From a cell-signaling perspective, short-duration intense muscular work is typically associated with resistance training and linked to pathways that stimulate growth. However, brief repeated sessions of high-intensity interval exercise training (HIT) induce rapid phenotypic changes that resemble traditional endurance training. Given the oxidative phenotype that is rapidly upregulated by HIT, it is plausible that metabolic adaptations to this type of exercise could be mediated in part through signaling pathways normally associated with endurance training. A key controller of oxidative enzyme expression in skeletal muscle is peroxisome proliferator-activated receptor gamma coactivator 1alpha (PGC-1alpha), a transcriptional coactivator that serves to coordinate mitochondrial biogenesis. Most studies of acute PGC-1alpha regulation in humans have used very prolonged exercise interventions; however, it was recently shown that a surprisingly small dose of very intense interval exercise, equivalent to only 2 min of all-out cycling, was sufficient to increase PGC-1alpha mRNA during recovery. Intense interval exercise has also been shown to acutely increase the activity of signaling pathways linked to PGC-1alpha and mitochondrial biogenesis, including AMP-activated protein kinase (alpha1 and alpha2 subunits) and the p38 mitogen-activated protein kinase. In contrast, signaling pathways linked to muscle growth, including protein kinase B/Akt and downstream targets p70 ribosomal S6 kinase and 4E binding protein 1, are generally unchanged after acute interval exercise. Signaling through AMP-activated protein kinase and p38 mitogen-activated protein kinase to PGC-1alpha may therefore explain, in part, the metabolic remodeling induced by HIT, including mitochondrial biogenesis and an increased capacity for glucose and fatty acid oxidation.

Peak cardiac output (Q˙ peak) can be measured noninvasively using inert gas rebreathing (IGR). There is no consensus on the optimal protocol to measure Q˙ peak using IGR, which requires a rebreathing period of ~10 s as close to “maximal” exercise as possible. Purpose This study aimed to compare Q˙ peak elicited by a constant load protocol (Q˙ CL) and an incremental step protocol (Q˙ step). Methods A noninferiority randomized crossover trial was used to compare Q˙ peak between protocols using a noninferiority margin of 0.5 L·min−1. Participants (n = 34 (19 female, 15 male); 25 ± 5 yr) performed two baseline V̇O2peak tests to determine peak heart rate (HRpeak) and peak work rate (W peak). Participants then performed the Q˙ CL and Q˙ step protocols each on two separate occasions with the order of the four visits randomized. Q˙ peak was measured using IGR (Innocor; COSMED, Rome, Italy). The Q˙ CL protocol involved a V̇O2peak test followed 10 min later by cycling at 90% W peak, with IGR initiated after 2 min. Q˙ step involved an incremental step test with IGR initiated when the participant’s HR reached 5 bpm below their HRpeak. The first Q˙ CL and Q˙ step tests were compared for noninferiority, and the second series of tests was used to measure repeatability (typical error (TE)). Results The Q˙ CL protocol was noninferior to Q˙ step (Q˙ CL = 17.1 ± 3.2, Q˙ step = 16.8 ± 3.1 L·min−1; 95% confidence intervals, −0.16 to 0.72 L·min−1). The baseline V̇O2peak (3.13 ± 0.83 L·min−1) was achieved during Q˙ CL (3.12 ± 0.72, P = 0.87) and Q˙ step (3.12 ± 0.80, P = 0.82). The TE values for Q˙ peak were 6.6% and 8.3% for Q˙ CL and Q˙ step, respectively. Conclusions The Q˙ CL protocol was noninferior to Q˙ step and may be more convenient because of the reduced time commitment to perform the measurement.

Brachial artery flow‐mediated dilation (FMD), an index of endothelial function, is predictive of cardiovascular disease risk. A wide range of FMD day‐to‐day variability has been reported, but whether this variability differs between men and women is unknown. We examined potential sex‐based differences in FMD and its day‐to‐day variability. Sedentary but otherwise healthy men (N=10, age: 22±3 years) and women (N=7, age: 21±2 years) completed two brachial FMD tests 24 hours apart. Women were tested during the mid‐follicular phase of their menstrual cycle (days 7‐12). Baseline arterial diameter (ADBL) and maximum reactive hyperemia diameter (ADRH) were determined, and relative FMD was calculated as FMD% = (ADRH–ADBL)/ADBL. Day 1 measures and between‐day differences were assessed with independent t‐tests. Day‐to‐day variability was assessed with the coefficient of variation (CV) and intraclass correlation coefficient (ICC). Compared to men, women had smaller arterial diameters (p<0.05), but a similar FMD response (p=0.828). No sex‐differences were observed in between‐day differences for any measures (p>0.05). Day‐to‐day variability was low for AD in both sexes (CV of 3‐5%), but high for FMD (men: 43%, women: 55%). Similarly, ICCs were substantial to almost perfect for AD in both sexes (ICC of 0.77‐0.92), but poor for FMD (men: 0.01, women: 0.07). These findings suggest that arterial diameter exhibits better day‐to‐day repeatability than relative FMD in both men and women. Despite women having greater FMD variability than men, between‐day differences were not statistically significant. A larger sample size may be required to detect potential sex‐differences.Funded by NSERC, OGS and GSSI.

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