5: Muscular System-Force Generation

Vocabulary flashcards covering force generation mechanisms, recruitment principles, muscle adaptations, and age-related changes from KINS 1131.

Graded force generation

The ability of healthy muscles to produce contractions that vary in strength in an incremental, step-by-step fashion.

Frequency of Stimulation

Refers to how fast a motor neuron can excite the muscle fibers in its motor unit; higher rates result in greater contraction strength.

Temporal summation

Occurs when a second stimulus is applied to a muscle before the relaxation period of the first stimulus is complete, adding the depolarizing effects together.

Unfused (incomplete) tetanus

A sustained but quivering contraction that occurs when a muscle is stimulated at an increasingly higher rate, shortening the relaxation time between twitches.

Fused (complete) tetanus

Occurs when stimulation frequency is so high that all relaxation disappears and twitches fuse into a smooth, continuous contraction.

Motor unit recruitment

The process of increasing the number of active motor units to increase the force of contraction.

Maximal stimulus

The strongest stimulus that increases contractile force, beyond which all available motor units are recruited and no further tension can be generated by recruitment alone.

Size principle of recruitment

The rule that motor units with the smallest fibers and most excitable neurons are activated first, while larger units are recruited only for powerful contractions.

Length-tension relationship

The phenomenon where the amount of force a muscle generates during an isometric contraction depends on the length of the sarcomeres.

Optimal resting length

The sarcomere length between $80-120\%$ of its resting length ($2.0$ to $2.25$ micrometer) that permits maximal cross-bridge formation.

Passive structural element theory

The theory that non-contractile structures provide extra force during eccentric contractions as they stiffen and create tensile strength.

Titin

A structural protein extending from the Z-disc to the M-line that contributes significantly to tensile strength in eccentric contractions.

Force-velocity relationship

An inverse relationship where increasing the velocity of a muscle contraction reduces the overall amount of cross-bridge overlap and force production.

Muscle tone

The small amount of tension a muscle exhibits even at rest, maintained by spinal reflexes that alternate the activation of motor units.

Hypertonicity

A state of increased muscle tone that results in stiff muscles and a lack of flexibility.

Hypotonicity

A state of decreased muscle tone resulting in floppy muscles, high flexibility, and difficulty maintaining posture.

Flaccid paralysis

The total loss of muscle tone that occurs when the motor neuron to a muscle is cut.

Muscle (peripheral) fatigue

The progressive decline in the ability to contract even while receiving stimuli, typically recovering after rest.

Central (psychological) fatigue

A condition where the nervous system fails to sustain muscle contraction regardless of the muscle's physical state.

Myosatellite cells

Quiescent cells located between the sarcolemma and basement membrane that activate, proliferate, and fuse to existing fibers for growth and repair.

Overload principle

The requirement that muscles must be subjected to greater demand than they are adapted to in order to produce further gains in strength or efficiency.

Muscular hypertrophy

The enlargement of existing muscle fibers through the addition of mitochondria, myofibrils, and glycogen, usually resulting from resistance training.

Muscular hyperplasia

An increase in the total number of muscle fibers; its extent in adult humans is a subject of debate.

Sarcopenia

The involuntary, age-related loss of muscle mass and strength, characterized by a decrease in the size and number of fibers, especially Type IIx.

Sarcopenic obesity

A condition in older adults where muscle mass loss is exacerbated by obesity, leaving the individual weaker but moving a larger body mass.