The relationship between test protocol and the development of exercise-induced hypoxemia (EIH) in highly trained athletes

Healthy male endurance-trained cyclists [n = 11, age = 27.3 (3.9) years; mass = 73.0 (9.3) kg; height = 180.5 (6.9) cm; maximal oxygen consumption ( = 71.1 (5.8) ml · kg^–1 · min^–1, mean ± (SD)] were recruited to assess the relationship between test protocol and the development of desaturation of arterial hemoglobin with oxygen, during incremental exercise tests to maximal aerobic capacity . All subjects demonstrated resting pulmonary function within normal limits [forced vital capacity (FVC) = 6.0 (0.9); forced expiratory volume (FEV_1.0) = 4.9 (0.6); FEV_1.0/FVC = 0.8 (0.1)] and completed three ramped tests (Mijnhardt KEM-3 electronically braked cycle ergometer) beginning at 0 W with increments of either 20, 30 or 40 W · min^–1. All periods of testing were separated by a minimum of 72 h. , peak minute ventilation (Medical Graphics, CPX-D), peak heart rate (ƒ_cpeak)), peak power output , and minimum percentage arterial oxyhemoglobin saturation (%S _aO_2min) (Omeda Biox 3740 pulse oximeter) were determined. There were no significant differences (p > 0.05) in [191.5 (26.2), 196.0 (24.4), 194.3 (23.9) 1 · min^–1] ƒ_cpeak [191.4 (7.0), 190.3 (5.5), 187.8 (5.9) beats · min^–1], [5.0 (0.5), 5.1 (0.4), 5.1 (0.5) 1 · min^–1] or %S _aO_2min [89.5 (1.5), 89.6 (1.3), 90.0 (2.3)] between protocols. The 20-W protocol [417 (27) W] demonstrated significantly lower (P < 0.05) than the 30-W [434 (36) W] and 40-W [453 (38) W] protocols, indicating that peripheral fatigue may play an important factor in response to these tests. The results of this study demonstrate that arterial desaturation occurs as a result of intense exercise in highly trained athletes independent of the rate of attainment of .     

Energy consumption of paraplegic locomotion using reciprocating gait orthosis

The energy cost of walking using a reciprocating gait orthosis (RGOII) with functional electrical stimulation (FES) was assessed in 14 patients with spastic complete paraplegia from six rehabilitation centres. Before and after training asing RGOII with FES, the subjects performed a progressive maximal test on an arm-crank ergometer to obtain their laboratory peak oxygen uptake heart rate (HR) and blood lactate concentration changes. At the end of the training session, oxygen uptake was measured during a walking test with orthosis at different speeds (6 min steady state at 0.1 m · s⁻¹, followed by 2-min stages at progressively increasing speeds up to exhaustion). Of the subjects 4 repeated this test using orthosis without FES. At a speed of 0.1 m · s⁻¹, represented 47 (SD 23)% of , mean HR was 137 (SD 21) beats · min⁻¹ and mean blood lactate concentration 2.4 (SD 1.4) mmol · l⁻¹. Maximal speed ranged from 0.23 to 0.5 m · s⁻¹. At maximal speed, was 91 (SD 18) % of mean HR reached 96 (SD 7)% and mean blood lactate concentration only 52 (SD 19)% of the maximal values measured during the laboratory test. Walking without electrical stimulation induced an increase in HR but there was no difference in and blood lactate compared to walking with stimulation. The training period did not result in any improvement in maximal physiological data. We concluded that the free cadence walking speed with orthosis remains much lower than that of able-bodied people or wheelchair users. The metabolic cost at a given speed is much higher even if, using a stimulation device, the cardiovascular stress is reduced.     

Fitness as a determinant of oxygen uptake response to constant-load exercise

Summary Exercise performed above the lactate threshold (Θ _La) produces a slowly-developing phase of oxygen uptake ( ) kinetics which elevates above that predicted from the sub-Θ _La -work rate relationship. This phenomenon has only been demonstrated, to date, in subjects who were relatively homogeneous with respect to fitness. This investigation therefore examined whether this behaviour occurred at a given absolute or whether it was a characteristic of supra-Θ _La exercise in a group of subjects with over a threefold range ofΘ _La (990–3000 ml O₂·min⁻¹) and peak (1600–5260 ml O₂·min⁻¹). Twelve healthy subjects performed: 1) exhausting incremental cycle ergometer exercise for estimation ofΘ _La ( ) and peak , and 11) a series of constant-load tests above and below for determination of the profile and efficiency of work. During all tests expired ventilation, and carbon dioxide production were monitored breath-by-breath. The efficiency of work determined during incremental exercise (28.1±0.7%, ,n=12) did not differ from that determined during sub- constant-load exercise (27.4±0.5%,p>0.05). For constant-load exercise, rose above that predicted, from the sub- -work rate relationship, for all supra- work rates. This was evident above 990 ml O₂·min⁻¹ in the least fit subject but only above 3000 ml O₂·min⁻¹ in the fittest subject. As a consequence the efficiency of work was reduced from 27.4±0.5% for sub- exercise to 22.6±0.4% (p<0.05) at the lowest supra- work rate (i.e. +20 W, on average). The efficiency of work generally decreased further at the higher supra- work rates. We conclude that the response to constant-load exercise includes an additional slow phase of the kinetics for all exercise intensities above irrespective of the fitness of the subject. Consequently, measurements of the aerobic efficiency of work during constant-load exercise must rigorously constrain the exercise intensity to the sub- domain. Supported by grants from the John D. and Catherine T. Mac-Arthur Foundation, USPHS RR 00865-15, and NIH HL 07694-01     

Hormonal and metabolic response to three types of exercise of equal duration and external work output

Summary Five normal men, aged 20–30 years, participated in three types of exercise (I, II, III) of equal duration (20 min) and total external work output (120–180 kJ) separated by ten days of rest. Exercises consisted of seven sets of squats with barbells on the shoulders (I; Maximal Power Output _max=600−900 W), continuous cycling at 50 rev · min⁻¹ (II; _max=100−150 W) and seven bouts of intermittent cycling at 70 rev · min⁻¹ (III; _max=300−450 W). Plasma cortisol, glucagon and lactate increased significantly (P<0.05) during the exercise and recovery periods of the anaerobic, intermittent exercise (I and III) but not in the continuous, aerobic exercise (II). No consistent significant changes were found in plasma glucose. Plasma insulin levels decreased only during exercise II. The highest increase in cortisol and glucagon was not associated with the highest , , _max or HR; however it was associated with the anaerobic component of exercise (lactic acid). It is suggested that in exercises of equal duration and total external work output, the continuous, aerobic exercise (II) led to lowest levels of glucogenic hormones.     

A prediction equation for indirect assessment of anaerobic threshold in male distance runners

Summary The predictability of anaerobic threshold (AT) from maximal aerobic power, distance running performance, chronological age, and total running distance achieved on the treadmill (TRD) was investigated in a sample of 53 male distance runners, 17–23 years of age. The dependent variable was oxygen uptake ( ) at which AT was detected (i. e., @AT). A regression analysis of the data indicated @AT could be predicted from the following four measurements with a multipleR=0.831 and a standard error of the estimate of 2.66 ml · min⁻¹ · kg⁻¹: (67.9±5.7 ml · min⁻¹ · kg⁻¹), 1,500-m running performance (254.5±14.2 s), TRD (6.82±1.13 km), and age (19.4±2.2 years). When independent variables were limited to (X ₁) and 1,500-m running performance (X ₂) for simpler assessment, a multipleR=0.806 and a standard error of the estimate of 2.76 ml · min⁻¹ · kg⁻¹ were computed. A useful prediction equation with this predictive accuracy was considered to be @AT= 0.386X₁−0.128X₂+57.11. To determine if the prediction equation developed for the 53 male distance runners could be generalized to other samples, cross-validation of the equation was tested, using 21 different distance runners, 17–22 years of age. A high correlation (R=0.927) was obtained between @AT predicted from the above equation and directly measured @AT. It is concluded that the generalized equation may be applicable to young distance runners for indirect assessment of @AT. This study was supported by grants from The Descente Foundation for the Promotion of Sports Science, awarded to K. Tanaka     

Residual analysis in the determination of factors affecting the estimates of body heat storage in clothed subjects

Body heat storage can be estimated by calorimetry (from heat gains and losses) or by thermometry [from changes (Δ) in mean body temperature (T _b) calculated as a weighted combination of rectal (T _re) and mean skin temperatures (T _sk)]. If an invariant weighting factor ofT _re andT _sk were to be used (for instance, ΔT _b = 0.8 · ΔT _re + 0.2 · ΔT _sk under hot conditions), body heat storage could be over- or underestimated substantially relative to calorimetry, depending on whether the subject was wearing light or protective clothing. This study investigated whether discrepancies between calorimetry and thermometry arise from methodological errors in the calorimetric estimate of heat storage, from inappropriate weightings in the thermometric estimate, or from both. Residuals of calorimetry versus thermometric estimates were plotted against individual variables in the standard heat balance equation, applying various weighting factors toT _re andT _sk. Whether light or protective clothing was worn, the calorimetric approach generally gave appropriate estimates of heat exchange components and thus heat storage. One exception was in estimating latent heat loss from sweat evaporation. If sweat evaporation exceeded 650 g·h⁻¹ when wearing normal clothing, evaporative heat loss was overestimated and thus body heat storage was underestimated. Nevertheless, if data beyond this ceiling were excluded from the analyses, the standard 4:1 weighting matched calorimetric heat storage estimates quite well. When wearing protective clothing, the same 4:1 weighting approximated calorimetric heat storage with errors of less than approximately 10%, but only if environmental conditions allowed a subject to exercise for more than 90 min. The best thermometric estimates of heat storage were provided by using two sets of relative weightings, based upon the individual's metabolic heat production ( in kilojoules per metre squared per hour): {4 − [( )· ] 2}:1 for an initial, thermoneutral environment and {4 + [( ) · ] · 5}: 1 for a final, hot environment; the optimal value of lay between 450 and 500 kJ m⁻² · h⁻¹. We concluded that the accuracy of thermometric estimates of heat storage can be improved by modifying weighting factors ofT _re andT _sk according to the environment, type of clothing, and metabolic rate.     

Cardiorespiratory adaptation with short term training in older men

Summary The purpose of this study was to assess the rate of training-induced cardiorespiratory adaptations in older men [mean (SD), 66.5 (1.2) years]. The eight subjects trained an average of 4.3 (0.3) times each week. The walk/jog training was in two phases with 4 weeks (phase 1) at a speed to elicit 70% of pre-training maximal oxygen consumption ( ), and 5 weeks (phase 2) at 80%. Maximal exercise treadmill tests and a standardized submaximal protocol were performed prior to training, at weekly intervals during the training programme, and after training. (ml·kg^–1·min^–1) increased significantly over both phases: 6.6% after the first 4 weeks, and an additional 5.2% after the final 5 weeks. The weekly changes in over phase 1 were well fitted by an exponential association curve (r=0.75). The half-time for the rate of adaptation was 13.8 days, or 8.3 training sessions. Over phase 2, the change in did not plateau and a time course could not be determined. Submaximal exercise heart rate (f _c ) was reduced a significant 10 beats · min^–1 after the first 4 weeks, and a further 6 beats · min^–1 over the final 5 weeks. Thef _c reductions showed half-times of 9.1 days (phase 1) and 9.8 days (phase 2) (or 5-6 training sessions). The anaerobic ventilation threshold was increased 13.9% over the 9 weeks of training and the respiratory exchange ratio during constant load heavy exercise was significantly reduced; however, these changes could not be described by an exponential time course. Thus, short-term exercise training of older men resulted in significant and rapid cardiorespiratory improvements.     

Hyperpnoea and CO2 sensitivity of the respiratory centres during exercise

Summary The aim of this study was to specify whether exercise hyperpnoea was related to the CO₂ sensitivity of the respiratory centres measured during steady-state exercise of mild intensity. Thus, ventilation , breathing pattern [tidal volume (V _T), respiratory frequency (f), inspiratory time (T _I), total time of the respiratory cycle (T _TOT),V _T/T _I,T _I/T _TOT] and CO₂ sensitivity of the respiratory centres determined by the rebreathing method were measured at rest (SCO_{2 re}) and during steady-state exercise (SCO_{2 ex}) of mild intensity [CO₂ output =20 ml·kg⁻¹·min⁻¹] in 11 sedentary male subjects (aged 20–34 years). The results showed that SCO_{2 re} and SCO_{2 ex} were not significantly different. During exercise, there was no correlation between and SCO_{2 ex} and, for the same , all subjects had very close values normalized for body mass (bm), regardless of their SCO_{2 ex} ( =1.44 l·min⁻¹·kg⁻¹ SD 0.10). A highly significant positive correlation between SCO_{2 ex} andV _T (normalised for bm) (r=0.80,P<0.01),T _I (r=0.77,P<0.01) andT _TOT (r=0.77,P<0.01) existed, as well as a highly significant negative correlation between SCO_{2 ex} and (normalised for bm^−0.25) (r=−0.73,P<0.01). We conclude that the hyperpnoea during steady-state exercise of mild intensity is not related to the SCO_{2 ex}. The relationship between breathing pattern and SCO_{2 ex} suggests that the breathing pattern could influence the determination of the SCO_{2 ex}. This finding needs further investigation.     

Maximal mechanical power output and capacity of cyclists and young adults

Summary The maximal average power output ( ) has been examined in 10 male students, 22 pursuit and 12 sprint cyclists. In 24 of these subjects (8 students, 10 pursuit and 6 sprint cyclists), estimates of the maximal capacity ( ) of the short-term anaerobic energy yielding processes were made. The results show that the sprinters had a higher absolute (1241±266 W) and (16.7±4.9 kJ) than either the students (1019± 183 W, 14.7±2.8 kJ) or the pursuit cyclists (962±206 W, 14.0±2.9 kJ). However, the differences were removed when the values were standardised for muscle size. In the sprinters theW _max was attained at an optimal pedal frequency of 132±3 min⁻¹ and the estimated maximal velocity of pedalling ( ) was 262±8 min⁻¹. The comparable figures in the students and pursuit cyclists were 118±8 min⁻¹, 235±17 min⁻¹ and 122±6 min⁻¹, 242±12 min⁻¹ respectively. The coefficient of variation of duplicate measurements of was found to be ±9%. Using data of Wilkie (1968) for muscle phosphagen and glycolytic stores (27 mmol · kg⁻¹), it was estimated that the probable efficiency of the anaerobic processes during maximal cycling was 0.22. It was concluded that and are largely determined by body size and muscularity. The efficiency of anaerobiosis appears to be of the same order of magnitude as found for oxidative work.     

Skeletal muscle properties and performance in elite female track athletes

Summary Selected biochemical and physiological properties of skeletal muscle were studied in light of performance capabilities in 24 elite female track athletes. The feasibility of quantifying end point histochemistry and relating oxidative staining density (reduced nicotinomide adenine dinucleotide diaphorase: NADH-D) to whole body maximal oxygen consumption ( max) was also investigated, while muscle fiber types, classified according to alkaline APTase stains, were studied and related to muscle oxidative capacity (succinate dehydrogenase: SDH), max and “in vivo” torque-velocity properties. Muscle biopsies were taken from the vastus lateralis of each subject and maximal knee extensor torques were recorded at 30‡ from full extension at four selected velocities. While results confirm earlier reports on skeletal muscle properties and performance it was concluded that end point histochemistry could be reliably quantified and that an “oxidative” stain such as NADH-D correlates extremely well with max (r=0.86,p<0.001) whereas correlations between %slow twitch fibres (alkaline ATPase stain) and max were lower (r=0.44,p<0.05). Additionally, as knee extension velocity increased from 0–1.7 rad·s⁻¹ angle specific extensor torque production did not decline as observed in vitro and pentathletes displayed significantly larger torques at all velocities when compared to the other athletes. These data confirm that while myofibrillar ATPase staining correlates with force-velocity properties of muscle, max is better correlated with quantified oxidative staining. Supported in part by a Public Health Service Biomedical Research Support Grant to UCLA, Public Health Service Grant 10423 and by the National Aeronautics and Space Administration (NASA)     

Comparison of oxygen kinetics in young and old subjects

Summary Five older men (aged 60–69 yr) and five young men (aged 21–29 yr) with approximately equal levels of age-corrected max were compared with respect to oxygen kinetics at equal absolute workloads (100 watts) and at equal relative workloads (45% max) on a cycle ergometer. At 45% max, half times for response to instantaneous transition from unloaded pedalling were 30.0 s and 27.4 s for old and young respectively (t=0.260,p<0.80). No significant differences were found in the response and by inference none existed in O₂ extraction. Mean half times for heart rate responses at a workload of 100 W were 24.2 s and 20.6 s for old and young groups respectively (t=0.722,p<0.49). Mechanical efficiency estimated from steady state data at 100 W was 19.8% and 20.5% for old and young groups respectively (t=0.574). The close similarity in responses to submaximal work in old and young subjects of equivalent fitness suggests caution in the interpretation of agewise decrements observed in physiological variables which may be sensitive to physical fitness status.     

Oxygen consumption and metabolic strain in rowing ergometer exercise

Summary Oxygen consumption ( ) when rowing was determined on a mechanically braked rowing ergometer (RE) with an electronic measuring device. was measured by an open spirometric system. The pneumotachograph valve was fixed to the sliding seat, thus reducing movement artefacts. A multi-stage test was performed, beginning with a work load of 150 W and increasing by 50 W every 2 minutes up to exhaustion. Serum lactate concentrations were determined in a 30 s break between the work stages. 61 examinations of oarsmen performing at maximum power of 5 W · kg⁻¹ or more were analysed. and heart rate (HR) for each working stage were measured and the regression line of on the work load (P) and an estimation error (s_xy) were calculated: (ml · min⁻¹) (S_xy = ± 337 ml,r = 0.98) Good reproducibility was found in repeated examinations. Similar spiroergometry was carried out on a bicycle ergometer (BE) with 10 well trained rowers and 6 trained cyclists. of rowing was about 600 ml · min⁻¹ higher than for bicycling in the submaximal stages for both groups. The of RE exercise was 2.6% higher than for oarsmen on BE, and the cyclists reached a greater on BE than the oarsmen. No differences were found between RE and BE exercise heart rate. The net work efficiency when rowing was 19% for both groups, experienced and inexperienced: when cycling it was 25% for cyclists and 23% for oarsmen.     

Ventilatory anaerobic threshold in healthy children

Summary The ventilatory anaerobic threshold (VAT) during graded exercise was defined as the oxygen uptake ( ) immediately below the exercise intensity at which pulmonary ventilation increased disproportionally relative to . Since VAT is considered to be a sensitive and noninvasive measure for evaluating cardiorespiratoy endurance performance, the purpose of the present study was to determine normal values in children. We examined 257 healthy children (140 boys and 117 girls) varying in age from 5.7 to 18.5 years, during treadmill exercise. The data were analyzed in relation to sex and age. In boys the lowest (ml · min⁻¹ · kg⁻¹) was found in the youngest age group (5–6 year). In girls, on the other hand, no significant increase occurred with age. For VAT, expressed as ml O₂ · min⁻¹ · kg⁻¹ or as a percent of a significant decrease was found in boys and girls with age. This suggests an increase in lactacid anaerobic capacity during growth. In contrast to observations in adults, only low correlations were found between and VAT (r=0.28 in boys and r=0.52 in girls), which suggests that the development of the underlying physiological mechanism does not occur at the same rate in growing children. These data provide normal values for VAT that can be used for clinical exercise testing in the pediatric age group.     

Effects of passive heat adaptation and moderate sweatless conditioning on responses to cold and heat

Summary Two series of experiments were performed in physically untrained subjects. In series A (heat adaptation, HA), seven male subjects were adapted to dry heat (five consecutive days at 55‡ C ambient air temperature (T_a) for 1 h · day⁻¹) under resting conditions. Before and after HA, the subjects' shivering responses were determined in a cold test (T_a+10 to 0‡ C). In series B, eight male subjects underwent mild exercise training (five consecutive days at a heart rate, HR, of 120b · min⁻¹) under T_a conditions individually adjusted (T_a + 15 to +5‡ C) to prevent both sweating and cold sensations. Before and after “sweatless training≓, the subjects were subjected to a combined cold and heat test. During HA the thresholds for shivering, cutaneous vasodilatation (thumb and forearm) and sweating were shifted significantly (p<0.05) towards lower mean body temperatures ( ). The mean decrease in threshold was 0.36‡ C. “Sweatless training≓ resulted in a mean increase in work rate (at HR 120b · min⁻¹) and oxygen pulse of 13 and 8%, respectively. However, “sweatless training≓ did not change the threshold for shivering or sweating. Neither HA nor “sweatless training≓ changed the slopes of the relationships of shivering and sweating to . It is concluded that the previously reported lowering of shivering and sweating threshold in long-distance runners is not due to an increased fitness level, but is essentially identical with HA. The decreased shivering threshold following HA is interpreted as “cross adaptation≓ produced by the Stressors cold and heat. This study was supported by the Deutsche Forschungsgemeinschaft (Br 184/16-3)     

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.     

Isometric fatigue induced by different levels of rhythmic exercise

Summary Three subjects were trained in leg extensor isometric contractions and in cycling. They then cycled for three consecutive bouts, each of 2.75 min at a constant level of , from 20 to 80% max. Fifteen seconds after each bout of cycling the subjects exerted an isometric contraction of the right leg at 40% of the maximum voluntary contraction. In each experiment, the duration of the three successive isometric contractions decreased as in hand-grip contractions. There was also a linear reduction in isometric endurance as the severity of the preceding rhythmic exercise increased. In other experiments, after three bouts of rhythmic exercise at 20% max (each followed by a fatiguing contraction at 40% MVC), further bouts of cycling at increasing levels of severity up to 60% max resulted in a linear fall in isometric endurance which could be reversed by interposing a lighter level of cycling. The heart rates during these experiments showed a steady increase during the isometric exercise, to about 150 beats·min⁻¹, as the bouts of preceding rhythmic exercise became progressively more severe. The isometric contractions had little influence on the heart rate during cycling. But the rhythmic cycling exercise markedly increased the heart rate achieved at the end of the isometric contractions while decreasing theincrement in heart rateduring the contraction. Supported by HEW Contract HSM 99-71-21, and U.S.N. Grant N00014-77-0640     

Passive temperature lability in the elderly

Thermoregulatory responses of nine healthy elderly [seven men and two women; mean age (SD) 73.9 (4.8) years] were compared to those of nine young adult men [26.6 (5.2) years]. They exercised on a cycle ergometer for 20 min at an intensity inducing a heart rate equivalent to 65% of their predicted maximum, and were thereafter immersed in 28°C water. The exercise was conducted to elevate tympanic temperature (T _ty) and initiate a steady rate of sweating. The post-exercise immersion period induced gradual cooling ofT _ty, and changes inT _ty relative to resting levels (ΔT _ty) at which sweating abated and shivering commenced were defined as the ΔT _ty thresholds for the cessation of sweating (T _sw) and onset of shivering (T _sh), respectively. In addition toT _ty, oxygen uptake ( ; 1 · min⁻¹), sweating rate (g · m⁻² · min⁻¹), and forehead skin blood perfusion were also measured during the trials. The mean (SD)T _sw occurred at a significantly (P <0.005) higher ΔT _ty [0.48 (0.18)°C] in the elderly than in the young adults [0.21(0.06)°C], while the Tsh occurred at significantly (P < 0.005) lower ΔT _ty in the elderly [ −0.64 (0.34)°C] than in young adults [−0.22 (0.10)°C]. Decreases in ΔT _ty below the shivering threshold were met with a significantly (P <0.01) reduced . The range of temperature lability between Ts, andT _sh, defined as the null-zone, was significantly greater in the elderly [1.12 (0.39)°C] than in the young adults [0.43 (0.12)°C], and the slope of the vasoconstrictor response in the null-zone was significantly (P <0.001) lower in the elderly subjects. The present study demonstrates a greater passive core temperature lability in older individuals, since the effector responses of sweating and shivering were initiated at higher and lower levels ofT _ty, respectively. The magnitudes of the effector responses beyond the thresholds were also significantly reduced, suggesting that the elderly may be more susceptible to hypo-/hyperthermia during periods of endogenous and/or exogenous thermal stress.     

Heart rate changes caused by varying the oxygen supply to isolated hind legs of rats

Summary In a rat with an isolated hind leg circulation perfused with varying tyrode solutions, heart rate (HR) changes were studied in dependence of in the isolated hind leg and of , [K⁺], pH and lactic acid concentration ([Lac]) measured in the venous outflow of the isolated hind leg. In experimental series I the inflow was kept constantly high (either about 65 or 72 kPa). The perfusion pressure alternated between 16 and 24 kPa leading to flow rates in isolated hind legs ( ) from 30 to 50 ml · 100 g⁻¹ · min⁻¹. The depended on the momentary (flow-limited oxygen uptake). The [K⁺] and [Lac], the pH and the remained nearly constant while the was lower at small flow rates. The HR decreases some 4 min after initial enhancement of and . Series II comprised experiments with low flow rates and a medium oxygen supply ( =2.5−17.4 ml · 100 g⁻¹ · min⁻¹), =17.5−62.7 kPa). The ranged between 0.02 and 0.2 ml · 100 g⁻¹ · min⁻¹. The [K⁺] and [Lac], the and the HR increased while the pH decreased. The [Lac] in the outflow showed a strong dependence on oxygen uptake and — at a weak oxygen supply — on the time. Cross-correlation analyses between the parameters confirmed that the HR was best temporally correlated to the [Lac] in the outflow. In series III a 17 min perfusion of normoxic solution ( =65.3 kPa) was followed by perfusion with a hypoxic tyrode solution ( =8.7 kPa). was 30 ml · 100 g⁻¹ · min⁻¹. The [Lac], the and the HR increased accompanied by a decrease in pH. However a HR increase was observed only when the actual values of [Lac], and pH exceeded their normal ranges for a resting muscle. The results support the hypothesis that heart rate is additionally influenced by metabolic muscle receptors measuring lactic acid concentration in working muscle.     

Time of day effects on sympathoadrenal and pressor reactivity to exercise in healthy men

Summary To investigate the influence of time of day on sympathoadrenal and pressor reactivity during exercise, eight trained men [age, mean (SD), 24 (0.5) years; maximal oxygen uptake ( ), 4.7 l·min^–1] performed bouts of static (ST) and dynamic (DYN) exercise at 0600–0800 hours (AM) and at 1600–1800 hours (PM). The ST protocol utilized a two-leg isometric contraction at 30% maximum voluntary contraction until failure, and was monitored by a strain gauge interfaced from a leg extension apparatus to a computer. Heart rate (f_c) and blood pressure ( ) responses were recorded at rest, after 1 and 2 min of exercise, and at failure. Epinephrine (EPI) and norepinephrine (NE) levels were recorded before exercise, and after 2 min of exercise. The DYN exercise protocol involved stationary. cycling for consecutive 6-min periods at 60% and 80% . f_c, , EPI, and NE were recorded before exercise and at each workload. No differences were observed in preexercise or exercise f_c under any condition. Preexercise did not differ under any condition. The response to DYN was significantly higher at 80% during PM only. was significantly higher in ST-PM at 1 min, 2 min, and failure. Elevations in both systolic and diastolic P _a contributed to this difference. Preexercise EPI-ST-AM was significantly elevated vs PM, but no other preexercise data were significantly different. Absolute exercise levels were significantly higher for EPIST-PM vs AM only, but the percentage change from baseline was significantly (P<0.01) higher in ST-PM for EPI (+231% PM vs + 32% AM) and NE (+352% PM vs +216% AM). The EPI and NE responses to DYN exercise tended to be higher in AM, but were not significantly different. These data support a time of day pattern in sympathoadrenal and pressor reactivity to exercise that is dependent on the type of activity involved but independent of baseline patterns.     

Cardiac output during exercise in paraplegic subjects

Summary The purpose of this study was to measure the cardiac output using the CO₂ rebreathing method during submaximal and maximal arm cranking exercise in six male paraplegic subjects with a high level of spinal cord injury (HP). They were compared with eight able bodied subjects (AB) who were not trained in arm exercise. Maximal O₂ consumption ( O_2max) was lower in HP (1.1 1·min^–1, SD 0.1; 17.5 ml·min^–·kg, SD 4) than in AB (2.5 1·min^–1, SD 0.6; 36.7 ml·min^–1·kg, SD 10.7). Maximal cardiac output was similar in the groups (HP, 141·min^–1 SD 2.6; AB, 16.81·min^–1 SD 4). The same result was obtained for maximal heart rate (f _c,max (HP, 175 beats·min^–1, SD 18; AB, 187 beats·min^–, SD 16) and the maximal stroke volume (HP, 82 ml, SD 13; AB, 91 ml, SD 27). The slopes of the relationshipf _c/ O₂ were higher in HP than AB (P<0.025) but when expressed as a % O_2max there were no differences. The results suggests a major alteration of oxygen transport capacity to active muscle mass in paraplegics due to changes in vasomotor regulation below the level of the lesion.