What is running threshold, actually? Getting sucked down a lactate/lactic acid buffer plug hole in Kay Jay’s quest for the sub 3.45 marathon

Yep…. Nearly 2 months since I last managed a blog post. Work is most definitely getting in the way of my writing plans! Sadly, I need the cash so I’m going to have to put up with it for the foreseeable. And I like my biology students. I just hate everything else….

I haven’t got any further with my planned mythology posts about the Morrigan and Manannan Mac Lir – my two favourite Celtic gods, but as a runner and certified (able) biology geek, I have got something to say.

A bit of background

Last Saturday, (or it was when I started writing this 3 weeks ago) after being woken up at 1 am by a classic south coast gale, I ran my second half marathon since my recalcitrant (and still weird and irritating) hamstring injury.

I ran a time identical to the one I ran at Oxford in October – just under 1.45 hrs with no significant leg issues (hooray).

Now, in October I was ecstatic with that time. But for the first 8 miles of this one – the Victoria park event organised by Run Through (very friendly all round; no major ego problems shouting ‘move’, and a great, fun morning despite the freezing wind) – I was on to get a PB, or at least close to my pre-injury PB of 1.43…..

Then at mile 9, my pace dropped, and I couldn’t do anything to make my body speed up. The guy I’d been playing overtake-tag with for 8 miles (nice friendly guy) disappeared into the distance. Then my pace dropped again! My last mile was 8.27 mins – just faster than my target marathon pace, and 40-45 seconds slower than I’d run the first eight miles. So what was going on?

I was obviously running at my threshold. And by mile 9, my body was not ‘fit’ enough to deal with it. My muscles were overwhelmed.

But what by?

The biology geek in me just has to understand what’s likely to be going on! I am not a sports scientist, but I do have a Ph.D in Microbiology, and I love a bit of research trawling. It reminds me about stuff I’d forgotten and gives me interesting anecdotes to insert into my biology classes…sometimes it’s nice to get away from the A-level exam question treadmill….

Energy for muscles

During all exercise, muscle cells will be respiring to produce ATP for energy.

I won’t go into the details here, but basically, respiration is the process by which cells transfer energy from ‘food’ into a usable form – ATP.

Cells prefer glucose to do this, but they can also use fats, amino acids and lactate…. In a roundabout way.

In all circumstances, muscles generate ATP using a combination of aerobic (requiring oxygen), anaerobic (no oxygen) respiration and the creatine phosphate route, but the harder the exercise, the more anaerobic respiration there will be.

This is because the circulatory system can’t get enough oxygen to muscles quickly enough during intense exercise, so muscle cells respire without oxygen to keep producing some ATP.

But what is ‘threshold’ then?

At a basic level, threshold would seem to be the point at which too much anaerobic respiration is occurring, and the muscles are effectively overwhelmed.

A lot of the research coming up when I search on this is depressingly old, but 1980’s and 1990’s research does show that as blood lactate accumulates, sporting performance drops.

But that doesn’t mean that the lactate is causing the fatigue….

In fact, I have put lactate in bold because it isn’t lactic acid – the byproduct of anaerobic respiration that has traditionally been blamed for muscle soreness and reduced performance during sustained, intense exercise – and its accumulation is only part of the story

Lactic acid v lactate

I am no chemist and what I’ve discovered very quickly here is that my ageing brain is no more up for buffer calculations now than it was at 16, so I’m going to gloss over anything complicated to do with Pk/dissociation etc.

Let’s just say that the lactic acid produced during anaerobic respiration separates (dissociates for any chemists cringing at my word choice) into lactate and H+ ions (aka protons).

The protons are a potential problem because they cause acidity/lower pH, and changes in pH alter protein function.

The lactate can be a useful energy source…… Especially for the heart…. and apparently the brain.

Protons (H+ ions); buffers; oxygen transport; proteins and other delicately-balanced systems

One of the things I hope my biology students grasp early, is that proteins do everything in living organisms.

They are the enzymes that digest the food we ingest, the transporters that allow us to get digestion-products into our blood, the hormones that control which cells take up these ‘nutrients’ and yet more enzymes that make new muscle cell proteins/membranes/DNA etc.

So, if proteins get damaged by a change in pH (hydrogen ions from acids) we are dead. Literally, not figuratively.

Consequently, our bodies ‘mop up’ H+ ions pretty much immediately. They are exported from cells into the blood and run into bicarbonate.

Bicarbonate is the main buffer in the blood according to what I’ve read, but there are others.

The Haemoglobin in red blood cells is one. It binds H+ions and when it does, Oxygen is dislodged … hmmmm my easily diverted brain pursued that avenue of research for a few hours, but drew a blank.

So it’s not the acid then?

Who knows!

Some researchers suggest that H+ ions can reduce the functioning of enzymes involved in anaerobic respiration (glycolysis) before cells can kick these troublesome protons out into the blood; others suggest these same H+ions stop transport proteins working, so mitochondria can’t get hold of NAD (I so don’t want to go into what this is here because my arm is about to fall off) and lactate that they can use in aerobic respiration.

Not all muscles and organs are short on oxygen during intense exercise after all … Our heart and brains require a constant oxygen supply and receive it in preference to other tissues. So they could use the lactate produced in other tissues, as long as the proteins that take it up from the blood and into the mitochondria work properly.

Lactate uptake and use as a fuel

Lactate is used by the heart, liver and it appears the brain as a fuel, but it has to get into cells.

It goes through transport proteins as discussed, and some of these require sodium.

I am an extremely salty sweater. I lose loads of sodium (and other electrolytes), so could that be a factor….. Hmmmmm……

Others have suggested that phosphate released when ATP is used for muscle contraction inhibits transport proteins too. There’s a readable review here. It covers most of this.

Does this happen out of a tissue culture dish in vivo? I don’t know and that’s someone else’s PhD project.

But there are other theories…


Other researchers have suggested that chemicals required to produce ATP, such as ADP and NAD may also run short in cells and their mitochondria.

In addition, phosphate released when ATP is used to provide energy, has also been implicated as a cause for muscle failure during exercise. It seems to have some effect on contraction force, glycolysis rate, and how much muscle fibres shorten. But then so do H + ions….

The brain and central governor

There’s also a fair bit of evidence supporting the role of the brain in reducing exercise intensity before things become critical.

Swill your mouth with sugary drink and carbohydrate receptors send an impulse to your brain that tells it fuel is coming. This has been shown to reduce fatigue in studies. A similar response has been observed with vinegar and salt, apparently, though that’s an anecdote I heard en route to a cross country race last week and I haven’t researched it yet. Could there be an electrolyte sensing system direct from the mouth to the brain too?

So, who knows. But what does seem to work is training your body to deal with threshold, whatever may cause it. Which comes back to me not being fit enough 😂 so that was a good use of writing time then….

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