Is it correct to apply the rule from Rossi et al. (2009) that 100% rMT=120% aMT?
At the beginning I used TMS intensity at 120%RMT when studying Cortical Silent Period and I called it AMT. For cortical SP u should activate muscle and the main thing is to be conseqvent and respect your own rules before and after the intervention protocol, in your case theta burst TMS.
My intervention was eccentric exercise in ankle flexor and extensor muscles, so I have checked RMT before exercise in TA and I used 120% of it when calculated Cortical Silent Period, applied to motor cortex with dorsiflexion of the muscle.
Now the problem is: at what amount of muscle contraction intensity..? 25%, 50%, 75%...And, infact, this is considered AMT (at what intensity of cortical TMS) correspond these forces. Exactly measured in muscle MEP I do not know (for RMT is 3 of 6 trials positives with amplitude of at least 50uV).
Normally people use three levels: low, medium and high force. But you can measure it with a dynamic myometer and be constant before and after intervention.
At the end of your intervention you should calculate again the RMT! You will have another...This alone constitute a measure of cortical excitability! But if you calculate again cortical Silent period you should apply 120% from the new RMT and repeated at the same force intensity as at before intervention.
I am not sure what kind of measure do you want to use for cortical excitability or you want to measure intracortical dynamic (SICI and ICF)?? I-wave.. These are also parameters which show cortical changes (I-wave for subcortical).
I will look to other colleagues answer,
Best whishes, Ovidiu C Banea
From Barcelone
We have a large dat series (approx 250 subjects), where we have looked at this. The AMT/RMT relationship was AMT= 83% RMT, but 100% RMT varies from between 75-85% AMT @ 20% MVC between individuals. For each individual therefore, the population average of 80% does not hold, and the outcome of iTBS can be affected substantially if stimulation intensity is not appropriate. see http://dx.doi.org/10.1016/j.clinph.2014.10.003
Thank you very much for your answers. I understand that possible variations of differences between RMT and AMT is a factor which does not allow to simply convert one value from the other. My major concern is, however, that when the muscle is contracted it is easy to take a wrong peak as a response even if the subject tries to maintain the same level of force (which is usually very difficult in case of neurological patients). This is a potential source of significant errors in MT evaluation leading to wrong stimulation intensity in iTBS. Is there any strict procedure to follow that may help with that?
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