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