Class Notes on Biomechanics and Proprioception

Class Overview

  • Today’s class will finish with the biomechanics of skeletal muscle before moving on to the biomechanics of the trunk.
  • There is a planned in-class activity subsequent to the review, aimed at deepening understanding.
  • A quiz on the biomechanics of skeletal muscle will follow on Wednesday.

Clarifications from Previous Lectures

  • Students are encouraged to ask questions about prior content.
  • Topics of confusion addressed include:
    • Organization of muscles and their movement.
    • Horizontal abduction and adduction, referencing the vertical and horizontal axes of the body.
    • Muscle spindles and their function.
    • Length-tension relationship of sarcomeres and the significance of cross-bridge formation.

Length-Tension Relationship

  • Concept Explanation:
    • In a lengthened position, the sarcomere is too stretched for effective cross-bridge formation.
    • In a shortened position, actin overlaps, leading to blocked binding sites for myosin heads.
    • Ideal Length: A slightly stretched position beyond resting length facilitates optimal cross-bridge formation and force production.
  • Example in practice: Ideal positioning in a bicep curl.
  • Mechanical Advantage: Can vary based on muscle position and moment arm.[^1]

Biarticular Muscles

  • Definition: Muscles that cross two joints.
  • Function: Can maintain a consistent length while moving through their range, leading to consistent force production.
    • Example: If a muscle shortens at one joint and lengthens at another, its functional length remains similar.

Proprioception and Proprioceptors

  • Definition of Proprioception: Non-visual awareness of body positioning in space.
  • Proprioceptors: Include muscle spindles and Golgi tendon organs (GTOs).
    • Muscle Spindles: Stretch receptors located deep in the muscle belly, detecting both the amount and rate of stretch.
    • Function involves sending afferent signals to the CNS during rapid or excessive stretch to protect against injury.
    • GTO: Found in tendons, detecting muscle tension and preventing damage by sending inhibitory signals back to the muscle.

Muscle Spindles

  • Primary Functions:
    • Detect stretch in muscles quickly.
    • Send sensory (afferent) signals to the spinal cord, which can initiate a reflex response without sending information to the brain.
    • Responses can be reflexive to prevent injury based on the stretch of the muscle.
  • Reciprocal Inhibition:
    • When one muscle (agonist) is excited to contract, the antagonist muscle receives an inhibitory signal, promoting relaxation.
    • This mechanism aids in enhancing muscle function by preventing opposition of muscle actions that could hinder performance.

Golgi Tendon Organs (GTO)

  • Location: Found in tendons, detecting tension rather than stretch or length.
  • Functionality: Similar to muscle spindles, GTOs also send sensory signals to the CNS, triggering reflex actions that inhibit the muscle generating excessive tension (autogenic inhibition).
    • Results: Protection against muscle damage from excessive force, leading to a subsequent decrease in force production if not properly trained.

Stretch-Shortening Cycle

  • Connection to Plyometric Exercise:
    • Integrates the function of muscle spindles and the series elastic component during eccentric muscle actions that precede concentric contractions.
    • Phases:
    • Eccentric phase: Muscle lengthens, stretching both the muscle spindle and series elastic component, storing elastic potential energy.
    • Amortization phase: Short pause to transition between eccentric and concentric; should be kept short to avoid dissipating elastic energy.
    • Concentric phase: Muscle shortens, utilizing stored energy and muscle spindle activation to increase force production (also called potentiation).
  • Real-World Example: Vertical jumps utilizing counter-movement.

Reciprocal Inhibition and Altered Reciprocal Inhibition

  • Reciprocal Inhibition: Vital for coordinated movement; prevents muscles from opposing each other.
  • Altered Reciprocal Inhibition: Occurs when one muscle is consistently overactive (tight) while the antagonist is underactive (weak), often leading to compensatory movement patterns and injuries.

Proprioceptive Neuromuscular Facilitation (PNF) Stretching

  • Overview: Utilizes knowledge of proprioception to enhance flexibility and range of motion.
  • Method: Involves resistance followed by stretching to inhibit muscle action through GTOs.
    • Steps include:
    1. Stretch a muscle to slight discomfort for 10–30 seconds.
    2. Engage the muscle by resisting against a partner or force.
    3. Relax and stretch further to exploit GTO function, leading to increased stretch.

Conclusion Structure of the Trunk

  • Transitioning to biomechanics of the trunk after reviewing skeletal muscle biomechanics.
  • Importance of axial skeleton: Skull, vertebrae, ribs, sternum, sacrum, and coccyx; provides foundational support for overall structure and function.
  • Reminder: Resources available for additional review in anatomy, emphasis on solidifying foundational knowledge for future topics.

[^1]: Discussion of mechanical advantages and disadvantages based on muscle positions during activities such as bicep curls, influencing training methods.