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Physiology Lab: Week 7 – Muscle Phys

MUSCLE PHYSIOLOGY

ELECTROMYOGRAPHY


Anatomy of Skeletal Muscles

• Levels of Organization

• Skeletal muscles consist of muscle fascicles

• Muscle fascicles consist of muscle fibers

• Muscle fibers consist of myofibrils

• Myofibrils consist of sarcomeres

• Sarcomeres consist of myofilaments

• Myofilaments are made of actin and myosin


NOTE: Each muscle fiber is innervated by only ONE neuron

I. Introduction


Defined as…One motor neuron and all the muscle fibers it innervates

Motor Unit

I. Introduction

Asynchronous recruitment…occurs when motor units alternate, allowing some to relax while others contract – only works for submaximal contractions.


• The number of muscle fibers in a motor unit varies from one

muscle to another

• Eye muscles vs. leg muscles

Precise movement – 6 fibers per nerve

vs.

More strength – 1,000 fibers per nerve

Motor Units vary in size

I. Introduction

Question: How do muscles control the force of contraction?


Answer: Graded contractions are achieved by:

1. Changing the strength (voltage) of the stimulus

(motor unit recruitment)

2. Changing the frequency of stimulation

(summation)

I. Introduction Graded Responses

Question: How do muscles control the force of contraction?


• The “activation of additional motor units” results

in an increased force of contraction

FEWER

MOTOR UNITS

MORE

MOTOR UNITS

Motor Unit Recruitment

I. Introduction


• Muscle twitch response is all-or-none

• Single twitches do not accomplish anything useful in the body

• Increasing the frequency of the stimulus increases the force of contraction

I. Introduction Summation

Increasing frequency


THRESHOLD AND MAXIMAL

RESPONSE

Experiment 1


II. Threshold and Maximal Response Maximum stimulus

Figure on p 146

Threshold

stimulus

Maximum contraction

Fo

rce

The minimum stimulus that

will cause a muscle twitch

The lowest voltage

that results in a

maximum contraction

Supramaximal

stimuli

All muscle fibers are active


II. Threshold and Maximal Response

• Phases of a SINGLE twitch contraction

– latent period

• first few milliseconds after stimulation

– contraction phase

• muscle shortens

– relaxation phase

• return to resting length

Figure on p 148


A. B. C.


SUMMATION, TETANUS AND

FATIGUE

Experiment 2


II. Summation, Tetanus and Fatigue Increasing Frequency of Stimulus

• Single Twitch – not useful

• Wave summation – one twitch is added to another

• Complete tetanus – relaxation phase is eliminated

• Fatigue – (next)

Physiology Lab: Week 7 – Muscle Phys Figure on p 147

• Wave summation – one twitch is added to another

• Complete tetanus – relaxation phase is eliminated

• Fatigue – decrease in the muscle’s ability to generate force

II. Summation, Tetanus and Fatigue Increasing Frequency of Stimulus

Tetanus


• Decrease in the muscle’s ability to generate force

• All motor units being used – none alternating

Due to:

• Energy sources used faster than ATP can be

generated

• Reduced blood flow

• Lactic acid build up (? New research says no)

Psychological vs. Physiological Fatigue

II. Summation, Tetanus and Fatigue


You should understand these terms:

• Motor unit

• Motor unit recruitment

• Threshold

• Summation

• Tetanus

• Fatigue

• What are some factors that increase the

strength of a contraction?

• What are some factors that lead to fatigue?


Electromyography

• Recording of electrical activity

of the muscle =

electromyogram (EMG)

• Measurement of Power =

Dynamometry

• Resting skeletal muscles exhibit

“tonus” (readiness)

- the detection, amplification, and recording of changes in skin

voltage produced by underlying skeletal muscle contraction


EMG Data

electromyogram

Force (Kg)

EMG (mV)

IEMG (mV)

Force = actual measurement of the strength of clench

EMG = actual recording of the mV

IEMG = absolute intensity of the mV


• Q1: Which gender is stronger

• Q2: Which arm is stronger (dominant vs. non-dominant)

• Q3: Which gender fatigues faster

Muscle Physiology

Experimentation


Test your force…

• Measure Maximum clench force • Max. ÷ 5 (ex. 30kg/5 = 6kg)

• Measure fatigue

• Repeat for 2nd arm

• Data analysis to fill in Tables

• Put values on excel

• Homework: perform t-tests on all 3

• Write a report: support/refute Hypothesis, t- values, conclusion etc.


Let’s get started…

• Place electrodes on Dominant forearm as shown

• Attach colored leads to electrodes as shown

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