Praxis Physical Education Content and Design (5095)

This chapter supplies the structural vocabulary behind movement questions: bones and joints, muscles and their actions, neural control, and the heart and lungs. Typical items name the joint type or prime mover used in a skill, trace blood through the heart, or apply the cardiac output formula.

The adult skeleton has 206 bones. It supports the body, protects internal organs, provides levers for movement, stores minerals such as calcium and phosphorus, and produces blood cells in red bone marrow. The axial skeleton (skull, vertebral column, ribs, and sternum) forms the central axis; the appendicular skeleton comprises the limbs and the shoulder and pelvic girdles.

Long bones grow in length at the epiphyseal (growth) plates, layers of cartilage near each end of the bone that close by late adolescence or early adulthood. Until it closes, a growth plate is weaker than the surrounding bone, tendon, and ligament, so it is vulnerable to fracture and to overuse injury from repetitive stress such as year-round pitching. Teachers therefore vary activities and limit repetitive high-load practice. Weight-bearing activity remains beneficial, because bone adapts to load and the school years are the peak period for building bone mass.

A joint (articulation) is the junction of two or more bones. Fibrous joints such as the skull sutures are immovable, cartilaginous joints such as those between the vertebral bodies are slightly movable, and synovial joints are freely movable. In a synovial joint, cartilage covers the bone ends, a capsule holds lubricating synovial fluid, and ligaments reinforce the joint.

The body has three types of muscle tissue. Skeletal muscle is striated and voluntary; it attaches to bone and produces movement. Cardiac muscle is striated but involuntary and forms the heart wall. Smooth muscle is non-striated and involuntary and lines the blood vessels and hollow organs. Tendons attach muscle to bone, and ligaments attach bone to bone. A sprain injures a ligament; a strain injures a muscle or tendon.

Skeletal muscles move bones by contracting. According to the sliding filament theory, myosin cross-bridges within each sarcomere pull the actin filaments toward the center, so the filaments slide past one another and the sarcomere shortens. The process requires ATP for energy and calcium ions, released when a nerve impulse arrives, for activation.

The central nervous system (CNS) consists of the brain and spinal cord; the peripheral nervous system (PNS) is the network of nerves connecting it to the rest of the body. Sensory (afferent) neurons carry information to the CNS, and motor (efferent) neurons carry commands to muscles. The autonomic division regulates involuntary functions: its sympathetic branch speeds the heart for activity, and its parasympathetic branch slows it at rest.

A motor unit is one motor neuron and all the muscle fibers it stimulates; when the neuron fires, every fiber in the unit contracts. Muscles that require fine control, such as those of the fingers, have few fibers per unit, and large muscles such as the quadriceps have many. Force is graded by recruitment: small, slow-twitch units are activated first, and larger, fast-twitch units are added as demand rises.

A reflex arc is the simplest neural pathway: a receptor detects a stimulus, a sensory neuron carries the signal to the spinal cord, and a motor neuron returns a command to the muscle without waiting for the brain. Voluntary movement begins in the motor cortex, and the cerebellum coordinates it by correcting timing, balance, and smoothness.

The heart is a four-chambered muscular pump. The two atria receive blood and the two ventricles pump it out, with valves keeping flow in one direction. The right side pumps oxygen-poor blood to the lungs (pulmonary circulation), and the left side pumps oxygen-rich blood to the body (systemic circulation). The left ventricle has the thickest wall because it works against the greatest resistance.

Arteries carry blood away from the heart under high pressure and have thick, elastic walls. Veins return blood at low pressure and contain one-way valves; contracting leg muscles squeeze them and assist the return of blood, which is one reason for an active cool-down. Capillaries have walls one cell thick, where oxygen, nutrients, and wastes are exchanged. The pulmonary vessels reverse the usual pattern: pulmonary arteries carry oxygen-poor blood and pulmonary veins carry oxygen-rich blood.

Air passes from the nose or mouth to the alveoli, the tiny air sacs of the lungs. Gas exchange occurs there by diffusion across the thin walls of the alveoli and the surrounding capillaries: oxygen moves into the blood and binds to hemoglobin, and carbon dioxide moves from the blood into the alveoli to be exhaled.

The diaphragm is the principal muscle of breathing. When it contracts it flattens and moves downward while the external intercostal muscles lift the ribs; the chest cavity enlarges, pressure inside falls, and air flows in. Quiet exhalation is passive. During vigorous exercise it becomes active as the abdominal and internal intercostal muscles force air out.

Maximal oxygen consumption (VO₂max) is the gold standard measure of cardiorespiratory fitness, usually expressed in milliliters of oxygen per kilogram of body mass per minute. It depends on central factors (delivery of blood by the heart) and peripheral factors (extraction and use of oxygen by muscle), and it improves with regular aerobic training.

Movement is the product of systems working together: the nervous system signals, the muscles pull on the skeleton, and the heart and lungs supply oxygen and remove wastes. Each system shows an immediate response to one bout of activity and a long-term adaptation to regular training.