Gut check: Unveiling the influence of acute exercise on the gut microbiota.

The human gastrointestinal microbiota and its unique metabolites regulate a diverse array of physiological processes with substantial implications for human health and performance. Chronic exercise training positively modulates the gut microbiota and its metabolic output. The benefits of chronic exercise for the gut microbiota may be influenced by acute changes in microbial community structure and function that follow a single exercise bout (i.e., acute exercise). Thus, an improved understanding of changes in the gut microbiota that occur with acute exercise could aid in the development of evidence-based exercise training strategies to target the gut microbiota more effectively. In this review, we provide a comprehensive summary of the existing literature on the acute and very short-term (<3 weeks) exercise responses of the gut microbiota and faecal metabolites in humans. We conclude by highlighting gaps in the literature and providing recommendations for future research in this area. NEW FINDINGS: What is the topic of this review? The chronic benefits of exercise for the gut microbiota are likely influenced by acute changes in microbial community structure and function that follow a single exercise bout. This review provides a summary of the existing literature on acute exercise responses of the gut microbiota and its metabolic output in humans. What advances does it highlight? Acute aerobic exercise appears to have limited effects on diversity of the gut microbiota, variable effects on specific microbial taxa, and numerous effects on the metabolic activity of gut microbes with possible implications for host health and performance.

Limitations associated with thermoregulation and cardiovascular research assessing laborers performing work in the heat.

PURPOSE: To quantify the current literature and limitations associated with research examining thermoregulatory and cardiovascular strain in laborers working in the heat. METHODS: PubMed, SCOPUS, and SPORTDiscus were searched for terms related to the cardiovascular system, heat stress, and physical work. Qualifying studies included adult participants (18-65 years old), a labor-intensive environment or exercise protocol simulating a labor environment, a minimum duration of 120 min of physical work, and environmental heat stress (ambient temperature ≥26.0°C and ≥30% relative humidity). Studies included at least one of the following outcomes: pre- and peak physical work, core temperature, heart rate (HR), systolic blood pressure, diastolic blood pressure, HR variability, and rate pressure product. RESULTS: Twenty-one out of 1559 potential studies qualified from our search. There was a total of 598 participants (mean = 28 ± 50 participants per study, range = 4-238 participants per study), which included 51 females (8.5%) and 547 males (91.5%). Of the participants, 3.8% had cardiovascular risk factors (diabetes: n = 10; hypertension: n = 13) and 96.2% were characterized as "healthy". Fifty-seven percent of the included studies were performed in a laboratory setting. CONCLUSIONS: Studies were predominantly in men (91.5%), laboratory settings (57%), and "healthy" individuals (96.2%). To advance equity in protection against occupational heat stress and better inform future heat safety recommendations to protect all workers, future studies must focus on addressing these limitations. Employers, supervisors, and other safety stakeholders should consider these limitations while implementing current heat safety recommendations.

Efficacy of two intermittent cooling strategies during prolonged work-rest intervals in the heat with personal protective gear compared with a control condition.

INTRODUCTION: Personal protective equipment (PPE) inhibits heat dissipation and elevates heat strain. Impaired cooling with PPE warrants investigation into practical strategies to improve work capacity and mitigate exertional heat illness. PURPOSE: Examine physiological and subjective effects of forearm immersion (FC), fan mist (MC), and passive cooling (PC) following three intermittent treadmill bouts while wearing PPE. METHODS: Twelve males (27 ± 6 years; 57.6 ± 6.2 ml/kg/min; 78.3 ± 8.1 kg; 183.1 ± 7.2 cm) performed three 50-min (10 min of 40%, 70%, 40%, 60%, 50% vVO2max) treadmill bouts in the heat (36 °C, 30% relative humidity). Thirty minutes of cooling followed each bout, using one of the three strategies per trial. Rectal temperature (Tcore), skin temperature (Tsk), heart rate (HR), heart rate recovery (HRR), rating of perceived exertion (RPE), thirst, thermal sensation (TS), and fatigue were obtained. Repeated-measures analysis of variance (condition x time) detected differences between interventions. RESULTS: Final Tcore was similar between trials (P > .05). Cooling rates were larger in FC and MC vs PC following bout one (P < .05). HRR was greatest in FC following bouts two (P = .013) and three (P < .001). Tsk, fluid consumption, and sweat rate were similar between all trials (P > .05). TS and fatigue during bout three were lower in MC, despite similar Tcore and HR. CONCLUSION: Utilizing FC and MC during intermittent work in the heat with PPE yields some thermoregulatory and cardiovascular benefit, but military health and safety personnel should explore new and novel strategies to mitigate risk and maximize performance under hot conditions while wearing PPE.

The Influence of Active Hamstring Stiffness on Markers of Isotonic Muscle Performance.

BACKGROUND: Previous research demonstrates hamstring muscle-tendon stiffness (HMTS) influences isometric strength, landing biomechanics and architectural tissue properties. However, the influence on kinetics & kinematics during other modes of strength testing (isotonic dynamometry) has yet to be established. PURPOSE: Investigate how HMTS influences kinetics and kinematics during a novel isotonic muscle performance test which has never been done for the hamstrings. Previous work using dynamometry has been limited to isometric or isokinetic contractions, so the novelty arises from our custom isotonic protocol which allows quantitative assessment of the stretch-shortening cycle. METHODS: Twenty-six recreationally active individuals (15 males, 11 females, 23.8 ± 2.5 years) completed baseline testing for anthropometry and maximum isometric hamstring strength (MVIC). At least 48 h later, subjects completed a measure of HMTS (damped oscillation technique) followed by an isotonic knee flexion test (eccentric velocity 180°/s; concentric torque 25% of MVIC). Separate linear regression models with examination of residuals were conducted between HMTS and each muscle performance variable. Standardized coefficients determined the magnitude of the relationships. RESULTS: Significance was found for all outcome variables tested. HMTS and rate of torque development demonstrated the strongest relationship followed by isotonic concentric peak torque. The weakest relationship observed was with isometric peak torque. CONCLUSIONS: These findings build off previous work quantifying HMTS by showing HMTS more strongly relates to dynamic versus static muscle testing and identifies the potential clinical utility of isotonic dynamometry.

Self-reported sleep quality is associated with central hemodynamics in healthy individuals.

BACKGROUND: Insufficient sleep is associated with arterial stiffness and elevated cardiovascular disease risk. Central hemodynamics are influenced by arterial stiffness, yet independently predict cardiovascular risk. Relationships between sleep characteristics and central hemodynamic parameters are largely unexplored. We aimed to characterize the relationship between self-reported sleep quality and central hemodynamics in healthy individuals. To explore the secondary hypothesis that impairments in glucose metabolism may underlie relationships between sleep and central hemodynamic variables, we also explored associations between self-reported sleep quality and fasting blood glucose values. METHODS: Thirty-one healthy study subjects (20 to 69 years, 17 men) were free from metabolic or cardiovascular disease and did not take sleep medication. Self-reported sleep quality was obtained using the Pittsburgh Sleep Quality Index (PSQI) with normal sleepers defined by PSQI scores 0-5 and poor sleepers by PSQI score > 5. Relationships were assessed between PSQI, central hemodynamic profiles (systolic and diastolic blood pressures, pulse and augmentation pressures, augmentation index) estimated from oscillometric pulse wave analysis, and blood glucose values. RESULTS: Central pulse pressure was significantly elevated in poor (PSQI score > 5) compared with that in normal (PSQI scores 0-5) sleepers (P < 0.05). Linear regression models, adjusted for age, gender, and body mass index, demonstrated PSQI score to be an independent predictor (P < 0.05) of both central pulse (ß = 0.469) and augmentation (ß = 0.364) pressures. Global PSQI scores were not related to fasting blood glucose values (r = 0.045; P > 0.05). CONCLUSIONS: Significant relationships between central pulse and augmentation pressures and self-reported sleep quality highlight the importance of considering sleep when examining lifestyle contributors to central hemodynamics.

Commercially Available Capsaicin Supplement Fails to Enhance Time-to-Exhaustion During Cycling.

This study examined whether a commercially available low-dose (1.2 mg), chewable capsaicin supplement could enhance endurance cycling performance. Thirteen young (8M/5F), recreationally active individuals (age = 24.2 ± 2.9 yrs, body fat = 21.2 ± 6.1%) participated in the study. The study consisted of three visits, beginning with an initial evaluation of cardiorespiratory fitness (37.1 ± 5.5 ml/kg/min). During the second and third study visits, participants completed time-to-exhaustion (TTE) tests on a cycle ergometer at a workload eliciting ~90% VO2max, 45 minutes after ingesting either a 139 kcal capsaicin fruit gummy, or eucaloric placebo. Heart rate and rating of perceived exertion (RPE) were recorded every two minutes throughout the TTE sessions. Time-to-exhaustion was not significantly different (p > 0.05; d = 0.13) between placebo (487.8 ± 187.7 sec) and capsaicin (517.5 ± 258.4 sec) trials. Furthermore, heart rate responses and ratings of perceived exertion were similar (p > 0.05) between trials. These findings suggest that pre-exercise ingestion of a commercially available low-dose (1.2 mg), chewable capsaicin supplement fails to provide ergogenic benefits for time-to-exhaustion during cycling exercise. Higher doses may be necessary to elicit the performance-enhancing benefits observed during alternative exercise modalities (i.e., running) of comparable intensity.

Effect of a Commercially Available Low-Dose Capsaicin Supplement on Knee Extensor Contractile Function.

Capsaicin, the active pungent ingredient in chili peppers and various spicy foods, is demonstrated to influence a variety of physiological systems including skeletal muscle. The purpose of this study was to examine if a chewable capsaicin supplement (1.2 mg) could enhance isokinetic knee extensor contractile performance. Nine young, recreationally active individuals (5 females/4 males; 23.6 ± 1.5 yrs; 24.2 ± 3.3 kg/m2) participated in this randomized, single-blind crossover study. Following a familiarization session, participants completed two isokinetic knee extensor contractile function assessments, 45 minutes after ingesting either a capsaicin fruit gummy or eucaloric placebo, the order of which was randomized. Knee extensor peak torque (strength), summed torque (endurance) and fatigue index (fatigue) were compared between trials. Knee extensor peak torque was significantly greater (p < 0.05; d = 0.80) in the capsaicin (126.0 ± 40.4 N·m-1) than the placebo (118.8 ± 41.3 N·m-1) trial. No significant differences (p > 0.05) were found for summed torque (8012 ± 2771 vs. 7823 ± 2611 N·m-1; d = 0.45) or fatigue index (56.0 ± 17.1 vs. 48.7 ± 21.0 %; d = 0.46) between capsaicin and placebo trials, respectively. These findings, in a relatively modest and mixed-gender sample, suggest that pre-exercise capsaicin ingestion may benefit knee extensor muscle strength but does not appear to affect parameters of skeletal muscle endurance or fatigue.