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A redox reaction, short for oxidation-reduction reaction, is a chemical reaction in which oxidation and reduction occur together. Oxidation involves the loss of electrons or an increase in oxidation state, while reduction involves the gain of …
Reducing equivalents are a way of describing transferable reducing power in oxidation-reduction reactions. They represent electron equivalents that can be donated to another chemical species, causing that species to be reduced. In biological reactions, these electrons …
Reduction is a chemical process in which a substance gains electrons or in which the oxidation state of one or more of its atoms decreases. In organic chemistry, reduction is also commonly associated with gaining hydrogen …
A reference interval is a range of laboratory test values derived from measurements in a defined group of reference individuals. It is bounded by a lower and an upper reference limit and is used to help …
A reflex arc is the neural pathway that mediates a reflex, an automatic response to a sensory stimulus. In its basic form, the pathway includes a sensory receptor that detects the stimulus, an afferent sensory neuron …
Rehydration is the correction of dehydration by replacing water and, when necessary, electrolytes that have been lost from the body. Dehydration develops when body-water losses exceed replacement and can occur with varying degrees of electrolyte loss. …
Relapse describes a sustained return to a previous problematic behavior, while a brief lapse does not necessarily lead to that outcome.
Relapse prevention addresses high-risk situations and responses to setbacks, helping distinguish a brief lapse from a sustained return to behavior.
Renal calcium reabsorption is the process by which the kidneys return filtered calcium from the tubular fluid to the bloodstream. Plasma calcium that is bound to proteins is not readily filtered, whereas ionized and small-molecule-complexed calcium …
The renal cortex is the outer region of the functional tissue, or parenchyma, of the kidney, lying beneath the renal capsule and surrounding the inner renal medulla. It contains most of the structures involved in the …
Repetitions in reserve estimate remaining repetitions before failure, offering a practical effort measure whose accuracy varies with the task and lifter.
Resting energy expenditure, or REE, is the amount of energy the body expends while a person is awake and resting, under conditions designed to minimize the effects of physical activity, recent food intake, and environmental temperature. …
Resting heart rate counts heartbeats at rest, but the number varies with the person and measurement conditions rather than staying fixed.
Reverse cholesterol transport (RCT) is the set of processes that moves excess cholesterol from peripheral tissues toward the liver and facilitates its eventual elimination from the body. It is particularly relevant to macrophages, which can accumulate …
Riboflavin, also called vitamin B2, is a water-soluble vitamin that forms part of two important coenzymes: flavin mononucleotide, or FMN, and flavin adenine dinucleotide, or FAD. These molecules help enzymes carry out chemical reactions involved in …
Ribose-5-phosphate (R5P) is a five-carbon sugar phosphate that functions as an important intermediate in cellular metabolism. Chemically, it consists of ribose with a phosphate group attached to its fifth carbon. Its best-known role is providing the …
Running economy measures the metabolic cost of running at a specified speed, rather than judging visible technique or maximal aerobic capacity alone.
SMART goals are goals written according to a set of criteria intended to make an objective clear and trackable. In health and exercise settings, SMART commonly stands for Specific, Measurable, Achievable, Relevant, and Time-bound, although alternative …
The sacroiliac joint, or SI joint, is the articulation between the sacrum at the base of the spine and the ilium of the pelvis. There is one sacroiliac joint on each side of the sacrum. Together, …
The sacrum is a large, triangular bone at the base of the vertebral column and the back of the pelvis. In most adults, it is formed by the fusion of five sacral vertebrae, designated S1 through …
Salivary amylase, also called salivary alpha-amylase or ptyalin, is a digestive enzyme produced by the salivary glands and released into saliva. Its main digestive role is to begin the breakdown of dietary starch as food is …
A sarcomere is the repeating contractile unit of striated muscle and the basic structural unit of a myofibril. Sarcomeres occur in skeletal and cardiac muscle and are arranged end to end along the length of each …
The sarcoplasmic reticulum, or SR, is a specialized form of the endoplasmic reticulum in muscle cells that stores, releases, and reabsorbs calcium ions. By controlling calcium concentration around the contractile machinery, it plays a central role …
Satiation is the set of processes that develops during eating and contributes to bringing an eating episode to an end. As food is consumed, signals generated by its sensory properties and by its passage through the …
Satiety is the temporary inhibition of further eating that develops after food has been consumed. It is associated with reduced hunger and a lower motivation to begin eating again. In appetite research, satiety mainly describes the …
The scapula, commonly called the shoulder blade, is a flat, triangular bone located on the posterior thoracic wall. Along with the clavicle, it forms the pectoral girdle, which supports and positions the upper limb. The scapula …
Scapular retraction is movement of the scapula posteriorly and medially around the thoracic wall, bringing it closer to the vertebral column. It is the opposite of scapular protraction, in which the scapula moves anteriorly and laterally …
Scapular upward rotation is an angular movement of the scapula in which the glenoid fossa turns more superiorly while the inferior angle of the scapula moves laterally and upward along the thorax. It is one of …
The sciatic nerve is the largest nerve in the human body. It forms from nerve fibers arising from spinal levels L4 through S3 and travels from the pelvis into the back of the thigh. It contains …
Selenium is an essential mineral that the human body uses as a component of specialized proteins called selenoproteins. These proteins participate in processes including thyroid hormone metabolism, DNA synthesis, reproduction, and protection against oxidative damage. Selenium's …
Self-efficacy describes confidence in carrying out a specific behavior, so perceived capability can differ across tasks and situations.
A sensory receptor is a specialized cell, nerve ending, or cellular structure that detects a particular type of physical or chemical stimulus and converts it into a signal that can be processed by the nervous system. …
A sesamoid bone is a bone that is partially or completely embedded within a tendon, usually near a joint or where the tendon passes around a bony prominence. Rather than simply forming part of a chain …
Short-chain fatty acids (SCFAs) are fatty acids with short carbon chains. In the human large intestine, the principal SCFAs are acetate, propionate, and butyrate. They are produced mainly when gut microorganisms ferment carbohydrates that have escaped …
A single-joint exercise is an exercise in which the primary movement occurs at one joint. In resistance training, exercises are commonly classified as single-joint or multi-joint according to the number of joints contributing substantially to the …
Skeletal muscle is one of the three major types of muscle tissue, alongside cardiac and smooth muscle. It is composed of long, cylindrical muscle cells called muscle fibers, which typically contain many nuclei. Most skeletal muscles …
Skeletal-muscle fibers, also called myofibers, are the individual contractile cells that make up skeletal muscle. They are long, cylindrical cells that can extend for many centimeters and typically contain many nuclei positioned near the cell membrane. …
Skinfold testing is an anthropometric method that measures the thickness of a fold of skin and underlying subcutaneous adipose tissue at standardized locations on the body. The skinfold is grasped and lifted away from the underlying …
The sliding-filament mechanism is the fundamental model explaining how striated muscle produces force and, when mechanical conditions allow, shortens. During shortening, the thin actin filaments and thick myosin filaments of each sarcomere slide past one another, …
Slow oxidative fibers are skeletal muscle fibers characterized by relatively slow contraction and a high capacity for oxidative metabolism. In humans, the term generally corresponds to Type I muscle fibers, which express the slow Type I …
A small-intestinal biopsy is a procedure in which small samples of tissue are removed from the lining of the small intestine for microscopic examination. In routine diagnostic practice, samples are commonly obtained from the duodenum, the …
Smooth muscle is one of the three major types of muscle tissue, alongside skeletal and cardiac muscle. It is typically made of elongated, spindle-shaped cells that contain the contractile proteins actin and myosin. Unlike skeletal and …
Sodium is a chemical element and an essential mineral nutrient. In body fluids it occurs as sodium ions, written Na+, which carry a positive electrical charge. Sodium is the main positively charged ion in the fluid …
Sodium–glucose cotransport is a membrane transport process in which the movement of sodium ions down their electrochemical gradient provides the energy to move glucose into a cell. It is carried out by sodium–glucose cotransporters (SGLTs). In …
Sports injury prevention uses strategies to reduce injury risk or burden, with effects that depend on the program and population studied.
Static stretching is a stretching technique in which a joint is moved to a position that places the relevant soft tissues under stretch, typically until passive resistance, a stretching sensation, or discomfort is felt, and that …
The sternoclavicular joint is the synovial joint between the medial end of the clavicle and the upper part of the sternum, primarily the clavicular notch of the manubrium. The clavicle also articulates with the first costal …
The stretch reflex, also called the myotatic reflex, is an automatic contraction of a skeletal muscle in response to its being stretched. The reflex is initiated by muscle spindles, sensory receptors located within the muscle that …
