宮本 忠吉

Although Gaussian white noise (GWN) inputs offer a theoretical framework for identifying higher-order nonlinearity, an actual application to the data of the neural arc of the carotid sinus baroreflex did not succeed in fully predicting the well-known sigmoidal nonlinearity. In the present study, we assumed that the neural arc can be approximated by a cascade of a linear dynamic (LD) component and a nonlinear static (NS) component. We analyzed the data obtained using GWN inputs with a mean of 120 mmHg and standard deviations (SDs) of 10, 20, and 30 mmHg for 15 min each in anesthetized rats (n = 7). We first estimated the linear transfer function from carotid sinus pressure to sympathetic nerve activity (SNA) and then plotted the measured SNA against the linearly predicted SNA. The predicted and measured data pairs exhibited an inverse sigmoidal distribution when grouped into 10 bins based on the size of the linearly predicted SNA. The sigmoidal nonlinearity estimated via the LD-NS model showed a midpoint pressure (104.1 ± 4.4 mmHg for SD of 30 mmHg) lower than that estimated by a conventional stepwise input (135.8 ± 3.9 mmHg, P < 0.001). This suggests that the NS component is more likely to reflect the nonlinearity observed during pulsatile inputs that are physiological to baroreceptors. Furthermore, the LD-NS model yielded higher R2 values compared with the linear model and the previously suggested second-order Uryson model in the testing dataset.NEW & NOTEWORTHY We examined the input-size dependence of the baroreflex neural arc transfer characteristics during Gaussian white noise inputs. A linear dynamic-static nonlinear model yielded higher R2 values compared with a linear model and captured the well-known sigmoidal nonlinearity of the neural arc, indicating that the nonlinear dynamics contributed to determining sympathetic nerve activity. Ignoring such nonlinear dynamics might reduce our ability to explain underlying physiology and significantly limit the interpretation of experimental data.

Background: Alterations in central blood volume (CBV) play a pivotal role in the functionality of the respiratory and circulatory systems. This study elucidates the adaptive changes in respiration, cerebral circulation, and cardiovascular function in response to orthostatic stress in male volleyball players compared to healthy non-athletes. The athletes’ unique physiological adaptability may help them cope with the frequent CBV changes related to their sports activities. Methods: Fourteen male university students participated in this study; seven were volleyball players and seven were non-athletes. Participants underwent a maximal ramp exercise test and a lower body negative pressure (LBNP) test conducted to examine the cardiorespiratory response under LBNP and no-LBNP conditions. Respiratory, metabolic, hemodynamic, and cardiac measurements were collected and analyzed. Results: Volleyball players (Ath group) were significantly taller and exhibited higher maximal oxygen uptake (VO2max) and maximum work rate compared to non-athletes (Non-Ath group). Under the LBNP condition, end-tidal CO2 partial pressure (PETCO2) decreased by 4.1% in the Non-Ath group, but was maintained stable in the Ath group. The CBV reduction rate due to LBNP was notably less in the Ath group (−12.5%) compared to the Non-Ath group (−24.5%). In all participants, a significant correlation was observed between the CBV and PETCO2 reduction rates. However, cerebral blood flow and cardiovascular responses to LBNP load did not differ between the two groups. Conclusion: Male volleyball players demonstrate distinctive adaptability in response to orthostatic stress, specifically in maintaining stable PETCO2 and attenuating CBV reduction rate under LBNP load. These findings suggest that the sport-specific training in volleyball may induce some protective mechanisms against abrupt changes in CBV, although cerebral blood flow and cardiovascular responses appear unaffected. Further research is warranted to understand the underlying mechanisms of these adaptations.

Purpose: High-intensity interval training (HIIT) may induce training-specific physiological adaptations such as improved respiratory and cardiovascular adjustments before and after the onset of high-intensity exercise, leading to improved exercise performance during high-intensity exercise. The present study investigated the effects of HIIT on time-dependent cardiorespiratory adjustment during maximal exercise and before and after initiation of high-intensity exercise, as well as on maximal exercise performance. Methods: 21 healthy male college students were randomly assigned to HIIT group (n = 11) or control group (n = 10). HIIT group performed training on a cycle ergometer once a week for 8 weeks. The training consisted of three bouts of exercise at 95% maximal work rate (WRmax) until exhaustion. Before and after the HIIT program, dynamic cardiorespiratory function was investigated by ramp and step exercise tests, and HIIT-induced cardiac morphological changes were assessed using echocardiography. Results: HIIT significantly improved not only maximal oxygen uptake and minute ventilation, but also maximal heart rate (HR), systolic blood pressure (SBP), and time to exhaustion in both exercise tests (p < 0.05). Time-dependent increases in minute ventilation (VE) and HR before and at the start of exercise were significantly enhanced after HIIT. During high-intensity exercise, there was a strong correlation between percent change (from before to after HIIT program) in time to exhaustion and percent change in HRmax (r = 0.932, p < 0.001). Furthermore, HIIT-induced cardiac morphological changes such as ventricular wall hypertrophy was observed (p < 0.001). Conclusion: We have demonstrated that HIIT at 95% WRmax induces training-specific adaptations such as improved cardiorespiratory adjustments, not only during maximal exercise but also before and after the onset of high-intensity exercise, improvement of exercise performance mainly associated with circulatory systems.