Recently I updated my post on the effects of skin temperature elevation on DFA a1 related thresholds. Since then, several threshold tests have come my way from athletes who did not use a fan while testing indoors and consequently will have results that won't apply to normal conditions outside. Today, during some unrelated literature review, I came across a study done almost 20 years ago that is very appropriate and informative to this discussion.
The authors of this study wanted to see what happened to heart rate (HR), lactate and VO2 during a 30 minute cycling session at an intensity near the MLSS - either with and without a cooling fan.
An initial VO2max ramp test was done (with a cooling fan) - then 2 sessions at "MLSS" power - one with and one without a fan.
"All subjects performed three exercise tests in an air conditioned laboratory (19 C = 66 F), each separated by 1 week"
Tests 2 and 3 were 30-min constant-power tests. After
the warm-up the power output was progressively increased
by two equal 5-min steps until the power output
corresponding to the 4-mmol lactate threshold,
determined from the prior graded exercise test, was
reached. This power output was assumed to correspond
to a quasi-maximum lactate steady state intensity (Heck
et al. 1985) and was to be maintained for 30 min or until
exhaustion, whichever occurred first.
Note - the 4 mmol lactate figure is felt to correlate with performance as does MLSS, but is not the current way to test for the MLSS.
During the preliminary graded
exercise test and during one constant-power test two
fans (40 cm in diameter) were placed in front of the
cyclist to generate laminar airflow around the subject’s
body at an air velocity of 3 m s/1. One fan was set to
ventilate the lower body, whilst the other fan was adjusted
to target the upper body. Thus, one of the constant-
power tests was performed in the presence (AV) of
air ventilation, whereas air ventilation was absent
(NAV) in the other test.
Temperature measurement method:
temperature was measured (Braun, IRT 3520, Kronberg,
Germany) twice in each ear before the warming up
and at exhaustion, as well as at 10-min intervals during
the constant-load test. The mean of the four measurements
was recorded as body temperature.
Note - older technology and does not segregate skin vs core temp
Results:The red refers to p<.05 on t testing Fan vs No Fan
Not everyone was able to finish the 30 min test. This is most likely due to the erroneous calculation of "MLSS" based on fixed lactate. However if we look at the numbers, fewer in the "no fan" finished than the "fan".
- Exercise time was 25.1 min in AV (Fan) versus 17.1 min in NAV (No Fan) (P<0.05).
- Seven of the 14 subjects
were able to complete the full 30-min constant-power
trial in AV. In NAV significantly more subjects (n=12)
stopped the test due to premature exhaustion (P<0.05).
- Note - only 2 were able to finish without using a fan!
VO2/Lactate did not differ between Fan vs No Fan:
- There were no significant differences between AV and
NAV for blood lactate concentration and VO2 either at
minute 10 or at the end of the trial.
This is important! It tells us that factors other than VO2 or lactate were preventing these athletes from finishing.And that factor appears to be Temperature (core vs skin is unclear from this assessment):
- Compared with AV, body temperature was 0.5 C (range: 0.5 to +1.8C) higher in NAV (No Fan) (P<0.05).
Observations about HR
- Peak HR was the same and VO2 did not differ between groups.
- HR during the MLSS test rose more rapidly in the No Fan group (with similar VO2) - leading one to conclude that stroke volume was reduced (since cardiac output, as inferred by VO2, was the same). This is the same behavior I described previously.
- The elapsed time of the first 10 minute measurement precludes any dehydration effect. Not enough time for that to happen.
- Given what we know about cardiac drift in the heat, HR and stroke volume - the results are consistent with a primary, autonomic driven HR rise and subsequent exercise limitation from central fatigue.
Observations with CORE temp monitor
Over the past few weeks I've used the Core temp monitor to observe skin and core body temps. I've already presented what happens without a fan in the skin temp post, but wanted to share some additional data points. Here are some side by side comparisons of skin and core temps, cycling in the Florida summer heat vs indoors (with AC) with a fan:
- Both Core and Skin temps are similar, as long as the fan is on. There is a definite rise in skin temp, without the initial use of a fan. I usually do not turn the fan on indoors until about 10 minutes into riding (the room is cool enough up to that point) - and we see how the temp was climbing higher than outdoors at that point.
Take home points:
- Do not expect DFA a1 HRVT results obtained from indoor exercise ramps performed without fan cooling to reflect behavior of DFA a1 seen outside, on the road. The tests in the above study were done in a very cool room (66F), much cooler than the average home (except in winter). The fans made all the difference!
- Physiologic measures seen during "no fan exercise" such as lactate or VO2 might be useful for non HR threshold metrics (speed/power) but can be misleading when it comes to HR/HRV. Both VO2 and lactate during the time trial were the same with and without a fan - yet time until exhaustion and test completion were markedly reduced.
- Since many athletes rely on HR for training zone threshold identification, extension of results from "no fan" ramp testing could lead to misleading targets.
- The autonomic system is sensitive to subtle changes in skin temp. This should be considered in DFA a1 observation.