Neuoromuscular Neuoromuscular SystemSystem(continued)(continued)

EXS 558EXS 558

Lecture #2Lecture #2

Wednesday September 7, 2005Wednesday September 7, 2005

Review Question #1Review Question #1

Which muscle structure surround the Which muscle structure surround the cellular contents of each muscle fiber?cellular contents of each muscle fiber?a.) perimysiuma.) perimysium

b.) fasciculusb.) fasciculus

c.) sarcolemmac.) sarcolemma

d.) endomysiumd.) endomysium

Review Question #2Review Question #2

The sarcomere, functional unit of the The sarcomere, functional unit of the muscle cell, is measured from:muscle cell, is measured from:a.) Z-disk to Z-diska.) Z-disk to Z-disk

b.) A-band to A-bandb.) A-band to A-band

c.) actin to myosinc.) actin to myosin

d.) H-zone to Z-diskd.) H-zone to Z-disk

Review Question #3, 4Review Question #3, 4

What type of activity would a motor unit What type of activity would a motor unit that contains only a few muscle fibers be that contains only a few muscle fibers be best suited for?best suited for?

True/False: All fibers associated with a True/False: All fibers associated with a motor unit are contracted when motor unit are contracted when innervated. innervated.

Review Question #5Review Question #5

What is released from the presynaptic What is released from the presynaptic side of the neuromuscular junction (NMJ) side of the neuromuscular junction (NMJ) to trigger a muscular contraction?to trigger a muscular contraction?a.) adenosine triphosphate (ATP)a.) adenosine triphosphate (ATP)

b.) calcium (Cab.) calcium (Ca2+2+))

c.) sodium c.) sodium

d.) acetylcholine (Ach)d.) acetylcholine (Ach)

Review Question #6Review Question #6

Explain the role of calcium in the Explain the role of calcium in the production of a muscular contraction.production of a muscular contraction.

Regulation of Tension ProductionRegulation of Tension Production

Motor Unit (MU) = motor neuron and all Motor Unit (MU) = motor neuron and all myofibers in innervatesmyofibers in innervates

When motor neuron delivers impulse ALL When motor neuron delivers impulse ALL fibers maximally contractfibers maximally contract

2 mechanisms by which CNS controls muscle 2 mechanisms by which CNS controls muscle tensiontension1.) 1.) RATE CODINGRATE CODING - - ↑ or ↓ rate of firing of neuron↑ or ↓ rate of firing of neuron

2.) 2.) RECRUITMENTRECRUITMENT - - ↑ or ↓ # of MU’s activated↑ or ↓ # of MU’s activated

Which dominates?Which dominates?

With With small homogenous musclesmall homogenous muscle, initially , initially rely upon recruitment (at 50% MVC, all rely upon recruitment (at 50% MVC, all MU’s recruited), additional force MU’s recruited), additional force production caused by production caused by ↑ ↑ rate codingrate coding

With With heterogeneous larger muscleheterogeneous larger muscle, first , first see see ↑ ↑ rate coding of low threshold MU’s, rate coding of low threshold MU’s, then see then see ↑ ↑ recruitment until reach 90% recruitment until reach 90% MVC, then MVC, then ↑↑ rate coding to reach 100% rate coding to reach 100% MVC.MVC.

Size Principle of MU RecruitmentSize Principle of MU Recruitment

Principle of orderly recruitment states that motor units are activated in a fixed order, based on their ranking in the muscle.

Size principle states that the order of recruitment is directly related to their motor neuron size

Slow-twitch fibers, which have smaller motor neurons, are recruited before fast-twitch fibers.

Size Principle of MU RecruitmentSize Principle of MU Recruitment(continued)(continued)

Why do FT MU develop more force?Why do FT MU develop more force?

FT develop electrical impulses to FT develop electrical impulses to myofibers more quicklymyofibers more quickly

FT MUs have myofibers with larger CSAsFT MUs have myofibers with larger CSAs FT MUs have greater # of associated FT MUs have greater # of associated

myofibers than ST MUs myofibers than ST MUs

Functional Classification of MusclesFunctional Classification of Muscles

Agonists—prime movers; responsible for the movement

Antagonists—oppose the agonists to prevent overstretching of them

Synergists—assist the agonists and sometimes fine-tune the direction of movement

Muscle ActionMuscle Action

Neuromuscular AdaptationsNeuromuscular Adaptations

““An understanding of the type of An understanding of the type of alterations seen with a given training alterations seen with a given training program will help the coach or athlete program will help the coach or athlete develop the most appropriate training develop the most appropriate training program and set the most realistic program and set the most realistic training goals”training goals”

- Hoffman - Hoffman

Mechanisms of Gains in Muscle StrengthMechanisms of Gains in Muscle Strength

Neural AdaptationsNeural Adaptations Synchronization and recruitment of additional motor units Coactivation of agonist and antagonist muscles Rate coding—the firing frequency of motor units

Muscle HypertrophyMuscle Hypertrophy Fiber hypertrophy Fiber hyperplasia

Muscle Fiber HypertrophyMuscle Fiber Hypertrophy

Neural Effects of Muscle Neural Effects of Muscle PerformancePerformance

Bilateral deficit: maximum force produced Bilateral deficit: maximum force produced by simultaneous contraction of both limbs by simultaneous contraction of both limbs less than total force produced with each less than total force produced with each limb acting singlylimb acting singly EMG activity EMG activity ↓↓ when activating both limbs when activating both limbs

concurrently as compared to singlyconcurrently as compared to singly

Why?Why? May be limit of neural drive from higher May be limit of neural drive from higher

centers in CNS (central drive)centers in CNS (central drive) Bilateral deficit associated with greater EMG Bilateral deficit associated with greater EMG

activity to antagonistactivity to antagonist Pre-contraction of antagonist can Pre-contraction of antagonist can ↓↓ recruitment of recruitment of

antagonistsantagonists

Neural Effects of Muscle Neural Effects of Muscle PerformancePerformance(continued)(continued)

Effects of Resistance TrainingEffects of Resistance Training

Early Early ↑↑ in strength not accompanied by in strength not accompanied by muscle hypertrophymuscle hypertrophy

Caused by neural adaptations ( Caused by neural adaptations ( ↑↑ EMG activity) EMG activity) See See ↑ in strength in untrained contralateral limb ↑ in strength in untrained contralateral limb

((↑↑ EMG activity) EMG activity) [cross training effect][cross training effect] ↑↑ EMG activity indicated improved MU activation EMG activity indicated improved MU activation

(MUA)(MUA)

Effects of Resistance TrainingEffects of Resistance Training

Maximal force development requires complete MUAMaximal force development requires complete MUA For many max effort does not induce complete MUAFor many max effort does not induce complete MUA Training Training ↑↑ ability to reach full MUA with max voluntary effort ability to reach full MUA with max voluntary effort Training Training ↑↑ ability to keep threshold MUs activated ability to keep threshold MUs activated Training Training ↑ duration at which high and low threshold MUs can ↑ duration at which high and low threshold MUs can

sustain optimal firing ratessustain optimal firing rates Training ↓ decrement between tension seen with MVC and Training ↓ decrement between tension seen with MVC and

tension induced with supramaximal stimulation of nervetension induced with supramaximal stimulation of nerve Training ↑ motor neuron excitability during voluntary effortTraining ↑ motor neuron excitability during voluntary effort

↑ ↑ ability to recruit MUs and disharge them at higher frequenciesability to recruit MUs and disharge them at higher frequencies Training ↑ synchronization in activation/firing rate of MUsTraining ↑ synchronization in activation/firing rate of MUs

Does NOT ↑ max force production, but may ↑ rate of max force Does NOT ↑ max force production, but may ↑ rate of max force productionproduction

Resistance Training FactResistance Training Fact

Once your goals for strength development have been achieved, you can reduce training frequency, intensity, or duration and still prevent losses in strength gained for at least 12 weeks. However, you must continue training with a resistance maintenance program that still provides sufficient stress to the muscles.

Effects of Endurance TrainingEffects of Endurance Training

Training causes Training causes ↑ in MU activation occurs in ↑ in MU activation occurs in early stages, ↑ skill acquistion/coordinationearly stages, ↑ skill acquistion/coordination

Delays fatigueDelays fatigue

Training causes rotation of activity among Training causes rotation of activity among synergists and among MU of prime moversynergists and among MU of prime mover

Delays fatigueDelays fatigue

Training ↑ consistency of firing rates of motor Training ↑ consistency of firing rates of motor neuronsneurons

Finer control of muscleFiner control of muscle Delays fatigueDelays fatigue

Are muscle fiber type conversions possible?Are muscle fiber type conversions possible?

Early studies showed no change in fiber type but changes in characteristics of muscle fibers

Cross-innervation studies and chronic stimulation studies demonstrate changes

Possible change from FTb to FTa, and from FTa to ST with endurance training, and FTb to FTa with resistance training

A combination of high intensity resistance training and short-interval speed work can lead to a conversion of ST to FTa fibers