Anatomy and Physiology of Children and Adolescents - M. R. Sapin 2007

Musculoskeletal System
Muscular System
Muscle Accessory Structures — Muscle Tone and Muscle Fatigue

Muscle tone. At rest, every human muscle is in a state of continuous involuntary contraction, known as tone, which is maintained reflexively by nerve impulses flowing into the muscle. This slight muscle tension is essential for maintaining their baseline state, resisting stretch, and ensuring readiness for action. A prolonged, spasmodic Muscle contraction that persists despite the cessation of the stimulus is called a contracture.

Movement control. The ability of animals, including humans, to move and perform various actions under the control of The Nervous System is one of the key features distinguishing animals from plants. Muscle fiber contraction occurs under METABOLISM/18.html">The Influence of impulses arriving from the Brain AND SPINAL cord via nerve fibers (processes of motor Neurons). By contracting, Muscles participate in the Movements of the body and its parts. At the same time, muscles never work in isolation or on their own. The execution of any movement is achieved through the coordinated action of muscle groups, both flexors and extensors. For example, maintaining an upright human posture involves up to 150 muscles.

Depending on the direction of the force developed by muscles, they are generally divided into synergists and antagonists. Muscles that act on a joint in the same direction (for example, flexing the wrist) are called synergistic muscles, whereas muscles with the opposite action are antagonistic muscles. During any movement, not only the muscles performing it contract, but also their antagonists, which counteract the pull and thereby impart precision and smoothness to the movement. Within each group of muscles, one can distinguish prime movers, which carry out the movement, and auxiliary muscles, which refine and "model" the movement, giving it individual characteristics.

Skeletal human muscles are capable of contracting under conscious control. Such movements are called voluntary. Movements of this type differ from reflexive (involuntary movements), which occur independently of human will. During voluntary movements, nerve impulses travel to the skeletal muscles from the motor centers of the Cerebral Cortex. Involuntary movements are regulated by the corresponding centers in the Brainstem and Spinal Cord.

Muscle fibers contract only upon the command of motor neurons. A motor neuron and its long process, the axon, together with the muscle fibers whose Functions it controls, are referred to as a motor unit.

The activation of neurons in the motor zone of the cerebral cortex is driven by visual, auditory, cutaneous, and muscular information transmitted to the cortex from Sensory Organs. Based on this input, the motor zone of the cortex forms a conscious motor act.

Fatigue is defined as a temporary decrease in performance capacity that is restored following rest. The Development of fatigue in the motor apparatus during prolonged or strenuous work depends on several factors. First and foremost, fatigue is associated with processes occurring within the nervous system and the neural centers involved in regulating motor activity.

A number of causes of fatigue are linked to processes taking place within the muscle itself. These include the accumulation of metabolic products (such as lactic acid), which exert an inhibitory effect on muscle fiber performance, and the depletion of their energy reserves (Glycogen).

The rate of fatigue development during muscular work depends on two indicators: physical load and work rhythm, i.e., the frequency of muscle contractions. As the load increases or the contraction rhythm accelerates, fatigue sets in rapidly. Muscular work reaches its optimal level under moderate loads and moderate contraction speeds.

Physical fatigue is a normal physiological phenomenon. Following rest, performance capacity is not only restored but often exceeds the initial level. Performance recovers more rapidly through active rest than through complete repose.

Review and Self-Assessment Questions:

1. Name the functions and properties of skeletal muscles.

2. Describe the Classification of Muscles. What is it based on?

3. Describe the Accessory structures of Introduction/39.html">Muscles and their functions.

4. What types of muscle work do you know? Give Examples.

5. What is meant by muscle strength, and what factors does it depend on?

6. Provide the anatomical and Functional Characteristics of first- and second-Class levers in biomechanics.

7. What state of a muscle is referred to as muscle tone?

8. What causes muscle fatigue to appear? What type of rest is most effective for restoring their performance capacity?