RESEARCH: Studies reviewed this week: 6 September 2021 to 12 September 2021

PHYSIOLOGY: Cardiac Autonomic Responses during Exercise and Post-exercise Recovery Using Heart Rate Variability and Systolic Time Intervals-A Review

In this studies the authors look at a different metric (not HRV) to understand if it can reflect sympathetic activity - they explain the concept in the introduction:

Cardiac parasympathetic activity may be non-invasively investigated using heart rate variability (HRV), although HRV is not widely accepted to reflect sympathetic activity. Instead, cardiac sympathetic activity may be investigated using systolic time intervals (STI), such as the pre-ejection period.

The findings showed that:

Exercise intensity is the primary factor influencing HRV, with a greater intensity eliciting a lower HRV during exercise up to moderate-high intensity, with minimal change observed as intensity is increased further.
Post-exercise, a greater preceding intensity is associated with a slower HRV recovery, although the dose-response remains unclear. A longer exercise duration has been reported to elicit a lower HRV only during low-moderate intensity and when accompanied by cardiovascular drift
STI responses during exercise and recovery have seldom been reported, although limited data suggests that intensity is a key determining factor.
Concurrent monitoring of HRV and STI may be a valuable non-invasive approach to investigate autonomic stress reactivity; however, this integrative approach has not yet been applied with regards to exercise stressors.

PRACTICAL TAKEAWAY - while STI appears to be an interesting metric, it is not yet fully understood and doesn't appear to provide meaningful data that can be acted on.

PHYSIOLOGY: Cardiovascular Drift and Maximal Oxygen Uptake during Running and Cycling in the Heat

At the moment I'm particularly interested in learning about cardiac drift (see the studies from last week). This study set out to understand if there was a difference in cardiac drift depending on modality with the authors setting out "to test the hypothesis that a greater magnitude of CV drift, accompanied by a greater decrement in V˙O2max, occurs during cycling compared to running in hot conditions".

The authors found that:

Heart rate increased 19% and 17% and stroke volume decreased 20% and 15% between 15 and 45 min during running and cycling, respectively, but modes were not different.
CV strain (indexed as CV drift) during prolonged exercise in the heat corresponds to reduced V˙O2max, irrespective of exercise mode or the thermal gradient.

PRACTICAL TAKEAWAY - cardiac drift in the heat appears to be the same for cycling and running so it's possible to cross-reference across activity types and when reviewing studies which use different modalities.

HYDRATION: Optimizing the restoration and maintenance of fluid balance after exercise-induced dehydration

Rehydrating after training is important especially if you have another session planned later in the day. This review looked at the research available on this topic and shared the following recommendations:

Volume replacement during recovery should exceed that lost during exercise to allow for ongoing water loss; however, ingestion of large volumes of plain water results in a prompt diuresis, effectively preventing longer-term maintenance of water balance.
Addition of sodium to a rehydration solution is beneficial for maintenance of fluid balance due to its effect on extracellular fluid osmolality and volume. The addition of macronutrients such as carbohydrate and protein can promote maintenance of hydration by influencing absorption and distribution of ingested water, which in turn effects extracellular fluid osmolality and volume.

PRACTICAL TAKEAWAY - adding sodium to rehydration formula is useful and rehydration requires drinking more than the volume lost. [see this previous recommendation: if you need to hydrate quickly after an event, a strategy of ingesting a CHO-electrolyte solution at 60%, 40% and 20% in the first three hours is more effective than drinking equal amounts over 5 hours].

ALTITUDE: Defining the "dose" of altitude training: how high to live for optimal sea level performance enhancement

This study investigated altitude training camps at different altitudes. The intervention protocol was as follows:

After 4 wk of group sea level training and testing, 48 collegiate distance runners (32 men, 16 women) were randomly assigned to one of four living altitudes (1,780, 2,085, 2,454, or 2,800 m). All athletes trained together daily at a common altitude from 1,250-3,000 m following a modified live high-train low model.

The results showed that:

On return from altitude, 3,000-m time trial performance was significantly improved in groups living at the middle two altitudes (2,085 and 2,454 m), but not in groups living at 1,780 and 2,800 m.
EPO was significantly higher in all groups at 24 and 48 h, but returned to sea level baseline after 72 h in the 1,780-m group.
When completing a 4-wk altitude camp following the live high-train low model, there is a target altitude between 2,000 and 2,500 m that produces an optimal acclimatization response for sea level performance.

PRACTICAL TAKEAWAY - altitude training camps should be performed at altitudes from 2000 to 2500m.

ALTITUDE: Timing of return from altitude training for optimal sea level performance

One of the important considerations when going to an altitude training camp is when to compete after returning from altitude. There are a number of different recommendations and protocols so I have been trying to find the best science to understand this topic better. The authors of this review outline why this is so important:

Many endurance competitions, like a marathon or multistage cycling race, are in essence one-shot events, where the athlete can only effectively attempt one or perhaps two races of this nature within a calendar year. Altitude training camps require significant costs for travel and housing and often create logistical challenges with relocating for >4 wk from family, work, coaching, and medical support. In short, a mistimed return to sea level from an altitude camp could result in missing out on professional accomplishment and financial gain for the athlete.

In their discussion the authors note that:

There is no reason to think that the deacclimatization response with return to sea level would show any less individual variability. From an applied standpoint, athletes who experience a high level of ventilatory acclimatization or mechanical limitations to ventilatory flow may be better off with a period of time at sea level before competing.
For an athlete with a faster than normal decline in red blood cell mass with time at sea level [perhaps compared with the mean response of the Prommer et al. (38) cohort], competing as soon as possible upon return to sea level may be the most beneficial.

PRACTICAL TAKEAWAY - return from altitude remains a highly individual consideration and "ultimately, the best time to return from altitude training prior to a major competition for peak performance remains undocumented from a physiological standpoint".

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