Electromyographic Recordings
EMG activity of transversus abdominus (TrA) (lower
portion), obliquus internus (OI), and externus (OE) were
recorded using bipolar intramuscular electrodes fabricated
from Teflon-coated stainless steel wire 75 m in diameter
(AM Systems, Carlsborg, WA). One millimeter of insulation
was removed from the cut ends of the wire, and the tips
were bent back approximately 1–2 mm from the end to form
a hook. The electrodes were threaded into a hypodermic
needle (0.70 38 mm) and sterilized.
The electrodes were inserted into TrA, OI, and OE under
the guidance of real-time ultrasound imaging (7.5-MHz
curved linear array transducer, Dornier Performa, Acoustic
Imaging Technologies Corp., Phoenix, AZ) (8). Electrodes
were inserted on the symptomatic side (or most severe side
if bilateral) and on the right side for control subjects. The
fine-wire electrodes for TrA and OI were inserted below the
level of the anterior superior iliac spines to ensure that the
lower fibers of the muscles were recorded.
EMG activity was recorded using pairs of surface Ag/
AgCl electrodes (Graphics Control Corporation, c/o Medical
Equipment Services PTY.LTD, Richmond, Australia)
placed over the muscle bellies of rectus femoris (RF)