Resting echocardiographic parameters after cessation of regular endurance training

Resting echocardiograms were examined in nonathletic healthy young men (controls, n = 16), in highly trained endurance athletes (n = 20), and in endurance athletes who stopped regular training (n = 40). The relative muscular wall thickness (Rel. MWTd), left ventricular internal diameters both in diastole and in systole (LVIDd, LVIDs), thus also the end-diastolic and end-systolic volumes (LVEDV, LVESV), and the stroke volume index (SVI) were greater in the endurance athletes still in training than in the nonathletes. The ejection fraction (EF), heart rate (HR), cardiac index (CI), and mean circumferential shortening velocity (Vcf) were significantly lower in the athletes. During the 60 days of detraining no change was seen in the Rel. MWTd, LVEDV, LVESV, and HR. The SVI became even greater; EF and Vcf rose up to the control level while CI exceeded it. The cardiovascular regulation is therefore assumed to undergo a peculiar shift during detraining in that a persisting cardiac enlargement and bradycardia is associated with a temporarily unstable autonomous control. This imbalance often leads to a hyperkinesis-like syndrome when an athlete stops endurance training abruptly.     

The effect of age and gender on heart rate variability after endurance training

PURPOSE: This research investigated the age and gender differences in cardiovascular adaptation to a standardized/quantified endurance-training program that included two taper periods. METHODS: The latter was analyzed from spectral analysis of electrocardiogram records of heart rate variability (HRV) at rest in groups of young (19-21 yr) and middle aged (40-45 yr), mixed gender groups (6 males and 6 females), pre- and poststandardized training. All subjects were recreational runners who completed the same 12-wk running program. Before, and subsequent to training, HRV was measured during supine rest and submaximal cycling. RESULTS: There was a significant decrease in heart rate both at rest (2.7 +/- 0.45 beats x min-1) and during submaximal exercise (8.1 +/- 0.67 beats x min-1) in both age groups after training. After training, total spectral power increased (560.7 +/- 308.9 ms2), as well as high-frequency power (362.3 +/- 405.5 ms2), in both age groups at rest. The young group showed a greater increase in total power (849.0 +/- 308.7 ms2) after the training program. CONCLUSION: It is concluded that a well-designed 12-wk endurance-training program will decrease resting and submaximal heart rate in both younger and older adults. The significant increase in HRV, total power, and high-frequency power in all groups after endurance training indicates that HRV measurement appears to provide an effective, noninvasive assessment of cardiovascular adaptation to aerobic training.     

Influence of short-term endurance exercise training on heart rate variability

PURPOSE: To examine the influence of 2 wk (eight sessions) of endurance training on cardiac autonomic modulation, as measured by heart rate variability (HRV). METHODS: Twenty-four males (mean age: 23.1 yr) were randomized to an exercise (EX; N = 12) or control group (CT; N = 12). EX trained for eight sessions (4x wk-1, 40 min, 80-85% HRreserve) on a cycle ergometer. ECG tracings were collected during 5 min of paced breathing (12 breaths x min-1 (PB)), 5 min of spontaneous breathing (SB1), 5 min of 70 degrees head-up tilt (TILT), and a second 5-min period of spontaneous breathing (SB2). Data were collected before (test 1), during (tests 2-4), and 48 h after (test 5) the 2-wk period. HRV was reported as the standard deviation of RR intervals, and as natural logarithm of the normalized units (NU) of high- and low-frequency power (lnHF and lnLF). RESULTS: EX exhibited a significant increase in peak oxygen consumption (8%). During PB and TILT conditions, ANOVA revealed a group x time interaction such that EX exhibited lower lnLFNU and lnLF/lnHF during test 5 compared with test 1. CONCLUSION: These data suggest that eight endurance exercise-training sessions performed over 2 wk enhance the relative vagal modulation of the heart during PB and TILT, but not during SB.     

The effect of heart rate controlled low resistance circuit weight training and endurance training on maximal aerobic power in sedentary adults

The effects of a 12-week low resistance circuit weight training (CWT) on cardiovascular and muscular fitness were studied in 90 healthy sedentary adults. The subjects were randomized into three equally fit groups: CWT, Endurance (END) and Control (CON) according to their maximal aerobic power (VO2max). Both training groups exercised for 12 weeks, 3 days a week in sessions of 40 min, with a heart rate (HR) level of 70-80% HRmax. The CWT group trained with air resistance machines. Heart rate was controlled by setting the speed of movement. The END group walked, jogged, cross-country skied or cycled. The net differences (between pre- and posttraining changes) between the CWT and CON groups was statistically significant for VO2max (2.45 ml x min(-1) x kg(-1), 95% CI 1.1; 3.8), for abdominal muscles (3.7 reps, CI 0.3; 7.1), for push-ups (1.1 reps, CI 0.2; 2.1), and for kneeling (2.25 reps, CI 0.01; 4.5). The net difference (between pre- and posttraining changes) in the END and CON groups was statistically significant for VO2max (2.75 ml(-1) x min(-1) x kg(-1), 95% CI 0.9; 4.6), and kneeling (3.0 reps, CI 0.7; 5.3). Low resistance CWT with moderately hard HR level has effects comparable to an equal amount of endurance training on the cardiovascular fitness of sedentary adults. The CWT model was benefical also on muscular fitness. Based on the results, this type of exercise can be recommended for beginners because of its multilevel effects.     

Strength/endurance training versus endurance training in congestive heart failure

PURPOSE: The aim of this study was to compare the effects of endurance training alone (ET) with combined endurance and strength training (CT) on hemodynamic and strength parameters in patients with congestive heart failure (CHF). METHODS: Twenty male patients with CHF were randomized into one of two training regimens consisting of endurance training or a combination of endurance and resistance training. Group ET had 40-min interval cycle ergometer endurance training three times per week. Group CT combined endurance and strength training with the same interval endurance training for 20 min, followed by 20 min of strength training. Left ventricular function was assessed at baseline and after 40 training sessions by echocardiography and radionuclide ventriculography. Work capacity was measured with cardiopulmonary exercise test (CPX) and lactate determination. Strength was measured with an isokinetic dynamometer. RESULTS: After 40 sessions, the ET group improved functional class, work capacity, peak torque, and muscular endurance. However, peak O2 remained unchanged. Left ventricular ejection fraction (LVEF) and fractional shortening (FS) decreased, whereas left ventricular end-diastolic diameter (LVED) increased. The CT group improved NYHA score, working capacity, peak O2, and peak lactate; peak torque and muscular endurance, LVEF, and FS increased, whereas LVED decreased. Compared with ET, CT was significantly (P < 0.05) better in improving LV function.CONCLUSION: Combined endurance/strength training was superior to endurance training alone concerning improvement of LV function, peak VO2, and strength parameters. It appears that for stable CHF patients, a greater benefit can be derived from this training modality.     

Exercise training and heart rate variability in older people.

PURPOSE: Heart rate variability (HRV), a characteristic that is potentially increased by physical activity, has been associated with incidence of cardiac events and total mortality. Since the incidence of cardiac events among older people is high and their physical activity levels and HRV are generally low, it is important to investigate whether regular physical activity can modify HRV in this age group. The purpose of the study was to investigate the effect of regular physical activity on HRV in older men and women. METHODS: In a randomized controlled trial, the effect of six months' training on HRV was investigated in a group of 51 older men and women (67.0 +/- 5.1 yr). The training group gathered three times per week for 45 min supervised training. RESULTS: At the end of the intervention period, HRV was higher primarily during the day. During daytime, the SD of all normal intervals (+6%) as well as the low frequency component (+ 15%) and the very low frequency component (+ 10%) of HRV were significantly increased (P < 0.05) as compared with the control group. Effects of training were most pronounced in subjects inactive in sports at baseline. CONCLUSION: This study demonstrates that regular physical activity increases HRV (specifically in the very low and low frequency components) in older subjects. Hence, in older subjects, physical training may be an effective means to modify positively a factor that is associated with increased incidence of cardiac events.     

Respiratory muscle endurance after training in athletes and non-athletes: A systematic review and meta-analysis

The objectives of this systematic review was to evaluate the effects of respiratory muscle training (RMT) on respiratory muscle endurance (RME) and to determine the RME test that demonstrates the most consistent changes after RMT. Electronic searches were conducted in EMBASE, MEDLINE, COCHRANE CENTRAL, CINHAL and SPORTDiscus. The PEDro scale was used for quality assessment and meta-analysis were performed to compare effect sizes of different RME tests. Twenty studies met the inclusion criteria. Isocapnic hyperpnea training was performed in 40% of the studies. Meta-analysis showed that RMT improves RME in athletes (P = 0.0007) and non-athletes (P = 0.001). Subgroup analysis showed differences among tests; maximal sustainable ventilatory capacity (MSVC) and maximal sustainable threshold loading tests demonstrated significant improvement after RMT (P = 0.007; P = 0.003 respectively) compared to the maximal voluntary ventilation (MVV) (P = 0.11) in athletes whereas significant improvement after RMT was only shown by MSVC in non-athletes. The effect size of MSVC was greater compared to MVV in studies that performed both tests. The meta-analysis results provide evidence that RMT improves RME in athletes and non-athletes and MSVC test that examine endurance over several minutes are more sensitive to improvement after RMT.     

Monitoring and adapting endurance training on the basis of heart rate variability monitored by wearable technologies: A systematic review with meta-analysis

ObjectivesTo systematically perform a meta-analysis of the scientific literature to determine whether the outcomes of endurance training based on heart rate variability (HRV) are more favorable than those of predefined training.DesignSystematic review and meta-analysis.MethodsPubMed and Web of Science were searched systematically in March of 2020 using keywords related to endurance, the ANS, and training. To compare the outcomes of HRV-guided and predefined training, Hedges' g effect size and associated 95% confidence intervals were calculated.ResultsA total of 8 studies (198 participants) were identified comprising 9 interventions involving a variety of approaches. Compared to predefined training, most HRV-guided interventions included fewer moderate- and/or high-intensity training sessions. Fixed effects meta-analysis revealed a significant medium-sized positive effect of HRV-guided training on submaximal physiological parameters (g = 0.296, 95% CI 0.031 to 0.562, p = 0.028), but its effects on performance (g = 0.079, 95% CI −0.050 to 0.393, p = 0.597) and V̇O2peak (g = 0.171, 95% CI −0.213 to 0.371, p = 0.130) were small and not statistically significant. Moreover, with regards to performance, HRV-guided training was associated with fewer non-responders and more positive responders.ConclusionsIn comparison to predefined training, HRV-guided endurance training had a medium-sized effect on submaximal physiological parameters, but only a small and non-significant influence on performance and V̇O_2peak. There were fewer non-responders regarding performance with HRV-based training.     

Intense endurance training on heart rate and blood pressure variability in runners.

Physical training with incomplete recovery times can produce significant fatigue. A study of cardiovascular responses showed that there is a sympathetic and a parasympathetic form of fatigue. PURPOSE: The purpose of this experimentation was to measure the effects of intense endurance training on autonomic balance through a spectral analysis study of the heart rate (HR) and systolic blood pressure (SBP). METHODS: Eight elite runners were tested twice: after a relative rest period (RRP) of 3 wk and after an 12-wk intense training period (ITP) for endurance. At the end of each phase, the subjects were tested by means of a VO2max test and a tilt-table test. RESULTS: The resting heart rate (HR) variability was lower (P < 0.001) in the intensive training phase. Likewise, there were differences in the low-frequency (0.04-0.150 Hz; LF) and high-frequency (0.150-0.500 Hz; HF) components and the LF/HF ratio of the HR spectral analysis. The LF spectral power was significantly lower in the supine position (P < 0.05) during ITP. Upright tilting was accompanied by a 22.6% reduction in HF values during the rest period, whereas in ITP the HF spectral power rose by 31.2% (P < 0.01) during tilt, characterizing a greater parasympathetic system control. Sympathetic control represented by the LF/HF ratio regressed markedly (P < 0.01) in response to the tilt test in ITP. CONCLUSIONS: The spectral analysis of SBP in the high frequencies shows that the changes in cardiac parameters are coupled with a decrease in sympathetic vasomotor control (-18%) and a reduction in diastolic pressure (-3.2%) in the response to the tilt test at the end of ITP. Spectral analysis could be a means of demonstrating impairment of autonomic balance for the purpose of detecting a state of fatigue that could result in overtraining.     

Heart rate and blood pressure changes with endurance training: the HERITAGE Family Study

PURPOSE: The purpose of this study was to determine the magnitude of change in resting and exercise heart rate (HR) and blood pressure (BP), by race, sex, and age, after a 20-wk endurance training program in 507 healthy and previously sedentary subjects from the HERITAGE Family Study. METHODS: After baseline measurements, subjects exercised on cycle ergometers 3 d x wk(-1) for a total of 60 exercise sessions starting at 55% of VO2max for 30 min x session(-1) and building to 75% of VO2max for 50 min x session(-1) for the last 6 wk. HR and BP at rest and during exercise (50 W, 60% of VO2max maximal exercise) were each determined in duplicate on two different days both before and after training (resting values at 24-h and 72-h posttraining). RESULTS: After the period of training, there was a small decrease in resting HR (-2.7 to -4.6 beats x min(-1) across groups at 72-h posttraining), and small changes (i.e., < 3 mm Hg) in resting systolic (SBP), diastolic (DBP), and calculated mean BP (MBP), which varied by race, sex, and age. During exercise at the same absolute work rate (50 W), HR, SBP, DBP, and MBP were all significantly reduced, with greater reductions in HR in women compared with men, and greater reductions in BP in blacks and older subjects compared with whites and younger subjects, respectively. At the same relative work rate (60% VO2max), HR, DBP, and MBP were reduced, but SBP remained unchanged. Blacks had a greater reduction in DBP, but whites had a greater reduction in HR. Finally, at maximal exercise, there was a small decrease in HR, with men and whites decreasing more than women and blacks; an 8 mm Hg increase in SBP, with men increasing more than women; a 4 mm Hg decrease in DBP, with blacks decreasing more than whites; and no change in MBP. CONCLUSION: In conclusion, the reductions in resting HR and BP with training were generally small, but the reductions during exercise were substantial and clinically important, with the older and the black populations experiencing greater reductions.     

Aerobic capacity of older adults: a training study

The effects of a four month aerobic conditioning program on heart rate, blood pressure, maximal oxygen consumption (VO2max), and physical work capacity of 55-70 year old sedentary individuals were evaluated. Twenty-eight men and women participated in either 4 months of supervised fast walking or jogging at a prescribed target heart rate or stretching exercises for one hour, three days per week. Gains in VO2max (ml/kg/min) obtained during a Balke maximal treadmill test in aerobic and exercise control subjects were 27% and 9%, respectively. At posttesting subjects in both groups demonstrated improved maximal work rate, increased treadmill time, and experienced lower resting and recovery heart rates, lower resting systolic blood pressure, and fewer premature ventricular depolarizations during exercise testing. In 67 physician-supervised maximal exercise tests, only one subject did not achieve VO2max due to exercise induced arrhythmias. No events of morbidity or mortality occurred as a result of the exercise testing and training. Subjects were contacted 4 years after study participation to determine if they were adhering to an exercise program. Sixty-four percent reported exercising at least 3 days per week in large muscle activities. We conclude that four months of supervised aerobic and nonaerobic exercise training is sufficient to improve aerobic capacity and other indicators of fitness in older, sedentary men and women, and that these previously sedentary people are likely to continue exercising on an individual basis once they have experienced improved physical capacity.     

A comparison of heart rate methods for predicting endurance training intensity.

Comparing results of various training studies is often confounded by use of different indices of exercise intensity. Two frequently used indices are: 1) the % HR max method (exercise at a HR corresponding to a chosen percentage of maximal HR), and 2) this study, % net VO2max was employed as the criterion measure of exercise intensity and a comparison was made between the prediction of % net VO2-MAXby the above two methods. Experiments were performed on nine males (M +/- SD age = 23.7 +/- 2.6 yrs) and consisted of double determinations of resting VO2 and multiple measurements of resting HR under five different conditions. Duplicate determinations of maximal VO2 and HR preceded a third treadmill test consisting of four, 5-minute workloads representing approx 25, 45, 65, and 85% net VO2max. The results demonstrated that the KM's prediction of exercise intensity was not significantly different (P greater than 0.05) from measured intensity at any of the four workloads. Additionally, the % HR max procedure yielded large, significant overpredictions of exercise intensity (29, 22, 16 and 8% overpredictions for the four submaximal intensities, respectively. These results suggest that the KM yields a training HR that reflects exercise intensity within reasonable limits of accuracy.     

Substrate source use in older, trained males after decades of endurance training

PURPOSE: The purpose of this study was to compare substrate source use in older, long-term exercising, endurance-trained males with sedentary controls. METHODS: [U-C]palmitate and [6,6-H2]glucose tracers were applied to assess plasma free fatty acid (FFA) and glucose oxidation rates, and to estimate muscle- and/or lipoprotein-derived triacylglycerol (TG) and muscle glycogen use. Subjects were 10 long-term exercising, endurance-trained males and 10 sedentary controls (age 57 +/- 1 and 60 +/- 2 yr, respectively). Muscle biopsy samples were collected before and after exercise to assess muscle fiber type-specific intramyocellular lipid and glycogen content. RESULTS: During exercise, plasma palmitate Ra, Rd, and Rox were significantly greater in the trained subjects compared with the controls (Ra: 0.36 +/- 0.02 and 0.25 +/- 0.02; Rd: 0.36 +/- 0.03 and 0.24 +/- 0.02; Rox: 0.31 +/- 0.02 and 0.20 +/- 0.02 mmol.min, respectively, P < 0.01). This resulted in greater plasma FFA and total fat oxidation rates in the trained versus sedentary subjects (P < 0.001). Muscle- and/or lipoprotein-derived TG use contributed 10 +/- 2 and 11 +/- 3% in the trained and control groups, respectively (NS). No significant net changes in muscle fiber lipid content were observed. CONCLUSIONS: Older, endurance-trained males oxidize more fat during moderate-intensity exercise than do sedentary controls. This greater total fat oxidation rate is attributed to a higher plasma FFA release, uptake, and oxidation rate. In contrast, intramyocellular triacylglycerol does not seem to represent a major substrate source during 1 h of moderate-intensity exercise in older trained or sedentary men.     

Analysis of heart rate variability after a ranger training course

We studied the effects of prolonged physical activities on resting heart rate variability (HRV) during a training session attended by 23 cadets of the French military academy. This course lasts 1 month and is concluded by a 5-day field exercise simulation with physical and psychological stress. Data collection took place before (B) and immediately at the end (E) of the course. It included HRV recordings during a stand test (5 minutes lying down and 5 minutes standing), with a Polar R-R monitor, followed by blood sampling to assay plasma testosterone. The results (B and E) showed that the testosterone level fell by approximately 28.6 +/- 7%, indicating a high level of fatigue. During the stand test, the total power (TP) of the HRV spectrum increased in a supine position. The TP of B was 5,515.7 ms2 (SE, 718.4) and of E was 13018.9 ms2 (SE, 2,539.2; p < 0.001). High-frequency (HF) normalized values increased and low-frequency (LF) normalized values fell, regardless of position (HF normalized values and LF normalized values: supine, p < 0.01, p < 0.05; standing, p < 0.05, p < 0.01, respectively). LF:HF ratio fell 66.2 (SE, 12.9%; p < 0.01) in a lying position. During the time-domain analysis of HRV, differences between adjacent normal R-R intervals more than 50 milliseconds, expressed as a percentage, and differences between the coupling intervals of adjacent normal RR intervals increased in the lying position (p < 0.001). These results as a whole suggest that parasympathetic nervous system activity increases with fatigue.     

Percentages of maximal heart rate, heart rate reserve and VO2max for determining endurance training intensity in male runners

The use of 60%-95% of maximal heart rate (HR), heart rate reserve (HRR) and VO2max as exercise training intensities was examined in male runners, and these intensities were related to VO2 observed at the lactate threshold (LT) and fixed blood lactate concentrations (FBLC) of 2.0, 2.5, and 4.0 mM. Thirty-one subjects (means age = 29.9 +/- 9.1 yrs; means ht = 177.3 +/- 8.2 cm; means wt = 69.2 +/- 9.9 kg) completed a level running treadmill protocol. The mean values at LT, FBLC of 2.0, 2.5, 4.0 mM and max for VO2 were 52.7, 56.4, 58.0, 61.2 and 63.5 ml/kg.min -1, respectively: for velocity they were 237.4, 252.2, 260.6, 274.4 and 286.5 m/min, respectively; and for HR were 165.7, 172.7, 176.5, 182.3 and 187.4 bts/min, respectively. The majority of subjects were not above LT (N = 20), until an intensity of 90% HR max was attained. At 95% HR max the majority of subjects were above 2.0 mM (N = 23) and 2.5 mM (N = 17) but below 4.0 mM (N = 26). For HRR, 85% HRR was necessary for the majority of subjects to be above LT (N = 20), 90% HRR resulted in the majority of subjects being above 2.0 mM (N = 19), while 95% HRR was required for the majority of subjects to be above 2.5 mM (N = 23). At 95% HRR 14 subjects were above 4.0 mM. For % VO2max, the intensities required for the majority of subjects to be above LT, FBLC of 2.0, and 2.5 mM were 90%, 95% and 95% VO2max, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)     

Strength training in older adults: an empowering intervention

PURPOSE: This study evaluated the benefits of adding a psychological empowerment intervention (PEI) to traditional strength training (TST) on social cognitive variables in community-dwelling older adults. METHODS: Thirty-eight participants were randomly assigned to either a PEI or a TST intervention for 6 wk. Before random assignment and following training, participants completed measures of self-efficacy for upper and lower body strength as well as the desire to be able to lift specific amounts of weight. Both treatments involved two sessions per week of center-based training and one session per week of home-based training. The PEI condition also included a group-mediated intervention that was designed to increase self-efficacy for physical strength and the desire for upper and lower body strength. RESULTS: General linear models on difference scores revealed that the two groups experienced differential gains in the desire for upper body strength (P = 0.023, effect size (ES) = 0.79) and were marginally different in gains for upper body strength self-efficacy (P = 0.065, ES = 0.63). On a four-point scale, the adjusted mean (+/-SE) differences for the PEI group on the desire for upper body strength was 0.71 (+/-0.12) as compared to 0.27 (+/-0.13) for the TST group, whereas the PEI group's improvement in self-efficacy for upper body strength was 25.70 (+/-3.02) as compared to 17.18 (+/-3.19) for the TST group. CONCLUSIONS: Empowerment-based exercise programs may be particularly motivating for older adults by creating a more meaningful physical activity experience for them.     

Evidence and possible mechanisms of altered maximum heart rate with endurance training and tapering

Exercise physiologists, coaches and athletes have traditionally used heart rate (HR) to monitor training intensity during exercise. While it is known that aerobic training decreases submaximal HR (HRsubmax) at a given absolute exercise workload, the general consensus is that maximum HR (HRmax) is relatively unaltered regardless of training status in a given population. It has not been seriously postulated as to whether HRmax can change modestly with aerobic training/detraining. Despite several sources stating that HRmax is unaltered with training, several studies report that HRmax is reduced following regular aerobic exercise by sedentary adults and endurance athletes, and can increase upon cessation of aerobic exercise. Furthermore, evidence suggests that tapering/detraining can increase HRmax. Therefore, it is plausible that some of the same mechanisms that affect both resting and HRsubmax may also play a role in altered HRmax. Some of the proposed mechanisms for changes in HRmax that may occur with aerobic training include autonomic (extrinsic) factors such as plasma volume expansion and(enhanced baroreflex function, while some nonautonomic (intrinsic) factors are alteration of the electrophysiology of the sinoatrial (SA) node and decreased beta-adrenergic receptor number and density. There is a high correlation between changes in both maximal oxygen uptake (VO2 max) and HRmax that occurs with training, tapering and detraining (r= -0.76: p < 0.0001; n = 314), which indicates that as VO2max improves with training, HRmax tends to decrease, and when detraining ensues, HRmax tends to increase. The overall effect of aerobic training and detraining on HRmax is moderate: effect sizes based on several studies were calculated to be -0.48 and +0.54, respectively. Therefore, analysis reveals that HRmax can be altered by 3 to 7% with aerobic training/detraining. However, because of a lack of research in the area of training on HRmax, the reader should remain speculative and allow for cautious interpretation until further, more thorough investigations are carried out as to the confirmation of mechanisms involved. Despite the limitations of using HR and HRmax as a guide to training intensity, the practical implications of monitoring changing HRmax are: (i) prescribed training intensities may be more precisely monitored; and (ii) prevention of overtraining may possibly be enhanced. As such, it may be sensible to monitor HRmax directly in athletes throughout the training year, perhaps at every macrocycle (3 to 6 weeks).     

Thyroidal changes associated with endurance training in women

The associations between endurance training, body composition, and the pituitary-thyroid axis were studied in 17 healthy, young women. Body composition and plasma concentrations of T4, T3, rT3, resin T3 uptake, TSH, and TRH-stimulated TSH were examined at baseline and after each subject's weekly distance had increased 48 km (delta 48) and 80 km (delta 80) above baseline. Total body weight did not change at delta 48 or delta 80. Mean (+/- SE) lean weight in kg increased from 42.9 +/- 1.2 at baseline to 44.8 +/- 1.2 at delta 80 (P = 0.002). We have reported previously that at delta 48 the subjects had evidence of mild thyroidal impairment, which consisted of decreased T3 and rT3, and an exaggerated TSH response to TRH. With more prolonged training (delta 48 to delta 80) there were significant increases in T4, rT3, and unstimulated TSH, while the ratios of T4/rT3 and T3/rT3 and the TSH response to TRH decreased significantly. Some of the thyroidal changes that occurred between delta 48 and delta 80 are similar to those seen in other stressful non-thyroidal conditions.