Normal postural responses preceding shoulder flexion: co-activation or asymmetric activation of transverse abdominis?

It is suggested that activation of the transverse abdominis muscle has a stabilizing effect on the lumbar spine by raising intra-abdominal pressure without added disc compression. However, its feedforward activity has remained a controversial issue. In addition, research regarding bilateral activation of trunk muscles during a unilateral arm movement is limited. The aim of this study was to evaluate bilateral anticipatory activity of trunk muscles during unilateral arm flexion.

Eighteen healthy subjects (aged 25 ± 3.96 years) participated in this study and performed 10 trials of rapid arm flexion in response to a visual stimulus. The electromyographic activity of the right anterior deltoid (AD) and bilateral trunk muscles including the transverse abdominis/internal oblique (TA/IO), superficial lumbar multifidus (SLM) and lumbar erector spine (LES) was recorded. The onset latency and anticipatory activity of the recorded trunk muscles were calculated.

The first muscle activated in anticipation of the right arm flexion was the left TA/IO. The right TA/IO activated significantly later than all other trunk muscles (P < 0.0005). In addition, anticipatory activity of the right TA/IO was significantly lower than all other trunk muscles (P < 0.0005). There was no significant difference in either onset latency or anticipatory activity among other trunk muscles (P > 0.05).

Healthy subjects showed no bilateral anticipatory co-activation of TA/IO in unilateral arm elevation. Further investigations are required to delineate normal muscle activation pattern in healthy subjects prior to prescribing bilateral activation training of transverse abdominis for subjects with chronic low back pain.