In MATLAB, you can extract a submatrix from a larger matrix using logical indexing. Here's an example:

Let's say you have a matrix A: 

A = [1 2 3;     4 5 6;     7 8 9];

You can create a logical index by specifying a condition that returns true or false for each element in the matrix. For example, let's create a logical index for all elements in A that are greater than or equal to 5: 

logical_index = A >= 5;

The logical_index variable will be a logical matrix of the same size as A, with true values where the condition is met, and false values where it is not. 

logical_index = [false false false;
                  false  true  true;
                   true  true  true];

To extract the submatrix that satisfies the condition, you can simply use the logical index as an index for A: 

submatrix = A(logical_index);

The submatrix variable will be a column vector containing all the elements of A that satisfy the condition: 

submatrix = [5;             6;             7;             8;             9];

Alternatively, you can use the logical index to extract the submatrix directly: 

submatrix = A(A >= 5);

This will give you the same result as before.