neuromuscular adaptations to conditioning chapter 2
TRANSCRIPT
Neuromuscular Adaptations to
Conditioning
Chapter 2
The Nervous System
• Central (CNS)– Brain– Spinal cord
• Peripheral– Nerves– Axons
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)
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
Phospholipid Bilayer
Slow or Block Nerve Conduction
• Demyelination– Multiple sclerosis– Guillain-Barre syndrome– Parkinsons– ALS
Neural Components of Muscle Activation
• Motor unit• Acetylcholine (ACH)- primary
neurotransmitter at the neuromuscular junction
• Frequency of nerve impulses– Twitch– Summation– Tetanus
Electrical Stimulation
• Motor nerve innervation• Latent period (.01)• Contraction phase (.04)• Relaxation phase (.05)• Fast vs. slow time varies
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.
Temperature
• Heat increases speed and force output.
• Cooling increases relaxation time.• Heat may increase speed by 20%.
Size Principle of Muscle Recruitment
TYPE I
TYPE IIa
TYPE IIb
% FIBERS USED
MUSCULAR FORCE
Reflexes
• Sensory receptors send a signal to a motor neuron
• Motor neuron sends signal to the effector
• Stretch shortening cycle (SSC)?
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.
Fatigue
• Repeated contractions diminish relaxation time.
• Neural signals continue to propagate.
• Contracture occurs at the muscle site.
70
75
80
85
90
95
100
105
110
1 5 9 13 17 21 25 29 33 37 41 45 49 53
Mechanical Factors
• Angle of pull is optimum at right angles or 90 degrees to the bone.
• Length is optimum at midpoint or resting length.
Exercise Modes
• Isokinetic=constant velocity.• Isotonic=constant resistance
(DCER).• Isometric=static and without
muscle movement.
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
Moritani and deVries Hypertrophy vs. Learning
Trained
0
20
40
60
80
100
2 4 6 8
Weeks
% C
ontr
ibut
ion
Neural
Hypertrophy Untrained
0
20
40
60
80
100
120
2 4 6 8
Weeks
% C
ontr
ibut
ion
Neural
Hypertrophy
Atrophy vs. Hypertrophy
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).
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.
EMG and Fatigue
• EMG increases with fatigue.• Recruitment responsible.• Local fatigue is a function of
individual muscle and joint.
Resistance Training and Aerobic Power
• Resistance training does not improve aerobic power
• Resistance training does not impair an individual’s 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
Next Class
• Chapter 6 Endocrine