Locomotion & Energetics

Overview

The Muscle Physiology page covered how a single muscle fiber generates force and regenerates ATP; this closing page scales that mechanism up to whole-animal energetics — how metabolic rate scales with body size across species, why swimming is cheaper than running per unit distance, why animals switch gait at predictable speeds, and how elastic tissue lets some animals recycle mechanical energy rather than paying its full metabolic cost each stride.

Key Concepts

Metabolic Scaling and Kleiber’s Law

Basal metabolic rate (see Digestive & Metabolic Physiology) does not scale linearly with body mass across species — Kleiber’s law describes an empirical relationship of BMR ∝ mass^0.75 (a three-quarter-power scaling law), meaning a 10,000-fold increase in body mass (a mouse to an elephant) corresponds to roughly a 1,000-fold increase in metabolic rate, not 10,000-fold. A simpler surface-area argument (heat loss scales with surface area, mass scales with volume, and surface-area-to-volume ratio itself scales as mass^(2/3)) predicts a 2/3-power exponent, close to but not exactly matching the empirically observed 3/4; the discrepancy between the simple surface-area prediction and the actual 3/4 exponent remains an active area of physiological research (proposed explanations include the fractal geometry of internal resource-distribution networks such as the circulatory system), but the practical, testable consequence is consistent either way: metabolic rate per unit mass (mass-specific metabolic rate) decreases as body size increases — a mouse’s tissue burns energy far faster per gram than an elephant’s tissue does, which is why small endotherms must eat disproportionately more relative to their body mass and lose heat disproportionately faster (higher surface-area-to-volume ratio, see Comparative Thermoregulation).

Source: ResearchGate, fig. 3

Net Cost of Transport

The net cost of transport (COT) — energy expended per unit body mass per unit distance traveled — differs substantially by locomotion mode, largely for physical rather than physiological reasons:

This is a direct case of physical constraint shaping physiological strategy: swimming’s low cost of transport is why aquatic migration (e.g., whale migration) can cover vastly greater distances on a comparable energy budget than an equivalent-mass terrestrial migration.

Source: Wikimedia Commons, traced from Schmidt-Nielsen (1972) as reproduced in Biewener, Animal Locomotion (Oxford University Press, 2003) — public domain, genuinely confirmed open licensing. Shows the full body-mass-scaling relationship for each locomotion mode rather than a single-mass bar comparison, richer than the original spec.

Gait Transitions as Energy Minimization

Terrestrial animals do not simply move faster within a single gait as speed increases — they switch gaits (walk → trot → gallop in horses, for instance) at fairly predictable speed thresholds, and this transition is itself an energy-minimization behavior: at any given speed, one gait pattern minimizes metabolic cost per unit distance more than the alternatives, and measured oxygen consumption in animals allowed to choose their own gait freely shows a characteristic sawtooth pattern — cost per distance rising within a gait as speed increases, then dropping sharply at the moment of a gait transition, as the animal adopts a mechanically more efficient stride pattern for the new speed range, before rising again. This provides direct experimental evidence that an animal’s freely chosen gait transition points are not arbitrary but track the specific speed at which the next gait becomes more economical than continuing the current one.

    graph LR;
    A["Speed increases within one gait"] --> B["Cost per distance rises"];
    B --> C["Threshold reached: next gait becomes cheaper"];
    C --> D["Gait transition occurs"];
    D --> E["Cost per distance drops"];
    E --> A;
  

Elastic Energy Storage

Some locomotor systems reduce the metabolic cost of movement further by storing and returning mechanical energy elastically rather than dissipating and re-generating it via muscle contraction each cycle: tendons (particularly the Achilles tendon in running mammals, and specialized tendon structures in hopping kangaroos) stretch under load during one phase of a stride (storing elastic strain energy, much like a compressed spring) and recoil during the next phase, returning a substantial fraction of that stored energy to assist the following stride — reducing the muscular/metabolic work that would otherwise be required to re-accelerate the limb or body each cycle. This is why a kangaroo’s cost of transport rises only modestly with increasing hopping speed compared to a similarly sized quadruped’s running cost, since more of the mechanical energy at higher speed is being recycled elastically through the tendon rather than newly supplied by muscle metabolism.

Source: bioRxiv, preprint 2021.05.08.443257

Muscle Fiber Recruitment Across the Speed Range

Energetic strategy at the whole-animal level connects directly back to the fiber-type recruitment pattern covered on the Muscle Physiology page: sustained, submaximal locomotion (e.g., long-distance running or steady swimming) draws primarily on Type I (slow oxidative) fibers and the oxidative phosphorylation energy system, while a sudden burst of maximal-speed locomotion (a sprint, or a rapid escape response) recruits Type IIx (fast glycolytic) fibers running on the ATP-PCr and anaerobic glycolysis systems — meaning an animal’s locomotor energy strategy at any given moment is really a direct expression of which fiber types and energy systems from that earlier page are currently dominant.

Comparative Structures

Common Exam Questions

• “State Kleiber’s law and explain why mass-specific metabolic rate decreases with increasing body size.”
• “Explain why swimming has a lower net cost of transport than running, referencing the physical role of buoyancy.”
• “Explain why an animal switches gait at a specific speed rather than simply moving faster within its current gait, using the concept of cost-per-distance minimization.”
• “Explain how elastic energy storage in tendons reduces the metabolic cost of hopping locomotion at higher speeds.”
• “Connect an animal’s choice of muscle fiber type during a sprint versus a marathon-distance effort to the corresponding whole-body cost-of-transport strategy.”

Visual Reference

Interactive

• Kleiber’s law scaling plot (Plotly, log-log axes) — body mass vs. basal metabolic rate plotted on log-log axes across several example species, with the 0.75-power-law regression line shown and a draggable point letting a student place a hypothetical new species’ mass and read off its predicted BMR — makes the power-law relationship a manipulable, quantitative tool rather than a memorized exponent.

• Cost-of-transport-by-gait sawtooth chart (Plotly) — oxygen consumption per unit distance vs. speed, showing the characteristic sawtooth pattern across walk/trot/gallop with the actual gait-transition speeds marked, directly visualizing the energy-minimization argument made in the text.

Static (placed inline in Key Concepts above, next to the concept each one illustrates, rather than collected here — no image was found for the walk/trot/gallop gait transition footfall diagram; that concept remains text-only)

Practice Problems

1. If a species’ body mass increases 16-fold, predict the approximate fold-increase in its basal metabolic rate using Kleiber’s law.
2. Explain why a swimming animal of a given mass can typically migrate a much greater distance on the same energy budget than a running animal of similar mass.
3. A horse increases speed and, at a specific point, switches from trot to gallop. Explain why this transition itself reduces the horse’s cost of transport rather than simply reflecting its top trotting speed being reached.
4. Explain why a kangaroo’s cost of transport increases only modestly with hopping speed, referencing tendon elasticity.
5. An animal transitions from a steady jog to a maximal sprint. Predict the shift in dominant muscle fiber type and energy system, referencing the Muscle Physiology page.