D4 Metabolic and Physiological Analysis

Topic

Metabolic and physiological analysis quantifies energy expenditure and the functional state of organ systems during exercise, providing the thermodynamic dimension of locomotor performance. Indirect calorimetry allows for the inference of the total metabolic heat produced by an animal based on its rates of oxygen consumption and carbon dioxide production, without the need to measure heat directly; this is possible because the stoichiometric relationships between the metabolic substrates burned by the body and the oxygen and carbon dioxide exchanged with the environment are known with high precision.

The ratio of carbon dioxide production to oxygen consumption reveals the predominant energy substrate the body is burning at any given moment: a ratio close to 0.7 indicates that the body is burning primarily fats; a value close to 1.0 indicates it is burning primarily glucose; and a value above 1.0 indicates that the muscle is producing lactate—meaning a significant portion of metabolism has become anaerobic.

For every animal, oxygen consumption has an aerobic ceiling: the maximum oxygen consumption the organism can sustain during the most intense exercise. When comparing animals of different sizes, this aerobic ceiling does not increase in direct proportion to body mass, but rather in proportion to a power of body mass slightly less than one—close to three-quarters. Consequently, an animal ten times heavier than another does not have an aerobic ceiling ten times higher, but rather one that is considerably lower.

The efficiency with which an animal converts oxygen consumption into movement is expressed as the "cost of transport": the amount of metabolic energy the animal expends per unit of body mass and per unit of distance traveled. Expressed in this way, the cost of transport allows for a comparison of locomotor efficiency between species of vastly different sizes—something that oxygen consumption alone, without normalization for mass and distance, does not allow for directly.

The functional state of the organ systems mentioned at the beginning can also be assessed directly via the skin using infrared thermography; this technique measures the body's surface temperature with spatial resolution, exploiting the fact that any body emits infrared radiation at a rate that increases much more rapidly than its own temperature. Temperature differences appearing across various body regions reflect—depending on where they are measured—patterns of blood flow, localized inflammatory processes, or generalized thermal stress.

Lactate appearing in the blood when metabolism shifts to an anaerobic state—along with other biomarkers such as cortisol and creatine kinase—follows simple dynamics within the bloodstream: the rate at which the concentration of each biomarker rises or falls at any given moment is proportional to its concentration at that time. Consequently, following the stimulus that triggers their release, their plasma concentrations rise and subsequently fall along exponential trajectories over time.

During submaximal exercise, the previously described gait economy proves to be a more sensitive predictor of locomotor performance than the animal's aerobic ceiling, precisely because most actual movement occurs well below that limit. The point at which blood lactate concentration ceases to rise gradually and begins to accelerate sharply marks the animal's anaerobic threshold—the transition to modes of locomotion that the organism cannot sustain for long periods without exhausting its metabolic capacity, the very same energy expenditure with which this metabolic and physiological analysis began.

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