, 3, 6, and 9) attaining V O2max during both running and cycling. C: data for 2 subjects (subjects 1 and 10) who did not reach V O2max during running or cycling.

5 2122 THE V O2 SLOW COMPONENT DURING RUNNING AND CYCLING Fig. 4. Relationship between difference in ( ) inv O2 and lactate between last minute and minute 3 of exercise during test at 90% V O2max for both running (open symbols) and cycling (closed symbols). There was a significant correlation between V O2 and La for running and cycling (solid line, y 96.47x , r 0.529, P 0.02) and for cycling only (dashed line, y x , r 0.667, P 0.05) but not for running. The difference in the V O2 slow component between cycling and running ( ml of O 2 /min) was not correlated with the difference of time to fatigue for running (18 82 s) at the supra-lactate threshold workload (r 0.25, P 0.50). Time to fatigue and blood lactate accumulation. Blood lactate was not correlated with the time to fatigue for both running and cycling (r 0.07, P 0.78, n 20); however, when each type of exercise is considered in isolation, maximal blood lactate and time to fatigue were correlated for running (r 0.79, P 0.005) but not for cycling (r 0.33, P 0.41). The V O2 slow component and cardioventilatory variables. The maximal heart rate at the end of the constant test was not significantly different between cycling and running ( vs beats/ min, P 0.81). Minute ventilation, however, was significantly higher in cycling ( vs l/min, P 0.002) but with a minute ventilation-to-v O2 ratio similar to that during cycling. DISCUSSION In the present study, an exhaustive test performed at 90% WR V O2max was used to examine the influence of the type of exercise (cycling vs. running) on the V O2 slow component and its relationship with the time to fatigue (i.e., failure to sustain a required power output). Our results were twofold. First, for a similar relative and absolute intensity, the V O2 slow component depends on the type of exercise. Second, the V O2 slow component was correlated neither with the time to fatigue nor with blood lactate accumulation, when running and cycling were considered separately. Moreover, in contrast to cycling, the time to fatigue was unrelated to the blood lactate accumulation for running. The V O2 slow component depends on the type of exercise. Despite the same relative intensity of exercise, the V O2 slow component was higher for cycling than for running. The level of work rate chosen for the test was well above each triathlete s lactate threshold because the intensity of exercise was above the blood lactate accumulation of between 3 and 5 mm (2). The constant work rate exercise corresponded to a severe-exercise domain, defined as the intensity at which it is no longer possible to reach a new V O2 steady state, consistent with exercise above the critical power (12). Most of the studies that have reported a V O2 slow component were performed during cycling (12). However, the V O2 slow component has also been reported during running but only for prolonged tests (20 30 min) (18, 25). No data are available on the behavior of the V O2 slow component during a submaximal running test leading to fatigue. We observed that 8 of the 10 subjects reached their V O2max in cycling and only 4 in running. A precise analysis of the main differences between cycling and running may help us to understand some of the mechanisms responsible for the V O2 slow component. By simultaneously measuring pulmonary and leg V O2 during cycle ergometry, Poole et al. (21) demonstrated that 86% of the increment in pulmonary V O2 beyond the third minute of exercise (i.e., the V O2 slow component) could be accounted for by the increase in leg V O2. These data indicate that the majority of the V O2 slow component is attributable to factors within the working limbs (12). Different mechanisms may account for such additional increase in V O2. It has been suggested that the V O2 slow component may primarily be related to motor unit recruitment patterns during exercise, depending on the contribution of lower efficiency, fast-twitch motor units (15). More recently, Barstow et al. (3) and Poole et al. (20) have linked the V O2 slow component with the type of fibers, hypothesizing that both slow- and fast-twitch fiber types are recruited simultaneously at the onset of heavy exercise. However, because of their slower kinetics, the V O2 requirement of the fast-twitch fibers may become manifest only after several minutes. The hypothesis that the V O2 slow component arises from the recruitment of a fast-twitch fiber population with slow kinetics is consistent with the notion that V O2 kinetics are limited by fiber mitochondrial content (20). Moreover, it has been shown that isolated mitochondria from type II fibers exhibit a 18% lower phosphate-tooxygen ratio (30). Similarly, Whipp (29) considers that a, if not the, major contributor to the V O2 excess is likely to be the high energy cost of contraction of the type II fibers recruited at a proportionally higher level of work rate and requiring a large high-energy phosphate cost for force production. In the present study, because triathletes exercised at the same relative intensity for cycling and running (between their lactate threshold and the work rate associated with V O2max ), there was no reason to attribute the difference in the V O2 slow component between these two types of exercises to a difference in the percentage of fast-twitch fibers recruited in cycling and in running, unless it is assumed that the contraction regimen recruits different types of fibers at a given work rate, a result actually supported by many studies. For instance, Gaesser (11) reported that the V O2 slow component was significantly higher for cycling at 100

6 THE V O2 SLOW COMPONENT DURING RUNNING AND CYCLING 2123 rpm than at 50 rpm. In our study, the subjects were free to choose their most comfortable rate and usually adopted a frequency of 80 rpm, which may have increased the magnitude of the slow component. Recently, Takaishi et al. (27) demonstrated that the optimal pedaling rate estimated from neuromuscular fatigue in working muscles was coincident not with the pedaling rate at which the smallest V O2 was obtained but with the preferred pedaling rate of the subjects. They suggested that the reason that cyclists preferred a higher pedaling rate was closely related to the development of neuromuscular fatigue in the working muscles. In contrast, during running, the most efficient step rate is virtually the same as the freely chosen step rate (9). Finally, the isometric component of the contractions during cycling should be considered. Indeed, the type of muscle contraction is not homogeneous during the pedaling cycle, and an isometric-like component occurs during various phases of this cycle. This could indeed greatly affect the actual cost of pedaling in terms of V O2 at a high level of work rate. The V O2 slow component, time to fatigue, and blood lactate. Our second finding was that the V O2 slow component was correlated neither with time to fatigue nor with blood lactate accumulation. Whipp (29) suggested that the more rapidly the slow component projects toward V O2, the shorter the tolerable duration of the exercise test. In the study by Nagle et al. (18), the subjects did not reach their V O2max and were not completely exhausted. In the present study as well, many subjects did not reach their V O2max during running but were completely exhausted at the end of the test. In cycling, however, more subjects reached their V O2max but were exhausted as well after the same duration. Jones (13) has suggested that the sensation of effort presumably reflects the magnitude of the voluntary motor command generated. The second source of sensory information, described as a sense of force or tension, is derived from peripheral receptors in muscles, tendons, and the skin and is assumed to reflect the actual force exerted by the muscle. There may be different reasons for stopping cycling and running, even if the delay of fatigue is not significantly different. However, the nature of the link between the V O2 slow component and the fatigue process remains unclear (20). The relationships between lactate and the V O2 slow component have also been the focus of attention. Barstow and Molé (4), for example, suggested that the magnitude of the V O2 slow component correlated temporally with the changes in blood lactate. Roston et al. (23) also reported a significant correlation between changes in blood lactate and the V O2 slow component during heavy exercise. It has been hypothesized that the V O2 slow component could be induced by the Hb-O 2 dissociation curve shifted to the right by acidosis (Bohr effect) (28). Therefore, the Bohr effect allowed further unloading of V O2 from Hb for uptake by the muscle cells at a constant (minimum) capillary PO 2 (26). However, the temporal relationship between blood lactate and the V O2 slow component has been convincingly shown not to be one of cause and effect (see Ref. 20 for review). Our results corroborate this conclusion because they clearly show a dissociation between blood lactate and the V O2 slow component. However, one should consider that because blood lactate concentration is the difference between rates of blood lactate production and disappearance (7), it might be possible that the same blood lactate concentration for cycling and running corresponds to a different lactate turnover reflected by the higher V O2 slow component in cycling. In conclusion, the type of dynamic exercise performed at the same intensity (%WRV V O2max ) and absolute V O2 was found to significantly affect the characteristics of the V O2 response during heavy exercise. Not only was the V O2 slow component dependent on the type of exercise, it was also not correlated with the time to fatigue. Address for reprint requests: V. Billat, Centre de Médecine du Sport CCAS, 2 Ave. Richerand, Paris, France. Received 17 January 1997; accepted in final form 31 March REFERENCES 1. Astrand, P. O., and K. Rodahl. Textbook of Work Physiology: Physiological Bases of Exercise. New York: McGraw-Hill, 1986, p Aunola, S., and H. Rusko. Reproducibility of aerobic and anaerobic thresholds in year old men. Eur. J. Appl. Physiol. 53: , Barstow, T. J., A. M. Jones, P. H. Nguyen, and R. Casaburi. Influence of muscle fiber type and pedal frequency on oxygen uptake kinetics of heavy exercise. J. Appl. Physiol. 81: , Barstow, T. J., and P. A. Molé. Linear and nonlinear characteristics of oxygen uptake kinetics during heavy exercise. J. Appl. Physiol. 71: , Billat, V. L., J. C. Renoux, J. Pinoteau, B. Petit, and J. P. Koralsztein. Hypoxémie et temps limite à la vitesse aérobie maximale chez des coureurs de fond. Can. J. Appl. Physiol. 20: , Bosco, C., G. Montanari, I. Tarkka, F. Latteri, M. Cozzi, G. Iachelli, M. Faina, R. Colli, A. Dal Monte, M. La Rosa, R. Ribacchi, P. Giovenali, G. Cortili, and F. Saibene. The effect of pre-stretch on mechanical efficiency of human skeletal muscle. Acta Physiol. Scand. 131: , Brooks, G. A. Anaerobic threshold: review of the concept and direction for future research. Med. Sci. Sports Exerc. 17: 31 35, Cavagna, G. A., G. Citterio, and R. Margaria. The additional mechanical energy delivered by contractile component of the previously stretched muscle. J. Physiol. (Lond.) 251: 65 66, Cavanagh, P. R., and K. R. Williams. The effect of stride length variation on oxygen uptake during distance running. Med. Sci. Sports Exerc. 14 : 30 35, Farrel, P. E., J. H. Wilmore, E. F. Coyle, J. E. Billing, and D. L. Costill. Plasma lactate accumulation and distance running performance. Med. Sci. Sports Exerc. 11: , Gaesser, G. A. O 2 uptake during high intensity cycling at slow and fast cadences (Abstract). Physiologist 35: 210, Gaesser, G. A., and D. Poole. The slow component of oxygen uptake kinetics in humans. Exerc. Sport Sci. Rev. 24: Jones, L. A. The senses of effort and force during fatiguing contractions. In: Fatigue, edited by S. C. Gandevia, R. M. Enoka, A. J. McComas, D. G. Stuart, and S. K. Thomas. New York: Plenum, 1995, p Komi, P. V., and H. Kyröläinen. Mechanical efficiency of stretch-shortening cycle exercise. In: Human Muscular Function During Dynamic Exercise, edited by O. Marconnet, B. Saltin, P. Komi, and J. Poortmans. Basel: Karger, 1996, vol. 41, p (Med. Sport Sci. Ser.)

7 2124 THE V O2 SLOW COMPONENT DURING RUNNING AND CYCLING 15. Kushmerick, M. J., R. A. Meyer, and T. R. Brown. Regulation of oxygen consumption in fast- and slow-twitch muscle. J. Appl. Physiol. 263: C598 C606, Kyle, C. R., and V. J. Caiozzo. Experiments in human ergometry as applied to the design of human powered vehicles. Int. J. Sport Biomech. 2: 6 19, Moritani, T., A. Nagata, H. A. De Vries, and M. Muro. Critical power as a measure of physical working capacity and anaerobic threshold. Ergonomics 24: , Nagle, F., D. Robinhold, E. Howley, J. Daniels, G. Baptista, and K. Stoedefalke. Lactic acid accumulation during running at submaximal aerobic demands. Med. Sci. Sports. Exerc. 2: , Patterson, R. P., and M. I. Moreno. Bicycle pedalling forces as a function of pedalling rate and power output. Med. Sci. Sports Exerc. 22: , Poole, D. C., T. J. Barstow, G. A. Gaesser, W. T. Willis, and B. J. Whipp. V O2 slow component: physiological and functional significance. Med. Sci. Sports Exerc. 26: , Poole, D. C., W. Schaffartzik, D. R. Knight, T. Derion, B. Kennedy, H. J. Guy, R. Prediletto, and P. D. Wagner. Contribution of exercising legs to the slow component of oxygen uptake kinetics in humans. J. Appl. Physiol. 71: , Poole, D. C., S. A. Ward, G. W. Gardner, and B. J. Whipp. Metabolic and respiratory profile of the upper limit for prolonged exercise in man. Ergonomics 31: , Roston, W. L., B. J. Whipp, J. A. Davis, D. A. Cunningham, R. M. Effros, and K. Wasserman. Oxygen uptake kinetics and lactate concentration during exercise in humans. Am. Rev. Respir. Dis. 135: , Sjödin, B., and I. Jacobs. Onset of blood lactate accumulation and marathon running performance. Int. J. Sports Med. 2: 23 26, Steed, J., G. A. Gaesser, and A. Weltman. Rating of perceived exertion and blood lactate concentration during submaximal running. Med. Sci. Sports Exerc. 26 : , Stringer, W. S., K. Wasserman, R. Casaburi, J. Porszasz, K. Maehara, and W. French. Lactic acidosis as facilitator of oxyhemoglobin dissociation during exercise. J. Appl. Physiol. 76: , Takaishi, T., Y. Yasuda, T. Ono, and T. Moritani. Optimal pedaling rate estimated from neuromuscular fatigue for cyclists. Med. Sci. Sports Exerc. 28: , Wasserman, K., J. E. Hansen, and D. Y. Sue. Facilitation of oxygen consumption by lactic acidosis during exercise. News Physiol. Sci. 6: 29 34, Whipp, B. J. The slow component of O 2 uptake kinetics during heavy exercise. Med. Sci. Sports Exerc. 26: , Willis, W. T., and M. R. Jackman. Mitochondrial function during heavy exercise. Med. Sci. Sports Exerc. 26: , 1994.