RESEARCH: Studies reviewed this week: 30 August 2021 to 5 September 2021

PHYSIOLOGY: The balance of muscle oxygen supply and demand reveals critical metabolic rate and predicts time to exhaustion

This study investigated whether muscle oxygen saturation levels can help to determine intensity zones. The authors tested two protocols using constant work rates or times to exhaustion. They found that:

Time to exhaustion predictions during continuous and intermittent exercise were either not different or better with %SmO2 for quad, versus running velocity.
Muscle O2 balance provides a dynamic physiological delineation between sustainable and unsustainable exercise (consistent with a “critical metabolic rate”) and predicts real-time depletion and repletion of finite work capacity and time to exhaustion.
These results highlight the matching of O2 supply and demand as a primary determinant for sustainable exercise intensities from those that are unsustainable and lead to exhaustion.

PRACTICAL TAKEAWAY - muscle oxygen saturation can be used to determine the second threshold (CP, MLSS, LT2, etc.).

If you'd like to learn more about this topic I highly recommend reading Bruce Rogers' post:

PACING: Distinct pacing profiles result in similar perceptual responses and neuromuscular fatigue development

I found this study quite interesting as I've been investigating different pacing strategies. The authors wanted to understand "the effect of distinct pacing profiles (i.e. U, J, and inverted J) in the perceptual responses and neuromuscular fatigue (NMF) development following a 4-km cycling time trial (TT)". The results were not what I expected:

TT performance was not different amongst pacing profiles.
RPE, ⩒O2 and HR increased similarly throughout the TT regardless the pacing strategy.
Therefore, endurance athletes with similar training status showed the same perceived responses and NMF development regardless the pacing profile spontaneously adopted.

The authors speculated that:

It was suggested that these responses occurred in order to preserve a similar rate of change in systemic responses (i.e. RPE, ⩒O2 and HR) and NMF development, ultimately resulting in same TT performance.

PRACTICAL TAKEAWAY - it appears that for short TT efforts the pacing profile does not make a difference to performance outcomes.

CHO: The Low-Carbohydrate Diet: Short-Term Metabolic Efficacy Versus Longer-Term Limitations

A low-CHO diet (LCD) has been recommended as a potentially optimal diet for endurance athletes. This study considered the low-CHO diet overall in a narrative review with the goal "to explore the efficacy, limitations and potential safety concerns of the LCD".

The authors concluded that:

The LCD usually has short-lived metabolic benefits, with limited efficacy and practicality over the longer term.
Dietary modification needs tailoring to the individual, with careful a priori assessments of food preferences to ensure acceptability and adherence over the longer term, with avoidance of dietary imbalances and optimization of dietary fibre intake (primarily from plant-based fruit and vegetables), and with a posteriori assessments of the highly individual responses to the LCD.

PRACTICAL TAKEAWAY - the LCD may be effective in some cases, but it needs careful consideration if you choose to implement it.

PACING: Pacing Strategy Affects the Sub-Elite Marathoner's Cardiac Drift and Performance

This study considered how to factor cardiac drift into pacing data and decisions. The authors considered using the cardiac cost (CC: HR divided by running velocity) "to establish the relationship between recreational marathoners' racing strategy, cardiac drift, and performance".

The first category that the authors found they called the fallers as they significantly decreased their speed during the race:

The asymmetry indicator of the faller group runners' speed is negative, meaning that the average speed of this category of riders is below the median, indicating that they ran more than the half marathon distance (56%) above their average speed before they "hit the wall" at the 26th km.

They then showed that:

Furthermore, we showed that marathon performance was correlated with the amplitude of the cardiac drift (r = 0.18, p = 0.0018) but not with those of the increase in HR (r = 0.01, p = 0.80).

Leading to the conclusion:

For addressing the question of the cardiac drift in marathon, which is very sensitive to the running strategy, we recommend to utilize the cardiac cost, which takes into account the running speed and that could be implemented in the future, on mobile phone applications.

PRACTICAL TAKEAWAY - cardiac cost may be a useful metric to help pace athletes.

If you're interested in this metric there is some useful ideas available here:

CHO: Graded reductions in pre-exercise glycogen concentration do not augment exercise-induced nuclear AMPK and PGC-1α protein content in human muscle

I've shared a previous study that suggested that there may be some benefits to training low (here). This study sought to answer the question: "What is the absolute level of pre-exercise glycogen concentration required to augment the exercise-induced signalling response regulating mitochondrial biogenesis?"

The authors found:

The magnitude of increase in PGC-1α mRNA was also positively correlated with post-exercise glycogen concentration.
In contrast, neither exercise nor carbohydrate availability affected the subcellular location of PGC-1α protein or PPAR, SCO2, SIRT1, DRP1, MFN2 or CD36 mRNA.

Leading to the conclusion that:

Using a sleep-low, train-low model with a high-intensity endurance exercise stimulus, we conclude that pre-exercise muscle glycogen does not modulate skeletal muscle cell signalling.

PRACTICAL TAKEAWAY - sleep-low, train-low may not be as effective as we have believed up to now.

PHYSIOLOGY: Cardiac drift during prolonged exercise with echocardiographic evidence of reduced diastolic function of the heart

In one of the studies above about pacing the authors looked into the cardiac cost of running. In this study the authors wanted to determine if "a continuous increase in heart rate (HR) during 4 h of ergometry cycling relates to cardiac fatigue or cardiomyocyte damage". The key findings were that related to ventricular function:

Left ventricular contractile function (end-systolic blood pressure-volume relationship [SBP/ESV]) and diastolic filling (ratio of early to late peak left ventricular filling velocities [E:A]) were calculated.
E:A decreased and returned towards baseline after 24 h, and individual changes were correlated to maximal oxygen uptake.

The authors concluded that:

During prolonged exercise cardiovascular drift occurred with echocardiographic signs of a reduced diastolic function of the heart, especially in those subjects with a high maximal oxygen uptake.

PRACTICAL TAKEAWAY - cardiac drift appears to be related to reduced heart muscle function.

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