***TASK*** the task is to summarize an input biomedical literature in six sentences 
    ***INPUT*** the input is a biomedical literature 
    ***OUTPUT*** the output is the summary of an input biomedical literature in six sentences 
    ***DOCUMENTATION*** 
    ***EXAMPLES*** 
    Input: BACKGROUND It is unclear from prior reports whether the relationships between self-ratings of anxiety or emotional stress and parasympathetic nervous system components of heart rate variability are independent of personality and cardiorespiratory fitness . We examined those relationships in a clinical setting prior to a st and ardized exercise test . METHODS AND RESULTS Heart rate variability ( HRV ) was measured during 5 min of supine rest among 92 healthy men ( N=52 ) and women ( N=40 ) who had above-average cardiorespiratory fitness as indicated by peak oxygen uptake measured during grade -incremented treadmill exercise . HRV data sets were decomposed into low-frequency ( LF ; 0.05 - 0.15 Hz ) and high-frequency ( HF ; 0.15 - 0.5 Hz ) components using spectral analysis . Self-ratings of trait anxiety and perceived emotional stress during the past week were also assessed . CONCLUSIONS There was an inverse relationship between perceived emotional stress during the past week and the normalized HF component of HRV ( P=0.038 ) . This indicates a lower cardiac vagal component of HRV among men and women who perceived more stress . That relationship was independent of age , gender , trait anxiety , and cardiorespiratory fitness . It was also independent of heart rate ; mean arterial blood pressure ; and respiration rate , factors which can influence HRV and might be elevated among people reporting anxiety and perceived stress . We conclude that vagal modulation of heart period appears to be sensitive to the recent experience of persistent emotional stress , regardless of a person 's level of physical fitness and disposition toward experiencing anxiety PURPOSE We analyzed HR variability ( HRV ) to detect alterations in autonomic function that may be associated with functional overreaching ( F-OR ) in endurance athletes . METHODS Twenty-one trained male triathletes were r and omly assigned to either intensified training ( n = 13 ) or normal training ( n = 8) groups during 5 wk . HRV measures were taken daily during a 1-wk moderate training ( baseline ) , a 3-wk overload training , and a 1-wk taper . RESULTS All the subjects of the intensified training group demonstrated a decrease in maximal incremental running test performance at the end of the overload period ( -9.0 % ± 2.1 % of baseline value ) followed by a performance supercompensation after the taper and were therefore diagnosed as F-OR . According to a qualitative statistical analysis method , a likely to very likely negative effect of F-OR on HR was observed at rest in supine and st and ing positions , using isolated seventh-day values and weekly average values , respectively . When considering the values obtained once per week , no clear effect of F-OR on HRV parameters was found . In contrast , the weekly mean of each HRV parameter showed a larger change in indices of parasympathetic tone in the F-OR group than the control group in supine position ( with a 96%/4%/0 % chance to demonstrate a positive/trivial/negative effect on Ln RMSSD after the overload period ; 77%/22%/1 % on LnHF ) and st and ing position [ 98%/1%/1 % on Ln RMSSD ; 99%/0%/1 % on LnHF ; 95%/1%/4 % on Ln(LF + HF ) ] . During the taper , theses responses were reversed . CONCLUSIONS Using daily HRV recordings averaged over each week , this study detected a progressive increase in the parasympathetic modulation of HR in endurance athletes led to F-OR . It also revealed that due to a wide day-to-day variability , isolated , once per week HRV recordings may not detect training-induced autonomic modulations in F-OR athletes Purpose of this study was to test utility of heart rate variability ( HRV ) in daily endurance exercise prescriptions . Twenty-six healthy , moderately fit males were r and omized into predefined training group ( TRA , n = 8) , HRV-guided training group ( HRV , n = 9 ) , and control group ( n = 9 ) . Four-week training period consisted of running sessions lasting 40 min each at either low- or high-intensity level . TRA group trained on 6 days a week , with two sessions at low and four at high intensity . Individual training program for HRV group was based on individual changes in high-frequency R – R interval oscillations measured every morning . Increase or no change in HRV result ed in high-intensity training on that day . If there was significant decrease in HRV ( below reference value [ 10-day mean-SD ] or decreasing trend for 2 days ) , low-intensity training or rest was prescribed . Peak oxygen consumption ( VO2peak ) and maximal running velocity ( Loadmax ) were measured in maximal treadmill test before and after the training . In TRA group , Loadmax increased from 15.1 ± 1.3 to 15.7 ± 1.2 km h−1 ( P = 0.004 ) , whereas VO2peak did not change significantly ( 54 ± 4 pre and 55 ± 3 ml kg−1 min−1 post , P = 0.224 ) . In HRV group , significant increases were observed in both Loadmax ( from 15.5 ± 1.0 to 16.4 ± 1.0 km h−1 , P < 0.001 ) and VO2peak ( from 56 ± 4 to 60 ± 5 ml kg−1 min−1 , P = 0.002 ) . The change in Loadmax was significantly greater in HRV group compared to TRA group ( 0.5 ± 0.4 vs. 0.9 ± 0.2 km h−1 , P = 0.048 , adjusted for baseline values ) . No significant differences were observed in the changes of VO2peak between the groups . We concluded that cardiorespiratory fitness can be improved effectively by using HRV for daily training prescription Daily aerobic training results in autonomic control of the heart toward vagal dominance . The constancy of vagal dominance after controlled training followed by a home-based training program in accordance with contemporary guidelines is not known . We set out here to study whether the vagal dominance induced by 8 weeks of controlled aerobic training is preserved after a 10-month home-based training program . For the controlled study , healthy men were r and omized as training ( n=18 ) and control subjects ( n=6 ) . The training was started by a supervised 8-week period with six training sessions a week [ 45 ( 15 ) min each ] at an intensity of 70–80 % of maximum heart rate , followed by a home-based training program for 10 months in accordance with the American College of Sports Medicine recommendations . Cardiovascular autonomic function was assessed by analyzing HR variability over a 24-h period and separately during the night hours ( midnight–6 a.m. ) . Maximal running performance improved during the controlled training 16 (7)% ( range 4–31 % , P<0.001 ) and remained 8 (8)% ( range −3 to 23 % , P<0.001 ) above the baseline level after the home-based training program . At night , the vagally mediated high-frequency ( HF ) power of R-R intervals increased during the controlled training from 6.7 ( 1.3 ) to 7.3 ( 1.1 ) ln ms2 ( P<0.001 ) and remained higher than the baseline after the home-based training [ 7.0 ( 1.3 ) ln ms2 , P<0.05 ] . The changes in running performance correlated with the changes in HF power at night ( r=0.41 , P<0.05 ) and over 24 h ( r=0.44 , P<0.05 ) after the home-based training program . Similarly , the changes in body mass index correlated with the changes in HF power over 24 h ( r=−0.44 , P<0.05 ) after the home-based training program . The high vagal outflow to the heart after the home-based training is associated with good physical performance and body mass control OBJECTIVES Heart rate kinetics are faster in well-trained athletes at exercise onset , indicating sensitivity to training status , but whether they track performance changes due to changes in training load is unknown . DESIGN R and omised , counterbalanced , cross-over . METHODS 17 cyclists completed two weeks of light and two weeks of heavy training . The day after each training period heart rate was recorded during 5 min cycling at 100 W to determine the maximal rate of heart rate increase . Participants then performed a 5 min cycle time-trial after which heart rate recovery was determined . RESULTS Work during 5 min cycle time-trial decreased 3.5 % ( P<0.04 ) in participants ( n=8 ) who increased training load ( completed light training then heavy training ) and , although maximal rate of heart rate increase did not change ( P=0.27 ) , within-individual changes in work were correlated with changes in maximal rate of heart rate increase ( r=0.87 , P=0.005 ) . Work during 5 min cycle time-trial increased 6.5 % ( P<0.001 ) in 9 participants who decreased training load ( completed heavy training then light training ) and maximal rate of heart rate increase increased 28 % ( P=0.002 ) but the changes in maximal work were not related to changes in rate of heart rate increase ( r=0.32 , P=0.40 ) . Heart rate recovery tended to track changes in 5 min cycle time-trial work following increases and decreases in training load ( r=0.65 - 0.75 , P=0.03 - 0.08 ) . CONCLUSIONS Maximal rate of heart rate increases during cycling at 100 W tracks reductions in exercise performance when training load is increased , but not performance improvements when training loads are reduced . Maximal rate of heart rate increase may be a useful adjunct to heart rate recovery for tracking changes in exercise performance Flaws in the design , conduct , analysis , and reporting of r and omised trials can cause the effect of an intervention to be underestimated or overestimated . The Cochrane Collaboration ’s tool for assessing risk of bias aims to make the process clearer and more The effects of intensified training in combination with a reduced training volume on muscle ion kinetics , transporters , and work capacity were examined . Eight well-trained cyclists replaced their regular training with speed-endurance training ( 12 × 30 s sprints ) 2 - 3 times per week and aerobic high-intensity training ( 4 - 5 × 3 - 4 min at 90 - 100 % of maximal heart rate ) 1 - 2 times per week for 7 wk and reduced training volume by 70 % ( intervention period ; IP ) . The duration of an intense exhaustive cycling bout ( EX2 ; 368 ± 6 W ) , performed 2.5 min after a 2-min intense cycle bout ( EX1 ) , was longer ( P < 0.05 ) after than before IP ( 4:16 ± 0:34 vs. 3:37 ± 0:28 min : s ) , and mean and peak power during a repeated sprint test improved ( P < 0.05 ) by 4 % and 3 % , respectively . Femoral venous K(+ ) concentration in recovery from EX1 and EX2 was lowered ( P < 0.05 ) after compared with before IP , whereas muscle interstitial K(+ ) concentration and net muscle K(+ ) release during exercise was unaltered . No changes in muscle lactate and H(+ ) release during and after EX1 and EX2 were observed , but the in vivo buffer capacity was higher ( P < 0.05 ) after IP . Expression of the ATP-sensitive K(+ ) ( KATP ) channel ( Kir6.2 ) decreased by IP , with no change in the strong inward rectifying K(+ ) channel ( Kir2.1 ) , muscle Na(+)-K(+ ) pump subunits , monocarboxylate transporters 1 and 4 ( MCT1 and MCT4 ) , and Na(+)/H(+ ) exchanger 1 ( NHE1 ) . In conclusion , 7 wk of intensified training with a reduced training volume improved performance during repeated intense exercise , which was associated with a greater muscle reuptake of K(+ ) and muscle buffer capacity but not with the amount of muscle ion transporters Numerous symptoms have been associated with the overtraining syndrome ( OT ) , including changes in autonomic function . Heart rate variability ( HRV ) provides non‐invasive data about the autonomic regulation of heart rate in real‐life conditions . The aims of the study were to : ( i ) characterize the HRV profile of seven athletes ( OA ) diagnosed as suffering of OT , compared with eight healthy sedentary ( C ) and eight trained ( T ) subjects during supine rest and 60 ° upright , and ( ii ) compare the traditional time‐ and frequency‐domain analysis assessment of HRV with the non‐linear Poincaré plot analysis . In the latter each R‐R interval is plotted as a function of the previous one , and the st and ard deviations of the instantaneous ( SD1 ) and long‐term R‐R interval variability are calculated . Total power was higher in T than in C and OA both in supine ( 1158 ± 1137 , 6092 ± 3554 and 2970 ± 2947 ms2 for C , T and OA , respectively ) and in upright ( 640 ± 499 , 1814 ± 806 and 1092 ± 712 ms2 for C , T and OA , respectively ; P<0·05 ) positions . In supine position , indicators of parasympathetic activity to the sinus node were higher in T compared with C and OA ( high‐frequency power : 419·1 ± 381·2 , 1 
    Output:
    
Conclusions Increases in vagal-related indices of resting and post-exercise HRV , post-exercise HRR , and HR acceleration are evident when positive adaptation to training has occurred , allowing for increases in performance .
However , increases in post-exercise HRV and HRR also occur in response to overreaching , demonstrating that additional measures of training tolerance may be required to determine whether training-induced changes in these parameters are related to positive or negative adaptations .
Resting HRV is largely unaffected by overreaching , although this may be the result of method ological issues that warrant further investigation .
HR acceleration appears to decrease in response to overreaching training , and thus may be a potential indicator of training-induced fatigue