5 Reasons You Can’t Run Faster!

Every runner knows what it feels like to reach his or her limits. But what really defines those limits? We decode the mysteries behind why our legs turn to lead and our will seems to vanish.

Alex Hutchinson |

Struggling to run faster? You don’t hit your limit just because lactic acid is scorching your muscles. The many sensations you feel while running each correspond to a different mini-crisis in your body, and they combine to determine whether you can hold your pace. Last year, exercise scientists gathered to share the latest results in their search for the limits of endurance. They discussed new topics such as metabolites and mental fatigue, as well as familiar foes including heat and hydration.

Here is what their findings reveal about what’s behind the pain of trying for a PB, and how you can push a little harder.

1. ‘I can’t catch my breath’

Cause: Oxygen deficit

Occurrence: Shortly after starting to run

Antidote: A ‘priming’ warm-up, including a sustained burst of intense running

The first rep of an interval workout always feels hard; the next rep, however, seems easier. ‘Your breathing rate doesn’t fall; you just kind of settle into it,’ says Dr Andrew Jones, an exercise physiologist. What you experience is the result of a brief mismatch between the oxygen your legs require and the oxygen your heart and lungs are able to deliver.

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When you start running, your muscles’ oxygen needs immediately spike, but the time it takes for the rest of your body to respond is dictated by your ‘oxygen kinetics’, or response time. This oxygen deficit triggers signals that cause your breathing and heart rate to speed up and your blood vessels to dilate, as well as activating oxygen-processing enzymes in the muscles. As a result, within two to three minutes, your muscles are getting enough oxygen.

The temporary oxygen shortage has lasting implications, though. To meet the energy shortfall, your muscles tap into their anaerobic (oxygen-free) fuel stores. That produces metabolic by-products that make your muscles feel fatigued – and it also leaves you with less energy for the final anaerobic sprint at the end of the run. ‘What you burn up in the first couple of minutes is never going to replenish unless you slow right down,’ says Jones.

To fight this oxygen deficit, Jones and others are studying an approach called ‘priming,’ which gets that first-rep fatigue out of the way before the race (or first rep). Ten to 20 minutes before the start, include a sustained burst of intense running in your warm-up, 45-60 seconds at 5K race pace, for example. This will activate enzymes and dilate blood vessels, while also allowing you time to recover before the race starts.

2. ‘I’m running harder, but I’m still not getting any faster’

Cause: Inefficient muscle-fibre recruitment

Occurrence: Sustained medium efforts (such as 10Ks and half marathons)

Antidote: Train your fast-twitch fibres to be more efficient

The early kilometres of a half marathon often feel pretty easy. You’re not running fast enough to accumulate high levels of lactate and other metabolites; and, unlike in a marathon, you’re not running far enough to empty fuel stores. So why does it eventually – and inevitably – get so hard?

The answer, according to studies from the University of Copenhagen, once again depends on oxygen kinetics. Over the course of a sustained run at half-marathon pace or faster, the amount of energy (and thus oxygen) needed to maintain that pace gradually centimetres upward. Over the course of 10-20 minutes, your oxygen consumption can drift upward by as much as 25 per cent, making it progressively harder for you to hold your pace.

This drift is the result of a shift to recruiting less-efficient muscle fibres. When you start running, you automatically recruit mostly slow-twitch muscle fibres, which are suited for long-distance running because they’re efficient and take a long time to fatigue. As time goes on, though, individual fibres begin to fatigue and run low on fuel. To replace them, your brain must recruit fast-twitch fibres, which demand more energy – and oxygen – to deliver the same output.

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One way to tackle this problem is to train your fast-twitch fibres, which are usually deployed for explosive movements, to be more efficient. ‘This might be one reason long runs are so important for marathoners,’ Jones explains. A two-and-a-half hour run, even at a slow pace, will eventually deplete slow-twitch fibres and force fast-twitch ones to practise delivering slow and steady power. In response, they’ll build endurance by ramping up mitochondrial content and adding capillaries to supply more blood.

3. ‘Help! My legs are on fire!’

Cause: Metabolite accumulation in your muscles triggers signals to your brain

Occurrence: Short or 5K races, fast surges or finishing sprints

Antidote: Short, fast interval workouts

Imagine the searing muscular discomfort of a hard interval workout, but focused entirely in your thumb. That’s the odd sensation that 10 lucky volunteers in a university lab experienced in 2014, when a research team led by professors Alan Light and Markus Amann injected a cocktail of metabolites – the chemical by-products that build up in your muscles during intense effort – into their thumbs. The results were extraordinary: they created sensations of fatigue in subjects who were not moving a muscle.

For decades, scientists and athletes have talked about ‘lactic acid burn’ triggered by intense exercise. When you run hard, you eventually reach a point where your aerobic energy system – the ultra-efficient fuel supply that relies on oxygen delivered by your heart and lungs – can’t supply energy to your muscles quickly enough. You turn instead to anaerobic (oxygen-free) energy sources, which provide much-needed fuel but also generate metabolites that build up in your muscles. One of those metabolites is, indeed, lactate (a molecule that’s closely related to lactic acid). But despite its nasty reputation, lactate, on its own, doesn’t make you tired.

Light and Amann tried injecting their volunteers with three different metabolites: lactate, protons (which make your muscle more acidic) and adenosine triphosphate, a form of cellular fuel. When the chemicals were injected alone or in pairs, nothing happened. But when they injected all three together – bingo! At first the subjects reported feelings akin to ‘fatigue’ and ‘heaviness’ in their thumbs, even though they were sitting still. Then, when researchers injected higher metabolite levels that would correspond to all-out exercise, the sensations shifted to ‘ache’ and ‘hot’– the so-called lactic burn, all created in a test tube.

These results show that, regardless of what it feels like, your muscles aren’t being dissolved by lactic acid. It’s only when special receptors in your leg muscles detect a particular combination of metabolites that they trigger a distress signal that travels up your spinal cord, which your brain interprets as a burning sensation. One solution? Train the receptors to be a little less sensitive by repeatedly triggering them in training. ‘The first time you do intervals after the off-season, you think you’re dying,’ notes Amann. But after just one or two workouts, ‘it already feels a bit better’.

4. ‘I can hardly lift my knackered legs’

Cause: Metabolite accumulation hinders muscle contraction

Near the end of hard races

Antidote: Prudent pacing

OK, so now we know that the ‘lactic burn’ is really just a sensation in the brain, triggered by nerve sensors in the muscles. Does that mean that the muscles themselves can keep going indefinitely if you somehow ignore those signals? To find out, Amann and his colleagues injected a nerve block called fentanyl into the spines of study volunteers, preventing signals from travelling up from the leg muscles to the brain, and asked them to ride 5K as hard as they could on a stationary bike. The results were dramatic. When the first subject finished and tried to step off the bike, he nearly collapsed on the floor before Amann and his researchers caught him. All subsequent subjects had to be helped off the bike. Some couldn’t unclip their feet from the pedals, Amann recalls, ‘and not a single one was able to walk’. They had all been given a gift that many athletes dream of – the ability to push as hard as they wanted without feeling much pain or fatigue – and now they were paying the price, with muscles that had essentially ceased to function.

However, despite their temporary superhuman status, the subjects didn’t ride faster than when they received a placebo injection. ‘They always feel great initially,’ says Dr Gregory Blain, one of Amann’s colleagues. ‘They’re flying. But we know they’re going to crash.’ By the halfway point, the cyclists still felt great, but they started to look puzzled, because their legs were no longer responding to the commands sent by their brains. Whatever advantage they gained from their fast start was soon lost as their legs stopped responding to instructions. In this case, the fatigue really is in the muscles rather than the brain. Without any warning signals in the brain, metabolites such as protons and phosphate ions accumulate far beyond levels that directly interfere with the ability of the muscle fibres to contract. In other words, the fatigue produced by metabolites isn’t ‘all in your head’ after all – instead, you experience a mix of ‘central’ (in the brain) and ‘peripheral’ (in the muscles) fatigue during hard runs. Push too hard at the start of a race and you’ll discover just how real those peripheral limits are.

5. ‘That’s it. I give up!’

Cause: Effort overload

Any time you’re pushing your limits

Antidote: Train your brain

It hurts too much. That’s the simplest way to explain why you don’t push a little harder in those final kilometres. But it’s not quite right. Pain isn’t what holds you back. When researchers ran an electric current through the brains of study volunteers to dull their sense of pain – using a technique called transcranial direct current stimulation – it didn’t improve how the subjects felt during exercise or how they performed in a ride to exhaustion on a stationary bike.

What matters, according to exercise physiologist Dr Samuele Marcora, one of the authors of the Kent study, is effort: the struggle to continue against a mounting desire to stop. All the other forms of fatigue – oxygen deficits, metabolite accumulation, overheating, dehydration, muscle damage, fuel depletion and so on – contribute to your overall sense of how hard it would be to maintain your pace or speed. Effort, in other words, combines all the different fatigue signals that emanate from every corner of your body, and the moment of truth in any race corresponds to maximum effort.

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Runners spend most of their training time trying to make their muscles, heart and lungs stronger and more efficient. But Marcora’s theory suggests that altering your subjective sense of effort is another way to run faster. Studies have successfully altered perceived effort – and endurance – using techniques such as subliminal messages (smiling faces flashed for a fraction of a second), electric brain stimulation (with electrodes positioned to alter perceived effort instead of pain), motivational self-talk (Feeling good!) and ‘brain endurance training’ (computerised tasks completed while exercising on a stationary bike).

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The big question, though, remains unanswered: what is effort, exactly? Is it a psychological state? Is it the tactile sensation of your muscles contracting? Or is it, as Marcora believes, our overall sense of how hard it is to maintain race pace? We’ve learned a lot about what happens in the body when we run, and come up with explanations for many of the sensations we feel and limits we encounter. The next training leaps will come from understanding the brain.

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