Muscle Oxygen Training: Ramp slope and HRV thresholds, Physiological Reports 2023

Have you ever wondered what is the best type of cycling incremental ramp (RI) to estimate DFA a1 based HRVT's? Is it better to use a ramp with a steep slope (rapid rise in cycling power) or one with a shallow slope (slow rise in power)? Or something in between? Although I have done some limited personal testing myself (N=1), that's a poor substitute for a formal investigation. A search of the literature reveals very little about the effects of ramp slope on HRV thresholds (actually, it reveals nothing).

Is this an important question to resolve, or just an esoteric research data point? In my opinion, it is of paramount importance since without this knowledge we may get inaccurate results and can't reliably compare studies looking at HRVT agreement to gold standards. For example - research group A looks at HRVT agreement in teen women using a ramp slope of 45 watts/min and finds a major discrepancy with historical values previously seen. However, there has never been any HRVT assessment at that ramp slope, so we don't know if the issue is the ramp increment being so high, or is it truly something related to females (estrogen status for example). Another example would be a meta analysis done on all HRVT trials (with different RI slopes) - if slope does matter, we can't hope to get an accurate analysis! Currently, the range of published works describing DFA a1 during RI have used slopes ranging from about 7 to 25 w/min.

Since the following study evaluating ramp slope was an offshoot of a topic near and dear to my interests (muscle O2 desats and HRV thresholds), ideally it would have been nice to have the "parent" article published before this came out. As I mentioned in the last post, some journals take longer to get through their queue of acceptances to finally be published. In our case, the "sequel" came out before the first book of the series. Having said that, I can give a small hint of what the original study goal was by way of this abstract from ECSS:

The full paper is accepted and should appear shortly in a popular sports physiology journal.
The accompanying post to that has been written for quite some time. 😁
And yes, the combo of HRVT2 and HHb BP to assess the critical intensity is article #1 (from here)
Would I like to say more - definitely - but it's not fair to the journal, so this is it.
I would again like to thank Juan Murias, the co authors and Pablo Fleitas-Paniagua for making this possible. The complexity of managing such a large pool of participants performing three different ramps was not trivial. Therefore, although future studies confirming these results would certainly be welcome, from a practical standpoint, they may not be quickly forthcoming.

Fortuitously, the data used for the prior HRVT/HHb BP Combo project was part of a larger set of material encompassing the same group of individuals performing 3 different RIs with slopes of 15, 30 and 45 w/min. Therefore, we have the ideal framework for evaluating the effects of RI slope on HRVT behavior. This has now been formally published in Physiological Reports.

Some highlights of the study are shown below:

Method highlights - the Polar H10 (with ECG guidance for belt placement) was used to record RRs with Fatmaxxer. We used the standard 2 minute measurement windows with every 5 sec recalculation to plot the a1 HRVT1 and 2.

This is an example of the a1 behavior vs HR during the 3 ramps in one individual:

The left plot is the 30 w/min ramp, and on the right are all three ramps in the same individual. As noted, they are fairly close.

Correlation and Bland Altman agreement between ramps were all excellent:

There seems to be no pattern of the degree of correlation or agreement with rising slope difference. In other words, the 15 vs 45 watt ramps were no worse off on a statistical basis than the 15 to 30 or 30 to 45 watt ramps.

Finally, a "longitudinal" look of HRVT HR and VO2 through the ramps per participant:

T testing between all paired groups (15 vs 30, 30 vs 45 and 15 vs 45 watt) showed no differences.

Did we have a pre study hypothesis?

We were requested by the reviewers to add a hypothesis since I did not have one in the original draft. My reasoning for not having one was based on the definition of hypothesis - "a supposition or proposed explanation made on the basis of limited evidence as a starting point for further investigation". The problem here is that there was no "limited evidence or starting point" to make the educated guess. In the end, we added something, but truly, the question of RI slope and HRV was a conundrum from the start.

On one hand, I was not surprised that the 15 vs 30 watt ramps were equivalent since they are relatively close, but was very surprised that the 45 w/min ramp was similar to both. The 45 w/min ramp represents a 90 watt change over the 2 minute DFA a1 measurement window. We discuss the significance of that in detail in the discussion:

"Over the past 20 years numerous studies evaluating DFA a1 behavior during dynamic exercise have been performed (17-18, 21-26, 30-32). However, despite showing potential as a marker defining exercise thresholds through RI testing, there has been no consensus as to what type of ramp protocol is optimal or desirable. Therefore, the intent of this study was to assess the behavior of DFA a1 related HRVTs during cycling RI with varying slopes. Literature has shown that fast ramps tend to have the greatest degree of discordance between measurements such as cycling power and corresponding gas exchange derived thresholds (12-15), unless a correction is used to account for the V̇O₂ mean response time and slow component (3,14). In the context of an established ANS marker such as DFA a1, conjecture as to the effect of ramp slope is complex. A slower incremental rise in work rate resulting in a longer ramp may lead to fatigue related effects (45-46) that could result in biased threshold estimation. On the other hand, a rapid intensity rise may not be able to truly describe an index encompassing a measuring window of 2 minutes. For example, over the 2-minute DFA a1 measuring window, a full 90 watts of external load increase will have occurred if the RI test was performed at a 45 W·min^-1 slope. Whether or not DFA a1 values done under such non steady state circumstances produce comparable results to those done under a more gradual rise in load is unclear.

Since DFA a1 calculations need about a 2-minute measurement window for validity (31,39-40), fast ramps lasting only several minutes may also present a challenge simply on the basis of limited available data points. In addition, even though the ANS response is believed to be rapid in relation to the various regulation factors (47-49), there could be a lag between these inputs and their effect on DFA a1 behavior during fast ramps. Many initial studies measured DFA a1 toward the end of a “step” interval of varying length but always longer than 2-minute steps (30-32). When DFA a1 was first proposed as a surrogate marker for ventilatory threshold determination (21), a new calculation technique was used, based on the “time varying” method available in Kubios HRV software. Time varying refers to the index being recalculated continually every 5 seconds throughout the exercise period. Before this technique, the index was determined either at the end of each interval step or at periodic, non-overlapping points during the exercise test. Since we are now able to easily measure DFA a1 on a more granular level over the course of increasing load, the question remains whether absolute ramp slope matters for both index behavior and HRVT determination.

The results of this study show that the V̇O₂ or HR reached at both HRVT1 and HRVT2 is relatively independent of the ramp slope during incremental exercise testing (for those slopes used in this report). There was excellent correlation between all three ramp protocols using ICC_3,1 with values between 0.88 and 0.93 and no mean differences across all groups with ANOVA. Pearson’s r was also highly correlated between paired ramp groups with values between 0.84 to 0.95 (Table 2). Bland Altman analysis showed small mean differences between ramp slopes (Table 2 and Figure 3). There were no statistical differences seen between any ramp slope series looking at either HR or V̇O₂ according to paired t testing. Importantly, there was no major discrepancy in correlation/agreement or t testing in comparing the 15 to the 45 W·min^-1 ramp slopes, despite the three-fold difference in power output rate increment. The observation that DFA a1 is capable of rapidly shifting during the 45 W·min^-1 ramp to match that of the 15 W·min^-1 ramp is a novel finding of interest. Like the HR response to RI testing (11-12), there appears to be a prompt matching of “organismic“ demand as represented by DFA a1, to the external exercise load. This makes sense as both HR and HRV responses are mediated by related and/or linked ANS, central nervous system (CNS) centers, vagal output and effects on the atrial pacemaker cells (16,27,47-49). However, it has been unclear whether an HRV measurement window encompassing a relatively large span of differing metabolic input would yield usable results. This similarity in DFA a1 response across disparate ramp slopes is illustrated in a detailed plot of HR vs DFA a1 of a typical participant during the 3 RI tests (Figure 1B). The pattern of DFA a1 decline as HR rises is similar across the differing ramp slopes. Since the 45 W·min^-1 group had similar agreement to that of the 15 or 30 W·min^-1 groups, it seems that DFA a1 measurement of a linear increasing load leads to comparable HR or V̇O₂ correspondence no matter the rate of rise (within tested limits). This has major practical significance as prior and possibly future studies evaluating DFA a1 behavior may employ RI with different slopes. Since it appears the RI slope does not affect the resultant HRVTs, these studies can be more easily compared and implemented.

How well did HRVT1/2 agree with gas exchange?

This group consisted of the same participants as the Combo NIRS HHb BP/HRVT2 study that appears in JSCR. In that assessment, the HRVT2 agreed quite well with the RCP/VT2 (with individual variation of course). However, the group HRVT1 did not agree well with the GET/VT1, with about a 20 bpm differential. This is discussed in the limitations section in more detail. Going forward, we will need to be more aware of potential subpopulation differences, device bias and even belt position differences. Although the significance of an a1 of 0.5 has been discussed (it represents an uncorrelated, random beat to beat state), the 0.75 is a more empirical guesstimate. Even though the bias was large, the correlation was still reasonable and similar to reported values:

This is a plot of the 30w/min ramp GET/VT1 vs HRVT1 HR from the same ramp.

Hopefully, as time goes on and we better understand the many factors involved with measuring a1 during exercise, this will become clearer.

A word about cycling power thresholds and RI slope

Although we discuss this briefly in the discussion, there is a wealth of data looking at the disconnect between RI gas exchange derived thresholds and their equivalence to constant power interval data. For an excellent review and discussion see this (by Dr Murias's team). However, an older study by Weston also provides some information on how thresholds will be skewed, especially with steep ramps in particular:

The cycling power difference between the 10 and 50 w/min ramp was 20 watts for VT1 and 60 watts for VT2.

Expected day to day variability

There is now solid evidence that ramp slope has minimal effect on the HR/VO2 of the HRVTs - but what about day-to-day repeatability/reliability of a1 in general? Recent study data point to minimal shifts in a1

Therefore, one may see some "shifting" of the HRVT1/2 when performing different ramp slopes just based on day to day variability (but not necessarily from the slope change).

Summary and Conclusions

DFA a1 related HRVT1 and HRVT2 (as HR or VO2) are not affected by ramp slope (at least through the range of 15 to 45 w/min).
Cycling power VT1/VT2 threshold is affected by ramp slope. If you are interested in an accurate assessment of power at a particular threshold, try to use shallow ramps. However, there is no difference in HR thresholds with ramp slope.
Since HRVTs behave as HR/VO2 gas exchange metrics during RI protocols, the power associated with HRVTs will be slope dependent - therefore, shallow slopes are preferred for threshold power agreement.
Although there appears to be no systematic bias in HRVT HR with slope, some ramp to ramp variation is expected.
Whether your own particular HRVT agrees perfectly with gold standard (or does not), one should be reassured that ramp slope is not a factor (for HR/VO2).
Cross comparisons with prior and future HRVT RI studies can now be reliably compared without slope as a concern.