Biomechanics of Muscle Contraction and Movement

Speed of Contraction

• Concentric contractions produce more force when slower, reaching peak force at isometric or zero-velocity contractions.
• Maximum crossbridge formation occurs at slow speeds; increased contraction speed reduces actin-myosin links and force.
• During eccentric contractions, muscle strength increases with speed, contrary to concentric contractions.

Moment Arm

• The moment arm relates to the perpendicular distance from the joint axis to the muscle's force vector, influencing joint rotation.
• Mechanics study the behavior of physical bodies under forces and their displacements.

Planes of Motion

• Movement occurs along three primary planes: sagittal, frontal, and transverse, defining how the body moves relative to its orientation.
• Examples of joint naming utilize the proximal bone first, e.g., RU, AA, SI joints.

Axes of Motion

• Imaginary lines around which body parts rotate, distinguished by being perpendicular to the movement planes.

Naming Movements at Joints

• Joints named by the accompanying bones.
• Movements include medial and lateral rotation at the hip, with rotation occurring around an anterior-posterior axis instead of a longitudinal axis.

Types of Motion

• Body segments exhibit either translatory (linear) or rotary (angular) motion.

Degrees of Freedom

• Degrees of freedom in joints reflect the number of plane movements allowable, maximized at three.

Joint Structure and Function

• Joints are classified into three types:
  • Synarthrodial (immovable, e.g., sutures)
  • Amphiarthrodial (slightly movable, e.g., pubic symphysis)
  • Diarthrodial (freely movable, e.g., ball-and-socket joints like the shoulder).

Circumduction vs. Rotation

• Circumduction involves circular or conical movement of a body part through a combination of flexion, extension, abduction, and adduction.
• Rotation involves turning around a central point or axis without changing the position of the joint itself.

Synarthrodial Joints

• Synarthrodial joints are composed of fibrous connective tissue, allowing minimal to no movement between bones.

Muscle Fibers and Action Potential

• Nerve impulses translate into action potentials transmitted over nerve and muscle fibers.
• Motor units contract maximally when activated.

Gradiation of Strength of Muscle Contraction

• The smallest motor units activate first (Size Principle), while increasing the number of motor units (Recruitment Principles) raises overall muscle tension.
• Excitatory input increases muscle tension by raising stimulation frequency.

Joint, Tendon, and Muscle Receptors

• Joint receptors continuously emit action potentials, giving the brain a sense of body position.
• Golgi Tendon Organs detect muscle force and tension.

Proprioception

• Proprioception integrates input from muscle spindles, tendons, and joints to assess joint position and movement.

Motor Control

• Motor control encompasses posture regulation and the intricate coordination of movements through muscle synergy.
• Movement functions within a heterarchical system, allowing diverse movement combinations.

Types of Muscle Activity

• Muscle activity is categorized into isotonic, isometric, and isokinetic movements, each defined by how muscle length and joint movement occur.
• Understanding muscle fiber types:
  • Type I fibers are postural, supporting stability against gravity.
  • Type II fibers are non-postural or mobility muscles, activated for rapid or forceful movements.

This quiz explores the principles of muscle contraction, focusing on the differences between concentric and eccentric contractions. It also delves into the mechanics of movement through various planes and axes, offering insights into the impact of force and moment arms on joint rotation.