Should I Trust My Calorie Readings?

Should I Trust My Calorie Readings?

— The 3 Minute Read —

Cycling devices play a crucial role in helping athletes monitor and understand their physical exertion, especially in terms of calorie expenditure. These devices typically employ a hierarchy system to calculate calories burned, prioritizing available data inputs. The hierarchy follows a logical sequence: power data takes precedence over heart rate, and if power data is unavailable, heart rate calculations come into play. If both power and heart rate are absent, the device resorts to using distance and time as a rough estimate of calorie burn. However, the accuracy of these methods diminishes as the hierarchy descends.

The challenge with relying solely on duration-based calculations is evident when distinguishing between scenarios such as coasting down a descent and pedaling uphill at threshold intensity. The lack of specific data inputs in these situations makes it challenging for devices to accurately gauge energy expenditure.

To understand how cycling devices quantify energy expenditure, it's essential to delve into the science behind it. Energy, in the context of cycling, involves stored chemical energy from food and mechanical/heat energy from work. These forms of energy adhere to the laws of physics, implying that they can neither be created nor destroyed, only transformed.

Calories calculated with power are often expressed in kilojoules (kJ), which conveniently converts to calories in a 1:1 ratio for cycling. However, the conversion isn't straightforward due to the human body's inefficiency in converting food's chemical energy to actual work on the bike. Approximately 75-80% of calories are lost as heat, making the actual energy applied to the pedals around 20-25%. Despite this efficiency factor, the 1:1 ratio of kilojoules to calories remains widely accepted in the cycling community.

The accuracy of power-based calculations makes it a preferred method for tracking energy expenditure. When a power meter reports, for example, 1000 kJ expended, it aligns with approximately 1000 calories burned.

On the other hand, heart rate-based calculations have evolved with technological advancements. These methods use algorithms derived from scientific metabolic testing, incorporating user input metrics like gender, height, weight, and activity level. Some devices even factor in beat-to-beat intervals to estimate MET (Metabolic Equivalent), improving accuracy. Although heart rate calculations are generally within 10-20% accuracy, they may not be as precise as power-based calculations.

Finally, time and distance become the primary metrics for calculating energy expenditure when no specific data is available. However, this method can be highly unreliable, with potential inaccuracies ranging from 20-60%. The lack of detailed information about effort levels, terrain, and wind conditions contributes to the variability.

In conclusion, the accuracy of energy expenditure calculations significantly improves when using direct measures such as power and heart rate. Power remains the most objective measure, providing precise data on work performed. Heart rate calculations, while reasonably accurate, depend on the quality of the reported data. Relying solely on time and distance for energy estimates is fraught with inaccuracies, impacting nutrition planning, fueling strategies, refueling expectations, and weight management. Cyclists are encouraged to use devices that incorporate a combination of these measures for a more comprehensive and accurate understanding of their energy expenditure.

Original article published on Trainingpeaks.