1. What is the best method to set the onset of muscle contraction during isometric strength testing? Is a set value of so many Newtons (e.g. 15 N) preferable?
2. When measuring peak force during an isometric muscle contraction (e.g. if data are obtained at 1000 Hz), what do we define as "peak force"? Is it the highest instantaneous value? If not, what is the best interval for averaging force?
1. Onset of muscle contraction. Measure the average force (noise) of the baseline and when the line stays 2 Standard deviations above the noise you can be assured that the contraction has started. This is more typically used with EMG signals but should apply to force as well.
2. Peak force is defined by some as the instantaneous peak force and others by a predetermined duration (i.e. 500 ms, 1 s) and the average over that duration. As long as it is well defined I do not think one analysis is better than the other unless you have slack in your system and the instantaneous peak force is significantly higher due to movement artifact (snapping effect of the cable for instance).
Hope that is helpful.
Dave B.
Dear professor. Very good question.
this is how we do in our lab.
1. What is the best method to set the onset of muscle contraction during isometric strength testing? Is a set value of so many Newtons (e.g. 15 N) preferable?
it depends on the exercise...during Leg Press we set at 15 N, but Bech Press, Squats maybe different...relative valeu to baseline force values may be an alternative. we are still working on it.
2. When measuring peak force during an isometric muscle contraction (e.g. if data are obtained at 1000 Hz), what do we define as "peak force"? Is it the highest instantaneous value? If not, what is the best interval for averaging force?
we use the stable part os the force signal. the last 1500 ms of the force-time curve, for example.
Perhaps this could help (see attached papers below)...
Article Consistency of rapid muscle force characteristics: Influence...
Article Identification of contraction onset during explosive contrac...
dear Gregory C. Bogdanis next site and research may help you
http://www.pt.ntu.edu.tw/hmchai/Biomechanics/BMmeasure/MuscleStrengthMeasure.htm#EMG
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2723712/
https://books.google.com.eg/books?id=5znew_FPvJcC&pg=PA266&lpg=PA266&dq=set+the+onset+of+muscle+contraction+during+isometric+strength+testing&source=bl&ots=IokP0YdMrj&sig=2l3Qhx7pxaSJyqLQxbYivd8Z4P8&hl=ar&sa=X&ved=0ahUKEwjd84btw97KAhVJXBQKHdK2AHkQ6AEIPjAD#v=onepage&q=set%20the%20onset%20of%20muscle%20contraction%20during%20isometric%20strength%20testing&f=false
http://jap.physiology.org/content/84/4/1341.abstract
http://jap.physiology.org/content/93/4/1318.short
http://link.springer.com/article/10.1007/BF00422902#page-1
http://e.guigon.free.fr/rsc/article/NewellCarlton85.pdf
http://www.ncbi.nlm.nih.gov/pubmed/16937959
Article Consistency of rapid muscle force characteristics: Influence...
Here is another approach to determine contraction onset. We define it as a point as the point in time where the first derivative of the force–time curve exceeds the baseline by 3% of its maximum value.
I agree with Olivera. We test muscle forces within very different ranges of magnitudes, so relative thresholds should work better. However, make sure that the relative threshold (e.g., 3% MVC, as Olivera suggested) is above the 'noise' of the force signal you have prior to the contraction initiation.
Good question Dr Bogdan,
I am adept at using mathematical values, because then we can accurately check the onset of contraction.
The suggestion of Dr. Knezevic I like very much. The sampling rate of 100Hz, we will have many points to make that assessment. You can use a routine within the MATLAB environment. On the noise in preload can smooth the data with a simple routine.
Good luck
I have had good success with the following procedures. Taking the fist derivative of the force trace and setting some percentage of the peak dF/dt (say 1-5%) is a sensitive marker for the onset of force. The onset of EMG is then some level of activity above baseline that is maintained for a certain period of time. RMS amplitude as a measure of the baseline noise has worked well. Whether is 1 to 2+ standard deviations for 10 ms all depends on your noise levels and how you filtered the signals. The search algorithm should stop before the derivative determined force onset. As for peak force, it is generally taken over an interval between 500 ms and 1s, with the window terminating before the middle of the contraction. Again, these are all just general guidelines that depend not only on the application, noise levels, but how you process the signals. There is very good advice in the posts above.
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