Background: Heart rate variability (HRV) data set has been used to identify HRV thresholds during exercise. However, the relationship between HRV thresholds and the first (LT1) and second lactate threshold (LT2), has been poorly investigated during maximal running test. In this regard, HRV may act as a useful, less expensive and time consuming method to estimate physiological thresholds. Objectives: The aim of the present study was to verify the validity and reproducibility of the heart rate variability (HRV) method to determine first and second lactate thresholds during a maximal running test.

Introduction. The purpose of this study was to evaluate the application of the Dmax method on heart rate variability (HRV) to estimate the lactate thresholds (LT), during a maximal incremental running test (MIRT). Methods. Nineteen male runners performed two MIRTs, with the initial speed at 8 km·h−1 and increments of 1 km·h−1 every 3 minutes, until exhaustion. Measures of HRV and blood lactate concentrations were obtained, and lactate (LT1 and LT2) and HRV (HRVTDMAX1 and HRVTDMAX2) thresholds were identified. ANOVA with Scheffe’s post hoc test, effect sizes (d), the bias ± 95% limits of agreement (LoA), standard error of the estimate (SEE), Pearson’s (r), and intraclass correlation coefficient (ICC) were calculated to assess validity. Results. No significant differences were observed between HRVTDMAX1 and LT1 when expressed for speed (12.1 ± 1.4 km·h−1 and 11.2 ± 2.1 km·h−1; p=0.55; d = 0.45; r = 0.46; bias ± LoA = 0.8 ± 3.7 km·h−1; SEE = 1.2 km·h−1 (95% CI, 0.9–1.9)). Significant differences were observed between HRVTDMAX2 and LT2 when expressed for speed (12.0 ± 1.2 km·h−1 and 14.1 ± 2.5 km·h−1; p=0.00; d = 1.21; r = 0.48; bias ± LoA = −1.0 ± 1.8 km·h−1; SEE = 1.1 km·h−1 (95% CI, 0.8–1.6)), respectively. Reproducibility values were found for the LT1 (ICC = 0.90; bias ± LoA = −0.7 ± 2.0 km·h−1), LT2 (ICC = 0.97; bias ± LoA = −0.1 ± 1.1 km·h−1), HRVTDMAX1 (ICC = 0.48; bias ± LoA = −0.2 ± 3.4 km·h−1), and HRVTDMAX2 (ICC = 0.30; bias ± LoA = 0.3 ± 3.5 km·h−1). Conclusions. The Dmax method applied over a HRV dataset allowed the identification of LT1 that is close to aerobic threshold, during a MIRT.

New Findings What is the central question of this study?What are the physiological mechanisms underlying muscle fatigue and the increase in the O2 cost per unit of work during high‐intensity exercise? What is the main finding and its importance?Muscle fatigue happens before, and does not explain, the trueV̇O2${\dot{V}}_{{{\rm{O}}}_{\rm{2}}}$ slow component (trueV̇normalO2sc${\dot{V}}_{{{\rm{O}}}_{\rm{2}}{\rm{sc}}}$), but they share the same origin. Muscle activation heterogeneity is associated with muscle fatigue and trueV̇normalO2sc${\dot{V}}_{{{\rm{O}}}_{\rm{2}}{\rm{sc}}}$. Knowing this may improve training prescriptions for healthy people leading to improved public health outcomes. Abstract This study aimed to explain the trueV̇O2${\dot{V}}_{{{\rm{O}}}_{\rm{2}}}$ slow component (trueV̇normalO2sc${\dot{V}}_{{{\rm{O}}}_{\rm{2}}{\rm{sc}}}$) and muscle fatigue during cycling at different intensities. The muscle fatigue of 16 participants was determined through maximal isokinetic effort lasting 3 s during constant work rate bouts of moderate (MOD), heavy (HVY) and very heavy intensity (VHI) exercise. Breath‐by‐breath trueV̇O2${\dot{V}}_{{{\rm{O}}}_{\rm{2}}}$, near‐infrared spectroscopy signals and EMG activity were analysed (thigh muscles). trueV̇normalO2sc${\dot{V}}_{{{\rm{O}}}_{\rm{2}}{\rm{sc}}}$ was higher during VHI exercise (∼70% vs. ∼28% of trueV̇O2${\dot{V}}_{{{\rm{O}}}_{\rm{2}}}$ reserve in HVY). The deoxygenated haemoglobin final value during VHI exercise was higher than during HVY and MOD exercise (∼90% of HHb physiological normalization, vs. ∼82% HVY and ∼45% MOD). The muscle fatigue was greater after VHI exercise (∼22% vs. HVY ∼5%). There was no muscle fatigue after MOD exercise. The greatest magnitude of muscle fatigue occurred within 2 min (VHI ∼17%; HVY ∼9%), after which it stabilized. No significant relationship between trueV̇normalO2sc${\dot{V}}_{{{\rm{O}}}_{\rm{2}}{\rm{sc}}}$ and muscle force production was observed. The τ of muscle trueV̇O2${\dot{V}}_{{{\rm{O}}}_{\rm{2}}}$ was significantly related (R2 = 0.47) with torque decrease for VHI. Type I and II muscle fibre recruitment mainly in the rectus femoris moderately explained the muscle fatigue (R2 = 0.30 and 0.31, respectively) and the trueV̇normalO2sc${\dot{V}}_{{{\rm{O}}}_{\rm{2}}{\rm{sc}}}$ (R2 = 0.39 and 0.27, respectively). The trueV̇normalO2sc${\dot{V}}_{{{\rm{O}}}_{\rm{2}}{\rm{sc}}}$ is also partially explained by blood lactate accumulation (R2 = 0.42). In conclusion muscle fatigue and O2 cost seem to share the same physiological cause linked with a decrease in the muscle trueV̇O2${\dot{V}}_{{{\rm{O}}}_{\rm{2}}}$ and a change in lactate accumulation. Muscle fatigue and trueV̇normalO2sc${\dot{V}}_{{{\rm{O}}}_{\rm{2}}{\rm{sc}}}$ are associated with muscle activation heterogeneity and metabolism of different muscles activated during cycling.

Resumo -O propósito deste estudo foi analisar o comportamento da frequência cardíaca (FC) versus a carga de trabalho crescente (CTC) em teste de esteira, utilizando três modelos matemáticos (linear, linear com dois segmentos de reta e sigmóide) e verificar qual o melhor modelo que possibilita a identificação de um limiar de FC que pudesse servir de preditor para os limiares ventilatórios (LV 1 e LV 2 ). Vinte e dois homens realizaram um teste incremental (re-teste: n=12), com velocidade inicial de 5,5 km.h -1 e incrementos de 0,5 km.h -1 a cada minuto, até a exaustão. Medidas contínuas de FC e trocas gasosas foram convertidas para médias de 5 e 20 segundos. Somatória dos resíduos quadrados e quadrado médio do erro foram usados para verificar o melhor ajuste. A relação FC/CTC foi melhor representada pelo modelo Lin 2 no grupo teste e re-teste (p<0,05). Foi possível identificar um ponto de deflexão de FC, utilizando o modelo Lin 2 (limiar de FC) em todos os indivíduos no teste (164 ± 16,6 bpm; 83,6% FC MÁX ) e no re-teste (162 ± 20,0 bpm; 83,9% FC MÁX ). O limiar de FC (Lin 2PDFC ) ocorreu a 9,2 ± 1,3 km.h -1 (67,9% Vel MÁX ) e foi menor que o LV 2 (LV 2 = 10,6 ± 1,5 km.h -1 ; 77,3% Vel MÁX ; p< 0,05), mas não diferente de LV 1 (8,4 ± 1,2 km.h -1 ; 61,6% Vel MÁX ; p> 0,05) . Durante teste incremental em esteira, a relação FC/CTC parece ser bem descrita por uma função linear com 2 segmentos de reta, a qual permite a determinação de um limiar de FC que se aproxima do LV 1. Palavras-chave: Ajuste sigmóide de Boltzmann; Ergometria; Teste de esteira rolante; Esforço físico; Frequência cardíaca (retest group: n=12) Abstract -The objective of this study was to analyze the heart rate (HR) profile plotted against incremental workloads (IWL) during a treadmill test using three mathematical models [linear, linear with 2 segments (Lin 2 ), and sigmoidal], and to determine the best model for the identification of the HR threshold that could be used as a predictor of ventilatory thresholds (VT 1 and VT 2 ). Twenty-two men underwent a treadmill incremental test

No abstract