week 8 Flashcards
what is EMG a representation of
the sum of all motor unit action potentials within the detecting range of electrodes
does EMG tell us muscle force
no it tells us about muscle effort but if we know the effort and the length we can estimate force
how is force increased in 2 ways
recruit more motor units
rate coding
how does motor unit recruitment increase force
size principle (small to large) and correlates with the fatigue resistance of the MUs
more metabolically efficient this way
how does rate coding increase force
how often a motor unit will fire eventually resulting in force teatnus
how does recruitment relate to the EMG signal
as more MUs fire or are active there is a greater chance multiple MUAPs will summate causing a greater signal
how does rate coding relate to EMG signal
as more motor units fire often a greater chance of MUAPs summating
what are the 6 EMG analysis techniques
- raw amplitude
- rectification
- linear envelope
- integrated EMG
- root mean square
- normalization
what is the EMG linear envelope (two steps)
- full-wave rectification (only positive values)
- low pass filter (2-10Hz)
4 parts to recording EMG signal
- via electrodes
- monopolar or bipolar arrangement
- placement
- skin resistance
what factors affecting surface EMG signal can we control
- electrode-skin interface
- signal conditioning
- inter-electrode spacing
- orientation of electrodes
4 basic features of signal analysis (signal properties)
frequency = how many waves over a time
amplitude = how big it is
vertical offset = bias removal
phase shift = horizontal starting at 0 or max
what is a time domain
amplitude of a signal at each point in time
what is a frequency domain
amplitude of signal at each frequency
how do we go from time domain to frequency domain
use Fourier transform
what does a fourier analysis allow us to do
breakdown a signal into sine waves and the sine waves tell us frequency, amplitude, and phase
frequency analysis (0-0.1)
bias
frequency analysis (0-1)
drift (bias will change overtime slowly)
frequency analysis (0-1000Hz)
white noise (contains all possible frequencies)
frequency analysis (0-10Hz)
human movement (tremor)
frequency analysis (10-500Hz)
surface EMG sin waves buried in the frequency that can be super rapid oscillations
frequency analysis (10-1000Hz)
indwelling EMG (needle in muscle)
frequency analysis (10-60Hz)
ECG
when converting an analog signal to digital how often do you need to take a measure?
must sample at least 2x + 1 the highest frequency present in the signal
ex. highest approximate frequency is 500Hz so 1001
what does power =
amplitude ^2
why aren’t filters perfect
remove some of what we want, and keep some of what we don’t want since we cant get rid of all the noise and we just want to reduce it. to preserve the signal we are interested in
what does a LPF of 2-10Hz do in the linear envelope
causes a delay (shifting the signal forward in time) to mimic the real electromechanical delay between AP and force
what is EMG amplitude influenced by
demand, muscle length, muscle velocity, rate coding, recruitment, joint position
slow twitch vs fast twitch in terms of EMG amplitude
in general, FT have larger diameter thus faster conduction velocity when something moves fast = fast frequency contents = increased Conduction velocity
depth of fibre with respect to electrode and EMG amplitude
larger MUs are more superficial within the muscle. causing an influence on the electrical activity recorded since they are closer to where they are being recorded. so they appear they are moving faster
what is fatigue
decreased ability of a muscle to produce force. but is recoverable
what does EMG look like to maintain MAX force
force will decrease over time and so will EMG amplitude
what does the effort to maintain subMAX force look like in terms of EMG
EMG amplitude will increase as recruitment of more MUs to maintain force and larger superficial MUs will begin to be recruited. also can get synchronous firing of MUs to yield a larger amplitude EMG signal
why/how does MUAP conduction velocity decrease with fatigue
due to alterations in metabolite concertation (pH) within muscle. Electrodes see MUAPs for longer giving the appearance the shape change and lower frequency
conduction velocity of the electrodes decreased as it moves sower in acidic environments and frequency content will be lower due to lower conduction velocity
how does MUAP duration increase and amplitude decrease
due to changes in Na+ and k+ gradients causing MUAP shape to change
what is mean power frequency
weighted average (higher frequencies are weighted heavier)
what is median power frequency
frequency that divides power spectrum in half
