Each summary that I prepare adds to my archives and is distributed as a newsletter. If you're interested in either of those you can find them here:
- All the studies I've shared are available on the RESOURCES PAGE.
- The weekly summary is distrubted by email through my SUBSTACK NEWSLETTER.
This week's quick summary:
- Changes in running economy after downhill running
- Polarized training and combining HIIT and SIT each week
- The effect of gut training for endurance exercise
- The best time of day for training
- Continuous glucose monitors during exercise
PHYSIOLOGY: Changes in running economy at different intensities following downhill running
Downhill running requires specific training to prepare for the large eccentric load. Fortunately, only small training doses are needed to start the adaptation through the repeated-bout effect (search for "downhill" on my resources page). This study is interesting because it looks at changes in running economy after downhill running. The authors set out "to investigate the effects of downhill running on running economy at different intensities, and whether the impact varies based on the intensity".
STUDY DETAILS
- Fifteen untrained young men participated in the study.
- Participants performed level running at 70%, 80%, and 90% of their maximal oxygen capacity (VO2 Max) for 5 minutes at each intensity.
- They underwent a 30-minute downhill run at a gradient of -16% at their pre-determined 70% VO2 Max intensity.
- Measurements included oxygen consumption, minute ventilation, respiratory exchange ratio, heart rate, rating of perceived exertion, and blood lactate concentration during the level runs.
- Kinematic measures such as stride length and frequency were assessed using high-speed video analysis.
- Downhill running led to a significant decrease in knee extensor strength, muscle soreness, and an increase in plasma creatine kinase activity and myoglobin concentration lasting for 5 days.
- Significant changes in running economy and kinematic measures were observed at 2 and 5 days after downhill running at 80% and 90% VO2 Max, but not at 70% VO2 Max.
PRACTICAL TAKEAWAY
Downhill running resulted in decreased running economy and the effect was greater when athletes were running at higher intensities. Therefore training for downhill is important and this may apply to a greater extent in shorter events where the athletes will be going harder. This is useful because athletes sometimes assume that for shorter races they can get away with less downhill training based on the idea that there will be less total eccentric load on the legs. My recommendation is to include downhill training to take advantage of the repeated-bout effect and a good starting point would be the practical takeaway which I shared in this newsletter.
TRAINING: The effect of polarized training (SIT, HIIT, and ET) on muscle thickness and anaerobic power in trained cyclists
Interval sessions and time-intensity distribution are two topics I share frequently. To provide some context about the study below, I would recommend reading the paper I shared last week comparing HIIT and SIT protocols. Which intervals to do and how to structure the training week are important subjects that generate significant interest in athletes so it is useful to learn from new research like this study. The authors' aims were "to investigate the effect of different training concepts on muscle thickness and anaerobic power in trained cyclists, with a focus on the benefits of a polarized training program".
STUDY DETAILS
- Twenty-six mountain bike cyclists were divided into an experimental group (E) and a control group (C).
- The experimental group underwent polarized training, consisting of sprint interval training (SIT), high-intensity interval training (HIIT), and endurance training (ET), while the control group performed HIIT and ET.
- The training program was conducted over a period of 9 weeks.
- Laboratory tests were conducted before and after the experiment, including ultrasound measurements of quadriceps femoris muscle thickness and a sprint interval testing protocol (SITP).
- Cyclists performed 4 maximal repetitions, each lasting 30 seconds, with a 90-second rest period between repetitions to measure maximal and mean anaerobic power.
- A decrease in muscle thickness with a concomitant increase in mean anaerobic power resulting from the polarized training program is beneficial in mountain bike cycling.
PRACTICAL TAKEAWAY
The results suggest that combining HIIT and SIT protocols led to increased anaerobic power in the participants. This makes sense intuitively as SIT training is an intensity that is more likely to target an anaerobic adaptation. Based on this paper, I suggest that athletes need to be deliberate about choosing which sessions to include in a training week and to note that a mixed approach with different types of intervals (rather than focusing on one type of session) may be valuable. I still prefer a block periodisation approach for many athletes, however, I can see that the type of protocol used in this study could be useful for certain athletes and for certain periods of the year such as periods outside of focused block training.
PHYSIOLOGY: The effect of gut‑training and feeding‑challenge on markers of gastrointestinal status in response to endurance exercise
This paper is a systematic review that "aimed to identify and synthesize research that investigates the impact of 'gut-training' or 'feeding-challenge' on markers of gastrointestinal integrity, function, and symptoms". This is useful to provide an up-to-date summary of the current research and to guide athletes on what the best understanding of gut training is.
STUDY DETAILS
- 304 studies were reviewed and eight studies were included after screening, focusing on gut-training or feeding-challenge interventions.
- Gut-training or feeding-challenge interventions involved carbohydrate provision alone (in various forms) during cycling or running (n=7), or carbohydrate with protein during intermittent exercise (n=1) for 4-28 days.
- A 2-week repetitive carbohydrate feeding protocol reduced gut discomfort by an average of 47% (n=2), while repeated fluid ingestion over five trials resulted in a 26% decrease (n=1).
- Repetitive carbohydrate feeding during exercise for 2 weeks reduced carbohydrate malabsorption by 45-54% (n=2), with inconclusive results in one study (n=1).
- Assessment of exercise-induced gastrointestinal symptoms (Ex-GIS) using different tools showed significant improvements in total, upper, and lower gastrointestinal symptoms in two studies (n=2), while four studies yielded unclear results. Gastric emptying rate remained unchanged (n=2), and markers of intestinal injury and permeability did not show significant changes (n=3). Plasma inflammatory cytokine concentration in response to increased carbohydrate feeding showed inconclusive results (n=2).
- Overall, gut-training or feeding-challenge around exercise may provide advantages in reducing gut discomfort, and potentially improve carbohydrate malabsorption and Ex-GIS, which may have exercise performance implications.
PRACTICAL TAKEAWAY
Gut-training appears to be effective in reducing discomfort and it may also improve CHO absorption. The interventions in the review were generally only two weeks long and involved using 60 to 90g/hr of CHO during the training sessions. This is both a relatively short period for adaptation and the doses of CHO are not that high, yet it was still effective. My recommendation would be to follow a more thorough and extended gut training protocol such as this one from Aitor Viribay Morales.
TRAINING: Best Time of Day for strength and endurance training to improve health and performance
I have seen a range of different studies on the best time for training during the day. While it is an interesting topic, I have not investigated it thoroughly as I believe athletes should exercise when they have the time and the ability to fit it into their schedule. This systematic review provides a current insight into the topic as the authors set out "to investigate whether the timing of exercise training influences the degree of improvements in physical performance or health-related outcomes".
STUDY DETAILS
- Studies included in the review had to conduct structured endurance and/or strength training with a minimum of two exercise sessions per week for at least 2 weeks. The studies also compared exercise training between different times of the day using either a randomized crossover or parallel group design.
- 26 articles met the inclusion criteria and were included in the systematic review.
- The qualitative and quantitative synthesis provided limited evidence neither for nor against the hypothesis that exercising at a specific time of day leads to greater improvements in performance-related or health-related outcomes compared to other times.
- However, there was some evidence suggesting that matching the time of day for both training and testing could yield benefits, particularly for performance-related outcomes.
- At this time, there is insufficient evidence to expand the current training recommendations containing the factors frequency, intensity, type, and time (duration), with the factor time of day of exercise.
PRACTICAL TAKEAWAY
It appears that there is not strong enough evidence to suggest that athletes should put in the effort to train at a specific time of day. This confirms my view above: athletes should fit training in when it suits them best. One actionable point that I would take away from this review is that there was some benefit to performance from training at the same time as the test. This would suggest that for course reconnaissance and race simulations it would help to do them at the same time as the race.
EQUIPMENT: Continuous glucose monitoring (CGM) in sports - a comparison between a CGM device and lab-based glucose analyser under resting and exercising conditions in athletes
Note that this paper is a preprint that has not been peer reviewed yet.
Previous research I've shared has suggested that "using CGM to monitor blood glucose concentration during exercise is not [yet] recommended". The reason for that was the high variability of values displayed on CGMs during exercise. This study investigated the quality of CGM measurements "to compare glucose concentrations in capillary blood (CB) samples analyzed in a laboratory with glucose concentrations measured in the interstitial fluid (ISF) using continuous glucose monitoring".
STUDY DETAILS
- Ten healthy, active subjects with an average age of 26±4 years, bodyweight of 67±11 kg, and 11±3 hours of physical activity per week were included in the study.
- The participants underwent six tests within 14 days, including: a) resting fasted (R/Fast), b) resting after consuming 1 g glucose/kg bodyweight (R/Glc), c) running for 60 minutes at moderate intensity after glucose intake (65/Glc), and d) running for 60 minutes at high intensity after glucose intake (85/Glc).
- Glucose concentrations in capillary blood were compared to simultaneous interstitial fluid glucose concentrations. The highest correlation coefficient (Pearson's r) was observed for R/Glc (.92), followed by R/Fast (.45), 65/Glc (.60), and 85/Glc (.70).
- Mean Absolute Relative Deviation (MARD) and Standard Deviation (SD) were smallest during the resting fasted condition and were similar among the other test conditions.
- Bland-Altman plot analysis revealed a wider range between the lower and upper limits of agreement (95% confidence interval) of paired data during exercise compared to resting conditions.
- Under resting postprandial and exercising conditions, respectively, there are differences between both methods.
PRACTICAL TAKEAWAY
This study shows that the quality of measurements taken during exercise may not be accurate enough to put to any practical use. In addition, the authors warn of the dangers of trying to interpret these values: "As lag-time under exercising conditions, data sensor-derived data could lead to misinterpretation by athletes. Therefore, application of CGM should be accompanied by medical and/or nutritional experts’ advise". I would suggest that at this time, using a CGM is not something I would recommend given the cost, the quality of the data, and the challenge of interpreting the results effectively.
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