6 (Final): Muscular System Part 3: Force Generation

Vocabulary practice cards covering force generation mechanisms, muscle tone, fatigue, and adaptations from KINS 1131.

Graded Force Generation

The process where muscle force is generated in an incremental, step-by-step fashion, allowing it to gradually increase.

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

An effect occurring when a second stimulus is applied to a muscle before the completion of the relaxation period of the first, adding their depolarizing effects together.

Unfused (Incomplete) Tetanus

A state of sustained but quivering contraction that occurs when stimulation frequency increases, shortening the relaxation time between twitches.

Fused (Complete) Tetanus

A state where stimulation frequency is so high that all relaxation disappears and twitches fuse into a smooth, maximal contraction.

Lockjaw

A common name for the bacterial condition called tetanus, which causes fused tetanus in the muscles of the neck and jaw.

Motor Unit Recruitment

The process of increasing the number of active motor units to control the force of contraction more precisely.

Maximal Stimulus

The strongest stimulus that increases contractile force; beyond this point, further increases in voltage produce no further increase in strength.

Size Principle

The principle stating that motor units with the smallest fibers and most highly excitable motor neurons are activated first.

Length-Tension Relationship

The property defining how the amount of force a muscle generates during an isometric contraction differs based on its length.

Optimal Resting Length

The sarcomere length between $80-120\%$ of its resting length ($2.0$ to $2.25\,\mu m$) where maximal tension is generated due to ideal myofilament overlap.

Titin

A structural protein extending from the Z-disc to the M-line that forms the core of the thick myofilament and contributes to tensile strength during eccentric contractions.

Force-Velocity Curve

The curve demonstrating the inverse relationship between the force a muscle can produce and the velocity of its contraction.

Muscle Tone

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

Hypertonicity

A condition resulting in stiff muscles and a lack of flexibility.

Hypotonicity

A condition resulting in floppy muscles, high flexibility, and difficulty maintaining postures.

Flaccid Paralysis

The complete loss of muscle tone that occurs if the motor neuron to a muscle is cut.

Muscle (Peripheral) Fatigue

The progressive decline in the ability to contract even though the muscle is still receiving stimuli, typically recovering after rest.

Central (Psychological) Fatigue

The failure of the nervous system to sustain muscle contraction.

Myosatellite Cells

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

Overload Principle

The concept that muscles must be stressed beyond their adapted capacity to produce further gains in strength or efficiency.

Hypertrophy

The enlargement of existing muscle fibers, which is the main mechanism underlying muscle growth from resistance training.

Hyperplasia

An increase in the total number of muscle fibers; its extent in adults is debated but linked to tissue repair.

Atrophy

The wasting of muscle tissue, often caused by inactivity.

Sarcopenia

The involuntary, age-related loss of muscle mass and strength, typically involving a $\sim 3-5\%$ mass loss per decade after age $30$.

Sarcopenic Obesity

A condition in an aging population where individuals are weaker due to muscle loss yet must move a larger body mass.