Influence of light physical activity on cardiac responses during recovery from exercise in humans

To examine the influence of light exercise on cardiac responses during recovery from exercise, we measured heart rate (HR), stroke volume (SV), and cardiac output (Q˙ _c) in five healthy untrained male subjects in an upright position before, during, and after 10-min steady-state cycle exercise at an exercise intensity of 170 W, corresponding to a mean of 68 (SD 4)% of maximal oxygen uptake. The recovery phase was evaluated separately for three different conditions: 10 min of complete rest (passive recovery), 7 min of pedalling at 20-W exercise intensity followed by 3 min of rest (partially active recovery), and 7 min of pedalling at 40-W exercise intensity followed by 3 min of rest (partially active recovery), on an upright cycle ergometer. The time courses of decreases in HR in the two active recovery phases at different exercise intensities were almost identical to those in the passive recovery phase. However, the subsequent HR reductions during the rest after active recovery at 20 W and at 40 W were mean 7.5 (SD 4.4) and mean 10.0 (SD 3.1) beats · min⁻¹, respectively, both of which were significantly larger (P<0.05 and P<0.005) than the corresponding reduction [1.4 (SD 2.5) beats · min⁻¹] for passive recovery. The SV values at the two exercise intensities during the active recovery periods were maintained at levels similar to that during 170-W steady-state exercise. In contrast, the SV during passive recovery decreased gradually to a level significantly below the initial baseline level at rest before exercise (P<0.05). The resultant time courses of CO values during active recovery were significantly higher (each P<0.05) than that during passive recovery. It was concluded from these findings that light post-exercise physical activity plays an important role in facilitating the venous return from the muscles and in restoring the elevated HR to the pre-exercise resting level. Accepted: 17 September 1997     

Part B: Revisions to the COMFA outdoor thermal comfort model for application to subjects performing physical activity

This study assessed the performance of the COMFA outdoor thermal comfort model on subjects performing moderate to vigorous physical activity. Field tests were conducted on 27 subjects performing 30 min of steady-state activity (walking, running, and cycling) in an outdoor environment. The predicted COMFA budgets were compared to the actual thermal sensation (ATS) votes provided by participants during each 5-min interval. The results revealed a normal distribution in the subjects’ ATS votes, with 82% of votes received in categories 0 (neutral) to +2 (warm). The ATS votes were significantly dependent upon sex, air temperature, short and long-wave radiation, wind speed, and metabolic activity rate. There was a significant positive correlation between the ATS and predicted budgets (Spearman’s rho = 0.574, P < 0.01). However, the predicted budgets did not display a normal distribution, and the model produced erroneous estimates of the heat and moisture exchange between the human body and the ambient environment in 6% of the cases.     

Effect of exercise-induced muscle damage on the blood lactate response to incremental exercise in humans

Eccentric muscle actions are known to induce temporary muscle damage, delayed onset muscle soreness (DOMS) and muscle weakness that may persist for several days. The purpose of the present study was to determine whether DOMS-inducing exercise affects blood lactate responses to subsequent incremental dynamic exercise. Physiological and metabolic responses to a standardised incremental exercise task were measured two days after the performance of an eccentric exercise bout or in a control (no prior exercise) condition. Ten healthy recreationally active subjects (9 male, 1 female), aged 20 (SD 1) years performed repeated eccentric muscle actions during 40 min of bench stepping (knee high step; 15 steps · min⁻¹). Two days after the eccentric exercise, while the subjects experienced DOMS, they cycled on a basket loaded cycle ergometer at a starting work rate of 150 W, with increments of 50 W every 2 min until fatigue. The order of the preceding treatments (eccentric exercise or control) was randomised and the treatments were carried out 2 weeks apart. Two days after the eccentric exercise, all subjects reported leg muscle soreness and exhibited elevated levels of plasma creatine kinase activity (P < 0.05). Endurance time and peak V˙O₂ during cycling were unaffected by the prior eccentric exercise. Minute volume, respiratory exchange ratio and heart rate responses were similar but venous blood lactate concentration was higher (P < 0.05) during cycling after eccentric exercise compared with the control condition. Peak blood lactate concentration, observed at 2 min post-exercise was also higher [12.6 (SD 1.4) vs 10.9 SD (1.3) mM; P < 0.01]. The higher blood lactate concentration during cycling exercise after prior eccentric exercise may be attributable to an increased rate of glycogenolysis possibly arising from an increased recruitment of Type II muscle fibres. It follows that determination of lactate thresholds for the purpose of fitness assessment in subjects experiencing DOMS is not appropriate. Accepted: 27 September 1997     

Heart rate deflection compared to 4 mmol?·?l?1 lactate threshold during incremental exercise and to lactate during steady-state exercise on an arm-cranking ergometer in paraplegic athletes

The deflection point (DP) of the heart rate in relation to the work rate (WR) of 8 male endurance-trained paraplegics and 11 male physically active sports students was investigated during nonsteady-state incremental arm cranking ergometry (IT) and compared to the 4 mmol · l⁻¹ blood lactate concentration threshold and to blood lactate concentration in steady-state exercise (SST). Heart rate, and lactate concentration from capillary blood, were determined at rest, during IT and SST. The DP was calculated by linear regression analysis of the heart rate during IT. The SST consisted of three consecutive exercise intensities over a period of 8 min at exercise intensities of 10 W below, and at 10 W above the work rate at deflection point (WR_DP). No difference was found between the paraplegics and non-handicapped subjects regarding heart rate and blood lactate concentration at rest and during exercise. A DP was established in all the paraplegics and in 72.7% of the non-handicapped subjects, but lactate accumulation was observed in 75% of the paraplegics and in 62.5% of the non-handicapped subjects at the lowest intensity of SST. In summary, endurance-trained paraplegics with an injury level below T₅ showed heart rate and blood lactate concentration values comparable to non-handicapped subjects during IT. A linear increase at moderate exercise intensities and a levelling-off at higher to maximal intensities could be identified in all the paraplegics and in 72.7% of non-handicapped subjects. The determination of the anaerobic threshold by DP should be applied with caution, since no causal relationship of DP and the anaerobic threshold was found and the WR_DP tended to overestimate threshold values. Accepted: 9 February 1998     

Effects of repeated exercise/rest sessions at –10°C on skin and rectal temperatures in men wearing chemical protective clothing

The development of thermophysiological responses during four consecutive exercise/rest sessions in the cold was studied in men wearing chemical protective clothing and a face mask. Six men repeated four exercise/rest sessions during 8 h at –10°C. Each session consisted of step exercise (240 W · m⁻²) for 60 min and rest for another 60 min. Rectal and skin temperatures were measured continuously and thermal sensations were obtained at 30-min intervals. Entering the cold from a warm environment and the onset of exercise resulted in a decrease in skin temperatures during the first session and the decrement in the temperatures of the extremities continued for 10–20 min during the following period of exercise. Torso skin temperature was at its lowest during the first rest period. After the first session of cold exposure the range and the level of variation in mean body temperature (Tˉ _b) followed a pattern which was repeated until the end of the experiment. However, the torso skin temperatures increased gradually until the fourth session, while the temperatures of the extremities, in contrast, tended to decrease up to the third session. In conclusion, the present results indicated that although Tˉ _b, reflecting the whole body heat balance, showed a typical pattern of change after the first session (2 h), the torso area was warming until the end of the cold exposure while the extremities continued to cool down up to the third session (6 h), obviously due to a prolonged redistribution of the circulation. Accepted: 29 May 1998     

The influence of cold stress and a 36- h fast on the physiological responses to prolonged intermittent walking in man

In a previous study, rectal temperature (T _re) was found to be lower, and oxygen consumption (V˙O₂) and the respiratory exchange ratio (R) were higher in a cold (+5°C), wet and windy environment (COLD), compared with a thermoneutral environment during intermittent walking at ≈30% of peak V˙O₂ (Weller AS, Millard CE, Stroud MA et al. Am J Physiol 272:R226–R233, 1997). The aim of the present study was to establish whether these cold-induced responses are influenced by prior fasting, as impaired thermoregulation has been demonstrated in cold-exposed, resting men following a 48-h fast. To address this question, eight men attempted a 360-min intermittent (15 min rest, 45 min exercise) walking protocol under COLD conditions on two occasions. In one condition, the subjects started the exercise protocol ≈120 min after a standard meal (FED/COLD), whereas in the other the subjects had fasted for 36 h (FASTED/COLD). The first two exercise periods were conducted at a higher intensity (HIGHER, 6 km · h⁻¹ and 10% incline), than the four subsequent exercise periods (LOW, 5 km · h⁻¹ and 0% incline). There was no difference in the time endured in FED/COLD and FASTED/COLD. In FASTED/COLD com pared with FED/COLD, R was lower during HIGHER and LOW, and T _re was lower during LOW, whereas there was no difference in V˙O₂, mean skin temperature and heart rate. Therefore, although the 36-h fast impaired temperature regulation during intermittent low-intensity exercise in the cold, wet and windy environment, it was unlikely to have been the principal factor limiting exercise performance under these experimental conditions. Accepted: 26 August 1997     

Part A: Assessing the performance of the COMFA outdoor thermal comfort model on subjects performing physical activity

This study assessed the performance of the COMFA outdoor thermal comfort model on subjects performing moderate to vigorous physical activity. Field tests were conducted on 27 subjects performing 30 min of steady-state activity (walking, running, and cycling) in an outdoor environment. The predicted COMFA budgets were compared to the actual thermal sensation (ATS) votes provided by participants during each 5-min interval. The results revealed a normal distribution in the subjects’ ATS votes, with 82% of votes received in categories 0 (neutral) to +2 (warm). The ATS votes were significantly dependent upon sex, air temperature, short and long-wave radiation, wind speed, and metabolic activity rate. There was a significant positive correlation between the ATS and predicted budgets (Spearman’s rho?=?0.574, P?<?0.01). However, the predicted budgets did not display a normal distribution, and the model produced erroneous estimates of the heat and moisture exchange between the human body and the ambient environment in 6% of the cases.     

Metabolic and ventilatory responses to steady state exercise relative to lactate thresholds

The metabolic and ventilatory responses to steady state submaximal exercise on the cycle ergometer were compared at four intensities in 8 healthy subjects. The trials were performed so that, after a 10 min adaptation period, power output was adjusted to maintain steady state VO2 for 30 min at values equivalent to: (1) the aerobic threshold (AeT); (2) between the aerobic and the anaerobic threshold (AeTAnT); (3) the anaerobic threshold (AnT); and (4) between the anaerobic threshold and VO2max (AnTmax). Blood lactate concentration and ventilatory equivalents for O2 and CO2 demonstrated steady state values during the last 20 min of exercise at the AeT, AeAnT and AnT intensities, but increased progressively until fatigue in the AnTmax trial (mean time = 16 min). Serum glycerol levels were significantly higher at 40 min of exercise on the AeAnT and the AnT when compared to AeT, while the respiratory exchange ratios were not significantly different from each other. Thus, metabolic and ventilatory steady state can be maintained during prolonged exercise at intensities up to and including the AnT, and fat continues to be a major fuel source when exercise intensities are increased from the AeT to the AnT in steady state conditions. The blood lactate response to exercise suggests that, for the organism as a whole, anaerobic glycolysis plays a minor role in the energy release system at exercise intensities upt to and including the AnT during steady state conditions.     

Exercise in a cold environment after sleep deprivation

Seven subjects exercised to thermal comfort in a cold environment (O degrees C, 2.5 m X s-1) after normal sleep (control) and following a 50-h period of sleep deprivation. Resting core temperature (rectal) taken before the subject entered the cold environment was significantly lower (-0.5 degrees C, P less than 0.05) following the 50-h period of wakefulness. However, rectal temperature was not different after 15 min of exercise during the two exposures, suggesting that the subjects stored heat more rapidly during the first 15 min of exercise after sleep deprivation. No significant differences in self-chosen exercise intensity, significant differences in self-chosen exercise intensity, heart rate, metabolic rate, or exercise time were evident between the control and sleep deprived exposures. Fifty hours of sleep deprivation failed to alter the core temperature response during exercise in severe cold stress, and subjects chose identical work rates to minimize fatigue and cold sensation. The results suggest that the 50-h sleep deprivation period was not a true physiological stress during exercise in a cold environment. (Supported by Contract #DAMD 17-81-C1023.)     

Implications of moderate altitude training for sea-level endurance in elite distance runners

Elite distance runners participated in one of two studies designed to investigate the effects of moderate altitude training (inspiratory partial pressure of oxygen ≈115–125 mmHg) on submaximal, maximal and supramaximal exercise performance following return to sea-level. Study 1 (New Mexico, USA) involved 14 subjects who were assigned to a 4-week altitude training camp (1500–2000 m) whilst 9 performance-matched subjects continued with an identical training programme at sea-level (CON). Ten EXP subjects who trained at 1640 m and 19 CON subjects also participated in study 2 (Krugersdorp, South Africa). Selected metabolic and cardiorespiratory parameters were determined with the subjects at rest and during exercise 21 days prior to (PRE) and 10 and 20 days following their return to sea-level (POST). Whole blood lactate decreased by 23% (P < 0.05 vs PRE) during submaximal exercise in the EXP group only after 20 days at sea-level (study 1). However, the lactate threshold and other measures of running economy remained unchanged. Similarly, supramaximal performance during a standardised track session did not change. Study 2 demonstrated that hypoxia per se did not alter performance. In contrast, in the EXP group supramaximal running velocity decreased by 2% (P < 0.05) after 20 days at sea-level. Both studies were characterised by a 50% increase in the frequency of upper respiratory and gastrointestinal tract infections during the altitude sojourns, and two male subjects were diagnosed with infectious mononucleosis following their return to sea-level (study 1). Group mean plasma glutamine concentrations at rest decreased by 19% or 143 (74) μM (P < 0.001) after 3 weeks at altitude, which may have been implicated in the increased incidence of infectious illness. Accepted: 19 March 1998     

Changes in amino acid concentration in plasma and type I and type II fibres during resistance exercise and recovery in human subjects

Eight male subjects performed leg press exercise, 4 × 10 repetitions at 80% of their maximum. Venous blood samples were taken before, during exercise and repeatedly during 2 h of recovery. From four subjects, biopsies were taken from the vastus lateralis muscle prior to, immediately after and following one and 2 h of recovery. Samples were freeze-dried, individual muscle fibres were dissected out and identified as type I or type II. Resistance exercise led to pronounced reductions in the glutamate concentration in both type I (32%) and type II fibres (70%). Alanine concentration was elevated 60–75% in both fibre types and 29% in plasma. Glutamine concentration remained unchanged after exercise; although 2 h later the concentrations in both types of fibres were reduced 30–35%. Two hours after exercise, the plasma levels of glutamate and six of the essential amino acids, including the branched-chain amino acids were reduced 5–30%. The data suggest that glutamate acts as an important intermediate in muscle energy metabolism during resistance exercise, especially in type II fibres.     

Effects of hydration state on hormonal and renal responses during moderate exercise in the heat

The effects of hydromineral hormones and catecholamines on renal concentrating ability at different hydration states were examined in five male volunteers while they performed three trials. Each of these trials comprised a 60-min exercise bout on a treadmill (at 50% of maximal oxygen uptake) in a warm environment (dry bulb temperature, 35°C; relative humidity, 20–30%). In one session, subjects were euhydrated before exercise (C). In the two other sessions, after thermal dehydration (loss of 3% body mass) which markedly reduced plasma volume (PV) and increased plasma osmolality (osm_pl), the subjects exercised either not rehydrated (Dh) or rehydrated (Rh) by drinking 600 ml of mineral water before and 40 min after the onset of exercise. During exercise in the Dh compared to C state, plasma renin, aldosterone, arginine vasopressin (AVP), noradrenaline and adrenaline concentrations were increased (P < 0.05). A reduction in creatinine clearance and urine flow was also observed (P < 0.05) together with a decrease in urine osmolality, osmolar clearance and sodium excretion, while free water clearance increased (P < 0.05). However, compared to Dh, Rh partially restored PV and osm_pl and induced a marked reduction in the time courses of both the plasma AVP and catecholamine responses (P < 0.05). Values for renal water and electrolyte excretion were intermediate between those of Dh and C. Plasma atrial natriuretic peptide presented similar changes whatever the hydration state. These results demonstrate that during moderate exercise in the heat, renal concentrating ability is paradoxically reduced by prior dehydration in spite of high plasma AVP levels, and might be the result of marked activation of the sympatho-adrenal system. Rehydration, by reducing this activation, could partially restore the renal concentrating ability despite the lowered plasma AVP. Accepted: 23 April 1997     

Case study of skin temperature and thermal perception in a hot outdoor environment

Focusing on the understanding and the estimation of the biometeorological conditions during summer in outdoor places, a field study was conducted in July 2010 in Athens, Greece over 6 days at three different sites: Syntagma Square, Ermou Street and Flisvos coast. Thermo-physiological measurements of five subjects were carried out from morning to evening for each site, simultaneously with meteorological measurements and subjective assessments of thermal sensation reported by questionnaires. The thermo-physiological variables measured were skin temperature, heat flux and metabolic heat production, while meteorological measurements included air temperature, relative humidity, wind speed, globe temperature, ground surface temperature and global radiation. The possible relation of skin temperature with the meteorological parameters was examined. Theoretical values of mean skin temperature and mean radiant temperature were estimated applying the MENEX model and were compared with the measured values. Two biometeorological indices, thermal sensation (TS) and heat load (HL)—were calculated in order to compare the predicted thermal sensation with the actual thermal vote. The theoretically estimated values of skin temperature were underestimated in relation to the measured values, while the theoretical model of mean radiant temperature was more sensitive to variations of solar radiation compared to the experimental values. TS index underestimated the thermal sensation of the five subjects when their thermal vote was ‘hot’ or ‘very hot’ and overestimated thermal sensation in the case of ‘neutral’. The HL index predicted with greater accuracy thermal sensation tending to overestimate the thermal sensation of the subjects.     

Thermoregulatory responses during exercise and a hot water immersion and the affective responses to peripheral thermal stimuli

Tympanic (Tty), mean skin (¯Tsk) and mean body (¯Tb) temperatures and heart rate (HR) increased more in low Vo₂ max group (LG) than in high Vo₂ max group (HG) during exercise. The regression coefficient of body temperatures (Tty and ¯Tb) on HR and the increased rate of heat storage were larger in LG than in HG during exercise. The local sweat rate (per min/cm²) during a hot water bath exhibited a considerable large quantity in comparison with the amount during exercise. Internal and skin temperatures during a hot water bath increased more immediately than those during exercise. The levels of comfort sensation during the preovulatory phase in women and pre-exercise period in men were higher at 40C than at 20C as peripheral thermal stimulus. The levels during the postovulatory and post-exercise phases in the same subjects were higher with the cool stimuli than with the warm stimuli. Above results suggest that thermoregulatory responses during submaximal exercise are different according to physical fitness and that these responses are different from those during hot water immersion. In addition, these suggest that the scores of thermal sensation with warm and cool stimuli are different during the pre- and post-ovulatory phases and the pre- and post-exercise periods.     

Calorimetric study of the effect of salicylate in man during heat exposure and exercise (author's transl)]

1. The effect of sodium acetylo-salicylate (2 g per os) on the thermoregulatory responses of 10 male subjects was studied by direct and indirect calorimetry during two tests : heat exposure at 37 degrees C and exercise (50 W) at 25 degrees C. Both test were performed twice : with salicylate treatment and with a placebo. 2. During heat exposure at 37 degrees C for 75 min, the rise in tympanic temperature (Tty) and in mean skin temperature Ts, the time course of heat losses by radiation (R), convection (C) and evaporation (E), and the metabolic rate (M), measured by oxygen consumption, were not altered by salicylate treatment. 3. During exercise, salicylate treatment did not affect the time course of Tty and Ts, (R + C) and M. However, salicylate treatment decreased the delay for triggering the evaporative response (E) to the thermal load; similarly, the increase in cutaneous blood flow was triggered sooner in subjected receiving salicylate than in controls. 4. In conclusion, these results suggest that, during exercise, the thermal controller triggers thermoregulatory responses during passive hyperthermia by heat exposure.     

Thermoregulatory and metabolic responses to repeated bouts of prolonged cycle-ergometer exercise in man

Changes in body temperature, oxygen uptake (VO2), heart rate (HR), sweating rate and plasma osmolarity were examined in 10 human subjects, performing four successive 30 min exercise-bouts of the same intensity (50% VO2 max) separated by 30 min rest periods. In spite of the rest intervals and replacement of body fluid loss there was a progressive increase in VO2. HR, rectal (Tre) and mean body (Tb) temperatures in consecutive exercise bouts. The thermoregulatory efficiency showed an increasing tendency, and a delay in the sweating response at the beginning of each exercise was shortened. It is concluded that a drift in metabolic and temperature responses to exercise, reported throughout a long-term continuous work, occurs also in the euhydrated subjects performing a prolonged intermittent exercise. It is not caused by an impaired thermoregulation during exercise but rather by insufficient restitution of metabolic processes during rest intervals.     

Effects of ambient temperature steps on thermal comfort requirements

The purpose of this study was to determine the thermal comfort requirements for steps in temperature. Thirty male subjects were exposed for 50 min to a 34 or 37°C condition, and then quickly transferred to a cooler environment of 31, 28, 25, and 22°C for 50 min. Mean skin temperature was continuously measured, and the subjects reported their thermal sensation and comfort sensation every 2 min. Just after the step changes, the mean skin temperature immediately decreased, while the thermal sensation overshot and gradually rose again. Both the skin temperature and the thermal sensation seemed to reach a constant level within about 20 min. However, there were differences in the mean skin temperature and the neutral temperature derived from the correlation between the ambient temperature and the thermal sensation even 50 min after the steps, due to the thermal environmental condition before the changes of temperature. The change in the neutral temperature with time was expressed as two attenuating equations. These equations indicate that there is an obvious difference between the neutral temperatures due to the thermal condition before step changes, and that it takes >50 min after the step changes to reach the steady state. It is expected that these equations predict in quantitative terms the thermal comfort requirements within a given experimental condition.     

Correlation between heat tolerance during exercise and maximum aerobic work capacity

Observation of the physiological responses during exercise in a hot environment and measurement of maximal work capacity were made on eight young male subjects, ages 20--22. Exercise was performed on a bicycle ergometer at a constant work load of 450 kg . m/min at a cycling rate of 50 rpm for 30 min in a climatic chamber at 30 degree C with 70% relative humidity. The maximum work capacity was measured by bicycle ergometer exercise. Heat tolerance during exercise was assessed by the magnitude of physiological strain expressed by the combination of relative rise in rectal temperature, relative water loss and relative salt loss. Heat load during exercise was calculated using metabolic rates at rest and during exercise, assuming heat loss through the respiratory tract to be 10 percent of metabolic rate. Fairly good correlations were found between the ratio of work done to maximum work capacity and rise in rectal temperature, ratio of body weight loss to body weight and heat tolerance during exercise. Close correlations were found among relative heat load during exercise and rise in rectal temperature, relative body weight loss and heat tolerance. Heat tolerance during exercise in a hot environment correlated well to capacity of heat dissipation and maximum work capacity.     

Increases in factor VIII complex and fibrinolytic activity are dependent on exercise intensity

Components of the factor VIII complex increase and activation of the fibrinolytic system occur during exercise. The relation between the duration and intensity of exercise and the relative changes in the VIII complex and fibrinolytic system have not been previously examined. Five healthy male subjects were exercised with three protocols: a graded progressive exercise test to exhaustion on a cycle ergometer with 50-W increments every 4 min, steady-state exercise, 15 min at 5 and 125 W each, and an acute 30-s maximal exercise test on a cycle ergometer. Venous blood samples were drawn at base line, during the last 30 s of each power output in the graded exercise, at 5-min intervals for the steady-state exercise, and for up to 1 h after completion of exercise in all three protocols. At the maximum exercise intensities, increases in plasma lactate concentration ([La]), O2 uptake, and [H+] were observed. Components of the VIII complex [VIII procoagulant, VIII procoagulant antigen, VIII-related antigen (VIIIR:Ag), VIII ristocetin cofactor activity] abruptly rose at only the highest work intensities, whereas the whole blood clot lysis time began to gradually shorten much earlier at low work intensities. There were no qualitative changes in the factor VIIIR:Ag on crossed immunoelectrophoresis nor was there evidence of thrombin generation as determined by fibrinopeptide A generation. We conclude that during exercise the changes observed in the coagulation and fibrinolytic systems are related to the intensity of the exercise, which is reflected by increases in plasma [La] and [H+], and that the fibrinolytic system is activated before the changes in the VIII complex are observed.