emg 1 Flashcards
why do we measure EMG (5)
- to infer force
- find muscles activated during a task
- determine pathology
- calc conduction velocity
- calc power
what is a motor unit
motor neuron (nerve cell) and all fibres it innervates
E-C coupling
Excitation-Contraction coupling
- action potential in skeletal muscle cells is what causes muscle cell contraction
- calcium ions regulate whether or not contraction occurs
propagation of action potential
(from motor neuron to muscle
- AP down axon to endplate
- neurotransmitter (acetylcholine) diffuses across synaptic cleft and binds receptors on sarcolemma
- AP across sarcolemma to T-tubules
- release Ca2+ all over muscle
role of calcium in muscle contraction
- calcium binds troponin
- changes configuration of troponin-tropomyosin complex
- tropomyosin no longer blocks actin active site
- myosin can bind
what is EMG actually measuring
AP down the sarcolemma
EMG actual name
electromyography
rheobase
- measure of membrane excitability
- minimum current amplitude required that results in the depolarization threshold of cell membranes, such as APs or muscle contraction
*small MU = small rheobase
easiest to recruit
how do we know what type of motor unit we are seeing when we first start a ramp voluntary contraction
- why
- recruitment from smallest to largest
- easiest to recruit (smallest rheobase)
- fine motor control
3 types of motor units
- general characteristics
fast twitch
- largest fibres and more fibres
- high force production
- quick to fatigue
- more cross bridges = more force
intermediate
slow twitch
- small fibre size and fewer fibres
Henneman’s Size Principle
- the orderly recruitment of MUs is due to variations in “motor neuron size”
- smaller MUs with smaller motor neurons are recruited first
- pertains to ALL MUs in a motor neuron pool
- due to morphology issues (rheobase)
- derecruitment: sequential inactivation
two ways to control muscle force
- recruitment
- more motor units - rate coding
- make active MUs fire more often
- fused tetanus
*spatial and temporal summation
time from excitation to contraction in humans
0.2 seconds
EMG can be used to estimate ….
force production of specific muscles
benefits of small vs large motor units
small
- fine motor control
large
- greater force production
purpose and placement of a ground electrode when measuring EMG
make sure measuring only biological tissue
- place on a neutral location (ex. boney elbow)
why use two electrodes when measuring EMG
signal down muscle very small
- electrodes amplify signal
- could also amplify external noise
- external noise viewed “same time” in both electrodes
- conductance in muscle “different time” in 2 electrodes
- therefore amplify the difference only
where to place electrodes in EMG collection
midline of muscle belly
- location where the EMG signal with the greatest amplitude can be detected
crosstalk
EMG pics up the AP of other muscles not of interest
where NOT to place EMG electrodes (4)
neuromuscular junction
- too much EMG activity
side of muscle
- fewer fibres
- crosstalk
musculotendonous junction
- limited electrical activity
subcutaneous tissue
- acts as a filter
- smooths EMG signal, removes high freq component
what to know when placing EMG electrodes (5)
place close together
- certain bandwidth of frequency –> standardized across experiments
- limits crosstalk but doesnt pick up all motor units
clean skin
- remove impedance
minimize movement of electrode leads
- reduce movement artifact (non muscle activty)
use ground and a differential amplifier
- amplify difference, subtract some environmental noise
keep EMG leads away from main power supply
- reduce environmental noise
what contributes to EMG / force relationship?
- muscle characteristics
- size, fibre type, contribution of synergistic muscles - motor units recorded by EMG
- only measuring a fraction of the muscle - % of max force output
- rate coding near max still increases EMG with limited effect on force production - agonist vs antagonist muscle
- synergist increase force without increasing EMG - length of muscle
- more cross bridges formed increases force production - velocity of movement
- increase force with fast twitch fibres and slower speed
graphical EMG / force relationship
linear at lower force / exponential at higher force
- largest fibres contribute last, higher voltage needed
- increases in rate coding with contribute to EMG while most MU force is saturated
- slow twitch force production maxed out (fused tetnus) but still fire at higher frequency to summation of fast twitch muscle fibers
- only occurs in muscles with different fiber types
ex. biceps –> 80% linear
2 ways to increase force production
rate coding (wave summation) motor unit recruitment
purpose of rectifying the EMG data
all positive values
- removes negative values
- otherwise average would be zero
purpose of filtering data
filters out high frequencies
- smooths out the curve
types of data filtering
low pass
- every 4/10 points gives 1 point
- whole signal moves forward (time lag)
- good representation of the delayed onset of force to electrical stimulus
dual pass filter
- filters in both directions
- removes time lag
- used when representing EMG (not force)
amplitude normalization
- why
presenting the values to a standardized value (%)
why
- amplitude of signal will depend on electrode placement
- signal will differ across subjects and days
how to normalize data
maximum voluntary contraction (MVC)
- relative to a phase movement
- amplitude of EMG / amplitude of MVC
*must rectify and filter data first **
zeroing
- what to do in lab to ensure this can be done
subtract the resting baseline value from the entire data set
- move down the curve
- amplitude calculated represents the change in amplitude
*collect 1-2sec of background data
amplitude calculation
first rectify, filter and zero the data
average the data over peak 100ms
hint - sampling at 1000Hz, which is 1000samples per seconds, or one sample per ms
calculating power in lab
3 curls per weight
- record time (1s up, 1s down)
- record distance
- calculate with formula
