Body Composition - More Than Body Fat
Body Composition describes how total body mass is distributed between components such as fat mass, muscle mass and body water. Even though it is usually overlooked, this matters for performance diagnostics, because all three compartments, fat, muscle and water, affect the result of performance testing.
Body Composition should therefore be part of a complete performance assessment. It helps explain not only how much mass an athlete has to move, but also how much metabolically active tissue is available and how body fluids influence the interpretation of concentration-based measurements such as blood lactate.
Body Composition: three layers, one body mass
Muscle contains substantial water, so the layers overlap rather than divide the body into three separate parts.
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The Obvious Part: Fat Is Inactive Mass You Have to Carry
When athletes hear Body Composition, they usually think first about body fat. For many sports, that is an important performance consideration.
Fat mass contributes to total body mass but does not directly generate propulsive force. In weight-bearing locomotion and whenever the athlete has to move mass against gravity, for example in running or climbing on the bike, additional non-propulsive mass increases the energetic cost.
Body fat is the obvious part of Body Composition. But diagnostically, it is only a fraction of what we need to look at.
What extra mass costs, step by step
Centre of mass through the stride, about 8 cm up and down on every step
+4.4 W more vertical work, +2.8 % for +2.0 kg carried, and every step lifts the whole body mass, propulsive or not
This athlete carries 5.7 kg of fat mass, 8.0 % of body mass, on his own INSCYD report.
Strongest where mass has to be lifted: running, climbing on the bike. Smaller on flat ground, and in swimming buoyancy changes the picture.
The Overlooked Parts I: Muscle
Muscle mass influences how much active tissue is available to generate external workload.
More active muscle mass also means a larger total intramuscular energy reserve. One example is phosphocreatine. It is stored within muscle, so the total amount available to support especially sudden changes in energy demand and high-intensity efforts depends in part on the amount of active muscle mass.
The same principle holds true for glycogen, the storage form of glucose in the muscle and another, highly performance-relevant fuel. The amount of muscle that is active and available for the task contributes to the athlete’s available carbohydrate storage capacity.
This is why muscle mass is not simply a body-composition statistic. It provides context for how much energy can be stored in active tissue and how a given performance demand is distributed across that tissue.
The overlooked part I: muscle
More active muscle, more intramuscular store: phosphocreatine and glycogen are held in it. Store context, not a formula for force or power.
Body water is equally important but is often overlooked. Blood lactate is measured as a concentration in mmol/L, so amount of lactate divided by liters. Whenever we look at such a division, like normalizing power to watt/kg bodyweight or VO₂max in ml/min per kilogram body weight, we ask “how many kilograms body weight are there?” However not with lactate.
This creates an important diagnostic blind spot. Lactate produced by active muscle is transported and distributed beyond the tissue in which it appeared. The measured blood concentration is therefore the result of lactate appearance, utilization, transport and distribution, which depends on body water.
This means that two athletes, or the same athlete at two different points in time, can show different measured lactate concentrations even when the underlying metabolic interpretation cannot be reduced to concentration alone.
The overlooked part II: water
Body water is not a compartment apart from muscle: muscle itself contains substantial water. A communication visual, not a three-compartment model.
Body Composition Changes How Lactate Values need to be Interpreted
INSCYD also uses Body Composition as context when interpreting lactate measurements.
By incorporating the athlete’s body-composition information into the assessment, INSCYD accounts for differences in the distribution context behind measured blood lactate and relates the concentration to the athlete’s metabolic response.
This becomes particularly relevant when comparing different athletes or repeated assessments in which body composition or hydration-related conditions have changed. The objective is not simply to compare two raw lactate numbers, but to understand the metabolic processes represented by those measurements.
The same lactate, two bodies, two readings
The same amount of lactate, two different concentrations, because a concentration is an amount read through the body water it distributes in. A distribution context, not a volume: nothing is multiplied back out.
