CARDIORESPIRATORY AND METABOLIC RESPONSE TO DIFFERENT SINUSOIDAL WORK RATES

ABSTRACT Cardiorespiratory and Metabolic Response to Different Sinusoidal Work Rates Cardiorespiratory fitness is one of the most important health criteria in seemingly healthy individuals and nearly all patient populations. Alongside the traditional tests for cardiorespiratory functional evaluation, a test protocol was proposed based on the intensity of the exercise that varies according to a sine wave model. This approach engenders an incessantly fluctuating response in which the response to one cycle is repeated. Indeed, by the periodic occurrence of increasing and decreasing phases, an evaluation of the physiological response kinetic to similar work rate perturbations can be obtained in a single testing session. Sinusoidal protocol pattern depends on several factors: period (T), amplitude (AMP) and midpoint (MP), therefore, modulating one or all of these factors yield various responses of the heart-lung-muscle integrated system. So far, previous studies administered sine wave protocols to investigate changes in the physiological responses induced by changes in T, pedal frequency, or exercise intensity (below and above the lactate threshold). In particular, those studies focused on changes in the temporal responsiveness of the cardiorespiratory and metabolic parameters, measured as the time delay (TD, the latency between mechanical work rate and physiological responses). To date, no study has investigated the possible differences in the cardiorespiratory and metabolic kinetic induced by different sine waves AMP in two protocols sustained by the aerobic metabolism (exercise intensity below the critical power (CP)). Additionally, given that none of the previous studies has ever explored the influence of fatigue on sinusoidal responses, and considering that all the previously reported results were obtained by overlapping, rather than averaging, the analysis of each cycle, it is worth investigating if a cycle-by-cycle analysis may reveal any sign of fatigue with the cycle’s number progresses. Therefore, the aim of this study was two-folds: i) to assess the cardiorespiratory and metabolic response kinetic between two exhausting sinusoidal protocols differing in MP and AMP: 30CP-30 and 50CP-50, and ii) to compare the traditional data analysis approach with a cycle-by-cycle evaluation. In this latter, the possible impact of fatigue on the kinetics characteristics of cardiorespiratory and metabolic parameters will be explored. Ten active male (age: 25.7 ± 1.5 yrs; stature: 1.80 ± 0.06 m; body mass: 75.0 ± 2.7 kg; maximum oxygen uptake (V ̇_(O_2 max)): 3905±182 ml·min−1, CP: 216 ± 10 W) contributed to the study that was conducted in accordance with the Basic Principles of the Declaration of Helsinki. After determining individual V ̇_(O_2 max) and CP on a cycle ergometer, they randomly underwent two sinusoidal work rates (50CP-50: AMP= 50W, MP =CP-50W; 30CP-30: AMP= 30W, MP =CP-30W) with a period of 4 minutes, until exhaustion. Expiratory ventilation (V ̇_E), oxygen uptake (V ̇_(O_2 )), carbon dioxide output (V ̇_(CO_2 )), and heart rate (fH) responses were fitted by the sinewave function that minimized the residuals. AMP, MP and TD were assessed for all the physiological variables. The TD was determined for: i) peak (TD_MAX); ii) down-ward MP crossing (TD_DOWN); iii) nadir (TD_MIN); and iv) up-ward MP crossing (TD_UP) of all parameters for each cycle in 50CP-50 and 30CP-30. After examining the normality with Shapiro-Walk test, a two‐way ANOVA for repeated measures was applied to test the presence of differences between the two protocols and among cycles, taking the first cycle as the reference condition. To determine the presence of fatigue a regression analysis was also applied for exploring possible relationship between each variables and time. Lastly, a paired samples t-test also evaluated the differences between the traditional approach and the cycle-by-cycle analysis. Despite the lower AMP and higher MP values obtained on all the physiological parameters during 30CP-30 compared to 50CP-50, no difference was found in the TDs values in both protocols, regardless of which TD was considered. In some investigated variables, the impact of fatigue on AMP was dissimilar between the two protocols. Indeed, none of the physiological parameters showed any sign of fatigue in 30CP-30, while a significant increase in V ̇_E and V ̇_(CO_2 )was observed in 50CP-50. In regards to a possible impact of fatigue on MP, the V ̇_E, V ̇_(O_2 ) and V ̇_(CO_2 )responses remained steady, whereas a significant rise was observed in fH in both protocols. Refer to a possible effect of fatigue on TDs, no difference was found among cycles in both protocols.Taken together, these findings indicated that the respiratory and metabolic parameters responded similarly among all the cycles and were not influenced by the onset of fatigue. On the contrary, the fH showed a different response in the MP since the beginning of the protocol therefore, the traditional analysis ventures the lack of possible alterations in cardiac response. In conclusion, TDs of the cardiorespiratory and metabolic variables are not affected by the work rate intensity and oscillation AMP when the exercise is mostly aerobic (below CP). Additionally, a cycle-by-cycle analysis is recommended especially when the steadiness of all the cardiorespiratory and metabolic parameters cannot be ensured.