Original episode & show notes | Raw transcript
In cycling, the term “watts per kilo” (W/kg) is often treated as the ultimate measure of performance, especially for climbing. While it’s a valuable metric, it only tells part of the story. The debate over absolute power versus power-to-weight ratio touches on fundamental principles of biology, physics, and physiology. This document explores these principles to provide a comprehensive understanding of how an athlete’s size truly impacts their performance. We will cover:
Allometric Scaling: The science of how body characteristics change with size.
Aerobic Power (VO2 Max): Why bigger athletes have bigger “engines.”
Anaerobic Power & Strength: Why sprinters are built differently from climbers.
Dieting & Performance: The complex effects of weight loss on power output.
Allometric scaling is the study of the relationship between the size of an organism and the size of any of its parts or properties. Put simply, as an animal gets bigger, its characteristics do not scale up in a 1:1 linear fashion.
A classic example is the relationship between surface area and volume. If you imagine a simple cube:
Its surface area is proportional to the square of its side length (L2).
Its volume (and thus, mass) is proportional to the cube of its side length (L3).
This means that as the cube gets larger, its volume increases much faster than its surface area. This simple geometric principle has profound biological consequences.
Metabolic Rate and Heat Dissipation An animal’s metabolism generates heat. This heat must be dissipated through its surface (the skin). As an animal gets larger, its heat-producing mass (volume) grows much faster than its heat-dissipating surface area. To avoid overheating, larger animals must have a slower metabolic rate per unit of mass.
This is why, as the podcast notes, a gram of rat liver is about seven times more metabolically active than a gram of human liver. The rat, being much smaller, has a much higher surface-area-to-volume ratio and can dissipate heat more effectively, allowing for a “faster” metabolism.
Scientists express these scaling relationships with an exponent. For basal metabolic rate, it was found that it scales with body mass to the power of approximately 0.75 (or ¾). This is not linear (an exponent of 1.0) nor is it purely based on surface area (an exponent of 0.67 or ⅔). It’s a fundamental biological law that governs everything from single-celled organisms to blue whales.
While basal metabolism scales to the ¾ power, maximal aerobic power (VO2 max) behaves differently. The podcast highlights a crucial finding: across a vast range of mammals, from a 7-gram pygmy mouse to a 500-kg horse, absolute VO2 max (in liters/minute) scales with body mass to the power of ~0.87. In “athletic” species, this exponent is even higher, at ~0.94.
Implication: This value is very close to 1.0, meaning that an animal’s maximal aerobic power increases almost linearly with its size. A bigger animal has a proportionally bigger engine.
Why is this? At maximal exercise, the body’s metabolic demand is almost entirely driven by the working muscles. Across the animal kingdom, muscle mass scales almost 1:1 with total body mass. Therefore, the maximal metabolic rate required to power those muscles also scales almost 1:1.
The Cyclist’s Dilemma: Why Not Just Get Bigger? This leads to a critical question: If more muscle means a higher potential for absolute power, why don’t endurance cyclists just lift weights to get bigger?
The answer lies in the distinction between allometric scaling across species and training adaptations within an individual. The scaling law applies to the average animal of a given size. It doesn’t mean that an individual can increase their VO2 max by simply adding muscle mass.
VO2 max in a trained athlete is primarily limited by the central cardiovascular system—specifically, the heart’s ability to pump oxygenated blood (cardiac output). Adding muscle via weightlifting does not provide the specific stimulus (high-volume aerobic work) needed to increase the heart’s stroke volume. The new muscle mass becomes tissue that requires oxygen without a corresponding upgrade in the delivery system.
Furthermore, when we look at relative VO2 max (the classic W/kg metric), it scales with body mass to a negative exponent of -0.13. This means that, on average, as an animal’s size increases, its power-to-weight ratio decreases. This is the scientific basis for why smaller, lighter riders excel in climbing.
While aerobic power is about oxygen delivery, maximal strength and anaerobic power are about muscle force. Muscle strength is proportional to its cross-sectional area (scaling with an exponent of ~0.67 or ⅔), not its volume.
This is the same surface-area-to-volume problem in a different context. Because mass increases faster than strength, larger animals are weaker relative to their body weight. This is why a grasshopper can jump many times its body length, while a human cannot.
Implication for Cyclists:
Absolute Strength & Power: A larger athlete, having thicker muscles, will generally be able to produce more absolute force and higher peak power. This is why track sprinters are heavily muscled; their sport is a direct test of force production and acceleration (F=ma).
The Trade-off: The mass required for world-class sprinting is a significant disadvantage when gravity becomes a factor, such as in a long climb. This is why you see a clear physical divergence between sprinters and climbers.
This is where the science meets the real-world experience of the athlete. Many cyclists report that their FTP drops when they lose weight. If muscle mass isn’t the primary driver of aerobic power, why does this happen?
The issue is not the loss of muscle itself, but the physiological stress of being in a caloric deficit.
Glycogen Depletion: Dieting often leads to chronically low muscle glycogen stores. Glycogen is essential for high-intensity performance, and its absence impairs the ability to complete hard workouts.
Impaired Recovery: A caloric deficit is, by definition, a state of inadequate energy for full recovery. This accumulates fatigue and prevents positive training adaptations.
Dehydration: A significant portion of initial weight loss is water. Since plasma volume is a key component of aerobic fitness, dehydration directly compromises performance.
Guidelines for Healthy Weight Management:
Performance is the Primary Metric: A successful diet for an athlete is one where performance is maintained or even improved. A significant drop in FTP or sprint power is a red flag that the deficit is too aggressive.
Go Slow: A slow rate of weight loss allows the body to adapt, minimizes muscle loss, and reduces the risk of a “rebound” where weight is quickly regained.
Periodize Your Diet: The most effective time to be in a caloric deficit is during periods of lower-intensity, higher-volume training. High-intensity blocks (like VO2 max intervals) and race periods require adequate fueling to be effective.
Focus on Fat Loss, Not Just Weight Loss: The goal is to reduce non-functional fat mass while preserving muscle and, crucially, the ability to train hard.
As the podcast’s case study illustrates, it is entirely possible to lose weight and gain power simultaneously. However, it requires a carefully managed, periodized approach to both training and nutrition, often with professional guidance.