Muscular Failure
Muscular failure in resistance training refers to the point during a set when a person can no longer complete another repetition against the given resistance despite attempting to do so. A more precise term is momentary …
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Myelin
Myelin is a specialized, multilayered membrane that wraps around selected axons in the nervous system. It is produced by glial cells, specifically oligodendrocytes in the central nervous system and Schwann cells in the peripheral nervous system. …
Myokines
Myokines are signals produced by skeletal muscle, but a rise in a molecule's blood concentration does not establish that muscle supplied the change.
Myosin
Myosin refers to a large superfamily of actin-associated molecular motor proteins. Myosins use energy from adenosine triphosphate, or ATP, to generate mechanical force or movement relative to actin filaments, which are part of the cell's cytoskeleton. …
Myosteatosis
Myosteatosis describes excess fat in skeletal muscle, with imaging definitions that vary and cannot be reduced to muscle mass or total body fat.
NADPH
NADPH, or reduced nicotinamide adenine dinucleotide phosphate, is a coenzyme that carries reducing power between biochemical reactions. It is the reduced form of NADP+ and can donate electrons to other molecules during oxidation-reduction reactions. NADPH is …
NAD⁺ / NADH
NAD⁺ and NADH are the oxidized and reduced forms of nicotinamide adenine dinucleotide, a coenzyme that carries electrons in cellular oxidation-reduction reactions. NAD consists of two nucleotide units, one containing adenine and the other nicotinamide. Its …
Net Carbohydrate
Net carbohydrate subtracts selected components from total carbohydrate, but variable formulas can oversimplify how different fibers and sugar alcohols behave.
Neurodevelopment
Neurodevelopment is the process by which the nervous system forms, organizes, and matures. It begins during embryonic development and continues after birth as the nervous system undergoes structural and functional change. Neurodevelopment includes the production and …
Neurodynamic Mobilization
Neurodynamic mobilization is a group of movement-based techniques that influence nerves and their relationship with surrounding tissues. Also called neural mobilization, it uses selected movements or positions to alter neural movement and mechanical loading. The target …
Neuromuscular Junction
The neuromuscular junction, or NMJ, is the specialized chemical synapse where a motor neuron communicates with a skeletal muscle fiber. It consists of the motor neuron's presynaptic nerve terminal, the narrow synaptic cleft between the cells, …
Neuromuscular Training
Neuromuscular training combines movement-control exercises, with injury-prevention evidence that depends on the specific program studied.
Nicotinic Acetylcholine Receptor
A nicotinic acetylcholine receptor, or nAChR, is a membrane protein that acts as both a receptor for the neurotransmitter acetylcholine and a ligand-gated ion channel. When acetylcholine binds, the receptor changes shape and opens a pore …
Non-Exercise Activity Thermogenesis (NEAT)
Non-exercise activity thermogenesis captures energy spent on everyday movement, with a daily contribution that varies beyond planned exercise sessions.
Noncovalent Interactions
Noncovalent interactions are attractive or repulsive interactions between atoms or molecules that do not involve formation of a covalent bond between the interacting groups. In biological molecules, they help stabilize three-dimensional structures and allow molecules to …
Nondigestible Carbohydrate
A nondigestible carbohydrate is a carbohydrate that resists digestion and absorption in the human small intestine and therefore reaches the large intestine to some extent. Rather than being fully broken down into absorbable sugars by human …
Nonesterified Fatty Acids (NEFA)
Nonesterified fatty acids circulate mainly bound to albumin, forming a pool distinct from fatty acids stored or transported within triglycerides.
Nonheme Iron
Nonheme iron is dietary iron that is not incorporated into a heme molecule. It is one of the two major forms of dietary iron, the other being heme iron. Nonheme iron is the form present in …
Norepinephrine
Norepinephrine acts prominently at sympathetic nerve endings and in brain pathways, with effects shaped by receptors rather than identical to epinephrine.
Nutrient Density
Nutrient density describes the amount of one or more nutrients in a food or diet relative to a defined reference amount. In a traditional nutritional definition, it refers to nutrient content relative to the energy provided …
Nutrient Leaching
In food preparation, nutrient leaching is the transfer of nutrients out of a food and into surrounding liquid, such as cooking water. Water-soluble vitamins and mineral constituents can leave the solid food this way. The process …
Nutrient retention
Nutrient retention describes how much of a particular nutrient remains in food after cooking or another preparation step, relative to the amount present beforehand. It is specific to both the nutrient and the preparation process. Retention …
Nutrition
Nutrition is the process by which the body obtains and uses nutrients from food and beverages to support growth, development, maintenance, and normal physiological function. The term also refers to the scientific study of these processes …
Nutrition Facts Label
The Nutrition Facts label is a standardized nutrition information panel required on most packaged foods and beverages sold in the United States. It is designed to help consumers understand the nutritional composition of a food and …
Obsessive-Compulsive Tendencies
Obsessive-compulsive tendencies describe symptoms or traits informally, while an OCD diagnosis depends on their persistence, distress and functional effects.
Omega-3 Fatty Acids
Omega-3 fatty acids share a structural feature but differ in metabolism and biological roles, so evidence for one member cannot represent the entire family.
One-Repetition Maximum (1RM)
One-repetition maximum measures the greatest load lifted once with defined technique, so the exercise and testing setup shape the result.
Oral Food Challenge
An oral food challenge, or OFC, is a medically supervised test in which a person eats a suspected allergenic food in controlled, gradually increasing amounts while being observed for signs of an allergic reaction. It is …
Oral Nutritional Supplements
Oral nutritional supplements add formulated nutrition when ordinary food intake falls short, with products and outcomes that differ by clinical setting.
Orthorexia Nervosa
Orthorexia describes a proposed pattern of rigid healthy-eating rules that cause distress or harm, rather than an ordinary preference for nutritious food.
Osmoregulation
Osmoregulation is the physiological control of the osmotic concentration of body fluids. In humans, it helps keep plasma osmolality within a narrow range despite changes in water intake and water loss. This is important because changes …
Osmosis
Osmosis is the net movement of water across a selectively permeable membrane in response to a difference in solute concentration across that membrane. When the membrane allows water to cross more readily than the relevant solute, …
Oxidation
Oxidation is a chemical process in which a substance loses electrons or in which the oxidation state of one or more of its atoms increases. Historically, oxidation referred to reactions involving oxygen, and many oxidation reactions …
Oxidative Phosphorylation
Oxidative phosphorylation is the mitochondrial process that couples respiratory electron transfer to the synthesis of ATP from ADP and inorganic phosphate. In human cells that contain mitochondria, it takes place across the inner mitochondrial membrane and …
Oxidoreductase
An oxidoreductase is an enzyme that catalyzes an oxidation-reduction, or redox, reaction. In these reactions, one chemical species is oxidized by losing electrons or increasing its oxidation state, while another is reduced by gaining electrons or …
PNF Stretching
PNF stretching, short for proprioceptive neuromuscular facilitation stretching, is a group of stretching techniques that combines stretching with voluntary muscle contractions. Rather than simply holding a muscle in a stretched position, PNF methods use sequences of …
Pancreatic Bicarbonate
Pancreatic bicarbonate is bicarbonate ion, HCO₃⁻, secreted in large amounts as part of the watery fluid produced by the exocrine pancreas. It is released mainly by epithelial cells lining the pancreatic ducts, whereas pancreatic acinar cells …
Pancreatic Lipase
Pancreatic lipase, more precisely pancreatic triglyceride lipase, is a digestive enzyme produced by the exocrine pancreas and released into the small intestine in pancreatic juice. Its main function is to hydrolyze dietary triglycerides, which make up …
Pancreatic Proteases
Pancreatic proteases are protein-digesting enzymes produced by the exocrine pancreas and released into the small intestine. The major pancreatic proteases include trypsin, chymotrypsin, elastase, and carboxypeptidases A and B. Together, they hydrolyze peptide bonds in dietary …
Parathyroid Hormone (PTH)
Parathyroid hormone (PTH) is an 84-amino-acid peptide hormone secreted by the parathyroid glands, small endocrine glands usually located behind the thyroid gland. Its principal physiological role is the regulation of extracellular calcium and phosphate concentrations, making …
Paraxanthine
Paraxanthine forms during caffeine metabolism and remains pharmacologically active, but its properties and effects differ from those of caffeine itself.
Patella
The patella, commonly called the kneecap, is a small triangular bone located at the front of the knee. It is the largest sesamoid bone in the human body, meaning that it develops within a tendon. The …
Patellofemoral Joint
The patellofemoral joint is the articulation between the patella, or kneecap, and the trochlear surface at the front of the distal femur. Specifically, the cartilage-covered posterior surface of the patella contacts the femoral trochlea, an articular …
Pathogenic Bacteria
Pathogenic bacteria are bacteria capable of causing disease in a host. Pathogenicity refers to this disease-causing capacity. Most bacteria are not pathogenic, and whether disease develops depends on both characteristics of the bacterium and the susceptibility …
Peak Power
Peak power measures the highest mechanical output in a defined task, so comparisons require consistent testing protocols and calculation methods.
Pectoral Girdle
The pectoral girdle, also called the shoulder girdle, is the bony structure that connects each upper limb to the axial skeleton. In humans, each side of the pectoral girdle consists of two bones: the clavicle, or …
Pelvic Floor
The pelvic floor combines muscles and connective tissues that support pelvic organs while coordinating relaxation and control at the pelvic outlets.
Pelvic Girdle
The pelvic girdle, also called the hip girdle, is the part of the appendicular skeleton that attaches the lower limbs to the axial skeleton. It is formed by the right and left hip bones, or coxal …