Energy expenditure and heart rate response to breaking up sedentary time with three different physical activity interventions.

BACKGROUND AND AIMS: Prolonged sedentary behaviour is associated with increased cardiovascular disease risk and decreased energy expenditure (EE). Workplace interventions breaking up sedentary time have increased EE but the cardiovascular responses are unknown. The practicalities of these interventions, such as required costs and workplace adaptations, are questioned. Calisthenics exercises overcome such limitations, but have not been assessed. The aim of this study was to assess the EE and heart rate (HR) response when breaking up sedentary time with a short bout of standing, walking or calisthenics. METHODS AND RESULTS: Twenty healthy participants (15 male) completed four 30 min conditions: a) 30 min sitting, or breaking up this period with two minutes of b) standing, c) treadmill walking (4 km·h(-1)) or d) a set of calisthenics exercises (including squats and lunges). HR and EE (indirect calorimetry) were assessed throughout. During the activity break, calisthenics caused the highest HR (90 ± 12 bpm) compared to all other conditions (Sit: 70 ± 12 bpm; Stand:72 ± 13 bpm; Walk:84 ± 10 bpm; p < 0.001) and EE was the highest with calisthenics (13 ± 5 kcal) compared to all conditions except walking (Sit:3 ± 1 kcal; Stand:5 ± 1 kcal; p < 0.001). The recovery following calisthenics had highest total EE (27 ± 7 kcal) compared to walking (23 ± 6 kcal) and standing (22 ± 6 kcal) and also the longest elevation of HR (p < 0.001). CONCLUSION: Calisthenics led to a greater total EE and HR response compared to standing or walking interventions. Calisthenics may be a time efficient method to break up sedentary time without individuals leaving their work environment. Hence calisthenics could be utilised to disrupt workplace sedentary time and improve cardiovascular health and assist in weight management.

The validity of the ActiPed for physical activity monitoring.

The ActiPed (FitLinxx) is a uniaxial accelerometer, which objectively measures physical activity, uploads the data wirelessly to a website, allowing participants and researchers to view activity levels remotely. The aim was to validate ActiPed's step count, distance travelled and activity time against direct observation. Further, to compare against pedometer (YAMAX), accelerometer (ActiGraph) and manufacturer's guidelines. 22 participants, aged 28±7 years, undertook 4 protocols, including walking on different surfaces and incremental running protocol (from 2 mph to 8 mph). Bland-Altman plots allowed comparison of direct observation against ActiPed estimates. For step count, the ActiPed showed a low % bias in all protocols: walking on a treadmill (-1.30%), incremental treadmill protocol (-1.98%), walking over grass (-1.67%), and walking over concrete (-0.93%). When differentiating between walking and running step count the ActiPed showed a % bias of 4.10% and -6.30%, respectively. The ActiPed showed >95% accuracy for distance and duration estimations overall, although underestimated distance (p<0.01) for walking over grass and concrete. Overall, the ActiPed showed acceptable levels of accuracy comparable to previous validated pedometers and accelerometers. The accuracy combined with the simple and informative remote gathering of data, suggests that the ActiPed could be a useful tool in objective physical activity monitoring.

The effects of views of nature on autonomic control.

Previously studies have shown that nature improves mood and self-esteem and reduces blood pressure. Walking within a natural environment has been suggested to alter autonomic nervous system control, but the mechanisms are not fully understood. Heart rate variability (HRV) is a non-invasive method of assessing autonomic control and can give an insight into vagal modulation. Our hypothesis was that viewing nature alone within a controlled laboratory environment would induce higher levels of HRV as compared to built scenes. Heart rate (HR) and blood pressure (BP) were measured during viewing different scenes in a controlled environment. HRV was used to investigate alterations in autonomic activity, specifically parasympathetic activity. Each participant lay in the semi-supine position in a laboratory while we recorded 5 min (n = 29) of ECG, BP and respiration as they viewed two collections of slides (one containing nature views and the other built scenes). During viewing of nature, markers of parasympathetic activity were increased in both studies. Root mean squared of successive differences increased 4.2 ± 7.7 ms (t = 2.9, p = 0.008) and natural logarithm of high frequency increased 0.19 ± 0.36 ms(2) Hz(-1) (t = 2.9, p = 0.007) as compared to built scenes. Mean HR and BP were not significantly altered. This study provides evidence that autonomic control of the heart is altered by the simple act of just viewing natural scenes with an increase in vagal activity.

Cardiac vagal activity following three intensities of exercise in humans.

SUMMARY: Cardioprotective benefits of exercising at vigorous intensities are known, but reservations remain in prescribing such activity to the untrained population, due to a perceived risk of cardiac events. Few studies have investigated the recovery of the autonomic nervous system (ANS) after a single exercise bout, especially following vigorous exercise in healthy, young but untrained individuals. In this study, the recovery of the ANS, in particular indices of vagal activity were measured postexercise, at three intensities similar to current international recommendations for health. Thirteen individuals (six females, 22.2 +/- 3.1 years) performed three 20-min constant load tests lying supine on a modified bicycle ergometer at the following intensities: moderate (2 mmol l(-1) blood lactate concentration, BLC); hard (3 mmol l(-1)BLC); and vigorous (4 mmol l(-1)BLC) as derived from a maximal test. ECG data were collected during 5-min epochs at baseline then at: 5, 15, 30, 45 and 65-min postexercise. Heart rate variability (HRV) analysis was performed to obtain R-R interval, standard time [root mean square of successive differences (RMSSD)] and frequency measures [natural logarithm of high (lnHF) and low frequency (lnHF)]. RMSSD, lnHF, lnLF and total power were reduced 5-min postexercise following all three intensities (P<0.01). Decreases persisted up to 15-min postexercise following hard and vigorous exercise only (P<0.01). In untrained young adults, parasympathetic reactivation is reduced up to 5-min postexercise regardless of intensity, returning to baseline by 30 min even after vigorous exercise. In this population, the benefits of exercise outweigh any risks of cardiac events that may be evoked by a reduction in the influence of vagal activity.

The influence of small fibre muscle mechanoreceptors on the cardiac vagus in humans.

We have previously shown that activation of muscle receptors by passive stretch (PS) increases heart rate (HR) with little change in blood pressure (BP). We proposed that PS selectively inhibits cardiac vagal activity. We attempted to test this by performing PS during experimental alterations in vagal tone. Large decreases in vagal tone were induced using either glycopyrrolate or mild rhythmic exercise. Milder alterations in vagal tone were achieved by altering carotid baroreceptor input: neck pressure (NP) or neck suction (NS). PS of the triceps surae was tested in 14 healthy human volunteers. BP, ECG and respiration were recorded. PS alone caused a significant decrease (P < 0.05) in R-R interval (962 +/- 76 ms at baseline compared to 846 +/- 151 ms with PS), and showed a reduction in HR variability, which was not significant. The decrease in R-R interval with PS was significantly less (P < 0.05, n = 3) following administration of glycopyrrolate (-8.1 +/- 4.5 ms) compared to PS alone (-54 +/- 11 ms), and also with PS during handgrip (+10 +/- 10 ms) compared with PS alone (-74 +/- 15 ms) (P < 0.05, n = 5). Milder reductions in vagal activity (NP) resulted in a small but insignificant further decrease in R-R interval in response to PS (-107 +/- 17 ms compared to PS alone -96 +/- 13 ms, n = 5). Mild increases in vagal activity (NS) during PS resulted in smaller decreases in R-R interval (-39 +/- 5.5 ms) compared to PS alone (-86 +/- 17 ms) (P < 0.05, n = 8). BP was not significantly changed by stretch in any tests. The results indicate that amongst muscle receptors there is a specific group activated by stretch that selectively inhibit cardiac vagal tone to produce tachycardia.

Heart rate at the onset of muscle contraction and during passive muscle stretch in humans: a role for mechanoreceptors.

Previous evidence suggests that the heart rate (HR) increase observed with isometric exercise is dependent on different afferent mechanisms to those eliciting the increase in blood pressure (BP). Central command and muscle metaboreceptors have been shown to contribute to this differential effect. However, in experimental animals passive stretch of the hindlimb increases HR suggesting that small fibre mechanoreceptors could also have a role. This has not been previously shown in humans and was investigated in this study. Healthy human volunteers were instrumented to record BP, ECG, respiration, EMG of rectus femoris and gastrocnemius and contraction force of triceps surae. Voluntary isometric contraction of triceps surae elicited a significant HR change in the first three respiratory cycles at 40 % of maximum voluntary contraction whereas BP did not change significantly until after 30 s. This suggests that different mechanisms are involved in the initiation of the cardiovascular changes. Sustained passive stretch of triceps surae for 1 min, by dorsiflexion of the foot, caused a significant (P < 0.05) increase in HR (5 +/- 2.6 beats min(-1)) with no significant change in BP. A time domain measure of cardiac vagal activity was reduced significantly during passive stretch from 69.7 +/- 12.9 to 49.6 +/- 8.9 ms. Rapid rhythmic passive stretch (0.5 Hz for 1 min) was without significant effect suggesting that large muscle proprioreceptors are not involved. We conclude that in man small fibre muscle mechanoreceptors responding to stretch, inhibit cardiac vagal activity and thus increase HR. These afferents could contribute to the initial cardiac acceleration in response to muscle contraction.