Trunk flexor-extensor asymmetry has been implicated in the development of back pain; however, left-right trunk muscle asymmetry has received little attention. This study examined whether such left-right asymmetries exist and if these are related to differing sporting tasks.

Thirty-five subjects were recruited and written informed consent obtained; 12 subjects participated in unilateral (UL) sports e.g. racquet sports (mean age 21.6±0.7 (SEM) years), 13 in bilateral (BL) activities e.g. rugby (mean age 21.7±0.2) and 10 controls (C) not involved in sport (mean age 21.7±0.2) years). Isokinetic and isometric trunk flexions and extensions including a fatiguing isometric hold were performed in a Cybex isokinetic dynamometer synchronised with bilateral electromyographic (EMG) recordings from trunk extensors (erector spinae at L4), and flexors (rectus abdominis at T10). A ratio of left:right EMG activity was calculated for each set of muscles, to examine asymmetry.

No differences were seen in left:right extensor EMG ratios across any of the test protocols. However, the UL group had higher (P< 0.05) left:right flexor EMG ratios than the BL group during pre-fatigue (UL:1.32±0.15 vs. BL:0.84±0.07) and post-fatigue (UL:1.30±0.18 vs BL:0.84±0.07) isometric flexion. Torque data suggested that the trunk extensor-flexor ratio was larger (P< 0.05) in the BL group compared to the C in the isokinetic exercises at the 30°s⁻¹ (BL:1.27±0.05; C:1.00±0.06) and at the 90°s⁻¹ speeds (BL:1.28±0.05; C:0.95±0.08), but no differences were seen during isometric testing.

This study suggests that training for different sports can generate significant asymmetry in the trunk muscles, particularly in the flexors, the importance of which requires further research.