All or nothing principle: Once a muscle contraction starts, it must either contract with all possible force or none at all.

Isometric contraction: Muscle length remains the same but tension increases. These contractions do work but do not produce movement. Most movements are a combo of isometric and isotonic contractions.

Isotonic contraction: The tone or tension within the muscle remains the same. This occurs usually when there is little or no resistance. Can be concentric (reduces the angle of the joint or eccentric: increasing the angle and lengthening the muscle)

Motor unit: A motor neuron together with the muscle fibers (cells) it stimulates.

Myofibril: A threadlike structure, extending longitudinally through a muscle fiber (cell), consisting mainly of thick filaments (myosin) and thin filaments (actin, troponin, and tropomyosin).

Myofilament: Composed of four proteins: Myosin (makes up almost all of a thick filament, the “heads” known as cross-bridges are chemically attracted to actin), Actin (globular proteins that make up the bulk of thin filaments), Tropomyosin (blocks the active sites of actin), and Troponin (holds tropomyosin molecules in place at the binding site).

Neuromuscular junction: A synapse between the axon terminals of a motor neuron and the sarcolemma of a muscle fiber (cell).

Threshold stimulus: The minimum level of stimulation needed to cause a contraction.

Excitability: The ability of muscle fibers to receive and respond to stimuli; the ability of neurons to respond to stimuli and generate nerve impulses.

Contractibility: The ability of cells or parts of cells to actively generate force to undergo shortening for movements. Muscle fibers (cells) exhibit a high degree of this.

Extensibility: The ability of skin or muscle tissue to stretch when it is pulled.

Acetylcholine: A neurotransmitter liberated by many peripheral nervous system neurons and some central nervous system neurons. It is excitatory at neuromuscular junctions but inhibitory at some other synapses (e.g., it slows heart rate).

Graded strength principle: Muscle organs do not contract according to the all-or-nothing principle as do individual muscle cells (organs vs cells). Skeletal muscles can contract with varying degrees of strength at different times. This allows us to match the force of a movement to the demand of a certain task. It is influenced by:

1. Metabolic condition of muscle fibers

2. Number of muscle fibers contracting

3. Number of motor units recruited

4. Intensity and frequency of stimulation

Intercalated discs: An irregular transverse thickening of sarcolemma that contains desmosomes, which hold cardiac muscle fibers(cells) together, and gap junctions, which aid in conduction of muscle action potentials from one fiber to the next.

Hernias: The protrusion or projection of an organ or part of an organ through a membrane or cavity wall, usually the abdominal cavity.

Antagonist: A muscle that has an action opposite that of the prime mover (agonist) and yields to the movement of the prime mover.

Fixator: A muscle that stabilizes the origin of the prime mover so that the prime mover can act more efficiently.

Endomysium: Invagination of the perimysium separating each individual muscle fiber (cell).

Fascicle: A small bundle or cluster, especially of nerve or muscle fibers (cells).

Aponeurosis: A sheetlike tendon joining one muscle with another or with bone.

Insertion: The attachment of a muscle tendon to a movable bone or the end opposite the origin

Lever: A rigid rod or bar (bone)

Motor neuron: Neurons that conduct impulses from the brain toward the spinal cord or out of the brain and spinal cord into cranial or spinal nerves to effectors that may be either muscles or glands. Also called efferent neurons.

Neuromuscular junction: A synapse between the axon terminals of a motor neuron and the sarcolemma of a muscle fiber (cell).

Origin: The attachment of a muscle tendon to a stationary bone or the end opposite the insertion.

Perimycium: Invagination of the epimysium that divides muscles into bundles.

Peristalsis: Successive muscular contractions along the wall of a hollow muscular structure

Prime mover: The muscle directly responsible for producing a desired motion. Also called an agonist.

Sarcolemma: The cell membrane of a muscle fiber (cell), especially of a skeletal muscle fiber

Sarcomere: A contractile unit in a striated muscle fiber (cell) extending from one Z disc to the next Z disc.

Sarcoplasm: The cytoplasm of a muscle fiber (cell).

Sarcoplasmic reticulum: A network of saccules and tubes surrounding myofibrils of a muscle fiber (cell), comparable to endoplasmic reticulum; functions to reabsorb calcium ions during relaxation and to release them to cause contraction.

Synergist: A muscle that assists the prime mover by reducing undesired action or unnecessary movement.

Tendon: A white fibrous cord of dense regular connective tissue that attaches muscle to bone.

T tubules: Small, cylindrical invaginations of the sarcolemma of striated muscle fibers (cells) that conduct muscle action potentials toward the center of the muscle fiber.

Visceral: Pertaining to the organs or to the covering of an organ.

Tendon sheath: Tube-shaped fibrous connective tissue.

Posture: General Function of Muscles; Muscle tone allows to maintain this. Sitting, Standing, Etc.

Relaxation: After Ca++ is released, the sarcoplasmic reticulum (SR) begins to pump it back into sacs. As CA++ is removed, it frees up the troponin molecules and shuts down the contraction. Muscles require a constant supply of ATP to function. ATP is used to do the work of pulling the thin filaments during contraction. ATP binds to a myosin cross bridge and transfers its energy to it in order to perform a contraction. The process repeats, and muscles must continue to resynthesize ATP by breaking down creatine phosphate.

Fused Tetanus: Smooth, sustained muscle contractions. Pertaining to postures

Unfused Tetanus: Sustained but wavering contractions.

1. Example: Doing planks more than your limits and you shake. In this situation, the muscle does not have enough time between contractions to completely relax.

Treppe: Also called the staircase phenomenon. Shows that muscles contract more forcefully after they have been contracted a few times This is why athletes warm up. However, repeated stimulation can result in fatigue.

Cross bridges: The globular head of a myosin molecule that projects from a myosin filament in muscle and in the sliding filament hypothesis of muscle contraction is held to attach temporarily to an adjacent actin filament and draw it into the A band of a sarcomere between the myosin filaments.