Calculate 3D angles?

I have a 40x12 matrix in MatLab. The first three columns represent the (x,y,z) coordinates of the hip, columns 4-6 represent the (x,y,z) coordinates of the knee, and columns 7-9 represent the (x,y,z) coordinates of the ankle. I want to calculate the angle formed between the hip-knee and knee-angle segment.

I know that I can simply use Pythagorean theorem for x,y,z to get the lengths and then use acos to get the angle, but I'd like to do it in a more advanced way with vectors!!!

1) How can I turn certain rows and columns of data from my big matrix into a vector?

2) How can I use those vectors (hip-knee and knee-ankle) to calculate a 3D angle?

Thank you so much!

Answer 1

[KSSV]

See this link on how to calculate a 3D angle:

https://in.mathworks.com/matlabcentral/answers/101590-how-can-i-determine-the-angle-between-two-vectors-in-matlab

[mlsnyder08]

Thank you for the link. I saw that question before but wanted to know how to selectively choose certain rows and columns from my big matrix with which to populate the vector. At that point, I will be able to use the answer from the link you provided.

Answer 2

[Ryan Plucinski]

1). you can do this in numerous ways, depending on how you want you vector to be.

ex: this code takes the first 3 hip coordinates and seperates them to a different vector, HipX, HipY, and HipZ respectively

%data=excel file of 40x12
data=xlsread('excelfile.xls');
HipX=excelfile(:,1);
HipY=excelfile(:,2);
HipZ=excelfile(:,3);