variance (assuming a mean of zero) for each variable in the segmented time series
    (equals abs_energy divided by (seg_size - 1))

def emg_var(X):
    ''' variance (assuming a mean of zero) for each variable in the segmented time series
    (equals abs_energy divided by (seg_size - 1)) '''
    segment_width = X.shape[1]
    return np.sum(X * X, axis=1) / (segment_width - 1)