# How to Measure Fatigue in a Cycling Race

Fatigue accumulated before the decisive part of a cycling race is one of the most important factors for the way it is raced and the final outcome, but how is fatigue actually measured and interpreted? There have been multiple approaches in the past, such as taking the normalized power, relative power in watts/kg (w/kg) or total energy expenditure in kilojoules (kj) as unit for measuring the fatigue in a race, but I believe the most optimal unit for the right interpretation of the difficulty of a stage is kilojoules/kilogram/hour (kj/kg/h).

Kilojoules itself are a unit for energy expenditure that directly correlates to watts, as total joules (kj*1000) equal avg. watts * time in seconds. They can be accessed for a specific section of a ride using the sauce4strava browser extension, in the analysis view on Strava, as long as the rider has uploaded power data.

As heavier riders naturally develop more watts than lighter riders at the same speed, the kilojoules are divided by the weight of the rider, to create a normalized unit for riders of all weight classes. Recording the same kj/kg in two rides doesn’t automatically mean both rides were at the same intensity, as less kj/kg are produced over the same time on a longer ride, reducing the intensity. This is why kj/kg are divided by the time, creating kj/kg/h – a unit that effectively is w/kg multiplied by 3.6. The nature of it being a unit for energy expenditure makes it more relevant considering the effect of refuelling in races, which is the intake of energy. That is the advantage of the unit compared to average relative power or normalized power.

According to calculations of Dr. Michele Ferrari, published on his 53×12 website, Vincenzo Nibali, one of the best GC riders of the last decade either possesses an incredible glycogen preservation at moderate climbing pace in a long mountain stage or very large glycogen stores. It is expressed that efforts after very long and hard stages are his biggest strengths, compared to other GC riders.

“Almost 800g of CHO is an amount that far exceeds the maximum capacity of saturation of glycogen stores and therefore confirms the extraordinary ability of Nibali to use fat as fuel, saving the CHO for the final stage, where he made the difference over the rivals.”

Current GC riders with similar ability seem to include the Colombian Miguel Ángel López, who has performed very well on hard stages in recent times, such as the stages finishing on the Col de la Loze and the Alto d’El Gamoniteiru, that he both won with impressive efforts on a w/kg basis. Tadej Pogačar is another rider who seems to fare well in difficult conditions, proving his astounding resistance on stage 8 of the Tour de France, when he essentially won the race, putting 3 minutes and 20 seconds into the rest of the contenders, after an effort of 14,80kj/kg/h for 02:43h in the early parts of the stage. See our article by @CyclingGraphs with our full analysis of that momentous stage.

Sergio Higuita on the other hand has excelled on easier stages in the UAE Tour and Tirreno Adriatico, finishing third on Jebel Hafeet and Prati di Tivo as well as achieving a fourth place on Jebel Jais, while performing below par in the Tour de France. He was completely uncompetitive there and although admittedly not being the designated leader of the team, he couldn’t even offer sufficient support to Rigoberto Urán on the majority of mountain stages.

These variances between the riders could come down to physiological differences as suggested by Dr. Michele Ferrari or their ability to refuel properly during a race. The ability to refuel properly also depends on the parcours of a race. Races with many climbs following shortly after each other, with not much rest between them, not only drain energy, they also make it difficult to effectively refuel before the next obstacle on the route. The lack of energy, that is created by not being able to refuel sufficiently, leads to bigger gaps as well as gaps in unexpected places. One race in which this clearly applied is the World Championship Road Race in Leuven, with its numerous short punches along the route. Julian Alaphilippe created an enormous gap on the Sint Antoniusberg, a rise that can barely be considered more than a speed bump for the pros with its 0,18km at 6,11%, certainly aided by the very hard race up to that point and a parcours which makes it difficult to effectively refuel.

Below, you can see a basic overview over the difficulty of an effort before the decisive part of a stage in terms of kj/kg/h.

The primary example of an effort after a stage ridden at ‘easy’ tempo is the ascent to Jebel Hafeet on stage 3 of the UAE Tour 2021, a stage race that is known for producing the lowest kj/kg/h of the entire year. On this specific stage, Sepp Kuss, who was in the Peloton going into the climb, rode the 03:38h preceding the Jebel Hafeet climb at 9,85kj/kg/h. This very low intensity definitely was one of the reasons for the high watts/kg efforts of Adam Yates (6,43w/kg for 26,03min) and Tadej Pogačar (6,34w/kg for 26,03min) on the climb, despite the high temperatures that peaked at over 40°C and them likely not being in their best shape in February.

Examples for ‘medium’ intensity ahead of the decisive climbing effort include stage 4 of Tirreno Adriatico, when Tadej Pogačar triumphed ahead of Simon Yates and stage 4 of the Tour of Slovenia, where Pogačar set it up for a Diego Ulissi victory. The ascent to Prati di Tivo remains Pogačar’s most impressive climbing effort ever on a watts/kg basis, as he achieved a remarkable 6,32w/kg for 36:06mins. That stage admittedly was relatively easy for ‘medium’ intensity with 12,28kj/kg/h for 03:13h before Prati di Tivo, according to Egan Bernal’s power data and featured perfect temperatures for high performances of around 14°C on the climb.

On the Sveta Gora climb in the Tour of Slovenia, Pogačar was riding for Diego Ulissi, having already secured the GC. He still climbed the 10:01min long climb at 7,02w/kg, perfectly setting up Diego Ulissi who punched away for the stage in the last 150m. All of this was achieved after a long, hard day on which the peloton rode at 11,87kj/kg/h for 04:04h before the Sveta Gora climb.

Stage 18 of the Vuelta a España was one of the hardest stages of the year, when the group of favourites had no rest, riding at a rate of 14,44kj/kg/h for 04:06h, before tackling the equally brutal climb of the Alto d’El Gamoniteiru – 14,7km at 9,83%. Despite the demanding stage, Miguel Ángel López proved his status as the best climber in the world at climbs longer than 45mins, ascending the Asturian monster in 50:09min at an unbelievable 6,04w/kg – one of the performances of the season.

It has to be noted that kj/kg/h only show the fatigue accumulated on that specific day, which is why kj/kg/h vary strongly between one day races, one week stage races and the third week of a Grand Tour. Further research is needed to show how much fatigue accumulated over multiple days and kj/kg/h on the day influence the watt/kg performances at the end of a race exactly, but they create a way to compare the intensity with which different races are ridden to a certain degree.

Gabriel Stróżyk (@NaichacaCycling)