Objective: To investigate the reflex control of the internal oblique (IO) muscles

Design: Reflex activity in the IO muscles was evoked by (i) tapping each IO muscle in turn and (ii) tapping the abdomen in the midline to produce a stretch of equal magnitude in both muscles. Muscle activity was recorded using surface EMG.

Subjects: Seventeen asymptomatic subjects and one subject with scoliosis.

Outcome measures: Onset latency and normalised amplitude of reflexes

Results: Tapping the IO muscle on one side evoked large reflex responses in both ipsilateral and contralateral IO muscles. Across all subjects the reflexes in these two muscles were of equal amplitude (p=0.12). Measurements of onset latency suggest that both reflexes are monosynaptic in origin. Tapping in the midline also produced large amplitude responses. In two subjects, the response in one IO muscle was always larger than the other, irrespective of which of the three points was tapped, and this was repeatable. One of these subjects has a scoliosis.

Conclusions: The IO muscle is usually controlled from ipsilateral and contralateral muscle afferents. In a scoliotic subject this afferent activity was biased to the IO muscle on one side, and may therefore be associated with this condition