Logan Steele
Animals have muscles to enable movement and perform other bodily functions. Muscle contraction allows animals to move about and perform various tasks. Animals have different types of muscle fibres, including slow fibres with a low maximum velocity of shortening and fast fibres with a high maximum velocity of shortening. The ratio of cross-sectional area to volume is important for muscle strength, and larger animals have a relatively smaller ratio. Some animals, such as migratory birds and bears, undergo physiological changes in response to seasonal cues, which enhance their physical fitness and protect their muscles. Invertebrates like slugs and worms move using muscle systems without a skeletal component, and even vertebrates have muscular body parts without bones, such as the tongue.
| Characteristics | Values |
|---|---|
| Movement | Muscle contraction allows animals to move about and perform other actions. |
| Structure | Animals have different muscle fibre types: slow fibres with low Vmax and fast fibres with high Vmax. |
| Strength | Muscle strength comes from cross-sectional area, not length or volume. |
| Endurance | Animals like bears and migratory birds have muscle-protecting compounds in their blood, allowing them to endure long migrations or hibernation. |
| Efficiency | Muscle systems in soft-bodied animals, like worms, enable them to change shape while maintaining a constant volume. |
| Protection | Some animals, like mollusks, use muscles for protection, such as pulling their shell down over their body. |
| Adaptation | Animals with higher intelligence may be more susceptible to muscle atrophy due to sedentary behaviour. |
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What You'll Learn
Muscle contraction and movement
The mechanism of muscle contraction differs depending on the animal's body structure. For instance, in animals with a skeleton, muscles work with the skeleton to produce lever action for movement. On the other hand, invertebrates like slugs and worms, which lack a skeleton, rely solely on their muscles for mobility. These invertebrates possess longitudinal muscle fibres that run lengthwise along their bodies and circular fibres that encircle them. By contracting these longitudinal and circular muscles, the animals can alter their body shapes, allowing for movement and other functions. For example, jellyfish, one of the simplest animals to use muscles for rhythmic movements, swim by contracting their muscle fibres to reduce their bell diameter and expel water, propelling them forward.
Additionally, animals have different muscle fibre types, including slow fibres with a low maximum velocity of shortening (Vmax) and fast fibres with a high Vmax. The combination of these fibre types during locomotion enhances force generation, mechanical power production, and efficiency. The specific use of these fibres may vary depending on the animal's needs and the optimal conditions for mechanical power production.
The presence or absence of physical activity plays a significant role in muscle development and maintenance in animals. For instance, migratory birds like barnacle geese develop stronger flight muscles by sitting around, and their bodies prepare for the challenge of migration through seasonal changes. In contrast, animals forced into inactivity, especially less intelligent species, experience muscle atrophy. Hibernating bears are an exception, as they have adaptations to prevent muscle atrophy during extended periods of inactivity.
Furthermore, the ratio of muscle mass to body mass varies across species. For example, while humans have approximately 40% of their body mass as muscle, other animals like gorillas have a smaller fraction of their body mass as muscle but exhibit greater strength. This indicates that muscle strength is derived from cross-sectional area rather than length or volume. Additionally, some animals, like draft horses and race horses, build different muscles due to their unique activities and uses.
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Animals with no skeleton
Worm-like invertebrates, for instance, have longitudinal muscle fibres running lengthwise along their bodies and circular fibres encircling them. When the longitudinal muscles contract, the body becomes shorter and fatter. Conversely, when the circular and radial fibres shorten, the body elongates and thins out. This change in body shape allows these animals to move and adapt to their environment.
Jellyfish, belonging to the class Scyphozoa, are another example of invertebrates with unique muscle usage. They possess a simple muscle structure with a single layer of cells on the outer surface and a layer lining the gut cavity. The muscle fibres contract, reducing the diameter of the bell-shaped body and expelling water, enabling them to swim weakly. The mesoglea, a gelatinous substance, then causes the bell to return to its original shape through elastic recoil. These rhythmic contractions and recoils propel jellyfish through the water.
Sea anemones, part of the phylum Cnidaria, also utilise muscles for movement and shape alteration. They have two primary body forms: the cylindrical tentacled polyp and the bell-shaped or inverted saucer-shaped medusa. The muscle structure allows them to contract and extend, facilitating movement and protection.
In summary, while lacking a skeleton, these invertebrates have evolved muscle systems that enable movement, adaptation, and survival in their respective environments. Their muscle fibres' ability to contract and relax provides the necessary locomotion and shape modification for their unique lifestyles.
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Muscle strength and endurance
Animals have muscles to enable movement and to perform other bodily functions. Muscle fibres contract, allowing animals to move about and perform other bodily functions. For example, the shell muscle of the abalone Haliotis, a type of mollusc, connects the domed shell of the animal to its adhesive foot. When the muscle shortens, the shell is pulled down over the animal to protect it. When the muscle lengthens, the shell is raised, allowing respiratory water currents to circulate.
On the other hand, muscle endurance is the ability to perform repetitive motions over an extended period without getting tired. This is achieved through training with lighter weights and higher repetitions. In animals, muscle endurance can be seen in the jellyfish's swimming movements. The circular muscles of the jellyfish contract rhythmically, forcing out a stream of water to propel the animal forward. This requires sustained contractions over a longer duration, demonstrating muscle endurance.
The benefits of building muscle endurance include improving the aerobic capacity of muscles, maintaining good posture for long periods, and preventing injuries. By improving muscle endurance, animals can enhance their performance in activities that require sustained and repetitive movements, such as long-distance running or swimming.
Additionally, maintaining both muscle strength and endurance contributes to overall health and fitness. Peak oxygen uptake (VO2), evaluated as exercise tolerance, is a strong predictor of life prognosis. Higher peak VO2 levels are associated with greater muscle endurance and improved skeletal muscle oxygenation dynamics during exercise. This highlights the importance of balancing muscle strength and endurance training for optimal physical performance and overall well-being.
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Muscle fibre types
Animals have muscles to enable movement. For example, the contraction of muscle fibres allows humans to move about and to do nearly everything else the human body does. The bodies of most living vertebrates, or animals with backbones, are made up of 40 to 50% muscle.
Skeletal muscle fibres can be classified based on two criteria: the speed of contraction and the way they regenerate adenosine triphosphate (ATP). Using these criteria, there are three main types of skeletal muscle fibres: slow oxidative (Type I), fast oxidative (Type IIa), and fast glycolytic (Type IIx).
Slow oxidative fibres use aerobic metabolism to produce low-power contractions over long periods and are slow to fatigue. They contract relatively slowly and use aerobic respiration (oxygen and glucose) to produce ATP. Fast oxidative fibres use aerobic metabolism to produce ATP but generate higher-tension contractions than slow oxidative fibres. They have relatively fast contractions and primarily use aerobic respiration to generate ATP.
Fast glycolytic fibres use anaerobic metabolism to produce powerful, high-tension contractions but fatigue quickly. They have a large diameter and possess large volumes of glycogen, which is used in glycolysis to generate ATP quickly. Because of their reliance on anaerobic metabolism, these fibres do not possess substantial numbers of mitochondria, have a limited capillary supply, and have low amounts of myoglobin, resulting in a white coloration for muscles containing large numbers of these fibres. Fast glycolytic fibres are used for short periods of rapid, forceful contractions associated with quick, powerful movements.
Most skeletal muscles in the human body contain all three types of fibres, although in varying proportions.
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Muscle atrophy
Animals have muscles to enable movement and to carry out other bodily functions. Muscle movement occurs when muscle fibres contract, allowing for changes in body form. For example, the muscle structure of molluscs like the abalone Haliotis allows it to pull its shell down over its body for protection.
The symptoms of muscle atrophy include a decrease in muscle mass, with one limb sometimes appearing smaller than the other, and numbness, weakness, and tingling in the limbs. Atrophy of the throat muscles may cause difficulty swallowing, while diaphragm atrophy can lead to breathing difficulties. The hallmark sign of muscle atrophy is the loss of lean muscle mass, which can be challenging to detect due to obesity or changes in fat mass.
Treatment for muscle atrophy depends on the underlying cause. Physiologic atrophy can often be reversed through exercise and improved nutrition. Exercise programs may include swimming or other rehabilitation exercises, while nutritional therapy can address muscle atrophy caused by malnutrition. In cases of neurogenic atrophy, treatment may involve physical therapy, ultrasound therapy, or surgery. Anabolic agents may also be considered for treatment, although they are not frequently used due to potential side effects.
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Frequently asked questions
Animals have muscles to enable movement. Muscle fibres contract, which allows animals to move about and perform other bodily functions.
Animals build muscle by repeating the same action over and over. For example, a draft horse will build different muscles to a racehorse due to their differing uses.
No. Slugs, worms, and many other invertebrate animals have no skeleton and therefore no muscles.
