Do you have an extra artery in your forearm? A recent Australian investigation discovered a significant increase in the prevalence of a third forearm artery since the late 19th Century. Formed during early gestation in the mother's womb, the median artery supplies blood to the forearm and hand of an embryo. Usually, this artery disappears and is replaced by the radial and ulnar arteries before birth, but in some people it remains - a case known as the 'persistent' median artery - leaving them with a third, additional vessel. Is this good news for some climbers? Does increased blood flow equal greater oxygen supply to the muscles, greater efficiency and better pump recovery? We asked some experts who explained that it's not so straightforward...
The investigation, carried out by researchers at the University of Adelaide and Flinders University, describes the increase in prevalence as a 'microevolutionary change' which shows that modern humans 'are evolving at a faster rate than at any point in the past 250 years.' The existence of the median artery has been studied since the 18th century, when prevalence was around 10% in people born in the mid-1880s, compared to 30% in those born in the late 20th century, as the investigation showed. This is 'a significant increase in a fairly short period of time, when it comes to evolution,' according to Dr Teghan Lucas, commenting in a press release about her study.
'This increase could have resulted from mutations of genes involved in median artery development or health problems in mothers during pregnancy, or both actually,' she adds.
The trio of scientists collected data published in anatomical literature and dissected cadavers donated to medical science in Adelaide. Their findings showed that the forearm artery is present in about one third of Australians, and the team estimate that everyone will have it by the end of the century if this process continues. 'When the median artery prevalence reaches 50% or more, it should not be considered as a variant, but as a 'normal' human structure,' the study's abstract reads.
Professor Maciej Henneberg, a member of the Institute of Evolutionary Medicine at the University of Zurich, Switzerland, says that the median artery is beneficial because it increases overall blood supply and can be used as a replacement in surgical procedures in other parts of the human body.
But what does this mean for climbers? It's tempting to think that any extra blood supply would result in greater metabolic efficiency and better recovery during exercise.
Dr Jeremy Windsor, Consultant in Anaesthetics and Critical Care at Chesterfield Royal Hospital and Senior Clinical Lecturer at the University of Sheffield, weighed in.
'When you take a look at this study I think there's at least three questions to ask yourself, with lots more to follow!' Jeremy told UKC. 'The first is, are we certain that there are growing numbers of adults with a median artery? The study seems to think so, but is the historical data accurate? Can modern data be compared with what's gone before? Perhaps they're studying a different population to what's been studied before, or these researchers are using a more sophisticated technique to find the median artery and as a result are finding "more" of them.'
If we accept that the proportion of people with a median artery is increasing, this leads to the next question, Jeremy explains. Is this evolution?
'An evolutionary process takes thousands of years, not tens or hundreds, to occur,' he explains. 'For evolution to take place in humans, some gene change has to occur that offers a reproductive advantage. This advantage leads to those who possess it thriving and reproducing at the expense of those that do not. This doesn't occur in just a few decades, or even centuries.'
Indeed, there may be other explanations for this apparent rise. There could be natural variations among different populations, or perhaps researchers are simply looking more closely at the vessel. 'Alternatively, changes in modern living may somehow be responsible,' Jeremy suggests. 'As an example, take the widespread use of folic acid supplements in pregnant women and how this has dramatically reduced the incidence of spina bifida.'
Leaving the reasons aside for the timebeing and assuming that there are growing numbers of adults with a median artery and that this is part of an evolutionary process, is this advantageous to humans and climbers in particular?
'If you want a muscle to work harder for longer, you need the mitochondria in the muscle cells to produce energy in the form of adenosine triphosphate (ATP),' Jeremy says. 'It makes sense to think that an increase in blood flow will help. After all, more glucose and oxygen will be arriving from 3 arteries compared to 2? Sounds feasible. But it's never that simple when it comes to physiology!'
It's possible that the three arteries will carry the same volume of blood as two, Jeremy explains. 'But let's assume not, what if blood flow was increased to the muscle cell - would it then work harder and for longer?' he asks. 'First of all, oxygen and glucose need to reach the cell, then mitochondria (the 'powerhouses' of the cell) need to be available to generate ATP (and the other raw materials, ADP and P), then the new ATP molecules need to reach the muscle fibres and trigger the contraction, before finally clearing the waste products and starting again! Yes, adequate flow is essential, but increasing flow may not lead to the improvements you'd hope to see!'
Blood enters the arm via the subclavian artery, which becomes the axillary artery and subsequently the brachial artery, which then divides into the radial and ulnar arteries. The volumes of blood and oxygen transported by the larger conducting vessels and the branched arteries are substantial, and likely exceed the volume required by the relatively small forearm muscles, so the sticking point for getting pumped and fatigued on a climb probably lies deeper within the muscle itself.
'It would most likely be at capillary/pre-capillary level, the smallest blood vessels which carry oxygen and waste products to and from the muscle cells, where blood flow becomes limited,' explains Dr Nigel Callender, a former international-level competitive boulderer and sports scientist, now turned anaesthetist at Northumbria NHS Trust and visiting research fellow at Leeds Beckett University. 'It's this interruption to oxygen delivery through occluded [shut-off] capillaries that is probably the primary performance-limiting factor in climbers, along with the ability to recover between bouts/contractions,' he says.
Blood flow at capillary-level in the muscle demonstrates something known as an occlusion threshold. 'This threshold is the highly individual-specific percentage of maximal contraction force at which muscle contraction force itself occludes the capillary bed, ceasing blood flow,' Nigel explains. One climbing-focused study of the occlusion threshold suggests that sport climbers might present different blood flow conditions at the same relative isometric contraction intensity.
'The body does various things to try and offset capillary occlusion,' Nigel says, 'including raising blood pressure, sometimes remarkably high, to try and compensate for this reduction in blood/oxygen delivery during higher-intensity contractions.' A 2020 study led by Nigel found that indoor climbing and associated training exercises such as campussing induce a pronounced exercise pressor response that substantially increases both intra-arterial pressure and heart rate - a normal response seen during most resistance-based activities.
In a 2014 study, Dr Simon Fryer, Senior Lecturer in Exercise Physiology at the University of Gloucestershire found that when climbers were categorised into ability groups - intermediate, advanced, elite and a control group - and asked to contract at 40% MVC (maximal voluntary contraction) until exhaustion, there were little differences in brachial artery diameter, velocity or flow. 'However, the level of muscle oxygenation was notably different between groups, Simon explains. 'The higher ability groups were able to extract more oxygen within the muscle during the exhaustive exercise.' In addition, the higher ability groups' recovery post-exercise was much greater. 'It's likely, but not confirmed, that these changes at muscle-level are related to an increase in mitochondria, which we tested via an assessment of oxidative capacity in later papers, and increased capillary density,' Simon adds. A paper published by Simon this year investigating whether muscle oxygenation during climbing could be improved using a New Zealand blackcurrant supplement found that it increased oxygenation, but it did not affect muscle performance.
In short, there's a lot more happening at metabolic and cellular levels in the muscles during physical exercise than a simple 'more blood in = more energy for working muscles' equation.
Circulating back to the heart of the study (via the veins), the presence of vestigial structures such as the median artery and physiological variation in the human body is not uncommon. 'There are lots of normal variants, particularly among nerve and vessel paths and divisions within the body,' Nigel explains. Although there are no studies relating to the median artery in climbers, there is some unpublished work looking at the palmaris longus tendon in the forearm and its influence on climbing ability and pinch strength, which found that it did not increase strength - a finding supported by more general studies into grip and pinch strength. 'The palmaris longus is another normal variant often billed as an evolutionary change and around 14% of us don't have it as it's no longer needed,' says Nigel. 'Strangely, I have it in my left arm, but not my right. To summarise: having this muscle/tendon does not affect your climbing ability if it's present or not.'
More research is needed to answer the many questions that this latest study has generated. In the meantime, best get back to those forearm-pumping endurance circuits - whether you possess a median artery or not...
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