Excitation vs. Activation

[Luke Jessup]

Hi all,

I'm curious as to whether you would expect to see the excitation and activation signals be as instantaneous as seen below/attached?
The main reason I ask is that we're seeing significant delays between the solutions from moco and experimental EMG (see also below/attached). This would be partly explained if there were a more significant delay between excitation and activation signals - excitation is more or less indicative of EMG, correct?
Could the lack of delay between excitation and activation be a result of appending the optimisation-friendly DeGrooteFregly muscles?

exc vs act.PNG (196.22 KiB) Viewed 248 times

act vs emg.PNG (88.35 KiB) Viewed 248 times

P.s. This is hopping data, and these findings are consistent between hop heights and frequencies.

Thank you!
Luke

[Ross Miller]

Hi Luke,

I think this looks pretty normal if you're using the default activation time constants for DGF (0.015 s for activation, 0.060 s for deactivation). They are on the faster side especially for "slower" muscles like soleus. They can be modified on a muscle-by-muscle basis (if you aren't doing that already). The XML tags are :

<activation_time_constant>0.015</activation_time_constant>
<deactivation_time_constant>0.060</deactivation_time_constant>

Winters & Stark (1985) I think uses the same activation dynamics as DGF and describes a procedure for specifying the time constants as functions of muscle mass and fiber type:

tau_act = 0.005 + 0.2*m*(1-FT)^2
tau_deact = 0.030 + 0.5*m*(1-FT)^2

m = muscle mass in kg
FT = fraction of fast-twitch fibers (from 0-1)
tau's will have units of seconds

https://doi.org/10.1109/TBME.1985.325498

Umberger et al. (2003) has a simpler approach (time constants are linear functions of FT only) but I think the activation dynamics formula in his muscle model is different from DGF; unsure if that is important here.

https://doi.org/10.1080/1025584031000091678
https://doi.org/10.1080/10255840612331393266

Hope this helps,
Ross

[Luke Jessup]

Much appreciated. Thank you, Ross!