There are 2 refs, although only abstracts:
Sapsford R R, Hodges P W, Richardson C A 1997 Activation of the abdominal muscles is a normal response to contraction of the pelvic floor muscles. International Continence Society Conference, Japan, abstract.
Subjects pewrformed maximal contractions of the pelvic floor while EMG of abdominal muscels was measured with fine wire electrodes.When subjects eprfromed the pelvic floor contractions, activation of TrA increased significantly.Some subjects had a selective increase in TrA only, others also had an increase in activation of other abdo muscles.

Sapsford R R, Hodges P W, Richardson C A, Cooper D A, Jull G A, MArkwell S J 1997 Activation of pubococcygeus during a variety of isometric abdominal exercises. Also in the International Continence Society Conference, Japan, abstract.
Fine wire electrodes were placed into pubococcygues through the vaginal wall in a second study while abdominal contractions were performed. Activating abdominal muscles resulted in an increased activation of pubococcygeus.