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What does this mean in biological terms?

Posted by SMcGrath on 19 Nov 2012 at 11:20 GMT

Assessment of the change in rMSSD over the entire testing period revealed a significant group x quadratic trend interaction [F(1,68) = 4.56, p = 0.04; ηp2 = 0.07]

Could you explain what the group x quadratic interaction term means in terms of a HRV difference between control and CFS subject?

No competing interests declared.

RE: What does this mean in biological terms?

ute_vollmer-conna replied to SMcGrath on 21 Nov 2012 at 05:07 GMT

As part of an analysis of repeated assessments, here of HRV, we first assessed whether there was a difference between the groups overall. Then we also look at whether the two groups differ in the pattern of their HRV readings across the different time-points or measurement occasions. This is called trend analysis, where a linear trend reflects a straight line going up or down, a quadratic trend reflects a line with one major bend in it; a cubic trend stands for a line that has 2 significant bends.
I guess the meaning of the group x quadratic interaction may be best explained by reference to Figure 3. The groups differ in the pattern of their HRV readings across the tests. The control groups shows a clear quadratic trend (the line goes first down and then straight), which is different form the pattern seen in the readings of the patients with CFS (their line first remains comparatively straight and then goes down). You could say that the patterns follow an opposite trend.
As discussed in the paper, biologically the pattern of the healthy control participants is what is expected when a person is confronted with a stressor. HRV generally goes down sharply during the initial confrontation (part of a normal stress response), but levels out when the person gets used to the challenge (this is called habituation). The pattern seen in the participants with CFS is unusual in that their response is slow to change and then does not show habituation (it continues to decline). The slow response of HRV measures may reflect that the HRV in participants with CFS is reduced already and may therefore lack dynamic flexibility when adjustments to a stressor are needed.
Sorry for the lengthy reply…..I hope this helps?

Competing interests declared: I am the senior author on this paper