Exercise Physiology Exam 3 Review: Muscle Mechanics, Reflexes, and Fatigue

Vocabulary flashcards covering muscle fiber types, excitation-contraction coupling, proprioceptors, reflex pathways, muscle contraction mechanics, and central versus peripheral fatigue mechanisms for Exam 3.

Type I fibers (Slow oxidative)

Muscle fibers characterized by slow myosin ATPase, many mitochondria and capillaries, and high myoglobin content; they are the most fatigue resistant and primarily support posture and endurance.

Type IIa fibers (Fast oxidative glycolytic)

Muscle fibers characterized by fast contraction speed and substantial oxidative and glycolytic capacity, providing intermediate fatigue resistance for repeated activity.

Type IIx fibers (Fast glycolytic)

The fastest muscle fiber type in the course comparison, characterized by less oxidative machinery and myoglobin, which fatigues rapidly during high-intensity efforts such as sprinting and jumping.

Myoglobin

An oxygen-binding protein located inside skeletal muscle fibers that gives oxidative fibers their red appearance and supports intracellular oxygen availability.

Phosphocreatine (PCr/CP)

A high-energy cellular phosphate source that rapidly regenerates ATP directly during the initial seconds of intense muscular work (about $8\,\text{seconds}$ in class).

Sarcomere

The structural and functional unit of skeletal muscle extending from Z line to Z line; during shortening, actin and myosin filaments slide past each other, causing the Z lines to approach and the I band and H zone to shrink while the A band remains unchanged.

Muscle spindle

A sensory receptor located in the muscle belly arranged in parallel with extrafusal fibers that detects muscle length and rate of stretch via Ia and II afferent sensory nerves.

Golgi tendon organ (GTO)

A sensory receptor located at the muscle–tendon junction in series with muscle fibers that detects muscle–tendon tension via Ib afferents and can inhibit the contracting muscle through spinal interneurons.

Alpha motor neuron

A motor neuron that innervates and stimulates force-producing extrafusal skeletal muscle fibers.

Gamma motor neuron

A motor neuron that innervates and contracts the intrafusal fiber ends of a muscle spindle, maintaining tension in the central sensory region during muscle shortening.

Proprioception

The sensory perception of body position and movement.

Kinesthesia

The conscious awareness of body and joint position and movement.

Free nerve endings

Abundant joint and tissue sensory receptors that detect touch, pressure, and pain, characterized by a strong initial response followed by a sustained discharge.

Golgi-type joint receptors

Slowly adapting sensory receptors situated in ligaments that provide sustained signals regarding joint position.

Pacinian corpuscles

Rapidly adapting sensory receptors located in joint tissues that primarily signal rapid joint movement and rotation.

Stretch reflex (Myotatic reflex)

A reflex that is monosynaptic at its core, in which rapid muscle stretch activates spindle Ia afferents that directly excite alpha motor neurons supplying the same muscle, causing it to contract.

Reciprocal inhibition

A reflex mechanism in which sensory afferents excite spinal inhibitory interneurons to suppress motor drive to the antagonist muscle during an agonist contraction.

Muscle chemoreceptors

Sensory receptors within skeletal muscle that detect local chemical variations, such as accumulations of $\text{H}^+$ and $\text{K}^+$, to coordinate cardiovascular and respiratory adjustments.

Isometric action

A muscle action characterized by active force production without a net change in overall muscle length or joint angle (e.g., holding a dumbbell curl steady).

Concentric action

A dynamic muscle action involving active shortening of the muscle while producing force (e.g., lifting the weight in a dumbbell curl).

Eccentric action

A dynamic muscle action involving active lengthening of the muscle under mechanical load (e.g., controlling the descent of a dumbbell curl).

Motor unit

A single alpha motor neuron together with all the skeletal muscle fibers it innervates, all of which share the same muscle fiber type in the course model.

Size principle

The orderly recruitment hierarchy of motor units according to neuron size, recruiting small, low-threshold Type I units first, then Type IIa units, and finally large Type IIx units as excitatory drive increases.

Power

The product of muscle force and shortening velocity ($\text{Power} = \text{force} \times \text{velocity}$), reaching its maximum at an intermediate combination of both variables.

Specific force

The force generated by a muscle divided by its cross-sectional area ($\text{force} / \text{cross-sectional area}$).

Summation

The additive mechanical effect of consecutive muscle twitches that occurs when repeated action potentials arrive before the muscle fiber has completely relaxed.

Tetanus

A sustained plateau of muscular contraction caused by high-frequency stimulation; it is unfused when tension fluctuates with partial relaxation, and fused when tension displays a continuous, smooth plateau.

Latent period

The brief time delay (about $5\,\text{ms}$ in the class model) between motor nerve/membrane stimulation and the initial onset of mechanical tension, reflecting excitation–contraction coupling processes.

Rhabdomyolysis

A rapid breakdown of damaged skeletal muscle tissue that releases intracellular components such as myoglobin and creatine kinase (CK) into the bloodstream, potentially causing kidney injury.

Central fatigue

A physiological reduction in voluntary neural drive from the brain and spinal cord to motor neurons, in which the muscle retains the capacity to generate additional force if stimulated electrically.

Peripheral fatigue

A reduction in muscular force output caused by physiological failures occurring at or beyond the neuromuscular junction, particularly within the muscle fibers themselves, despite incoming neural stimulation.

Inorganic phosphate ($\text{P}_i$)

A metabolic byproduct that accumulates during high-intensity ATP breakdown and contributes to peripheral fatigue by decreasing cross-bridge force production and impairing sarcoplasmic reticulum calcium release and availability.