I've been looking into the smaller muscles in the spine such as the intertransversarii, interspinales, and rotatores. To my understanding, they are capable of spine extension, rotation and lateral flexion. However, I have not been able to find any forward flexion muscles in a similar class.
The cervical region has some flexion capability thanks to muscles such as longus colli, which reaches across 5-6 vertebrae, which is larger than I am looking for. But even muscles like that appear to be absent in the lumbar region. The only muscle I could find for lumbar flexion was psoas, which is quite a large muscle indeed!
Are there any small muscles which support flexion in the lumbar on the size scale of 2 or 3 vertebrae, like the extension, rotation, and lateral flexion muscles above?