Foundations of Skeletal Muscle Function

Vocabulary flashcards reviewing skeletal muscle anatomy, biomechanical principles, lever systems, muscle architecture, and functional proteins based on HK 448 Lecture 3.

Sagittal plane

A cardinal plane that divides the body into left and right portions.

Frontal plane

A cardinal plane (also called coronal plane) that divides the body into anterior and posterior portions.

Transverse plane

A cardinal plane (also called horizontal plane) that divides the body into superior and inferior portions.

Axis of rotation

An imaginary line perpendicular to the plane of movement around which bones rotate at a joint.

Anterior-posterior axis

The axis of rotation around which movement occurs in the frontal plane.

Medial-lateral axis

The axis of rotation around which movement occurs in the sagittal plane.

Vertical axis

The axis of rotation around which movement occurs in the horizontal (transverse) plane.

Productive antagonism

The coordinated activation of agonist and antagonist muscles producing opposing actions around a joint to allow movement, control, and joint stability.

Torque

The rotational force generated around an axis of rotation, calculated as $\text{Torque} = \text{Force} \times \text{Moment Arm}$.

Moment arm

The perpendicular distance between the axis of rotation and the line of action of a force.

Internal torque

The product of internal force ($IF$) and the internal moment arm ($D$).

External torque

The product of external force ($EF$) and the external moment arm ($D_1$).

First-class lever

A lever system where the axis of rotation is located in the middle between the force and resistance (Force - Axis - Resistance), as seen in head/neck extension.

Second-class lever

A lever system where resistance is located in the middle between the axis of rotation and force (Axis - Resistance - Force), providing a mechanical advantage ($IMA > EMA$), as seen in a heel raise.

Third-class lever

A lever system where the force is applied in the middle between the axis of rotation and resistance (Axis - Force - Resistance), resulting in a mechanical disadvantage ($EMA > IMA$), as seen in elbow flexion.

Mechanical advantage

The ratio of the internal moment arm ($IMA$) to the external moment arm ($EMA$) when $IMA > EMA$.

Sarcomeres in series

An end-to-end arrangement of sarcomeres that results in $1\text{ unit of force}$ and $2\text{ units of velocity}$ for two sarcomeres.

Sarcomeres in parallel

A side-by-side arrangement of sarcomeres that results in $2\text{ units of force}$ and $1\text{ unit of velocity}$ for two sarcomeres.

Pennation angle

The angle formed between individual muscle fibers and the line of pull of the muscle's tendon.

Physiological cross-sectional area (PCSA)

The cross-sectional area of muscle fibers oriented perpendicular to their long axes, calculated as $\text{PCSA} = \left(\frac{\text{Muscle volume}}{\text{Fiber length}}\right) \times \cos(\theta)$.

A band

The dark band region within a sarcomere caused by the presence of thick myosin myofilaments.

I band

The light band region within a sarcomere caused by the presence of thin actin myofilaments.

H band

The region within the A band where thin actin and thick myosin filaments do not overlap.

M line

The midregion thickening of myosin myofilaments located in the center of the H band.

Z disc

The connecting structures between successive sarcomeres that anchor the thin actin myofilaments.

Myosin heavy chain

An active contractile protein that acts as a molecular motor for muscle contraction by binding with actin to generate contraction force.

Actin

An active contractile protein that binds with myosin to translate force and shorten the sarcomere.

Tropomyosin

An active regulatory protein that stabilizes actin filaments and regulates the interaction between actin and myosin.

Troponin

An active regulatory protein that binds with calcium ions and influences the position of tropomyosin.

Myosin light chain

An active regulatory protein that influences the contraction velocity of the sarcomere and modulates crossbridge cycling kinetics.

Nebulin

A structural protein that anchors actin filaments to Z discs.

Titin

A structural protein that stabilizes myosin within the sarcomere and acts as a large molecular spring conferring elasticity during recoil.

Desmin

A structural protein that connects Z-disks of adjacent myofibrils to stabilize longitudinal and lateral alignment.

Vimentin

A structural protein that helps maintain the periodicity of Z discs.

Skelemin

A structural protein that helps stabilize the position of M lines.

Dystrophin

A structural protein providing structural stability to the cytoskeleton and sarcolemma of the muscle fiber.

Integrins

Structural proteins that stabilize the cytoskeleton of the muscle fiber.