Working with point constraints

[Peter Bishop]

Hello,
I am investigating the use of Point Constraints in my model to constrain the motion of one segment to another. My model no longer would have a typical 'open' tree structure, but rather would have a closed part to it (like a four-bar linkage).

So for example, instead of the simple kinematic chain of humerus > ulna > radius > carpals, something like humerus > ulna > carpals + humerus > radius > carpals. The ulna-carpal joint is defined in the normal sense, but the radius-carpal 'joint' would involve a point constraint, such that a point on the distal radius is constrained to follow a point on the carpal structure, however the rest of the model moves.

My question is this: aside from being a very handy tool for tackling kinematics, can we get at other biomechanical aspects of a point constrained 'joint'? Is there a way of determining a muscle's moment arm about that 'joint', or can we determine the moments or contact/reaction forces at that 'joint' during a simulation? From what I can gather, I suspect the answer is "no" (at least in a straightforward way), but I thought I'd ask.

Thanks in advance for any enlightenment.

Yours sincerely,
Peter Bishop.

[jimmy d]

can we get at other biomechanical aspects of a point constrained 'joint'? Is there a way of determining a muscle's moment arm about that 'joint', or can we determine the moments or contact/reaction forces at that 'joint' during a simulation? From what I can gather, I suspect the answer is "no" (at least in a straightforward way), but I thought I'd ask.

You are correct, the answer would be no. You would need to define a joint to be able to compute a moment arm.

You can get reaction forces from the constraint, so that may be useful