FATIGUE RESISTANCE DEFEAT THE NEED TO SLOW DOWN
CYCLING WEEKLY|September 16, 2021
The best endurance cyclists are those who can stave off fatigue to keep pushing hard hour after hour. But what exactly is fatigue resistance and how is it developed? Dr Mark Burnley investigates
Dr Mark Burnley

It is difficult to overstate just how good Anna Kiesenhofer’s performance was to win Olympic gold in the women’s road race. She had to sustain her effort for almost four hours in blistering heat, pursued by the best riders in the world. Part of the reason they failed to catch her was, of course, tactical: everyone seemed to work to their nearest rival’s disadvantage while Kiesenhofer got on with the business of reaching the finish line. That meant being fit enough to sustain the speed to win. But it also meant having the physiological and psychological attributes to avoid slowing down after hours of effort. This aspect of performance is called fatigue resistance, and it does not get as much attention as it deserves.

Fatigue is a difficult concept to pin down because it has several meanings. The fatigue we are considering here is specifically physiological. Neuromuscular fatigue is the phenomenon Kiesenhofer was working against. Simply stated, this is a form of fatigue that is caused by muscle activity and which causes maximal performance (e.g. an all-out sprint) to decline, while being reversible through rest. It isn’t just maximal or hard effort that causes fatigue, of course. Fatigue can occur at any level of exercise. What matters is whether or not that fatigue slows you down. Kiesenhofer’s did not – and with the right training, you can do the same.

When the Austrian attacked in Tokyo, four other riders went with her. For the next 100km, they shared a well-organised breakaway. Gradually her breakaway companions became less inclined to pull on the front, and so she went on the offensive: “When I attacked my breakaway group, I was feeling really good,” Kiesenhofer tells me. “I could have done a VO2max interval at that point – I was feeling really fresh”.

She refers to this final third of the race as “a 40-kilometre time trial” – something that she had not planned. Fearing the chasing group would catch her, she kept pushing hard all the way to the finish. “In the last half hour, I was feeling really low on glycogen,” recalls the 30-year-old. “At least, that was my feeling. Something I don’t usually experience in training is that it was really hard to command my legs, so I felt the neuromuscular pattern start to change, that my body was trying to use whatever fibres were still there.”

What Kiesenhofer describes here is a form of fatigue that occurs if you exercise at a heavy exercise intensity between the so-called lactate threshold and the critical power or CP (equivalent to functional threshold power, FTP). In this intensity zone, exercise can continue for a number of hours but will be limited, one way or another, by muscle glycogen stores running critically low – that’s why Kiesenhofer’s legs felt empty. After two to four hours of sustained effort at this intensity, glycogen depletion is liable to substantially slow you down no matter what you do. But Kiesenhofer kept pushing hard.

What causes fatigue?

Exceeding FTP/CP by even 10-15 watts will cause fatiguing metabolites to accumulate, and muscle power-generating capacity will decline much faster than exercising below this threshold. These metabolites include inorganic phosphate, potassium and hydrogen ions. Lactate will also accumulate, but it does not appear to cause fatigue, at least not directly. Phosphorylcreatine, which is crucially important for maintaining energy transfer in the cell, will also eventually fall to critically low levels at these intensities. Collectively, metabolite depletion and accumulation ultimately forces the pace to slow dramatically (think domestiques peeling off after pacing their team leaders up a long climb). The further above FTP you ride, the more rapidly all of the above happens. To avoid overcooking it in her ‘40km TT’ effort, Kiesenhofer had to get her pacing spot-on.

The first 100km of Kiesenhofer’s race was performed below the lactate threshold, burning fat and sparing muscle glycogen. “It was actually quite controlled. It didn’t hurt. It was more like tempo riding and trying to stay on top of my nutrition and hydration,” she says. It was only when she attacked that the fatigue processes outlined above began to plot her own muscles’ downfall. But fatigue doesn’t take place only in the muscles; a big component of the feeling of fatigue comes from changes in the brain and spinal cord – which explains Kiesenhofer’s difficulty in driving her muscles as the race progressed.

As muscle fibres run low on glycogen, the power they produce drops and they need to be replaced by others to maintain the power ultimately going into the pedals. Recruiting new fibres requires more effort, and the ‘new recruits’ are fast-twitch fibres. With less powerful fibres being replaced with high-power fibres, the pedalling action becomes progressively harder to coordinate. The fast-twitch fibres also tire more quickly and require even more effort. At the same time, the brain gets less and less inclined to activate the muscle. Your muscles seem to stop responding to your desire to recruit them, and the lack of coordination makes you feel like you’re pedalling squares.

One of the most impressive things about Kiesenhofer’s performance was her ability to resist that fatigue as the race progressed, evidenced by the maintenance of a high cadence despite the undulating course.

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