350: Exam 2a (PART 3/5) [Muscle Contraction]

Principle of muscle contraction

all or none principle

motor unit

• Single motor neuron (efferent nerve) 

• All the muscle fibers it innervates

• Functions as a single unit 

Numbers of motor units activated variable

Relatively few to virtually all

all or none principle is dependent on

• Number of muscle fibers in each activated motor unit

• Number of motor units activated

all or none principle means

• Individual muscle fibers within a given motor unit will either fire & contract maximally, or not at all 

Muscle force may be increased by

• Activating more motor units & increasing the frequency of motor unit activation

  • Pulses per second (pps) 

Number of muscle fibers per motor unit varies significantly

true

• From less than 10 

  • Precise and detailed movement (ocular muscles)

• A few thousand 

  • Less complex activities

    • Quadriceps 

Phases of single muscle fiber contraction or twitch 

• Stimulus

• Latent period

• Contraction phase

• Relaxation phase 

  • Successive stimulus before relaxation lead to summation and tetanus (ie, high frequency stimulation with no relaxation) 

Factors affecting muscle tension development

• muscle force?

• number of muscle fibers, phases of single muscle fiber contraction or twitch, and treppe

treppe

• Multiple maximal stimuli at low frequency 

• Complete relaxation between contractions 

key factors of muscle length-tension relationship

• Maximal ability to a muscle to develop tension varies

• Eccentric > Isometric > Concentric

• Stretch-Shortening Cycle

Maximal ability to a muscle to develop tension varies

• Dependent upon length of muscle during contraction 

• Active tension is from actin-myosin

• Passive tension is from connective tissue (like a rubber band)

Eccentric > Isometric > Concentric

• Force is dependent on the number of cross-bridges formed

• Slow Concentric > Fast Concentric 

Sequencing & timing of contractions can enhance total amount of force produced during stretch-shortening cycle

• Basis of plyometric training 

• Requires integration of muscle spindles and GTO’s

• Eccentric stretch phase stores elastic energy 

  • Returned during the concentric muscle shortening phase

• Transition phase must be immediate 

  • Elastic energy is transient and will convert to heat 

A benefit of this cycle – that is not related to elastic properties or neuro-sensory mechanism (eg, muscle spindles, GTO); [HINT: 3 PARTS]

1. Eccentric force is developed during stretching phase

2. When concentric phase begins, muscle force is already large

3. Results in larger muscle work (force x distance) produced during concentric phase (than if the muscle was at rest to begin at the concentric phase) 

• Example: compare the squat jump vs countermovement jump

Types of Muscle Articulations

• uniarticular

• biarticular

• multiarticular

Uniarticular 

• Cross & act directly only on the joint they cross

Biarticular 

• Cross & act on two different joints

Biarticular advantages are [HINT: 2]

• Can cause/control motion at more than one joint

• Able to maintain relatively constant length by “shortening and lengthening” across different joints 

Multiarticular 

• Act on three or more joints due to line of pull

• Ex: flexor digitorum profundus = index finger

Lombard’s Paradox

• The paradox - these joint actions contradict the muscles that are activated for the task 

  • When rising from a chair or squatted position

    • Simultaneous hip extension and knee extension

    • Rectus femoris - hip flexion and knee extension

    • Hamstrings - hip extension and knee flexion 

muscular insufficiency includes

active insufficiency and passive insufficiency

Active insufficiency is reached

• when the muscle becomes shortened to the point that it can not generate or maintain active tension 

  • Ex: rectus femoris in the figure

Passive insufficiency is reached

• when the opposing muscle becomes stretched to the point where it can no longer lengthen & allow movement 

  • Ex: hamstrings in the figure