Muscle and Locomotion
How animals turn chemical energy into movement: the molecular machinery of muscle contraction and the mechanics of swimming, flying, running, and crawling.
Definition
Muscle is the contractile tissue that generates force and movement by the interaction of actin and myosin filaments, and locomotion is the self-propelled movement of an animal through its environment, achieved by the action of muscles on skeletal or hydrostatic supports.
Scope
This area covers the comparative physiology of muscle and movement: the structure of muscle and the sliding-filament mechanism of contraction, the energetics and types of muscle fibres, the biomechanics that translate muscle force into motion, and the diverse modes of locomotion and their efficiencies. It spans the molecular, cellular, and whole-animal levels and the way movement is matched to body size and medium. Coverage is comparative and mechanistic rather than clinical.
Sub-topics
Core questions
- How does muscle convert chemical energy into force and shortening?
- How do muscle fibres differ in speed, fatigue resistance, and energy supply?
- How is muscle force translated into useful movement by skeletons and limbs?
- What modes of locomotion have evolved, and what makes movement efficient?
Key theories
- Sliding-filament theory of contraction
- Muscle shortens not because its filaments shorten but because actin and myosin filaments slide past one another, a model proposed independently by two groups from microscopy of contracting muscle.
- Cross-bridge cycling
- Force and sliding are produced by myosin heads that repeatedly attach to actin, pull, detach, and reattach in a cycle powered by ATP hydrolysis and regulated by calcium, accounting for the mechanical properties of muscle.
Mechanisms
Striated muscle is built of sarcomeres in which interdigitating actin and myosin filaments slide past one another to shorten the fibre. Contraction is triggered when an action potential releases calcium from the sarcoplasmic reticulum, exposing binding sites on actin so that myosin heads cycle through attachment, power stroke, and detachment, each cycle consuming ATP. Muscle fibres differ in contraction speed and in whether they rely on aerobic or anaerobic metabolism, giving slow fatigue-resistant and fast powerful types suited to different tasks. Muscles act on skeletal levers or on hydrostatic skeletons to produce movement, and the resulting mechanics depend on body size, with locomotion across swimming, flying, running, and burrowing showing characteristic costs of transport. Comparative work relates these costs to body size and medium, revealing why each mode of movement is efficient under its own conditions.
Clinical relevance
The molecular understanding of contraction and the comparative study of muscle energetics underlie the analysis of muscle performance, fatigue, and the energetic cost of exercise and locomotion. This entry is educational and does not provide medical guidance.
History
A. V. Hill's thermodynamic studies of muscle and the independent proposals of the sliding-filament theory by Andrew Huxley with Rolf Niedergerke and by Hugh Huxley with Jean Hanson in 1954 established how muscle contracts. Comparative biomechanists such as Robert McNeill Alexander later analysed how muscle powers the diverse locomotion of animals.
Key figures
- Andrew Huxley
- Hugh Huxley
- Archibald Vivian Hill
- Robert McNeill Alexander
Related topics
Seminal works
- huxley1954
- huxleyhanson1954
- hill2016
Frequently asked questions
- What does the sliding-filament theory say?
- It says muscle shortens because the actin and myosin filaments slide past each other rather than shrinking, so the overlapping filaments increase their overlap during contraction.
- Why are some muscles fast and others slow?
- Muscle fibres differ in their contractile proteins and energy supply, with fast fibres built for quick powerful efforts and slow fibres for sustained, fatigue-resistant work.