RESEARCH: Studies from 15 to 21 Apr 24
Sharing research and insights from coaches, scientists and athletes to help us improve endurance performance.
This week’s quick summary:
Modelling thermoregulatory responses during high-intensity exercise
Effect of prior exercise on glucose metabolism in trained men
Caffeine ingestion compromises thermoregulation
Pacing by winners of a 161km mountain ultramarathon
Changes in running economy during a 65km ultramarathon
HEAT: Modelling thermoregulatory responses during high-intensity exercise in warm environments
The six-cylinder thermoregulatory model (SCTM) is a conceptual framework used in physiology to describe the mechanisms by which the body regulates its temperature. It represents the integration of various physiological processes involved in maintaining thermal balance. The “six cylinders” are metabolic heat production, insulation, circulation, evaporative heat loss, behavioral responses, and thermosensors and integration. This model has been validated for resting individuals and the authors of this review set out to update “the accuracy of the model for intense exercise in warm environments [to] help a wide range of individuals in athletic, recreational, and military settings”.
RESEARCH DETAILS
Three datasets were analysed to assess SCTM accuracy for intense exercise in warm conditions.
Dataset 1 involved cycling in temperate conditions (25.1°C) with varying metabolic rates and body sizes.
Dataset 2 replicated the cycling in warm/hot environments (36.2°C) with similar variations in metabolic rates and body sizes.
Dataset 3 included track trials in cool and warm environments, estimating metabolic rates and observing core temperature differences.
SCTM-predicted core temperatures closely matched observed core temperatures, with root mean square deviations ranging from 0.06 to 0.46°C, meeting the acceptance threshold of 0.5°C.
PRACTICAL TAKEAWAY
In this review, the authors confirmed the SCTM's accuracy in predicting core temperatures during intense exercise in warm conditions. The value of this research is primarily for scientists in predicting thermoregulatory responses to warm environments. However, for athletes there is still value in understanding that three of the six cylinders are under their control: insulation, evaporative sweat loss, and behavioral responses. Thus when facing hot conditions, athletes can look to reducing their insulation (removing clothing), improving evaporative sweat loss (wet the skin or removing clothing that may prevent sweat loss), and adjusting behaviors (slowing down). There are more strategies and protocols for coping with hot conditions in my archives.
PHYSIOLOGY: Effect of prior exercise on glucose metabolism in trained men
A previous study I shared showed that carbohydrate loading before an event may benefit from a short session of near maximal intensity prior to the loading phase. Another study showed that for optimal glycogen synthesis after exercise, glucose should be chosen over galactose. Both of these studies consider that glucose uptake is greater post exercise which is what the authors of this study set out to determine. Their goal was “to examine glucose kinetics during an oral glucose tolerance tests (OGTT) after exercise, shedding light on post-exercise glucose metabolism”.
STUDY DETAILS
Endurance-trained men participated in a 2-hour OGTT, 30 minutes post-exercise, and at rest 24 hours post-exercise.
The area under the plasma glucose curve was 71% higher post-exercise compared to rest (p=0.01).
Glucose disappearance rate increased by 24% after exercise (p=0.04).
Glucose appearance rate was 25% higher after exercise (p=0.03).
Cumulative oral glucose appearance was 30% higher post-exercise (p=0.03).
PRACTICAL TAKEAWAY
This study suggests that prior exercise may enhance glucose output from splanchnic tissues, leading to a higher glycemic response and increased whole-body glucose uptake. The practical takeaway is that the body is ready and in an optimal state to use glucose to synthesize glycogen after training. My recommendation is for athletes to include carbohydrates in their recovery meal after training (do not focus only on protein) and to consume this as quickly after training as possible.
SUPPLEMENT: Caffeine ingestion compromises thermoregulation and does not improve cycling time to exhaustion in the heat amongst males
I have shared a number of studies on caffeine (see Supplement - CAFFEINE) and I am convinced that it is one of the most effective supplements for endurance athletes. Included in that list of studies is a study that showed that “caffeine ingested alone or in combination with water or a sports drink was not thermogenic nor did it impair heat dissipation”. However, there is still some concern about caffeine use in hot conditions, and in this study the authors set out “to investigate the effects of acute caffeine supplementation on cycling time to exhaustion and thermoregulation in the heat”.
STUDY DETAILS
In a double-blind, crossover trial, 12 caffeine-habituated males cycled in the heat after ingesting either caffeine (5mg/kg) or a placebo.
Caffeine did not affect cycling time to exhaustion (TTE).
Caffeine increased oxygen uptake, heat production, sweat rate, and evaporative heat transfer, while decreasing skin blood flow and thermal comfort.
Core temperature rose slightly with caffeine.
Despite its thermogenic effect, 5mg/kg of caffeine didn't improve performance and intensified thermal strain.
PRACTICAL TAKEAWAY
The results of this study showed that when exercising at the pre-determined gas exchange threshold in the heat, 5mg/kg of caffeine did not provide a performance benefit and increased the thermal strain of participants. I find this outcome quite surprising because I would have expected a performance benefit from caffeine even if that did come at the cost of additional thermal strain. I’m not yet ready to dismiss the benefits of caffeine for endurance performance based on this study, however, I do recommend that athletes racing in hot conditions extensively test their reactions to caffeine in those conditions to determine their individual responses.
PACING: Pacing by winners of a 161km mountain ultramarathon
In the studies I’ve shared on pacing, a conservative strategy on courses with a lot of climbing and a more even pacing strategy on flat courses has proven to be most effective. In this study the author set out “to examine pacing among the most successful runners in the 161km Western States Endurance Run (WSER) to determine if variations in segmental speed relate to performance, ambient temperature, and calendar year”.
STUDY DETAILS
Segmental speeds were consistent among eventual winners and lead runners, with notable differences only seen between 1st and 2nd finishers in the latter race stages.
Winners displayed significantly lower speed variability (12%) compared to other top-5 finishers (14-15%).
Speed variability strongly correlated with finish time (r =0.80, P =0.006) for the fastest 10 finishers.
Multiple linear-regression analysis revealed a significant relationship between speed variability and maximum ambient temperature (coefficient =0.14, P < 0.05), as well as calendar year (coefficient =-0.086, P =0.034).
Overall, top performers excelled at maintaining consistent pacing throughout the race, adapting their strategy to changing conditions and optimizing performance.
PRACTICAL TAKEAWAY
The results of this study showed that a consistent pacing strategy throughout the race and adapting to changing conditions optimised performance. This is probably not a surprise for most readers, but it is useful to have validation of conservative pacing to help plan race strategies. My recommendation is for athletes to prepare their race plans with realistic pacing strategies that rely on the information they have from race simulations in training and from previous races. This should result in pacing that is conservative and consistent throughout the race.
PHYSIOLOGY: Changes in running economy during a 65km ultramarathon
Downhill running has been shown to reduce running economy especially when running at higher intensities. In this study, the authors set out “to investigate changes in running economy (RE) during a 65km ultramarathon (UM)” by examining RE before, during, and after a 65km UM.
STUDY DETAILS
15 male UM runners, averaging 45 years, underwent a standard exercise test.
The test determined individual running speeds (60% VO2max: mean 9.4 km/h).
Participants completed a 65km UM with varied elevations (±1093m), comprising three laps.
Pre'- and post-UM, indirect calorimetry measured RE at individual speeds on treadmills with UM-specific slopes.
O2 cost and energy cost of running (Cr) significantly increased pre- to post-UM. During uphill running, O2 cost and Cr showed a gradual, linear increase.
PRACTICAL TAKEAWAY
This study found that the O2 cost of running increased about 11% for uphill and downhill running and 10% for flat running at the end of the ultramarathon. This is a significant change in the cost of running over the course of the race which could have impacts on pacing and race management. My recommendations are to ensure that athletes are as well prepared as possible for their race - hopefully reducing the loss in running economy in the final stages of the race - and that they account for this reduced economy by following a conservative pacing strategy (as mentioned in the study above).
Quick summary from last week’s paid newsletter
Paid subscribers receive a newsletter every week and have full access to all newsletters listed in the archives (355 studies and practical takeaways). Last week, the newsletter covered studies on the following topics:
Effect of speed, surface gradient, and cadence on running injury location
Effect of post-exercise heat exposure on endurance performance
Determinants of endurance in well-trained cyclists
Nutrition and altitude
Effect of curcumin supplementation on functional strength outcomes