Exercise-induced plasma volume expansion and post-exercise parasympathetic reactivation

The purpose of this study was to investigate the effect of exercise-induced plasma volume expansion on post-exercise parasympathetic reactivation. Before (D₀) and 2 days after (D₊₂) a supramaximal exercise session, 11 men (21.4 ± 2.6 years and BMI = 23.0 ± 1.4) performed 6-min of submaximal running where heart rate (HR) recovery (HRR) and HR variability (HRV) indices were calculated during the first 10 min of recovery. Relative plasma volume changes (∆PV) were calculated using changes in hematocrit and hemoglobin measured over consecutive mornings from D₀ to D₊₂. Parasympathetic reactivation was evaluated through HRR and vagal-related indexes calculated during a stationary period of recovery. Compared with D₀, ∆PV (+4.8%, P < 0.01) and all vagal-related HRV indices were significantly higher at D₊₂ (all P < 0.05). HRR was not different between trials. Changes in HRV indices, but not HRR, were related to ∆PV (all P < 0.01). HRR and HRV indices characterize distinct independent aspects of cardiac parasympathetic function, with HRV indices being more sensitive to changes in plasma volume than HRR.     

Possibilities and limitations of the polar RS800 in measuring heart rate variability at rest

A growing trend among clinical studies is the use of heart rate monitors (HRMs) for assessment of heart rate variability (HRV). These instruments offer a convenient alternative to traditional electrocardiographs (ECGs) for recording and processing of R–R data. Reports on the validity of such systems are, however, conflicting. This study aimed to assess the validity of a commercial HRM on a large study sample, with emphasis on gender and age. Simultaneous recordings of R–R intervals were conducted with the Polar RS800 HRM and a 3-lead ECG on 341 individuals. Data editing was performed with individually designated software for each instrument. Agreement on SDNN, RMSSD, and HF- and LF power was assessed with intraclass correlations (ICCs), standard errors of measurement (SEMs) and Bland and Altman plots. The HRM was not able to identify 18 observations with non-sinus beats. For men, agreement between instruments ranged from good to excellent (ICC ≥ 0.8) on all HRV measures, and SEMs were generally small. For women the results were weaker, with unacceptable agreement between instruments on SDNN. Women over 60 years did not reach a critical ICC value of 0.75 on any of the HRV measures. Bland and Altman plots demonstrated that the RS800 generally overestimated HRV, and that uncertainty increased with higher values. Since the Polar system did not identify errors satisfactorily, or return valid values of HRV for certain groups, it is concluded that, whenever possible, traditional ECGs should be used for both gathering and editing of HRV data.     

Low vagal tone is associated with impaired post stress recovery of cardiovascular, endocrine, and immune markers

Reduced heart rate variability (HRV) and delayed blood pressure recovery are associated with increased cardiovascular risk. Besides this evident link, the vagus is thought to play an inhibitory role in the regulation of other allostatic systems, including inflammation and the hypothalamic–pituitary–adrenal (HPA) axis. However, human evidence is scarce. To further explore these associations and with special regard to the postulated mediating role of the vagus, we hypothesised that subjects with low vagal tone as indexed by reduced resting HRV would show impaired post-stress recovery of cardiovascular, endocrine and immune system markers involved in cardiovascular pathology. 44 healthy men underwent a standardised mental stress test. Besides continuous measurement of systolic and diastolic blood pressure (SBP, DBP), heart rate (HR), and HRV serum cortisol, tumour necrosis factor-alpha (TNF-alpha), and interleukin-6 (IL-6) were measured before, after, 20, and 60 min after stress. Low versus high HRV groups was defined by median split on resting HRV (RMSSD). The task elicited significant time effects for SBP, DBP, HR, HRV, cortisol, and TNF-alpha. Subjects with low baseline HRV showed almost no modulation of HRV coupled with overall reduced HRV levels, and impaired recovery of DBP, cortisol, and TNF-alpha. Confirming our hypothesis, low vagal tone was associated with impaired recovery of cardiovascular, endocrine, and immune markers in healthy males. The data support an inhibitory role of the vagus in the regulation of allostatic systems as described in the neurovisceral integration model. We posit reduced resting HRV as a risk marker for future cardiovascular and other stress-related disease.     

Effects of dynamic resistance training on heart rate variability in healthy older women

Twenty healthy women aged between 65 and 74 years, trained three times a week, for 16 weeks, on a cycle ergometer, to determine the effects of dynamic resistance training on heart rate variability (HRV). Subjects were allocated to two training groups, high (HI, n=10) and low (LO, n=10) intensity. The HI group performed eight sets of 8 revolutions at 80% of the maximum resistance to complete 2 pedal revolutions (2RM); the LO group performed eight sets of 16 pedal revolutions at 40% of 2RM. Subjects were tested twice before, as control period (−4 weeks and 0 weeks) and once after training (16 weeks) for HRV, maximum voluntary contraction (MVC) of knee extensors and peak power (P _p) of lower limbs by jumping on a force platform. HRV was measured using time and frequency domain parameters. Two-way ANOVA for repeated measures was performed on all variables (P<0.05). Results showed no differences between training groups. Following training HRV was not modified, while MVC and P _p significantly increased. The two proposed forms of dynamic resistance training were appropriate to improve muscle power and strength in elderly females without affecting HRV. More research should verify the effects of an isometric and more prolonged training stimulus on HRV in older subjects. Electronic Publication     

Cardiac vagal withdrawal and reactivation during repeated rest–exercise transitions

It is not known whether subjects that have higher cardiac vagal reactivation (CVR) during repeated exercise transitions also have higher cardiac vagal withdrawal (CVW) at the onset of exercise, which would lead to better heart rate (HR) regulation during exercise transitions. Therefore, our aims were to investigate: (a) the influence of CVR on CVW during repeated rest–exercise transitions; and (b) the influence of the sympathetic activity on CVR and CVW. Fifty-eight healthy men (22 ± 4 years) performed 20 rest–exercise transitions interspaced by 30 s. In addition, nine healthy men (24 ± 3 years) ingested either 25 mg of atenolol or placebo, on a crossover, double-blind, randomized design, then performed 20 rest–exercise transitions interspaced by 30 s. Cardiac vagal reactivation was assessed by a HR variability index (RMSSD) and CVW by the HR increase at the onset of a valid and reliable cycling protocol. The CVR and CVW responses were associated (partial r ranged from 0.60 to 0.66; p < 0.05). Participants with higher CVR over transitions maintained their CVW over repeated transitions [first transition (mean ± SEM) = 1.59 ± 0.04 vs. 20th = 1.50 ± 0.03 (a.u.), p = 0.24], while participants with lower CVR had a CVW decrease over repeated transitions [first transition (mean ± SEM) = 1.38 ± 0.04 vs. 20th = 1.19 ± 0.03 (a.u.), p < 0.01). In addition, the CVR and CVW over the rest–exercise transitions were similar during atenolol and placebo (ANCOVA interaction p = 0.12 and p = 0.48, respectively). In conclusion, the CVR among repeated rest–exercise transitions influenced the CVW at the onset of exercise, which was not affected by a partial β₁ cardioselective adrenoceptor blockade.     

Linear and nonlinear analysis of heart rate variability in healthy subjects and after acute myocardial infarction in patients

The objectives of this study were to evaluate and compare the use of linear and nonlinear methods for analysis of heart rate variability (HRV) in healthy subjects and in patients after acute myocardial infarction (AMI). Heart rate (HR) was recorded for 15 min in the supine position in 10 patients with AMI taking β-blockers (aged 57 ± 9 years) and in 11 healthy subjects (aged 53 ± 4 years). HRV was analyzed in the time domain (RMSSD and RMSM), the frequency domain using low- and high-frequency bands in normalized units (nu; LFnu and HFnu) and the LF/HF ratio and approximate entropy (ApEn) were determined. There was a correlation (P < 0.05) of RMSSD, RMSM, LFnu, HFnu, and the LF/HF ratio index with the ApEn of the AMI group on the 2nd (r = 0.87, 0.65, 0.72, 0.72, and 0.64) and 7th day (r = 0.88, 0.70, 0.69, 0.69, and 0.87) and of the healthy group (r = 0.63, 0.71, 0.63, 0.63, and 0.74), respectively. The median HRV indexes of the AMI group on the 2nd and 7th day differed from the healthy group (P < 0.05): RMSSD = 10.37, 19.95, 24.81; RMSM = 23.47, 31.96, 43.79; LFnu = 0.79, 0.79, 0.62; HFnu = 0.20, 0.20, 0.37; LF/HF ratio = 3.87, 3.94, 1.65; ApEn = 1.01, 1.24, 1.31, respectively. There was agreement between the methods, suggesting that these have the same power to evaluate autonomic modulation of HR in both AMI patients and healthy subjects. AMI contributed to a reduction in cardiac signal irregularity, higher sympathetic modulation and lower vagal modulation.     

Adaptation to chronic eccentric exercise in humans: the influence of contraction velocity

We compared changes in muscle fibre composition and muscle strength indices following a 10 week isokinetic resistance training programme consisting of fast (3.14 rad^.s^–1) or slow (0.52 rad^.s^–1) velocity eccentric muscle contractions. A group of 20 non-resistance trained subjects were assigned to a FAST (n=7), SLOW (n=6) or non-training CONTROL (n=7) group. A unilateral training protocol targeted the elbow flexor muscle group and consisted of 24 maximal eccentric isokinetic contractions (four sets of six repetitions) performed three times a week for 10 weeks. Muscle biopsy samples were obtained from the belly of the biceps brachii. Isometric torque and concentric and eccentric torque at 0.52 and 3.14 rad^.s^–1 were examined at 0, 5 and 10 weeks. After 10 weeks, the FAST group demonstrated significant [mean (SEM)] increases in eccentric [29.6 (6.4)%] and concentric torque [27.4 (7.3)%] at 3.14 rad^.s^–1, isometric torque [21.3 (4.3)%] and eccentric torque [25.2 (7.2)%] at 0.52 rad^.s^–1. The percentage of type I fibres in the FAST group decreased from [53.8 (6.6)% to 39.1 (4.4)%] while type IIb fibre percentage increased from [5.8 (1.9)% to 12.9 (3.3)%; P<0.05]. In contrast, the SLOW group did not experience significant changes in muscle fibre type or muscle torque. We conclude that neuromuscular adaptations to eccentric training stimuli may be influenced by differences in the ability to cope with chronic exposure to relatively fast and slow eccentric contraction velocities. Possible mechanisms include greater cumulative damage to contractile tissues or stress induced by slow eccentric muscle contractions. Electronic Publication     

Increased heart rate variability and executive performance after aerobic training in the elderly

This study examined the effects of two short physical training programs on various parameters of heart rate variability (HRV) and on executive performance in older people. Twenty-four sedentary men and women aged 65–78 years were randomly assigned to an aerobic exercise program or a stretching program three times a week for 12 weeks. Resting HRV was measured in time and frequency domains in each participant before and after the 12-week programs. Executive performance was measured with the Wisconsin card sorting test (WCST). Significant group–session interactions emerged for the standard deviation of normal beat-to-beat (R–R) intervals, the root-mean-square of successive R–R, and high frequency power. Only the aerobic training group increased vagal-mediated HRV parameters. Moreover, only the participants in the aerobic training group improved their performance on the WCST. These results highlight the role of aerobic exercise as an important cardiac and brain protective factor, and suggest a direct link between exercise, HRV, and cognition in the aged population.     

Cardiac vagal outflow after aerobic training by analysis of high-frequency oscillation of the R–R interval

This study was designed to assess the effect of aerobic training on the dynamics between the R–R interval length and the high-frequency (HF) oscillation of the R–R interval. Seventeen healthy males (26±2 years) participated in an 8-week aerobic training intervention. The mean HF spectral power (0.15–0.4 Hz) of the R–R interval and the mean R–R interval length were analyzed from 24-h recordings. HF power was also analyzed in 5-min sequences and plotted as a function of the corresponding mean R–R interval length. The relationship between the R–R interval length and the HF power was analyzed by a quadratic regression model. The relationship was defined as saturated if the distinct deflection point of the model occurred before the maximum R–R interval. Otherwise, the relationship was defined as linear. Additionally, the mean HF power was calculated from the linear portion of the R–R interval versus the HF power regression curve (HF index). Before the training intervention, seven subjects had a saturated HF power. After the intervention, five new cases of saturated HF power were observed. The mean HF power of the 24-h recording did not change in the group with a saturated HF power before training (7.4±0.8 vs. 7.6±0.8 ms²), but the HF index increased (6.7±0.7 vs. 7.1±0.7 ms², P<0.05). We conclude that enhanced vagal activity due to aerobic training increases the prevalence of the saturation of the HF oscillation of the R–R interval variability in healthy subjects. HF power calculated from unsaturated area detects more accurately subtle changes in the vagally mediated beat-to-beat variability of the R–R interval after aerobic training than the mean 24-h HF power.     

The effects of performing isometric training at two exercise intensities in healthy young males

No previous studies have examined the effects of isometric training intensity upon resting blood pressure (BP). The aims of this study were (a) to compare the effects of leg isometric training, performed at two intensities, upon resting systolic-SBP, diastolic-DBP and mean arterial-MAP BP; and (b) to examine selected cardiovascular variables, in an attempt to explain any changes in resting BP following training. Thirty-three participants were randomly allocated to either control, high- (HI) or low-intensity (LI) training for 8 weeks. Participants performed 4 × 2 min exercise bouts 3× weekly. Resting BP was measured at baseline, 4-weeks and post-training. SBP, DBP and MAP fell significantly in both groups after training. Changes were –5.2 ± 4.0, –2.6 ± 2.9 and –2.5 ± 2.2 mmHg [HI]; –3.7 ± 3.7, –2.5 ± 4.8 and –2.6 ± 2.5 mmHg [LI] for SBP, DBP and MAP, respectively. There were no significant changes in BP at 4 weeks. No significant changes were observed in any of the other cardiovascular variables examined. These findings suggest that isometric training causes reductions in SBP, DBP and MAP at a range of exercise intensities, when it is performed over 8 weeks. Furthermore, it is possible to reduce resting BP using a much lower isometric exercise intensity than has previously been shown.     

The repeated bout effect of eccentric exercise is not associated with changes in voluntary activation

The aim of this study was to compare the possible changes in muscle activation level between a first and second bout of damaging eccentric exercise performed at 2 weeks interval (i.e. repeated bout effect). To that purpose, ten physically active males took part in this study. The eccentric exercise consisted of 10 sets of 12 maximal voluntary contractions (MVC) produced by the knee extensors during movements performed at a constant speed of 160°s⁻¹. Changes in voluntary and electrically evoked torque in concentric and/or isometric conditions were assessed at the following time points: pre-exercise, and 2 min, 1 and 24 h after each eccentric exercise. At the same time points, voluntary activation was quantified by the superimposed electrical stimulation technique. Muscle soreness and plasma CK activity were measured within 48 h after the eccentric exercise. The results showed that the decrease in eccentric peak torque was linear throughout the exercise protocol. At the end of bouts 1 and 2, torque was significantly reduced by 27.7 ± 9.1 and 23.4 ± 11.2, respectively, with no difference between bouts (P > 0.05). At 24 h post-exercise, a lower reduction (P < 0.05) in MVC (17.8 ± 5.4%) and electrically evoked (16.7 ± 4.6%) isometric torque was observed for bout 2. In contrast, no statistical difference was found in the deficit in voluntary activation between the two bouts. In conclusion, our results indicate that the repeated bout effect of eccentric exercise appears to reduce muscle damage, but does not influence the level of voluntary activation.     

Inclusion of a rest period in diaphragmatic breathing increases high frequency heart rate variability: Implications for behavioral therapy

Heart rate variability (HRV) is associated with positive physiological and psychological effects. HRV is affected by breathing parameters, yet debate remains regarding the best breathing interventions for strengthening HRV. The objective of the current study was to test whether the inclusion of a postexhalation rest period was effective at increasing HRV, while controlling for breathing rate. A within‐subject crossover design was used with 40 participants who were assigned randomly to a breathing pattern including a postexhalation rest period or a breathing pattern that omitted the postexhalation rest period. Participants completed training on each breathing pattern, practiced for 6 min, and sat quietly during a 5‐min washout period between practices. Participants were given instructions for diaphragmatic breathing at a pace of six breaths/minute with or without a postexhalation rest period. Recordings of heart rate, breathing rate, HF‐HRV, RMSSD, LF‐HRV, and SDNN were collected before and during each of the breathing trials. HRV indices were derived from Lead 1 ECG recordings. Pairwise contrasts showed that inclusion of a postexhalation rest period significantly decreased heart rate (p < .001) and increased HF‐HRV (p < .05). No differences were found for breathing rates (p > .05), RMSSD (p > .05), and SDNN (p > .05). Results indicated that omission of the postexhalation rest period resulted in higher LF‐HRV (p < .05). A postexhalation rest period improves HF‐HRV, commonly associated with self‐regulatory control, yet the importance of a postexhalation rest period requires further exploration.     

Reductions in resting blood pressure after 4 weeks of isometric exercise training

There is some evidence to suggest isometric training can reduce resting blood pressure in a shorter period than the typical 8 weeks, reported most commonly. The purpose of the present study was to explore whether 4 weeks of bilateral-leg isometric training can reduce resting blood pressure, and whether these changes are associated with altered cardiac output or total peripheral resistance. Thirteen participants volunteered for a 4-week crossover training study, involving three sessions per week (each session involving 4 × 2 min bilateral-leg isometric exercise). The training intensity used (95% peak HR) was equivalent to 24% MVC. In addition to blood pressure, resting heart rate, cardiac output, stroke volume, and total peripheral resistance were measured. Results demonstrated that bilateral-leg isometric exercise training for 4 weeks caused significant reductions in systolic, diastolic, and mean arterial pressure. Changes were −4.9 ± 5.8, −2.8 ± 3.2, and −2.7 ± 2.4 mmHg, respectively. No differences were observed in the other resting measures. In conclusion, this study has shown that it is possible to induce reductions in arterial blood pressure after 4 weeks of bilateral-leg isometric exercise.     

Post-resistance exercise hypotension, hemodynamics, and heart rate variability: influence of exercise intensity

The occurrence of post-exercise hypotension after resistance exercise is controversial, and its mechanisms are unknown. To evaluate the effect of different resistance exercise intensities on post-exercise blood pressure (BP), and hemodynamic and autonomic mechanisms, 17 normotensives underwent three experimental sessions: control (C—40 min of rest), low- (E40%—40% of 1 repetition maximum, RM), and high-intensity (E80%—80% of 1 RM) resistance exercises. Before and after interventions, BP, heart rate (HR), and cardiac output (CO) were measured. Autonomic regulation was evaluated by normalized low- (LF_R–Rnu) and high-frequency (HF_R–Rnu) components of the R–R variability. In comparison with pre-exercise, systolic BP decreased similarly in the E40% and E80% (−6 ± 1 and −8 ± 1 mmHg, P < 0.05). Diastolic BP decreased in the E40%, increased in the C, and did not change in the E80%. CO decreased similarly in all the sessions (−0.4 ± 0.2 l/min, P < 0.05), while systemic vascular resistance (SVR) increased in the C, did not change in the E40%, and increased in the E80%. Stroke volume decreased, while HR increased after both exercises, and these changes were greater in the E80% (−11 ± 2 vs. −17 ± 2 ml/beat, and +17 ± 2 vs. +21 ± 2 bpm, P < 0.05). LF_R–Rnu increased, while ln HF_R–Rnu decreased in both exercise sessions. In conclusion: Low- and high-intensity resistance exercises cause systolic post-exercise hypotension; however, only low-intensity exercise decreases diastolic BP. BP fall is due to CO decrease that is not compensated by SVR increase. BP fall is accompanied by HR increase due to an increase in sympathetic modulation to the heart.     

Effects of long-term exercise training on cardiac autonomic nervous activities and baroreflex sensitivity

Reductions in tonic vagal controls of the heart and depressed baroreflex sensitivity (BRS) have been associated with a postural fall in blood pressure (BP) and the incidence of cardiac events among older people. We examined the hypothesis that BP regulation during orthostatic challenge as well as heart rate variability (HRV) at rest can be better maintained in long-term exercise-trained, healthy, older men (aged 60–70 years). Subjects were classified into two groups; long-term exercise-trained (LTET, n=14) and sedentary (SED, n=10) according to their history of physical activity. Prior to the dynamic BRS assessment, supine resting autonomic cardiac modulation was assessed by means of time domain HRV [standard deviation of ECG R–R interval (RRISD) and the coefficient of variation (CV)]. The BRS was assessed during 60° head-up tilting by simultaneously measuring beat-by-beat systolic blood pressure (SBP) and ECG R–R interval changes. The BRS gain was determined by the regression slope coefficient based on the extent of the SBP fall and the corresponding ECG R–R shortening during the orthostatic challenge. The results indicated that the LTET group manifested greater ECG R–R interval fluctuations with significantly higher resting RRISD and CV, compared with the SED group [59.5 (10.4) versus 27.7 (7.8) ms, p<0.05; 5.5 (0.8) versus 2.8 (0.7)%, p<0.05], respectively. Using dynamic BRS testing during the acute orthostatic challenge, the LTET group showed a significantly higher BRS gain than the SED [6.4 (0.8) versus 3.8 (0.6) ms·mmHg⁻¹, p<0.017] group. These results indicate that CV and BRS are well maintained in healthy, LTET older individuals when compared with their sedentary peers. Our data suggest that this augmented autonomic cardiac modulation reflects better parasympathetic responsiveness in LTET individuals. Data provide further support for long-term exercise training as another possible cardioprotective factor that might decrease susceptibility to ventricular fibrillation as well as assist arterial BP at the onset of an orthostatic challenge in older men. Electronic Publication     

Effect of a 24-h continuous walking race on cardiac autonomic control

This study investigated the relationships between walking speed and heart rate (HR) variability (HRV) in eleven subjects during a 24-h race. It was hypothesized that the nycthemeral rhythm on HR is preserved during the race. RR intervals and walking speed were measured. Fast Fourier transform was applied to samples of 1,024 successive RR intervals collected every hour from a HR monitor. Walking speed was averaged every hour and decreased (first lap: 8.8 ± 0.3 vs. last lap: 7.3 ± 0.8 km h⁻¹, P < 0.001) with HR also decreasing (max at 19:00 h: 143 ± 9 vs. min at 7:00 h: 117 ± 14 beats min⁻¹, P < 0.001) following a third order polynomial shape. HRV power spectral components followed distribution patterns similar to the mean RR during the race with a minimum in the early evening (19:00–20:00 h) and a maximum in the morning (5:00–8:00 h). Thus, as for mean RR, spectral components over time are also fitted to a third order polynomial regression. LF/HF ratio increased linearly (min = 0.5 ± 0.3, max = 2.8 ± 5.3, P = 0.02). Although mean HF peak did not decrease significantly over time, it was positively correlated with walking speed. In conclusion, this study showed that despite a constant decrease in walking speed, HR circadian rhythm is preserved during a continuous 24-h walking race. The short-term HRV components remain linked to HR whereas the LF/HF ratio increases linearly until the end of the race whatever HR is.     

Influence of cold water face immersion on post-exercise parasympathetic reactivation

The aim of the present study was to investigate the effect of cold water face immersion on post-exercise parasympathetic reactivation, inferred from heart rate (HR) recovery (HRR) and HR variability (HRV) indices. Thirteen men performed, on two different occasions, an intermittent exercise (i.e., an all-out 30-s Wingate test followed by a 5-min run at 45% of the speed reached at the end of the 30–15 Intermittent Fitness test, interspersed with 5 min of seated recovery), randomly followed by 5 min of passive (seated) recovery with either cold water face immersion (CWFI) or control (CON). HR was recorded beat-to-beat and vagal-related HRV indices (i.e., natural logarithm of the high-frequency band, LnHF, and natural logarithm of the square root of the mean sum of squared differences between adjacent normal R–R intervals, Ln rMSSD) and HRR (e.g., heart beats recovered in the first minute after exercise cessation) were calculated for both recovery conditions. Parasympathetic reactivation was faster for the CWFI condition, as indicated by higher LnHF (P = 0.004), Ln rMSSD (P = 0.026) and HRR (P = 0.002) values for the CWFI compared with the CON condition. Cold water face immersion appears to be a simple and efficient means of immediately accelerating post-exercise parasympathetic reactivation.     

The effect of strength training and short-term detraining on maximum force and the rate of force development of older men

This study examined the effect of strength training (ST) and short-term detraining on maximum force and rate of force development (RFD) in previously sedentary, healthy older men. Twenty-four older men (70–80 years) were randomly assigned to a ST group (n = 12) and C group (control, n = 12). Training consisted of three sets of six to ten repetitions on an incline squat at 70–90% of one repetition maximum three times per week for 16 weeks followed by 4 weeks of detraining. Regional muscle mass was assessed before and after training by dual-energy X-ray absorptiometry. Training increased RFD, maximum bilateral isometric force, and force in 500 ms, upper leg muscle mass and strength above pre-training values (14, 25, 22, 7, 90%, respectively; P < 0.05). After 4 weeks detraining all neuromuscular variables were significantly (P < 0.05) lower than after 16 weeks training but remained significantly (P < 0.05) higher than pre-training levels except for RFD which had returned to pre-training levels. These findings demonstrate that high-intensity ST can improve maximum force and RFD of older men. However, older individuals may lose some neuromuscular performance after a period of short-term detraining and that resistance exercise should be performed on a regular basis to maintain training adaptations.     

The incidence of stress symptoms and heart rate variability during sleep and orthostatic test

This study examined the relation of self-reported stress to cardiac autonomic modulation in real-life conditions. The participants for the study were healthy male (N = 59) and female (N = 40) employees (age 40 ± 10 years). A single-item question and a 14-item questionnaire on perceived stress were administered to the participants before the experimental night. RR-intervals (RRI) were recorded during night sleep and an orthostatic test after awakening at home. The RRI data were analyzed for heart rate (HR) and heart rate variability (HRV) in time and frequency domains. Nocturnal urinary stress hormone (cortisol, adrenal and noradrenal) secretion was also analyzed. Based on the self-reported stress, the participants were divided into either low or high stress group. The results showed that higher incidence of stress symptoms was significantly associated with lower HRV in the orthostatic test regardless of age and gender. Differences between the stress groups in HRV indices were approximately 20–50 and 30–75% in supine and standing positions, respectively. No difference was found in nocturnal HR, HRV, or stress hormone secretion between the stress groups. Higher incidence of stress symptoms was significantly associated with greater decrease of HRV from night sleep to the orthostatic test, as a response to awakening. In conclusion, the present findings support the view that autonomic modulation measured in the orthostatic test, but not during night sleep, is related to self-reported stress.     

Muscle oxygenation of vastus lateralis and medialis muscles during alternating and pulsed current electrical stimulation

This study compared between alternating and pulsed current electrical muscle stimulation (EMS) for muscle oxygenation and blood volume during isometric contractions. Nine healthy men (23–48 years) received alternating current EMS (2500 Hz) modulated at 75 Hz on the knee extensors of one leg, and pulsed current EMS (75 Hz) for the other leg separated by 2 weeks in a randomised, counter-balanced order. Pulse duration (400 μs), on–off ratio (5–15 s) and other stimulation parameters were matched between conditions and 30 isometric contractions were induced at the knee joint angle of 100° (0° full extension). Changes in tissue oxygenation index (∆TOI) and total hemoglobin volume (∆tHb) of vastus lateralis and medialis muscles over 30 contractions were assessed by a near-infrared spectroscopy, and were compared between conditions by a two-way repeated measures ANOVA. Peak torque produced during EMS increased over 30 contractions in response to the increase in the stimulation intensity for pulsed current, but not for the alternating current EMS. The torque during each isometric contraction was less stable in alternating than pulsed current EMS. The changes in ∆TOI amplitude during relaxation phases and ∆tHb amplitude were not significantly different between conditions. However, the decreases in ∆TOI amplitude during contraction phases from baseline were significantly (P < 0.05) greater for the pulsed current than alternating current from the 18th contraction (−15.6 ± 2.3 vs. −8.9 ± 1.8%) to 30th contraction (−10.7 ± 1.8 vs. −4.8 ± 1.5%). These results suggest that the muscles were less activated in the alternating current EMS when compared with the pulsed current EMS.