Maximal aerobic capacity for repetitive lifting: comparison with three standard exercise testing modes

Summary A multi-stage, repetitive lifting maximal oxygen uptake ( ) test was developed to be used as an occupational research tool which would parallel standard ergometric testing procedures. The repetitive lifting test was administered to 18 men using an automatic repetitive lifting device. An intraclass reliability coefficient of 0.91 was obtained with data from repeated tests on seven subjects. Repetitive lifting test responses were compared to those for treadmill, cycle ergometer and arm crank ergometer. The mean±SD repetitive lifting of 3.20±0.42 l · min^–1 was significantly (p<0.01) less than treadmill ( = 0.92 l · min^–1) and cycle ergometer ( = 0.43 l· min^–1) and significantly greater than arm crank ergometer ( = 0.63 l · min^–1). The correlation between repetitive lifting oxygen uptake and power output wasr = 0.65. correlated highly among exercise modes, but maximum power output did not. The efficiency of repetitive lifting exercise was significantly greater than that for arm cranking and less than that for leg cycling. The repetitive lifting test has an important advantage over treadmill or cycle ergometer tests in the determination of relative repetitive lifting intensities. The individual curves of vs. power output established during the multi-stage lifting test can be used to accurately select work loads required to elicit given percentages of maximal oxygen uptake.     

The effects of one- and two-legged exercise on the lactate and ventilatory threshold

Summary The purpose of this investigation was to compare differences between one- and two-legged exercise on the lactate (LT) and ventilation (VT) threshold. On four separate occasions, eight male volunteer subjects (1-leg =3.36 l · min^–1; 2-leg =4.27 l · min^–1) performed 1- and 2-legged submaximal and maximal exercise. Submaximal threshold tests for 1- and 2-legs, began with a warm-up at 50 W and then increased every 3 minutes by 16 W and 50 W, respectively. Similar increments occurred every minute for the maximal tests. Venous blood samples were collected during the last 30 s of each work load, whereas noninvasive gas measures were calculated every 30 s. No differences in (l · min^–1) were found between 1- and 2-legs at LT or VT, but significant differences (p<0.05) were recorded at a given power output. Lactate concentration ([LA]) was different (p<0.05) between 1-and 2-legs (2.52 vs. 1.97 mmol · l^–1) at LT. This suggests it is rather than muscle mass which affects LT and VT. for 1-leg exercise was 79% of the 2-leg value. This implies the central circulation rather than the peripheral muscle is limiting to .Supported by NSERC A7555     

Sex difference in maximal oxygen uptake

Summary Ten men and 11 women were studied to determine the effect of experimentally equating haemoglobin concentration ([Hb]) on the sex difference in maximal oxygen uptake ( ). was measured on a cycle ergometer using a continuous, load-incremented protocol. The men were studied under two conditions: 1) with normal [Hb] (153 g · L^–1) and 2) two days following withdrawal of blood, which reduced their mean [Hb] to exactly equal the mean of the women (134 g· L^–1). Prior to blood withdrawal, expressed in L · min^–1 and relative to body weight and ride time on the cycle ergometer test were greater (p<.01) in men by 1.11 L · min^–1 (47%), 4.8 ml · kg^–1 min^–1 (11.5%) and 5.9 min (67%), respectively, whereas expressed relative to fat-free weight (FFW) was not significantly different. Equalizing [Hb] reduced (p<.01) the mean of the men by 0.26 L · min^–1 (7.5%), 3.2 ml · kg^–1 min^–1 (6.9%) or 4.1 ml · kg FFW^–1 min^–1 (7.7%), and ride time by 0.7 min (4.8%). Equalizing [Hb] reduced the sex difference for less than predicted from proportional changes in the oxygen content of the arterial blood and arteriovenous oxygen content difference during maximal exercise. It was concluded that the sex difference in [Hb] accounts for a significant, but relatively small portion of the sex difference in (L · min^–1). Other factors such as the dimensions of the oxygen transport system and musculature are of greater importance.     

Effect of coupling the breathing- and cycling rhythms on oxygen uptake during bicycle ergometry

Summary The influence of the degree of coupling between the breathing and cycling rhythms (K) on oxygen uptake was examined in 30 volunteers. They cycled on an ergometer with a load equal to 50% of their work capacity 170 in two experimental runs with spontaneous breathing rhythm, and in a further two runs with acoustically triggered breathing. K was continuously ascertained. and other respiratory parameters were measured by an automatic breath-by-breath analysis system.In 16 subjects, -differences between runs were correlated with the differences in K. In the majority of these subjects (12), decreased significantly with increasing K. In 14 subjects, -and K-variations within individual runs were analyzed. Phases with higher K were regularly accompanied by a decrease in .It is concluded that coupling the breathing and cycling rhythms reduces for a given moderate work load, although the magnitude of the -reduction varies considerably between individuals.     

Effects of training at and above the lactate threshold on the lactate threshold and maximal oxygen uptake

Summary Thirty-three college women (mean age=21.8 years) participated in a 5 d·wk^–1, 12 week training program. Subjects were randomly assigned to 3 groups, above lactate threshold (> LT) (N=11; trained at 69 watts above the workload associated with LT), =LT (N=12; trained at the work load associated with LT) and control (C) (N=10). Subjects were assessed for , LT, LT/ , before and after training, using a discontinuous 3 min incremental (starting at 0 watts increasing 34 watts each work load) protocol on a cycle ergometer (Monark). Respiratory gas exchange measures were determined using standard open circuit spirometry while LT was determined from blood samples taken immediately following each work load from an indwelling venous catheter located in the back of a heated hand. Body composition parameters were determined before and after training via hydrostatic weighing. Training work loads were equated so that each subject expended approximately 1465 kJ per training session (Monark cycle ergometer) regardless of training intensity. Pretraining, no significant differences existed between groups for any variable. Post training the > LT group had significantly higher (13%), (47%) and LT/ (33%) values as compared to C (p<.05). Within group comparisons revealed that none of the groups significantly changed as a result of training, only the > LT group showed a significant increase in (48%) (p<.05), while both the = LT and > LT group showed significant increases in LT/ (= LT 16%, > LT 42% (p<.05)). No differences were found between or within groups post training for body composition parameters. It was concluded that training above the LT results in an improvement in LT and that large improvements in may not be required for large improvements in .Data were collected at the Human Performance Laboratory, University of Colorado     

Changes in ventilation at the start and end of moderate and heavy exercise of short and long duration

Summary These experiments examined the effect of exercise intensity and duration on the magnitude of the abrupt change in ventilation at the start ( ) and end ( ) of exercise. Five subjects performed constant load treadmill exercise at 50% and 80% of their maximum oxygen consumption ( ) for 6 and 10 min while inspiring atmospheric air. The subjects also completed additional exercise tests at 80% for 10 min while inspiring an oxygen-enriched gas mixture. During each exercise trial ventilation was measured breath-by-breath. The and were determined by using non-linear curve-fitting techniques. The results showed that was greater at the start of the 80-% exercise tests compared to the 50-% tests and that at each level of exercise was greater than . The results also demonstrated that was inversely related to the intensity and duration of exercise. Furthermore, the was not altered subsequent to the inspiration of oxygen-enriched air. These findings have led us to postulate that the stimulus responsible for is reduced during exercise and that the degree of reduction is related to the intensity and duration of exercise. In addition, it was concluded that these changes might occur independently of peripheral chemoreceptor activity.     

Maximum oxygen consumption in dogs during muscular exercise and cold exposure

Maximum oxygen consumption for a short exhaustive work (Ex max) and for a severe cold stress (Ex max) were investigated in 8 dogs. Heart rate, plasma catecholamines and substrate concentrations were measured under both conditions. Mean C was lower than mean Ex max. Heart rate and plasma lactate were also lower during cold exposure than during exercise. Average plasma epinephrine concentrations were not significantly different and average plasma norepinephrine concentrations were similar under C and Ex max conditions. A positive correlation was found between plasma lactate and epinephrine concentrations measured under both conditions.It may be assumed that maximum oxygen consumption during muscular exercise is higher than during shivering thermogenesis. This difference does not seem to be due to differences in the involvement of the sympathico-adreno-medullary system.     

Transfer effects in endurance exercise

Summary The effects of 8 weeks of bicycle endurance training (5 x /week for 30 min) on maximal oxygen uptake capacity ( ) during arm and leg ergometry, and on the ultrastructure of an untrained arm muscle (m. deltoideus), and a trained leg muscle (m. vastus lateralis) were studied. With the training, leg- for bicycling increased by +13%, while the capillary per fiber ratio and the volume density of mitochondria in m. vastus lateralis increased by +15% and +40%, respectively. In contrast, the untrained m. deltoideus showed an unchanged capillary per fiber ratio and a decreased mitochondrial volume density (−17%). Despite this decrease of mitochondrial volume arm- increased by +9%. It seems unlikely that the observed discrepancy can be explained by cardiovascular adaptations, since arm cranking did not fully tax the cardiovascular system (arm- /leg- : 0.74 and 0.71 before and after training, respectively). Thus neither cardiovascular adaptations nor local structural changes in the untrained muscles could explain the increased arm- . However, the enhanced capacity for lactate clearance after endurance training could be sufficient to account for the larger during arm cranking. We propose that an increased net oxidation of lactate might be responsible for the increased arm- found after bicycle endurance training. This work was supported by grant 3.128.81 from the Swiss National Science Foundation, and by Cilo S.A., Bicycles, Romanel/Switzerland     

Maximal cardiorespiratory responses to one- and two-legged cycling during acute and long-term exposure to 4300 meters altitude

Summary During exposure to altitudes greater than about 2200 m, maximal oxygen uptake ( ) is immediately diminished in proportion to the reduction in the partial pressure of oxygen in the inspired air. If the exposure lasts longer than a couple of days, an increase in arterial oxygen content (CaO₂), due to a hemoconcentration and an increase in arterial oxygen saturation, occurs. However, there is also a reduction in maximal cardiac output ( ) at altitude which offsets the increase in CaO₂ and, therefore, does not improve. The purpose of this investigation was to study the contribution of the increase in CaO₂ to the working muscles without the potentially confounding problem of a reduced . The approach used was to have seven male subjects (aged 17 to 24 years) perform one- and two-legged tests on a cycle ergometer at sea level (SL, PIO₂ = 159 Torr), after 1 h at 4300 m simulated altitude (SA, PIO₂ = 94 Torr) and during two weeks of residence on the summit of Pikes Peak, CO. (pP, 4300 m, PIO₂ = 94 Torr). Cardiac output limits maximal performance during two-legged cycling but does not limit performance during one-legged cycling. During the study, CaO₂ changed from 189±3 (mean ±SE) at SL to 161±4 ml·L^–1 during SA (SL vs. SA,p<0.01) and to 200±6 ml·L^–1 at PP (SL vs. PP,p<0.05; SA vs. PP,p<0.01). Two-legged decreased from 3.64±0.26 L·min^–1 at SL to 2.70±0.14 L·min^–1 during SA (p<0.01) to 2.86±0.16 L·min^–1 at PP (p<0.01). One-legged decreased from 2.95±0.22 at SL to 2.25±0.17 L·min^–1 during SA (SL vs. SA,p<0.01) but improved to 2.66±0.18 L·min^–1 at PP (SA vs. PP,p<0.05). Since only one-legged increased as more oxygen was made available to the working muscles, the altitude-induced reduction in can be implicated as being responsible for the reduction in during two-legged cycling.     

Aerobic performance capacity in paraplegic subjects

Summary To determine adaptation to prolonged exercise in paraplegics, maximal O₂ uptake ( ) and lactate threshold (LT) were evaluated during an arm cranking exercise in nine patients (P) and nine able-bodied (AB) subjects.Mean averaged 25.1 and 31.6 ml · min^–1 · kg^–1 in P and AB groups respectively. in P was found to be directly related to the level of spinal injury: the higher the lesion the lower the uptake. Lactate threshold expressed as a percentage of was higher in P (59%) than in AB (43%), and close to that observed in armtrained athletes.Since training has less effect on in paraplegics than in able-bodied subjects, attributable to a deficiency in the circulatory adaptation of paraplegics to exercise, the observed differences between AB and P in lactate threshold and submaximal exercise indicate that the possible effect of training in paraplegics is located at the level of intracellular chemistry, with a diminution in glycogenolysis (higher LT) and a higher rate of lipid utilization (lower RQ).     

Aerobic power and pubertal peak height velocity in Belgian boys

Summary To determine the cardiorespiratory response to maximal exercise before, during and after the pubescent growth spurt, thirty boys were tested at yearly intervals over a period of six consecutive years. For each individual, peak height velocity (PHV) was determined. The age at PHV (¯X= 13.6 years) was taken as a standard of maturation. The results from all subjects at 1.5 and 0.5 years before and 0.5 and 1.5 years after PHV are presented. The highest oxygen uptake ( ) obtained during an incremental bicycle ergometer test to voluntary exhaustion was taken as peak oxygen uptake ( peak). Across each of the four years studied, mean peak (min=49.6; max=52.5 ml·kg^–1·min^–1) and mean heart rate (HR) at peak (min=190; max=192) did not change significantly as a function of PHV. On the other hand, the respiratory quotient at peak increased considerably from mean minima and maxima of 0.99 and 1.01 before PHV to 1.07 and 1.10 after PHV. Ventilatory equivalent for ( ), taken as an indicator of ventilatory economy, seemed to be unaffected by the maturation process. The steepest increase in circumpubertal oxygen pulse was found one year after PHV. Average stability coefficients (¯r), calculated from the inter-years correlations were high for height (¯r=0.95), weight (¯r=0.92), HR at peak (¯r=0.74), peak in 1/min (¯r=0.71), oxygen pulse (¯r=0.68) and tidal volume (¯r=0.64).     

Effects of standing cycling and the use of toe stirrups on maximal oxygen uptake

Summary Twenty-eight subjects (6 normal men, 14 distance runners, and 8 rowers) were tested for maximal oxygen uptake ( ) and associated physiological measures during bicycle ergometer exercise with toe stirrups while standing (BEts) and during treadmill exercise (TM). Correlation between BEts and TM was high (r=0.901, p<0.05). No significant difference existed between the two values (60.3±8.9 vs. 60.5±9.7 ml · kg⁻¹ · min⁻¹; n=28). No differences were found even when three different subgroups were separately compared. It is concluded that the higher elicited during BEts as compared with normal sitting cycling may be attributed to the increased muscle blood flow and/ or involvement of a larger muscle mass, the latter being partly evidenced by the observation of greater electrornyographic activity during BEts.     

Body composition,work capacity,and work efficiency of active and inactive young men

On 46 healthy young men, of whom 18 took part in strenuous sport at least once a week, height, weight, total body fat (as % of body mass) and lean body mass (LBM) were determined. The subjects performed submaximal exercise on a bicycle ergometer and climbing on an upwardly inclined treadmill at work loads of 60, 110, and 140 watts. Oxygen consumption ( ), respiratory quotient (RQ), energy expenditure (), and heart rate (f _H) were measured at rest and at each work load, and maximum oxygen intake ( max) and physical work capacity (PWC₁₅₀, PWC₁₇₀) were calculated.Anthropometric parameters did not differ significantly between sportsmen and sedentary subjects. max, PWC₁₅₀, and PWC₁₇₀ had higher correlations with LBM than with the other anthropometric parameters. max expressed in terms of LBM (ml/kg LBM/min) was the parameter which showed the clearest distinction between sportsmen and sedentary individuals. The sportsmen had higher max on the treadmill test than on the bicycle ergometer. PWC₁₅₀ and PWC₁₇₀ were higher on the bicycle than on the treadmill and had high correlations with max. Work efficiency was of the same order in both groups and showed negative correlation with the degree of obesity on the bicycle ergometer and positive correlation on the treadmill.     

Factors which alter the relationship between ventilation and carbon dioxide production during exercise in normal subjects

The slope of the linear relationship between ventilation and carbon dioxide production has been thought to indicate that is one of the major stimuli to . A group of 15 normal subjects undertook different incremental treadmill exercise protocols to explore the relationship between and . An incremental protocol using 1 instead of 3-min stages of exercise resulted in an increase in the to ratio [26.84 (SEM 1.23) vs 31.08 (SEM 1.36) (P < 0.008) for the first stage, 25.24 (SEM 0.86) vs 27.83 (SEM 0.91) (P < 0.005) for the second stage and 23.90 (SEM 0.86) vs 26.34 (SEM 0.81) (P = 0.001) for the third stage]. Voluntary hyperventilation to double the control level of during exercise resulted in an increase in the to slope [from 21.3 (SEM 0.71) for the control run to 35.1 (SEM 1.2) for the hyperventilation run (P < 0.001)]. Prolonged hyperventilation (5 min) during exercise at stage 2 of the Bruce protocol resulted in a continuted elevation of and the slope. A steady state of and metabolic gas exchange can only be said to have been present after at least 3 min of exercise. Voluntary hyperventilation increased the slope of the relationship between and . End-tidal carbon dioxide fell, but remained within the normal range. These results would suggest that a non-carbon dioxide factor may have been responsible for the increase we found in during exercise, and that factors other than increased dead space ventilation can cause an increased ventilation to slope, such as that seen in some pathophysiological conditions, such as chronic heart failure.     

Effects of prolonged exercise at a similar percentage of maximal oxygen consumption in trained and untrained subjects

Summary Six trained male cyclists and six untrained but physically active men participated in this study to test the hypothesis that the use of percentage maximal oxygen consumption (% , as a normalising independent variable is valid despite significant differences in the absolute of trained and untrained subjects. The subjects underwent an exercise test to exhaustion on a cycle ergometer to determine and lactate threshold. The subjects were grouped as trained (T) if their exceeded 60 ml ·kg^–1 ·min^–1, and untrained (UT) if their was less than 50 ml · kg^–1 · min-^–1. The subjects were required to exercise on the ergometer for up to 40 min at power outputs that corresponded to approximately 50% and 70% The allocation of each exercise session (50% or 70% was random and each session was separated by at least 5 days. During these tests venous blood was taken 10 min before exercise (–10 min), just prior to the commencement of exercise (–10 min), after 20 min of exercise (20 min), at the end of exercise and 10 min postexercise (+ 10 min) and analysed for concentrations of cortisol, [Na⁺], [K⁺], [CI^–], glucose, free fatty acid, lactate [la-], [NH₃], haemoglobin [Hb] and for packed cell volume. The oxygen consumption ( ) and related variables were measured at two time intervals (14–15 and 34–35 min) during the prolonged exercise tests. Rectal temperature was measured throughout both exercise sessions. There was a significant interaction effect between the level of training and exercise time at 50% for heart rate ( _c:) and venous [la^–]. At 70% and ventilation ( ) for the T group and and carbon dioxide production for the UT group increased significantly with time and there was a significant interaction effect forf _c, ]Ia^–1], [Hb] and [NH₃]. The change in body mass at 50% and 70% was significantly greater in the T group. The present study found that when two groups of male subjects with different absolute exercised at a similar percentage of some effector responses were significantly different, questioning the validity of selecting % as a normalising independent variable.     

Relationship between record time and maximal oxygen consumption in middle-distance running

Summary The relationship between record time (t _r) and maximal oxygen uptake ( ) has been examined in 69 male physical education students who had taken part in 800-m and 1500-m footraces. It was found thatt _r and were inversely related. The relationshipst _r=f( ) have been fitted by two exponential equations:t _r(1500 m)=698e –0.0145 t _r(800 m) = 272e^–0.01 P<0.001. A mathematical formulation of the energy conservation principle in supramaximal running, based on the exponential increase of the oxygen uptake as a function of time with a rate constant of 0.025 s^–1 has been applied to thet _r calculation from . As calculatedt _r were highly correlated to measuredt _r (P<0.001), it was concluded that the relationshipst _r=f( ) can be interpreted on the basis of the model described in this study.     

A new approach to assessing maximal aerobic power in children: the Odense School Child Study

Summary In two experiments maximal aerobic power calculated from maximal mechanical power (W _max) was evaluated in 39 children aged 9–11 years. A maximal multi-stage cycle ergometer exercise test was used with an increase in work load every 3 min. In the first experiment oxygen consumption was measured in 18 children during each of the prescribed work loads and a correction factor was calculated to estimate using the equation . An appropriate increase in work rate based on height was determined for boys (0.16 W · cm^–1) and girls (0.15 W · cm^–1) respectively. In the second experiment 21 children performed a maximal cycle ergometer exercise test twice. In addition to the procedure in the first experiment a similar exercise test was performed, but without measurement of oxygen uptake. Calculated correlated significantly (p<0.01) with those values measured in both boys (r=0.90) and girls (r=0.95) respectively, and the standard error of estimation for (calculated) on (measured) wass less than 3.2%. Two expressions of relative work load (% and %W _max) were established and found to be closely correlated. The relative work load in % could be predicted from the relative work load in % W _max with an average standard error of 3.8%. The data demonstrate that calculated based on a maximal multi-stage exercise test provides an accurate and valid estimate of      

Respiratory frequency and the oxygen cost of exercise

Summary Although many studies indicate that the spontaneous breathing frequency minimizes breathing work, the consequences of this for exercise energetics have never been investigated. To see if the spontaneous exercise breathing frequency minimizes oxygen uptake, we compared during treadmill walking (2/3 max) at several alternative frequencies. The alternative frequencies ranged from the lowest sustainable to a frequency twice the spontaneous value. All eight subjects adjusted tidal volume to comfort. Exercise oxygen uptake was constant, independent of breathing frequency. At the same time, minute ventilation rose to be 65% greater at the highest frequency than at the lowest (P<0.01). We then reproduced the various exercise frequencies, tidal volumes, and ventilations during seated isocapnic hyperpnea to measure with locomotory muscles at rest. Once again, oxygen uptake was constant, independent of breathing frequency. We conclude that the spontaneous exercise breathing frequency fails to minimize during either exercise or resting reproduction of exercise ventilation.Supported in part by NIH Grant HL 26351     

Cerebral blood flow distribution and systemic haemodynamic changes after repeated hyperbaric oxygen exposures in rats

The effects of acute and repeated exposures to 500 kPa O₂ on the distribution of cerebral blood flow and systemic haemodynamics were assessed in awake rats. After habituation, the control rats (group 1,n=7) were restrained for 1 h daily for 8 days in air at 101 kPa, while the test rats (group 2,n=8) were exposed to 500 kPa O₂ for 1 h daily for 8 consecutive days. During a final exposure, both groups were exposed to 500 kPa O₂. Systolic (BP_s) and mean arterial blood pressure (BP _a), and heart rate (f _c) were measured continuously from implanted arterial catheters; while cardiac output and regional were measured by the microsphere method in air before the O₂ exposure, and after both 5 min and 60 min at 500 kPa O₂ in all the animals. The baseline measurements in air of BP_s andBP _a were higher andf _c was lower in group 2, while the acid-base chemistries were similar in the two groups. Total was similar in both groups. However in group 2, blood flows and calculated O₂ supplies to colliculi, hippocampus, hypothalamus, and most cerebral cortical regions were higher, but lower to pons and medulla oblongata. During O₂ exposure andf _c decreased, andBP _a, BP_s, and peripheral vascular resistance increased in all the rats. Arterial partial pressure of CO₂ and [HCO₃ ^–] decreased in group 1, but remained at baseline levels in group 2. Total and decreased in both groups, and the distribution was altered. Calculated O₂ supplies to different brain regions varied according to the changes, so that most regions sustained baseline O₂ delivery, although O₂ delivery to some regions may have been reduced. The decline of also indicated reduced removal of waste from the brain, so that CO₂ tension and temperature could have been elevated, thereby potentiating the toxic effects of O₂ on brain cells. In conclusion, repeated O₂ exposures induced heterogeneous and persistent changes in , as well as a persistent increase in arterial pressure.     

The energetics of middle-distance running

Summary In order to assess the relative contribution of aerobic processes to running velocity (v), 27 male athletes were selected on the basis of their middle-distance performances over 800, 1500, 3000 or 5000 m, during the 1987 track season. To be selected for study, the average running velocity corresponding to their performances had to be superior to 90% of the best French of the season. Maximum O₂ consumption and energy cost of running (C) had been measured within the 2 months preceding the track season, which, together with oxygen consumption at rest allowed us to calculate the maximalv that could be sustained under aerobic conditions: . The treadmill runningv corresponding to a blood lactate of 4 mmol·^–1 (v _la4), was also calculated. In the whole group, C was significantly related to height (r=–0.43;P<0.03). Neither C nor (with, in this case, the exception of the 3000 m athletes) were correlated to . On the other hand,v _{a max} was significantly correlated to over distances longer than 800 m. These were also correlated tov _la4. Howeverv _la4 occurred at 87.5% SD 3.3% ofv _{a max}, this relationship was interpreted as being an expression of the correlation betweenv _{a max} and . Calculation ofv _{a max} provided a useful means of analysing the performances. At the level of achievement studied, sustained over 3000 m corresponded tov _{a max}. The shape of the relationship ofv/v _{a max} as a function of the duration of the event raised the question of a possible change in C as a function of v during middle-distance running competitions.