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Correlation or causation?

Oscar Foulkes June 29, 2024 Blood Pressure Interventions No comments

The second week of my experiment kicked off with two mornings of lowish blood pressure readings, contributing to an early observation that there appears to be some correlation with high intensity exercise on previous days. Of course, satisfying the requirements for causation will take more investigation.

Regardless of the causality, it was nevertheless a little comforting to see numbers within spitting distance of what’s regarded as being ‘normal’.

In the context of several successive days of the systolic pressure having been under 140, Tuesday’s 153 was somewhat alarming, but I had additional information to parse:
– I had lamb chops for dinner on Monday night, having had fish on the previous three days.
– I had a stressful interaction just before going to bed.
– My sleep was interrupted and almost certainly affected by this.

Having had the leftover lamb chops for dinner the following night, I can probably rule them out as a causative factor. It’s hard to control for stress, so it was useful to have an identifiable instance to work with. At this point I’m agnostic about the role of ambient stress, but if it’s resulting in poor sleep quality, then I’ll certainly be paying more attention to how that can be managed.

Seeing as exercise is central to this phase of my experiment, I should dip into some basics. Since late 2023, most of my cycling has been in zone 2, of which Dr Iñigo San Millán is the most visible proponent. At this level of effort it is still possible (just) to speak in complete sentences, so it’s a relatively easy pace to maintain. The thesis is that this level of exercise builds mitochondria, which burns fat, and on multiple levels is also supportive of high performance. Watch this interview for more detail.

Before heading out for my ride on Sunday, I happened to watch this interview with another cycling coach, Olav Aleksander Bu. While not dismissing zone 2, he calls for greater nuance. He makes the point that one of the great benefits of zone 2 training is that it’s hard to mess up. On other hand, training at higher intensities requires more careful management to ensure it isn’t overdone. If an athlete is going to change the training mid-session, it should be to decrease – not increase – the load.

This is relevant to the state I found myself in on Sunday morning. I could feel that I was fatigued, so my intention was to have a chilled ride on the Missing Link trail. That went out of the window when I joined up with stronger riders as I started the trail (in other words, I broke his rule about upscaling effort mid-session). I managed to hang onto them until slightly beyond halfway, but then I blew. I’d put in so much effort – especially on the back of the week’s higher than usual workload – that my Garmin displayed a recovery time of 43 hours! Usually, this would under 24 hours.

This is why my Tuesday ride was at such a low level of effort. I felt that I needed to treat it as an easy recovery day. Normal service resumed on Thursday, with an interval session. These sessions involve four by four minute efforts, each at 90% of maximum heart rate.

As my schedule has worked out this week, by the end of Monday I would have ridden every day for five consecutive days. It will be interesting to see where this takes me.

The question I’m trying to answer is if there are identifiable lifestyle factors that have a predictable impact on my blood pressure. The ideal outcome is to find the ones that reduce it. At worst, to find the ones to absolutely avoid at all times.

The Week in Numbers

Sunday
BP: 135/78
Bike: 1:53 on trail, mostly zone 3 & 4

Monday
BP: 137/85

Tuesday
BP: 153/86
Bike: 1:10, mostly in zone 1

Wednesday
BP: 143/95

Thursday
BP: 153/90
Bike: 4 x 4 minutes at 90% of max HR, plus warm-up and cool-down

Friday
BP: 136/84
Bike: 00:53, equally split between zones 2 & 3

Saturday
BP: 144/83
Bike: 2:00 on trail, roughly half in zone 2, balance shared between zones 3 & 4

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