Sucrose
Sucrose is a disaccharide composed of one glucose unit and one fructose unit joined by a glycosidic bond. It is commonly known as table sugar and occurs naturally in many plants. Because it contains two linked …
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Sudden Cardiac Death
Sudden cardiac death (SCD) is an unexpected death caused or presumed to be caused by a cardiac condition and occurring over a short time. In witnessed cases, it is commonly defined as death occurring within about …
Sugar Alcohols
Sugar alcohols, commonly referred to as polyols in nutrition, are carbohydrate-derived compounds with multiple hydroxyl groups. Examples used in foods include sorbitol, mannitol, xylitol, erythritol, maltitol, lactitol, and isomalt. They occur naturally in some foods and …
Superset
A superset is a resistance-training set configuration in which two exercises are performed consecutively with little or no rest between them. After both exercises are completed, a longer recovery period is usually taken before the sequence …
Talk Test
The Talk Test is a simple method for estimating and monitoring aerobic exercise intensity based on how comfortably a person can speak while exercising. In a standardized version, the person recites a familiar passage or set …
Target Heart-Rate Zone
A target heart-rate zone is a range of heart rates used to represent a desired level of aerobic exercise intensity. Rather than prescribing one exact heart rate, the zone provides lower and upper boundaries within which …
Tendinous Intersections
Tendinous intersections are transverse bands of dense connective tissue that cross the rectus abdominis muscle and divide it into muscular segments. They are also called tendinous inscriptions. Most commonly, three intersections are present on each side, …
Thermogenesis
Thermogenesis is the production of heat by metabolic processes in the body. Metabolism continually releases energy as heat, and heat production contributes to maintaining body temperature, particularly when environmental conditions increase heat loss [R1][R2].Heat production can …
Thermoregulation
Thermoregulation is the set of physiological and behavioral processes that keep body temperature within a range compatible with normal cellular and organ function despite changes in environmental temperature or internal heat production. In humans, it depends …
Thyroid Hormones
Thyroid hormones are iodine-containing hormones that regulate metabolism, growth, development, and the function of many tissues. The two principal thyroid hormones are thyroxine (T4) and triiodothyronine (T3). They are produced from iodinated tyrosine residues within the …
Thyroxine
Thyroxine, or T4, is an iodine-containing hormone produced by the thyroid gland and the major thyroid hormone secreted into the circulation. It is also called tetraiodothyronine because each molecule contains four iodine atoms. T4 is synthesized …
Tidal Volume
Tidal volume is the volume of air moved into or out of the lungs during a single breath. It is usually abbreviated VT. During quiet breathing at rest, tidal volume represents the air inspired with each …
Tissue Stiffness
Tissue stiffness describes how strongly a biological tissue resists deformation when a mechanical load is applied. A stiffer tissue generally deforms less under comparable loading conditions than a more compliant tissue. Stiffness is relevant to tissues …
Tocopherols
Tocopherols are a family of fat-soluble compounds within the broader group known as vitamin E. Four naturally occurring forms are commonly recognized: alpha-, beta-, gamma-, and delta-tocopherol. They share a chromanol ring and a long, saturated …
Tocopheroxyl Radical
A tocopheroxyl radical is the radical formed when a tocopherol, such as alpha-tocopherol, loses the hydrogen atom from its phenolic hydroxyl group during an oxidation reaction. In biological membranes, this commonly occurs when alpha-tocopherol reacts with …
Tocotrienols
Tocotrienols are a family of fat-soluble compounds within the naturally occurring vitamin E family. Four forms are recognized: alpha-, beta-, gamma-, and delta-tocotrienol. Like tocopherols, they contain a chromanol ring and a long hydrocarbon side chain. …
Tolerable Upper Intake Level
The Tolerable Upper Intake Level, usually abbreviated UL, is the highest average daily intake of a nutrient that is likely to pose no risk of adverse health effects to almost all healthy people in a particular …
Training Load
Training load is a sport and exercise science concept used to describe the demands placed on a person during training or competition. It is a multidimensional construct rather than a single physical quantity, and different measurements …
Transversus Abdominis
The transversus abdominis is the deepest of the three flat muscles forming the anterolateral abdominal wall, lying beneath the internal oblique. Its fibers run predominantly horizontally around the abdomen rather than vertically or diagonally. The muscle …
Triglycerides
Triglycerides, also called triacylglycerols, are lipids composed of one glycerol molecule esterified with three fatty acids. The three fatty acids can be identical or different, so triglycerides represent a large family of molecules rather than a …
Triiodothyronine
Triiodothyronine, or T3, is an iodine-containing thyroid hormone and the more biologically potent of the two principal thyroid hormones in humans. Its molecule contains three iodine atoms, which gives rise to the name triiodothyronine. The thyroid …
Tripeptides
Tripeptides are peptides composed of three amino-acid residues joined by peptide bonds. In a typical linear tripeptide, three amino acids are connected by two peptide bonds, producing a molecule with an amino end, called the N-terminus, …
Trochlear Surface
The trochlear surface, in knee anatomy, is the cartilage-covered articular surface on the anterior part of the distal femur that articulates with the posterior surface of the patella, or kneecap. Together, these surfaces form the patellofemoral …
Tropomyosin
Tropomyosin is an elongated, filament-forming protein that lies along the surface of actin filaments. In skeletal and cardiac muscle, it is an essential component of the thin filament, together with actin and the troponin complex. Tropomyosin …
Troponin C
Troponin C, or TnC, is the calcium-binding component of the troponin complex, a group of proteins that regulates contraction in skeletal and cardiac muscle. The complex lies on the actin-containing thin filaments of the sarcomere and …
Trunk Stiffness
Trunk stiffness is the mechanical resistance of the trunk to deformation or movement when an external force or moment is applied. In biomechanics, it can be estimated from the relationship between an applied perturbation and the …
Trypsin
Trypsin is a digestive enzyme that hydrolyzes peptide bonds within proteins and peptides in the small intestine. It belongs to the serine protease family and is classified as an endopeptidase because it cleaves peptide chains internally …
Type I Muscle Fibers
Type I muscle fibers are a major type of skeletal muscle fiber characterized by relatively slow contraction and high resistance to fatigue. They are commonly called slow-twitch or slow oxidative fibers. Their contractile properties are associated …
Type II Fiber Atrophy
Type II fiber atrophy is a reduction in the size of Type II, or fast-twitch, skeletal muscle fibers. It is typically identified microscopically in muscle tissue by measuring fiber diameter or cross-sectional area after distinguishing fiber …
Type II Muscle Fibers
Type II muscle fibers are skeletal muscle fibers characterized by relatively fast contractile properties. They are commonly called fast-twitch fibers and are identified primarily by the fast myosin heavy chain isoforms they express. In adult human …
Type IIa Fibers
Type IIa fibers are a subtype of fast skeletal muscle fiber defined primarily by predominant expression of the myosin heavy chain IIa, or MyHC-IIa, isoform. They are one of the main fiber types found in adult …
Type IIx Fibers
Type IIx fibers are a subtype of fast skeletal muscle fiber defined primarily by predominant expression of the myosin heavy chain IIx, or MyHC-IIx, isoform. They are one of the principal fiber types identified in adult …
VO₂max
VO₂max, or maximal oxygen uptake, is the highest rate at which the body can take up and use oxygen during intense exercise involving large muscle groups. It reflects the integrated ability of the cardiorespiratory system to …
Ventilatory Control
Ventilatory control is the physiological system that regulates breathing so that ventilation adjusts to the body's changing needs while helping maintain arterial oxygen, carbon dioxide, and acid-base balance within appropriate ranges. It involves generation of the …
Ventilatory Threshold
Ventilatory threshold is an exercise intensity at which ventilation begins to increase disproportionately relative to oxygen uptake during progressively harder exercise. It is identified from changes in ventilation and respiratory gas exchange during a cardiopulmonary exercise …
Villi
Villi are small, finger-like projections of the mucosal lining of the small intestine. The singular form is villus. They project into the intestinal lumen and substantially increase the surface area available for absorption. Most of the …
Vitamin C
Vitamin C is a water-soluble vitamin that includes L-ascorbic acid and compounds with equivalent biological activity. In the body, ascorbic acid exists mainly in its ionized form, ascorbate. Humans cannot synthesize vitamin C because they lack …
Vitamin D
Vitamin D refers to a group of fat-soluble compounds that function mainly as precursors to a hormone involved in calcium and phosphate regulation. Its two principal forms are vitamin D3, or cholecalciferol, and vitamin D2, or …
Z-Disc
The Z-disc, also called the Z-line or Z-band, is a protein-rich structure that marks the boundary between neighboring sarcomeres, the repeating contractile units of striated skeletal and cardiac muscle. Each sarcomere extends from one Z-disc to …
Zymogens
Zymogens are inactive or weakly active precursor forms of enzymes that undergo a molecular change before acquiring their full catalytic activity. They are also called proenzymes. Zymogen formation is particularly common among proteases, enzymes that hydrolyze …