emg Flashcards
2 advantages of surface electrodes,
Convenient and more affordable, a little discomfort for the participant,
Preparation can be time consuming as the skin has to be prepared, only suitable for large muscle groups and not for deep muscles as cross talk from other muscles can alter data. Emg picks up noise.
2 advantages of indwelling
Reads deep muscle activity,
Suitable for smaller muscle groups, reduces cross talk
2 disadvantages of indwelling
Very invasive method, can risk damaging muscle which ultimately can change and affect how a participant moves and the muscle recruitment
why is optimal electrode placment impornant.
Optimal placements, help increase signal magnitude which increases the distance between peaks and troughs making data more clear from a bigger range. Correct placement reduces the signal-to-noise ration. We can reduce cross talk by placing electrodes on the correct area of muscle in interest.
Describe how EMG data can be normalised
Removal of DC Bias, find the mean value and subtract it from each data set to shift data to oscillate around 0. We then rectify data through full wave rectification, converting negative amplitudes into positive.
We then smooth the data, this is removing the non-reproducible parts of the signal, effectively the steep amplitude spikes are cut away, this gives a moving average.
Identify four factors that can increase the magnitude of the EMG signal we collect (they can be either intrinsic [participant related] or extrinsic factors [operator/equipment related]). (4 marks)
Electrode placement/location (exteinsic) & electrode orientation/configuration, ensuring correct positioning and orientation to increase magnitude, by decreasing distance between electrode and muscle fibre.
Adipose tissue can affect the magnitude signal, less adipose tissue increase signal.
Using bandpass filters(combination of high and low pass filters), which allows only signals within a set Hz range to pass through, removing noise, by filtering the Hz range noise is expected to be at. (extrinsic)
Fatigue can affect the magnitude signal, (intrinsic) the more fatigue the weaker the signal.
EMG data are typically used to determine when muscles are active and how active they are. Explain why EMG activity is not an appropriate measure of muscle force. (2 marks)
The force producing capabilities of a muscle depend on numerous other factors in particular the force-length and force-velocity relationship.
Describe one way in which EMG data can be normalised. (1 mark)
EMG can be normalised to a reference value like Maximal VOluntary COntraction (MVC) allowing for data comparison.
Divide EMG data by MVC and multiply by 100 to get percentage of EMG relative to the max.
What physiological activity within the muscle affects:
The magnitude of the EMG signal?
Number and synchronisation of active muscle units
Describe the optimal placement for the reference electrode and how this improves the EMG signal
Placement away from muscle belly, over flat bony surface.
Reference records background signal (noise) which is removed from EMG signal
Describe how you could determine the onset of EMG activity. (3 marks)
We could set a threshold value, this could be an absolute value, e.g 0.05 mv, or 0.10mV , and anything above this we accept as activity, we can also further filter this by saying only if reading is above for a certain amount of time is it active.
Or we can use a relative value, we can find the mean zero activity (point where muscle is relaxed), and then adding a SD value and using it as a threshold value, and anything above +1SD for example is accepted as activity.
Similarly, we can also use the peak value and say when a reading is greater than a % of this, it counts as active e.g above 2%.
What is the minimum sampling frequnecy and what can occur if not?
sampling frequency should be a minimum of 1000Hz, aliasing effects occur if the sampling frequency is to slow, and skes data and we miss peaks and troughs.
