Effects of altering pedal frequency on the slow component of pulmonary VO2 kinetics and EMG activity.

This study investigated the effects of pedal frequency on the slow component of pulmonary oxygen uptake ( V O(2)) kinetics during heavy exercise at the same relative intensity. We hypothesized that higher pedal frequency (expected to enhance fast-twitch muscle fiber recruitment) would be associated with greater slow component amplitude (A' (s)), surface electromyography (normalized root mean square; RMS) and blood lactate concentration ([lactate]). Eight subjects performed square-wave transitions to heavy exercise at 35 and 115 rpm. Furthermore, alternated cadences square-wave transitions (35-115 rpm) were performed to examine the potential effects of additional fast-twitch muscle fiber recruitment on the slow component. Significance was accepted when P<0.05. The A' (s) was greater at higher cadences (0.58+/-0.08 and 0.70+/-0.09 L.min (-1) at 115 and 35-115 rpm, respectively) than at 35 rpm (0.35+/-0.04 L.min (-1)). Greater EMG increase over time (DeltaRMS ((10-3 min))) and [lactate] were observed at 115 and 35-115 rpm compared with 35 rpm. There was a significant correlation between A' (s) and overall DeltaRMS ((10-3 min)) for all pedal frequencies combined (r=0.63; P=0.001). Pedal frequency had no effect on time constants or time delays. These findings are consistent with the concept that progressive recruitment of muscle fibers is associated with the V O(2) slow component.

Estimation of the lactate threshold from heart rate response to submaximal exercise: the pulse deficit.

In this study, we evaluated the validity of a sharp increase in pulse deficit (PD) as a noninvasive index for estimation of the first lactate threshold (LT (1)) in healthy individuals with various levels of aerobic fitness. Three groups of healthy male subjects participated in the study: 15 sedentary individuals, 14 students of physical education, and 13 competitive athletes. Each subject performed a maximal incremental exercise test on the cycle ergometer for the determination of the LT (1), the second lactate threshold, and peak power output. On different days, subjects performed several 8-min bouts of constant-load exercise on the cycle ergometer, corresponding to each of the power outputs of the maximal test, to evaluate PD, which was calculated as the total number of heart beats of the last 4 min minus the total number of heart beats in the first 4 min of exercise. The three groups presented similar blood lactate, heart rate and pulse deficit responses to exercise. For the first power output up to the LT (1), PD showed no significant changes. For the three groups, a sharp increase in PD was seen at the intensity immediately above LT (1). There was a significant correlation between PD and blood lactate changes from the rest to 4th min of submaximal exercise (r = 0.83, p < 0.05). The power output before a sharp increase in PD detected during constant-load exercise (112 +/- 38 W) and the power output corresponding to the LT (1) detected during the incremental test (111 +/- 37 W, p = 0.323) were similar and strongly correlated (r = 0.99, p = 0.0001). The absolute cut-point value of 25 beats for PD had a sensitivity of 100 %, a specificity of 95 %, and a positive predictive value of 90 % for the detection of LT (1). The determination of PD provides an accurate noninvasive estimate of the LT (1) in healthy young men with different levels of fitness. One 8-min submaximal exercise bout can establish if an individual is exercising above or below the LT (1).

Effects of gender and aerobic fitness on cardiac autonomic responses to head-up tilt in healthy adolescents.

Cardiovascular autonomic responses to orthostatic challenges are affected by gender and cardiorespiratory fitness in adults. However, little is know about the effects of these factors in healthy adolescents. We studied 41 adolescents (20 boys and 21 girls) aged 12-17 years, divided into aerobic fitness tertiles based on the results of a maximal treadmill exercise test. Cardiac autonomic modulation was assessed by heart rate variability (HRV) analysis of 5-minute RR interval recordings before and after 70 degrees head-up tilt maneuver. HRV was analyzed by time (TD) and frequency domain (FD) methods. TD was analyzed by standard deviation of the RR intervals and the root mean square of successive differences of RR intervals. The power spectral components were studied at low (LF) and high (HF) frequencies and as the LF/HF ratio. We did not find any differences in TD and FD measures before and after tilt in either gender or fitness groups, except for a higher heart rate response for boys. These results suggests that cardiac autonomic responses to head-up tilt in healthy adolescents are not affected by gender or aerobic fitness.