Neuromuscular Adaptations to Conditioning

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Neuromuscular Adaptations to Conditioning

• Chapter 2

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Objectives

• Discuss the concept of a motor unit
• Discuss different fiber types
• Discuss two ways to increase force production
• Discuss adaptations to the neural system

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The Nervous System

• Central (CNS)
• Brain
• Spinal cord
• Peripheral
• Nerves
• Axons

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Neuron and Motor Unit

• Neuron is a single nerve cell
• 1014 neurons in brain
• Synapses convey information via chemicals
• Afferent-from periphery to CNS
• Efferent-from CNS to periphery
• Neuron body, dendrites and axon (myelin sheath)

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Action Potential

• Alteration in permeability
• Sodium influx and potassium outflow
• Negative to positive
• Nerve conduction velocity
• 120 m/s or 270mph for myelinated
• 400 f/s
• 5 m/s or 2mph for unmyelinated
• 16 f/s

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Phospholipid Bilayer
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Slow or Block Nerve Conduction

• Demyelination
• Multiple sclerosis
• Guillain-Barre syndrome
• Parkinsons
• ALS

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Neural Components of Muscle Activation

• Motor unit
• Acetylcholine (ACH)- primary neurotransmitter at
  the neuromuscular junction
• Frequency of nerve impulses
• Twitch
• Summation
• Tetanus

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Electrical Stimulation

• Motor nerve innervation
• Latent period (.01)
• Contraction phase (.04)
• Relaxation phase (.05)
• Fast vs. slow time varies

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Threshold

• AP results from the quick and dramatic alteration
  to ionic permeability following chemical or
  electrical intervention. Muscle resting at -90
  millivolts
• After stimulation of an excitable cell membrane
  sodium ions move into the cell and the
  transmembrane potential is reduced - referred to
  as depolarization
• When a critical voltage level called the
  threshold is reached, voltage-sensitive sodium
  gates are opened followed by slower acting
  potassium gates (move out)
• At 35 millivolts the sodium channels and the
  potassium channels are fully opened, resulting in
  restoration of the negative transmembrane
  potential - called repolarization
• The amplitude of voltage changes in response to
  stimulation is constant from stimulus to stimulus
  and is described as "all or none"
• Electrical stimulation of excitable cells is
  possible up to 1000 pps.

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Temperature

• Heat increases speed and force output.
• Cooling increases relaxation time.
• Heat may increase speed by 20.

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Size Principle of Muscle Recruitment
F I B E R S U S E D
TYPE IIb
TYPE IIa
TYPE I
MUSCULAR FORCE
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Reflexes

• Sensory receptors send a signal to a motor neuron
• Motor neuron sends signal to the effector
• Stretch shortening cycle (SSC)?

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Stretch Shortening Cycle

• Concentric force is increased as a function of
  eccentric action or stretching.
• Increased force with speed of the motion.
• Stored elastic energy responsible.

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Fatigue

• Repeated contractions diminish relaxation time.
• Neural signals continue to propagate.
• Contracture occurs at the muscle site.

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Mechanical Factors

• Angle of pull is optimum at right angles or 90
  degrees to the bone.
• Length is optimum at midpoint or resting length.

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Exercise Modes

• Isokineticconstant velocity.
• Isotonicconstant resistance (DCER).
• Isometricstatic and without muscle movement.

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Neuromuscular Adaptations to Exercise

• Hypertrophy- enlargement and increase in number
  of muscle myofibrils (not fibers), increasing the
  size of actin and myosin
• Hyperplasia-increase in the number of fibers (not
  in humans, only in birds).
• Fast twitch muscle fibers hypertrophy to a
  greater extent than slow twitch muscle fibers
• Early increases in muscle strength have a large
  neural component
• Long term increases in strength also have a
  neural component

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Moritani and deVries Hypertrophy vs. Learning
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Atrophy vs. Hypertrophy
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Electromyography (EMG)

• Records electrical signals from the brain.
• EMG reflects muscle activation.
• Surface electrodes (summated) or fine needle
  electrodes (individual).
• Amplitude increases with recruitment (summation).
• Integration of signal equals true mean of firing
  (RMS).

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EMG cont

• Positive relationship between EMG and
  force/velocity.
• A measure of intensity.
• Efficiency of electrical activity stronger
  individuals require less activation.
• Learning curve demonstrates greater force with
  less EMG.

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EMG and Fatigue

• EMG increases with fatigue.
• Recruitment responsible.
• Local fatigue is a function of individual muscle
  and joint.

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Resistance Training and Aerobic Power

• Resistance training does not improve aerobic
  power
• Resistance training does not impair an
  individuals ability to develop maximal aerobic
  power
• Aerobic training does not enhance muscle strength
  or size
• Aerobic training may compromise the benefits of
  strength training on muscle force production

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Next Class

• Chapter 6 Endocrine