Download EMG Lab

EMG LAB I and II
• Electromyography (EMG)- recording of the electrical
activity associated with skeletal muscle contraction.
• Dynamometry is the study of power output of muscles.
• Todays lab consists of two parts.
1)
Design experiments in which your lab group will decide
which muscles to record EMGs from, and what activities
those muscles will perform.
2) Combine electromyography with dynamometry to
examine motor unit recruitment and skeletal muscle
fatigue.
Excitation-Contraction Coupling
Sliding Filament Theory
Muscle Contraction
• Tension – force muscle exerts on an object when
contracted

Responses are graded by:


Changing the frequency of stimulation
Changing the strength/intensity of the stimulus
Factors Affecting Tension
• Intensity of stimulus – number of motor units
▫ Motor unit – a single motor neuron and all the
muscle fibers it supplies
▫ Recruitment – calling
additional motor units,
stimulating more
fibers will increase
muscle tension
Factors Effecting Tension
• Intensity of stimulus
▫ Threshold stimulus – minimal stimulus needed to
invoke visible muscle contraction
▫ Maximal stimulus – all motor units are recruited,
strongest contraction produced
 Asychronous recruitment of motor units – alternates motor
units
Factors Affecting Tension
• Frequency of stimulation
▫ Twitch – single impulse, contraction followed by
relaxation
▫ Wave summation – when impulses are delivered in
succession the second twitch will be stronger then the first
▫ Complete tetanus – rapid stimulation results in sustained
smooth contraction without periods of relaxation
Factors Effecting Tension
• Frequency of stimulation
Types of Contractions
• Isotonic – tension remains constant during
contraction
▫ Muscle length shortens or lengthens during
contraction
▫ Concentric vs. Eccentric
Isotonic Contractions
• Concentric - muscle shortens and does work
▫ Examples: pick up pencil, kick soccer ball
Isotonic Contractions
• Eccentric – muscle contracts as it lengthens
▫ Helps counter act gravity or prevent joint injury
“muscle braking”
▫ Example: squats – quadriceps stretch but are
contracted to counter act gravity and control
movement
Types of Contractions
• Isometric – Tension increases but muscle length remains
the same
▫ Muscle is unable to produce enough force to overcome the
load
▫ Example: pushing against a stationary wall
Fatigue
• Fatigue is a reduced ability to perform work.
▫ Psychological fatigue results from reduced
input from the CNS and usually occurs before the
physical capabilities of the muscle are fully
depleted.
▫ Physiological fatigue results from the depletion
of ATP at a rate faster than it can be replenished
through respiration as well as a build up of lactic
acid (from anaerobic metabolism).
Lab Equipment
BioPac
Electrodes
Lab Procedure Part 1
• Design an experiment to measure how electrical activity
changes when a muscle contracts at 4 different degrees
of force.
• Design an experiment to demonstrate the actions of an
agonist and its antagonist by recording from the two
muscles simultaneously. Plug a second cable into
Channel 2.
• Design an experiment to demonstrate concentric and
eccentric contractions in the same muscle.
EMG Data
• To collect data select the I beam tool. Click and drag it
through the first EMG cluster. Note the p-p value above
the graph. This millivolt value can be used as an
indication of the number of muscle fibers stimulated.
Lab Equipment part 2
• Dynamometer- hand held bar that is
equipped with an electronic transducer to record
the force exerted by the muscles on the
transducer.
Lab Procedure Part 2
• Select the subject’s dominant forearm to
attach the electrodes to.
 Perform three muscle contractions using increasing
strengths
 Record a maximal strength contraction and sustain
your grip for as long as possible. Relax your grip only
when you reach fatigue.
Lab Procedure Part 2
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•
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Measure data
Record data on lab sheet
Analyze results
Answer questions
Muscle Twitch on a Myogram
Myogram
Calibration of
a 50g weight