Trying to understand your questions: are you trying to compute the mean of columns 1, 21, 41,..., 421 for each row, then repeat this exercise for columns 2, 22, ... , 422, and keep repeating until you have a 815 x 22 matrix of such means? This would actually give an 815 x 20 matrix...
Are you instead trying to take the average of columns 1,2,...,20 for each row, and repeat for columns 21,22,...,40, etc until you get to the mean of columns 421,422,...,440? This would give an 815 x 22 output as you mentioned.
