Calculating the muscle length difference



I am working on a arm model. (Free posture moving model).
I am trying to calculate the muscle length difference over time for a specific movement.
In this case, i am using wrist flexion movement. The following is the action that i used on the flexion.
AnyVector WristFlexion ={ 0.0,20.40,60,80,90,70,50};
And i am using the variable LmtDot variable in the chart.
At certain instances, i am getting a change in the muscle length in some muscle that only focuses on the finger flexion. I had a change in the length in muscles like extensor digiti minimi, flexor digitorum superficialis, flexor digitorum profundus, extensor pollicis brevis. Some of these muscles are active only for finger flexion. I am not sure why they change their length or in other words, become active for the wrist flexion.
My question is that when we perform wrist flexion in anybody, does it automatically perform finger to flex as well. I don’t think the finger flexes because i am not able to see that in the simulation.
If not, then why does the muscle that are supposed to be active for finger flexion become active during wrist flexion?

Please let me know.

The reason i am calculating the the muscle length difference is that I am using the muscle length difference to determine which muscle is active for a specific movement.

Thank you in advance.


Hi Raman,

The LmtDot property of a muscle will give you the velocity of the muscle line, nothing else. It can not be used to determine if a muscle is active or not. The muscle may be active regardless of it speed and position.

If the analysis shows a speed change it is just a result of the geometry, it do not tell anything on muscle activation.

Best regards


Hi Soron,

I am sorry for not being clear.
I am actually trying to find out the contraction of the muscle for a particular action.
The muscle basically performs actions by contracting and thereby moving the joints/bones.
From the below link, i found out that Lmt is the total length of the muscle-tendon unit and LmtDot is the rate of change of the length.
In other words, i am guessing that the LmtDot gives us information about how much the muscle has contracted over time.
If i am wrong about any of the things that i have mentioned above, please let me know.
Coming back to my previous question, my doubt is why does the muscle that are supposed to contract only for finger flexion is also contracting for the wrist flexion?
Please let me know.

Thank you.


Hi Raman,

LmtDot is the speed of entire length of the muscle unit it is a purely geometrical quantity, all muscle models has this quantity.

Lmdot on the other hand is the speed of the contractile element (only if the muscle is a 3 element hill type)

Hope it made it more clear.


Best regards